Ebook Principles of deformity correction: Part 2

(BQ) Part 2 book “Principles of deformity correction” has contents: Six-Axis deformity analysis and correction, consequences of malalignment, malalignment due to ligamentous laxity of the knee, ankle and foot considerations, sagittal plane knee considerations,… and other contents.

CHAPTER 12 1111 Six-Axis Deformity Analysis and Correction

In previous chapters, we defined deformity components

and divided them into angulation, rotation, translation,

and length Angulation and rotation are angular

defor-mities, measured in degrees Translation and length are

displacement deformities, measured in distance units

(e.g., millimeters, inches, etc.) In Chap 9, we discussed

how angulation (axis in the transverse [x-y] plane) and

rotation (axial [z] axis) deformities can be resolved

three-dimensionally and characterized by a single

vec-tor (ACA) inclined out of the transverse plane

(char-acterized by x,y,z coordinates) Similarly, translation

(displacement in the transverse plane) and length

(placement axially) can be combined into a single

dis-placement vector inclined out of the transverse plane

(characterized by x,y,z coordinates)

Deformity between two bone segments can be fully

characterized by three projected angles (rotations) and

three projected displacements (translations) Therefore,

six deformity parameters are required to define a single

bone deformity Mathematically, it is necessary to assign

positive and negative values to each rotation and each

translation, depending on the direction of rotation of

each angle and the direction of displacement of each

translation The signs (+I-) of these angles and

transla-tions are determined by the mathematical convention of

coordinate axes and the right-hand rule

The unique position of an object (bone segment) can

be determined by locating three non-collinear points on

that object One segment can be moved with respect to

another by translating along three orthogonal axes and

rotating about these same three axes The final position

after three orthogonal translations is independent of the

order undertaken The final position after three

orthog-onal rotations is dependent on the order or sequence

undertaken (~ Fig 12-1) Stated more formally, rotation

is not commutative

Deformity analysis, as discussed in previous

chap-ters, is conducted using AP and LAT view radiographs of

the bone deformity Considering that a radiograph is an

X-ray projection of objects onto a plane (section), the

mathematical field concerning projection and section

(the plane of observation) is called projective geometry

(Klirre 1955) Projective geometry is the mathematical

basis of interpreting radiographs of bone deformities

Yaw - Pitch - Roll

(first column) Each of the blocks is shown as undergoing a 90°

rotation in yaw (Y), a 90° rotation in pitch (P), and a 90° tion in roll (R), each in a different order Note the very different final orientations depending on the order in which the rota- tions were undertaken Rotation is not commutative

rota-Gerard Desargues, a self-educated engineer, lished the first known text on projective geometry in

pub-1639 Blaise Pascal, a French mathematician and losopher, added his theorem and published a text on conic sections and projective geometry in 1640 All printed copies of these works were lost Fortunately, a student of Desargues, Philippe de la Hire, made a man-uscript copy of Desargues's book Nearly 200 years later, this copy was found serendipitously in a bookshop by the geometer Michel Chasles (1793-1880) Along with other 19th century geometers, Chasles rediscovered and further developed projective geometry

phi-Chasles was the first to realize that the complex sitioning of an object in six axes (three translationsplus three rotations) could be duplicated by rotation of a threaded nut along a threaded shaft, the revolute The path of the nut in space is a curvilinear axis of correc-

repo-IIEJI CHAPTER 12 · Six-AxisDeformi!J AnalysisandCorrection

d

tion of all the rotations (angulation and rotation

defor-mities) and all the translations (translation and length

deformities) The central axis of this revolute in space is

the same as the vector that resolves the three rotations in

space When rotation occurs in each reference axis, the

revolute will be inclined to all three reference axes The

offset from the central axis (radius) of the revolute is

dependent on two translations, and the pitch of the

thread is dependent on the third translation

The Chasles axis can be developed as a vector, with

direction and magnitude The three contributions to the

vector are based on three angles (rotations): two from

radiographs (AP and LAT views of angulation) and the

third from clinical examination (axial rotation) or from

CT analysis of rotation deformity

By treating the rotation or Chasles axis as a vector

quantity, one is able to exactly locate this axis in any of

eight octants By invoking the right-hand rule, one can

readily determine the direction of rotation about this

axis to recreate the deformity In addition to recreating

observed angulation and rotation with a single oblique

axis, Chasles showed that this same axis, if displaced

from the center of the fragment, can also provide

trans-lation in two planes If the fragment is allowed to

progress along the shaft as it rotates (like a nut on a

threaded shaft), the third translation can be addressed

The exact positioning of this shaft is beyond the scope of

this book, but a few conceptual examples are provided

( Fig 12-2)

Fig 12-2 a-f Characterizing anatomic terms in their mathematical equiva- lents Ieads to improved understanding Choose the point of interest, or origin, as the zero position Assuming you are work- ing on yourself, anterior is positive, right is positive, and cepha- lad is positive Positiverotation ab out each of the axes is shown The fragment is shown in reduced position (a) It is then rotated ab out an oblique displaced axis and advanced along the same axis The fragment is shown in 40° and 2-cm increments (b-f) This spiral or revolute motion can reproduce ( or correct)

Strut

t

C HA PTE R 12 · Six-Axis Deformity Analysis and Correction 1111

Standard Struts Mini 60-75 mm X-short 75-96 mm Short 90-125 mm Medium 11 6 178 mm Long 169-283 mm

Fast Fx Struts X-short 91 - 121 mm Short 116 152 mm Medium 143-205 mm Long 195- 311 mm

Anti-master tab

Fig 12·3

The Taylor spatial frame construct is always the same: six struts

connected to every other tab on a full ring The master tab is

always on the proximal ring and faces anterior Looking down

on the proximal ring,as ifto put the ring on one's leg, the

num-bered struts are attached 1 through 6 starting at the master tab

in a counterclockwise configuration It is important to

remem-ber that this assembly does not change for either side of the

c

Fig 12-3

Taylor spatial frame adjusted to perform the same function as

the adjacent Ilizarov construct:

body or for proximal or distal reference frames The ter tab is the empty distal tab between struts 1 and 2 This tab

anti-mas-is a virtual tab in a danti-mas-istal two-thirds ring construct The able components consist of rings (full, half, and two-thirds), struts (Fast Fx [Smith & Nephew) and standard), foot plates, and butt plates

IlD CHAPTER 12 • Six-Axis Deformity Analysis and Correction

a Anteroposterior plane angulation

b Lateral plane angulation

c Axial plane angulation

d Anteroposterior plane translation (proximal reference)

e Lateral plane translation (proximal reference)

I Axial plane Iranslaiion

In Memphis, Tennessee, in 1994, J Charles and Harold

S Taylor first applied the Stewart platform and the

Chasles theorem to orthopaedics They modified the

Ilizarov external fixation system by connecting six

tele-scopic struts that are free to rotate at their connection

points to the proximal and distal rings This external

fix-ator is called the Taylor spatial frame In Germany, a

sim-ilar modification to the Ilizarov device, called the

hexa-pod, was developed (Seide et al 1999) By adjusting only

the strut lengths, one ring can be repositioned with

respect to the other Using a computerprogram that

cal-culates the strut lengths relative to deformity

parame-ters, the frame can be preconstructed to mirnie any

deformity A two-ring construct can simulate a

single-level deformity, and a three-ring construct with six

struts between each pair of rings can be preconstructed

for a two-level deformity Simple and complex

deformi-ties are treated with the same frame The same frame

construct - two rings and six struts- can simulate

vari-ous Ilizarov frame constructs (~ Figs 12-3 and 12-4)

The multiple angles and translations of a particular

deformity are addressed simultaneously by adjusting

the lengths of the struts only The Taylor spatial frame

Fig.12-Sa-f Six deformity parameters needed to fully define a dinical deformity

a Anteroposterior plane angulation

b Lateral plane angulation

c Axial plane angulation

d Anteroposterior plane translation

e Lateral plane translation

f Axial plane translation

fixator is capable of correcting all aspects of a six-axis deformity simultaneously This external fixator is very streng The angled six-strut construct Ioads each strut axially without applying bending forces to the inclined struts If one Iooks at only the points of attachment of the struts to the ring, the shape is a triangle instead of a circle The entire structure, including the side triangles formed by the struts and the two end triangles, has the same shape as the crystal structure of a diamond (octa-hedron) Not surprisingly, this is a very streng construct When compared with the Ilizarov external fixator, the spatial frame was 1.1 tim es as axially stiff, was 2.0 tim es

a AP view frame offset

c LAT view frame offset

to origin

0 = Center of reference ring

as stiff in bending, and had 2.3 tim es the torsional

stiff-ness The computational accuracy of the computer

pro-gram is 1/1,000,000 inch and 1/10,000° The real

mechanical accuracy using manual adjustment of the

struts for even a full six-axis deformity correction has

been measured to within 0.7° and 2 mm

To treat a specific deformity with the spatial frame,

one must determine the frame parameters, the

defor-mity parameters, and the mounting parameters The

frame parameters consist of the proximal and distal ring

diameters along with the strut type, sizes, and lengths

The deformity parameters consist of the

radiograph-ic and clinradiograph-ical measurements of the three rotations and

three translations, defined relative to a point designated

as the origin on the reference segment and its

corre-sponding point on the correcorre-sponding segment We

pre-sent an example of the six deformity parameters in

terms of a tibial model ( , Fig 12-5): (1) coronal plane

CHAPTER 12 · Six-Axis Deformity Analysis and Correction lllJI

b Axial frame offset

d Rotary frame offset (30° external rotation) Anteroposterior Mastertab

30°

Fig 12-6a-d

Four mounting parameters determine the position of the ter of the reference ring in space with respect to the assigned origin

cen-a AP view frame offset

b Axial frame offset

c LAI view frame offset

d Rotary frame offset (30" external rotation)

angulation, varus or valgus; (2) sagittal plane tion, procurvatum or recurvatum; (3) axial plane angu-lation, internal or external rotation; (4) anteroposterior plane translation, medial or lateral; (5) lateral plane translation, anterior or posterior; (6) axial plane trans-lation, short or long Measure the deformity parameters

angula-by characterizing the fragment-to-fragment deformity This characterization is independent of the selected frame size, but the translational parameters are depen-dent on how the frame is oriented to the fragments

Ei ther the proximal or distal fragment can be designated

as the reference fragment The origin may be niently chosen as any point along the reference frag-

conve-1111 CHAPTER 12 · Six-Axis Deformi!J Analysisand Correction

ment's axis, as long as its corresponding point can be

identified or determined The CORA is a good choice for

the origin in many cases Using the CORA as the origin

is the marriage of the CORA method to the method of

simultaneaus six-axis deformity correction The

corre-sponding point lies along the axis of the moving

frag-ment and is determined by various planning methods

discussed later in the chapter

The mounting parameters define the position of the

reference ring (proximal or distal) in space with respect

to the position of the origin In other words, the

mount-ing parameters determine the position of the center of

the reference ring in space to the position of the

as-signed origin Once the mounting parameters have been

assigned, the frame orientation to the limb can be

antic-ipated However, the frame usually is applied first and

the mounting parameters subsequently determined

Four measurements defining the relationship of the

ref-erence ring to the origin determine the mounting

para-meters The four mounting parameters are as follows:

(1) anteroposterior frame offset, medial or lateral offset

to the origin; (2) lateral frame offset, anterior or

poste-rior offset of the center of the reference ring to the

ori-gin; (3) axial frame offset, proximal or distal offset of the

reference ring to the origin; and ( 4) rotational frame

off-set, the degree of rotation between the master tab

(prox-imal reference) or anti-master tab (distal reference) to

the designated anteroposterior plane (usually patella

forward) (~ Fig 12-6) The rotational offset is either

external or internal With most applications, the intent is

to place the frame in a neutral position with no

rota-tional offset However, if rotarota-tional offset is present but

not accounted for, a secondary deformity will be created

during the initial correction For example, if a varus

deformity is being corrected and the frame has been

mounted with an internal rotational offset, a secondary

recurvatum deformity will be created during the varus

deformity correction This occurs because the frame is

correcting the varus deformity not in the

anteroposte-rior plane but in an oblique plane because of the

rota-tional offset that has not been accounted for On the

other hand, a rotational offset allows the freedom to

mount a frame in a better position for soft tissue

dear-ance or patient comfort An external rotational offset of

90° in a proximal femoral two-thirds ring allows

clear-ance for the opposite thigh and perineal area This same

construct with a distal reference will result in a 60°

exter-nal rotatioexter-nal frame offset due to the position of the

dis-tal anti-master tab (~ Fig 12-7)

_ _

Fig 12-7

oo Patella torward

rotational offset

= goo

Distal reference imaginary anti-master tab

/

External rotalienal offset

= 60°

An external rotational offset of 90° in a proximal femoral thirds ring allows clearance for soft tissues The same construct with a distal reference will result in a 60° external rotational frame offset due to the position of the distal (imaginary) anti-master tab The anti-master tab is imaginary in this construct because the distal ring is a two-thirds ring

two-Modes of Correction

Currently, three program modes of correction can be accomplished with the Taylor spatial frame: chronic deformity, residual deformity, and total residual defor-mity program modes However, since the advent of the Total Residual Program, the earlier Chronic and Resid-ual Programs have been used less frequently and are becoming of academic interest only In this chapter, we focus only on the total residual deformity mode For the total residual deformity mode, the rings are applied independently of each other Ideally, to facilitate the planning, the reference ring should be applied per-pendicular to the long axis of the reference bone seg-

ment Nevertheless, the planning can compensate for

nonorthogonal mountings After the two rings are

ap-plied, the six struts are connected to the rings and the

osteotomy is performed at a chosen Ievel The deformity

is defined for the computer by six deformity parameters:

AP view angulation, LAT view angulation, axial view

angulation (rotation), AP view translation, LAT view

translation, and axial view translation (shortening or

lengthening) The three angulations (rotations) can be

measured independently of the orientation of the

refer-ence ring The three translations are dependent on the

orientation of the reference ring For orthogonal

mount-ings of the reference ring, the measurement of

transla-tion can be made perpendicular to the long axis of the

bone for AP and LAT view translations and along the

long axis of the bone for axial translations If the

refer-ence ring is nonorthogonal, the translations are

mea-sured according to a virtual grid of lines parallel and

perpendicular to the reference ring The mounting

para-meters define the relationship of a chosen point on the

reference axis ( origin) to the center of the reference ring

These mounting parameters include offset of the center

of the reference ring from the origin in the

anteroposte-rior and lateral planes, axial offset of the reference ring

from the origin, and rotational offset of the reference

ring to the anatomic or designated neutral rotation

(usually patella forward) The position of the

corre-sponding or moving ring is defined by entering the

mov-ing rmov-ing size and strut length data into the computer The

moving ring does not need to be perpendicular to the

long axis of the moving segment

During surgery, the appropriate ring size (diameter)

and type (full, two-thirds, foot, etc.) are chosen for the

proximal and distal rings Six struts that can connect the

two rings are attached between the two rings The ring

size and type and the type and length of struts chosen

represent the frame parameters

CHAPTER 12 · Six·Axis Deformity Analysis and Correction IIfl

b

Fig 12-Sa,b The mounting parameters are influenced by the orientation of the reference ring The orange dots represent the center of the

reference ring The green dots represent the corresponding

We prefer placing the reference ring as orthogonal as possible for ease of non-digital planning This might, however, be a vestige of our bias based on extensive experience with the Ilizarov device, with which orthog-onal ring placement is critical

1111 CHAPTER 12 · Six-Axis Deformity Analysis and Correction

Planning Methods

J Charles Taylor developed the origin-corresponding

point method of planning (also called the fracture

method) It permits characterization of the deformity

and mounting parameters relative to two points in

space: the origin and its corresponding point John E

Herzenberg and Dror Paley simplified this method by

relating it to the CORA, coincidentally and conveniently

renaming these methods the CORAgin and

CORA-sponding point methods Shawn C Standard added the

virtual hinge method of planning The most recent

plan-ning method developed by J Charles Taylor is termed

the line of closest approach (LOCA) The LOCA is a

method of determining the location of osteotomy that

minimizes translation during the deformity correction

The five methods of planning are as follows: ( 1)

frac-ture method, (2) CORAgin method, {3) CORAsponding

point method, {4) virtual hinge method, and (5) LOCA

With the fracture method, the surgeon chooses both the

origin and corresponding points as points on opposite

sides of the fracture These designated points should

represent congruent points of the opposing fractured

fragments With the CORAgin method, the surgeon

chooses the origin at the CORA and then finds the

Fig.12-9

Fracture method: two corresponding points ( CP) on opposite sides of the fracture ( e g., at the ends of a recognizable spike and corresponding negative of the spike) are chosen as the ori-gin and corresponding point

corresponding point With the CORAsponding point method, the surgeon chooses the corresponding point first, at a CORA, and then finds the origin With the vir-tual hinge method, both the origin and corresponding points are located at a CORA, on the convex edge of the bone

Fracture Method

The fracture method brings two points in space ( origin and corresponding point) to the same location This method can be likened to docking a mobile object to a stationary object in space The fracture method is the simplest method to learn

Two corresponding points on opposite sides of the fracture ( e.g., at the ends of a recognizable spike and cor-responding negative of the spike) are chosen as the ori-

CHAPTER 12 · Six-Axis Deformity Analysis and Correction ß

AP view frame offsei LAT view frame offsei Axial frame offset

posterior

to origin

Frame proximal

The mounting parameters are calculated by determining the

position of the origin with respect to the center of the reference

ring It is important to note that the mounting parameters are

always measured as perpendicular distances from the

refer-ence ring

= 30° rotation

gin and corresponding point (JII> Fig 12-9) The origin is defined as the point on the reference fragment, and the corresponding point is defined as the point on the mov-ing segment The deformity parameters are determined

by calculating the angulation in the coronal and sagittal planes (from the midaxillary lines of the fragments), by measuring the displacement or translation between the origin and corresponding points (in the anteroposterior, lateral, and axial planes), and by estimating the rotation deformity based on clinical examination The mounting parameters are calculated by determining the position

of the origin with respect to the center of the reference ring (JII> Fig 12-10) Once these parameters are deter-mined, the strut settings areentered into the Total Resid-ual Program and the correction schedule is generated The new strut settings are gradually dialed into position and the fracture deformity reduced (JII> Fig 12-11)

B CHAPTER 12 · Six·Axis Deformity Analysis and Correction

Fig.12·11

Once all the deformity, frame, and mounting parameters are

determined and entered into the Total Residual Program, the

struts are dialed to the new settings and the fracture deformity

is reduced An important clinical strategy is to leave the

frac-ture shortened and aligned This reduces swelling,

compart-ment pressures, and pain Acute reductions with distraction

should be avoided The spatial frame schedule will provide

gradual reduction that is weil tolerated by the patient

CORAgin Method

In Situations in which no acute fracture with identifiable

bone ends that correspond to each other is present, the

fracture method cannot be used Such deformities are

called chronic deformities and include congenital,

devel-opmental, and posttraumatic residual (nonunion,

malu-nion) deformities With the CORAgin method, the

ori-gin is chosen to be the CORA, and the corresponding

point is determined by using locallength analysis or by

adding extrinsic length data (e.g., limb length discrep- Fig.12·12

ancy data per radiograph; ~ Fig 12-12) Local length With the CORAgin method, the origin is assigned to the CORA analysis is used when the desired correction is a pure

neutral wedge This analysis permits calculation of the

amount of shortening that is present because of the

a

W = 13 mm

111(

b

deformity The amount is added to determine the

loca-tion of the corresponding point When limb length

dis-crepancy data are chosen instead, this extrinsic

infor-mation is added in the same manner that the local

length analysis adds length along the reference axis line

Locallength analysis is conducted by measuring the

distance from the CORA to the convex surface of the

deformity (W line) This line segment, W, is then

pro-jected from the moving fragment's axis at a 90° angle

( , Fig 12-13a) The projected W line is then translated

down the moving fragment's axis until it contacts the

original W line The point on the moving fragment's axis

at which the projected W line is contacting the original

W line is assigned to be the corresponding point The

deformity parameters, especially the coronal, sagittal,

and axial plane translations, can then be determined

( , Fig 12-13 b,c} The corresponding point in the

sagit-tal plane is determined from the axial translation

calcu-lated from the coronal plane In the sagittal plane,

start-ing at the same level as the coronal CORAgin, the

dis-tance of the axial translation is measured on the

proxi-mal reference axis and a perpendicular line (line s) is

drawn This marks the level of the corresponding point

in the sagittal plane The point of intersection of the line

s and the moving axis is the corresponding point in the

sagittal plane( , Fig 12-13d)

An alternate way of determining the corresponding

point is by assigning a certain amount of length needed

during deformity correction The amount of length

needed is determined by the amount of planned

length-ening based on the safe limits of lengthlength-ening and the

limb length discrepancy This is considered extrinsic

information because it is not inherently obvious from

the radiograph of the deformed bone To factor in

short-ening of the bone with deformity correction, the amount

of shortening is added on the moving segment axis line

in a direction toward the reference fragment In the

example shown ( , Fig 12-14a), the shortening of the

moving segment is 20 mm By marking the

correspond-ing point as shown, it is as if the movcorrespond-ing segment were

CHAPTER 12 · Six-Axis Deformity Analysis and Correction IDII

Fig.12-lla-d

c

4 mm lateral translation

dis-(W line) This line segment, W, is then projected from the

moving fragment's axis at a 90° angle

b The projected W line is then translated down the moving fragment's axis until it contacts the original W line The point on the moving fragment axis at which the projected W line contacts the original W line is assigned the correspond- ing point ( CP)

c The deformity parameters, especially the coronal, sagittal, and axial plane translations, can then be determined

d The corresponding point in the sagittal plane is determined from the axial translation calculated from the coronal plane

In the sagittal plane, starting at the same Ievel as the coronal CORAgin, the distance of the axial translation is measured

on the proximal reference axis and a perpendicular line

(Line s) is drawn The point of intersection of the Line s and

the moving axis is the corresponding point in the sagittal plane

m CHAPTER 12 · Six-Axis Deformity Analysis and Correction

~~~

~~~ ~ -~ ~~ ~

Fig.12·14a,b

a The extrinsic information has determined that the

shorten-ing of the movshorten-ing segment is 20 mm By marking the

corre-sponding point ( CP) as shown, it is as if the moving segment

were 20 mm longer and shortened relative to the reference

segment

b When entering the amount of axial translation, one has to

measure the distance of the perpendicular line from the

ref-erence line to the corresponding point to the origin This will

be less than 20 mm

20 mm Ionger and shortened relative to the reference

segment When entering the amount of axial translation,

one has to measure the distance of the perpendicular

from the reference line to the corresponding point to the

origin This will be less than 20 mm (., Fig 12-14b)

CORAsponding Point Method

With the CORAsponding point method, the

corre-sponding point is chosen first and is assigned tobe at the

CORA instead of the origin This places the

correspond-ing point on the reference line because the CORA is the

one point at which both the corresponding point and the

origin are on the reference line This method is

espe-cially useful when extrinsic length needs to be added The length is added on the reference line by moving the origin along the reference line toward the corresponding fragment This is referred to as the extrinsic origin

(., Fig 12-15) One of the advantages of this method is that it eliminates anteroposterior and lateral translation deformity parameters The one downside is that it increases the distance of the origin to the reference ring, increasing the axial frame offset This becomes signifi-cant only if a large unaccounted for magnification error

is present The extrinsic origin is still a reproducible point in space because its distance from the CORA-sponding point is known The mounting parameters are based on the position of the extrinsic origin relative to the center of the reference ring (., Fig 12-16)

With the deformity and mounting parameters entered into the Total Residual Program, a correction schedule can be generated and the deformity corrected Even with the CORAsponding point method of plan-ning, some deformities include true translational defor-mities These deformities must be taken into account and entered into the deformity parameters With careful planning, these translational deformities will become obvious, as in the example shown (., Fig 12-17) Also, another subtle sign of underlying translational defor-mity is a CORA point that is at different Ievels in the coronal and sagittal planes A CORA at different Ievels signifies angulation and translation in different planes

CHAPTER 12 · Six-Axis Deformity Analysis and Correction EI

CORAsponamg po1n

\~~~ - EO -~~·

AP

Fig 12·15

The CORAsponding point is assigned to the CORA This places

the CORAsponding point on the reference axis line, which

appears as a red Une in the figure The CORA is the one point

that allows the CORAsponding point and the origin tobe

posi-tioned on the reference axis line This method is especially

use-ful when extrinsic length needs to be added The length is

of this method is that it eliminates anteroposterior and lateral translation deformity parameters, provided the reference ring

111 CHAPTER 12 · Six-Axis Deformity Analysis and Correction

a

I

AP

Fig 12-17 a, b

True translational deformities will be encountered even with

the CORAsponding method of planning With careful

defor-mity analysis, these true translational deformities will be

iden-tified EO, extrinsic origin

a An example of a tibial malunion with varus and posterior

translational deformity is shown Careful analysis of both

planes easily demonstrates the translational deformity

Another subtle sign of underlying translational deformity is

a CORA point that is at different Ievels in the coronal and

sagittal planes

b Detailed LAT view

b

LAT

Virtual Hinge Method

The virtual hinge method places the origin and sponding point at the same location in space By placing both the origin and corresponding point at the same location, a virtual axis of rotational correction - or vir-tual hinge - is created The ideal position of a virtual hinge is at the CORA The CORA at the intersection of the proximal and distal midaxillary lines can be chosen,

corre-or any other CORA point that lies along the transverse bisector can be designated the virtual axis of rotational correction point (~ Fig 12-18)

This planning strategy has several advantages By placing the origin and corresponding point at the same location, all translational deformities are eliminated Next, the virtual hinge can be used to create a pure open-ing wedge osteotomy when placed on the transverse bisector line at the convex surface of the bone deformity (~ Figs 12-19 and 12-20)

The virtual hinge can also be placed at the center of rotation of the knee or ankle joint This allows the joint

to be rotated about its normal axis of rotation The lor spatial frame can first be used to distract a joint with subsequent rotation ab out the virtual hinge

Tay-When planning Taylor spatial frame correction using the virtual hinge method, certain concepts must be kept

in mind First, when adding length with this method, the planning becomes the CORAgin method Second, if

The CORA at the intersection of the proximal and distal

midaxillary lines can be chosen, or any other CORA point that

lies along the transverse bisector line ( tBL) can be designated

the virtual axis of rotational correction point

CP and origin

the virtual hinge has been placed on the convex cortex

to create an opening wedge, concurrent axial rotation

should not be performed If rotational correction is

performed about this point, secondary translation will

occur Therefore, if secondary rotational correction is

needed after an opening wedge is completed, the origin

must be adjusted to the center of the reference

frag-ment's axis by changing the mounting parameters

c The virtual hinge method allows for pure opening wedge osteotomy correction

lf1l CHAPTER 12 • Six-Axis Deformity Analysis and Correction

AP

Fig.12-21

Reference fragment

LAT

The first step with the LOCA method is to assign two Ievels in

the coronal and sagittal planes These two Ievels are arbitrary

but should be reproducible to ensure the same Ievel in both the

coronal and sagittal radiographs These points should be

cho-sen at the ends of the bone

Fig.12-22 l'

The translational deformities between the two axis lines are

determined at each of the assigned Ievels and are plotted on a

graph representing the axial plane The two points on the graph

are connected and represent the deformed fragment in the

axial plane with respect to the reference fragment

AP

Line of Closest Approach (LOCA)

In chronic fracture deformities (nonunion, malunion), the CORA on the AP view radiograph does not neces-sarily correspond to the CORA on the LAT view radi-ograph This is because angulation and translation are in different planes In Chap 8, we considered various solu-tions to the level of osteotomy in such cases One other solution has been proposed by J Charles Taylor: to cor-rect the deformity at the level of the LOCA, which is the level at which the translation between the fragments is the least

The LOCA can be determined by a graphic method First, two levels are designated in both the anteroposte-rior and lateral planes (111> Fig 12-21) Second, the trans-lations between the reference and deformed fragments are determined at both levels The two points are plotted

on a graph representing the axial plane (111> Fig 12-22) Third, a line is drawn from the reference fragment per-pendicular to the deformed fragment on the axial graph This line is the LOCA, and the point of intersection of this line with the deformed fragment is the LOCA point The translations of the new LOCA point are determined and extrapolated to the anteroposterior and lateral planes These measurements from the LOCA point to the reference fragment represent the translational defor-mity parameters (111> Fig 12-23 a) The translations of the new LOCA point are used to determine the level

of osteotomy (111> Fig 12-23b) The points lying on the LOCA are the origin and corresponding points The translational deformity parameters, along with the other deformity parameters are now complete and can

be entered into the computer program If length is needed, the CORAgin method is used The correspond-

Anterior Posterior

fragment

Point 2

Posterior Deformity parameters

AP view angulation = 14° valgus LAT view angulation = 8° apex anterior

AP view Iranslaiion = 3 mm medial LAT view Iransialion = 3 mm posterior Axial translation = Local length analysis

or extrinsic length data

a A line is drawn from the reference fragment perpendicular

to the deformed fragment on the axial graph This line

rep-resents the LOCA The translations of the new LOCA point

(Point 3) from the reference fragment are measured- in this

example, 3 mm medialand 3 mm posterior These

measure-ments represent the translational deformity parameters CP,

I!IJ CHAPTER 12 • Six·Axis Deformity Analysis and Correction

\ Medial

Moving fragment

By entering the angulation data from the same example into

the Taylor spatial frame web-based program, a schematic

dia-gram of the deformity can be generated The axial view is the

diagram that was produced with the LOCA graphic method

d

Fig 12-23a-d

c In this example, 2 cm of lengthening is desired The new deformity translation parameters are determined and entered into the computer program

d Correction after 2 cm of lengthening was performed

ing point is translated along the moving segment's axis from the LOCA level to the point of desired lengthening (~ Fig 12-23c,d) Interestingly, the spatial frame pro-gram can be used to create the axial LOCA diagram (~ Fig 12-24) Also, the LOCA diagram can be used to determine the magnitude of the oblique plane defor-mity (~ Fig 12-25)

Most posttraumatic deformities can be defi.ned by using the LOCA In essence, the CORA is a special case

of a LOCA with the length of the LOCA equaling 0 ever, when translational and angular deformities place the CORA at different levels in the coronal and sagittal planes, the LOCA is the level at which the origin and cor-responding points are the closest, as stated above The end points of the LOCA comprise one possible pair of origin and corresponding points Therefore, by defi.ning the level of the LOCA, the origin can be placed on the ref-erence fragment at that level If the osteotomy is chosen

How-at the level of the LOCA, the amount of translHow-ation rection is minimized If length is needed, the corre-sponding point is translated along the moving frag-ment's axis and the CORAgin method is used

CHAPTER 12 · Six-Axis Deformity Analysis and Correction ~~~

The oblique plane angulation is calculated by forming a triangle The height of the triangle equals the distance between the two

designated Ievels The base of the triangle equals the length of the deformed fragment in the axial plane graph The triangle is

completed by drawing a hypotenuse and the angle measured (angle 9) The angle equals the oblique plane angular deformity

Taylor Computer-assisted Design (CAD) Software

A CAD program for Taylor spatial frame planning was

recently developed by Orthocrat Ltd (Tel Aviv, Israel)

This program allows for detailed and accurate deformity

and mounting parameter analysis using digital

radi-ographic images The information can be uploaded to

the Taylor spatial frameweb site to generate a deformity

correction schedule The CAD program also allows for

manipulation of the digital images for preoperative

"paper doll" planning A complete description and

demo version can be found at www.ortho-crat.com

Reference Concepts

During the planning of a Taylor spatial frame

correc-tion, the surgeon decides on the reference fragment and

reference ring The reference ring is critical when

deter-mining the mounting parameters and when positioning

the frame in space as it relates to the designated origin

The translational mounting parameters relate to the

center of the reference ring The rotational mounting

parameter relates to the master tab for proximal

refer-ence cases and the anti-master tab for distal referrefer-ence

cases The decision for proximal or distal referencing is

based on certain standard concepts The juxta-articular

ring usually is the reference ring The most orthogonal

ring may also be a good choice for referencing The

deci-sion for distal referencing creates a problern of

perspec-tive for orthopaedic surgeons Orthopaedic surgeons are

trained to describe deformities from a proximally based

perspective However, with distal referencing, this

per-Fig 12-26

AP view Proximal reference

With distal referencing, the standard orthopaedic perspective

is reversed, resulting in opposite translational deformity scriptions When the same deformity is characterized from two different perspectives, different descriptions occur This is termed parallactic homologues The example shows a distal tib- ial fracture that is displaced in a posterolateral direction This deformity would be described differently from a distal refer- ence perspective If the distal tibia and foot were looking at the rest of the body (a distal perspective), the foot would describe the body as being both anterior and medial Therefore, a distal reference would describe these translational deformities as a medial translation in the coronal plane and an anterior trans- lation in the sagittal plane Angulation, rotation, and axial translation deformities are unaltered by distal referencing

de-m CHAPTER 12 · Six-AxisDeformi~AnalysisandCorrection

spective is reversed, resulting in opposite translational

deformity descriptions When the same deformity is

characterized from two different perspectives, different

descriptions occur This is termed parallactic

homo-logues and is discussed in further detail later in the

chapter For example, a distal tibial fracture that is

dis-placed in a posterolateral direction will be described

dif-ferently from a distal reference perspective Using this

example, if the distal tibia and foot were looking at the

rest of the body, the foot would describe the body as

being both anterior and medial Therefore, a distal

ref-erence would describe these translational deformities as

a medial translation in the coronal plane and an anterior

translation in the sagittal plane Angulation, rotation,

and axial translation deformity parameters are

unal-tered by distal referencing ( , Fig 12-26)

Rate of Correction and Structure at Risk (SAR)

As with the Ilizarov system, the rate of correction is

based on the biology of distraction of the bone and

soft tissues With the spatial frame, this analysis can

be taken to a more sophisticated Ievel The surgeon has

the opportunity to determine the SAR and the rate of

distraction of the SAR The SAR might be the concave

side of the bone on the osteotomy line or the peroneal

nerve at the neck of the fibula, for example With the

Ilizarov method, we approximated the ideal correction

rate so that the SAR would not distract faster than 1 mm

per day This calculation was based on the arc length ( arc

length = 2nnx/360, where a is the magnitude of

c LAT view of tibia, orthogonal to knee forward The vatum deformity measures 25°

procur-d LAT view of tibia, orthogonal to ankle forward The vatum deformity measures 20°

procur-tion) Are length probably overestimates the amount of lengthening occurring at the SAR The shortest length or chord length between the SAR in the deformed state and the normal state is calculated by using the following for-mula: chord length = 2rsina/2 These calculations are adequate when only the three rotations are considered When displacement of the bone segments will occur, the totallinear displacement also should be considered: dis-placement = V(anteroposterior translation)2 + (lateral translation)2 + (axial translation)2 With the spatial frame calculations, the computer considers the SAR parameters and then determines the nurober of days of correction It will also generate an adjustment schedule for the patient, from the start position to the end posi-tion of the frame The designation of the SAR is not mandatory The surgeon has the option of determining the rate of deformity correction by entering the desired distraction rate or the nurober of days over which the correction will be achieved Three clinical examples of the use of the spatial frame are shown in , Figs 12-27 and 12-28

Fig 12·27 a-g

e AP view of tibia with a pre-constructed spatial frame

mounted The proximal segment was used as the

refer-ence fragment

f AP view of final correction The complex tibial deformity

was corrected, but the nonunion was not fully healed It

was therefore treated by intramedullary nailing, as shown

in Fig 8-18

g LAT view of final correction

Fig 12·28a- l

CHAPTER 12 · Six·Axis Deformi!Y Analysis and Corrertion

a AP view of tibial varus and rotational malunion with concurrent distal femoral valgus deformity

b Clinical photograph of thigh-foot axis viewed from foot end

c Preoperative AP view radiograph

111 CHAPTER 12 · Six-AxisDeformi~AnalysisandCorrection

Fig 12-28a-l

d Preoperative LAT view radiograph

e Long standing AP view radiograph

f Clinical AP photograph of tibial correction and

simulta-neous six-axis deformity correction using the Taylor spatial

frame

g AP view radiograph shows the correction

h Long standing AP view radiograph shows the correction

AP view radiograph shows fixator-assisted nailing of the tal femoral valgus deformity Note that the mechanical axis

dis-is properly aligned

j LAT view radiograph shows results

k Clinical photograph of the thigh-foot axis viewed from the foot end shows results afterfemoral and tibial correction

I Clinical photograph shows final correction

Parallactic Homologues of Deformity:

Proximal versus Distal Reference Perspective

In previous chapters we considered the projection of

angulation, translation, and rotation independently of

each other However, projective geometry is not so

sim-ple (Kline 1955) Taylor has observed that the six-axis

deformity parameters viewed from one reference

per-spective differ from those seen from another reference

perspective (Taylor 2004) In the example of a tibial

deformity, the deformity parameters viewed relative to

the anatomic frontal, sagittal, and axial planes of the

knee differ from those seen relative to the anatomic

frontal, sagittal, and axial planes of the ankle Both the

proximal and the distally referenced deformity

parame-ters accurately describe the same deformity The key is

that both sets of parameters are referenced to different

coordinate planes Taylor calls these parallactic

homo-logues The amount and even direction of the different

rotations and translations can differ when viewed from

different perspectives (~ Figs 12-29 and 12-30)

From a practical standpoint, it is important to keep

the reference segment in mind when evaluating and

operating on a deformity For example, the tibial

defor-mity associated with Blount's disease usually is

de-scribed as varus, procurvatum, and internal rotation of

the distal segment relative to the proximal segment The

magnitude of these deformities will differ if viewed

rel-ative to the knee or relrel-ative to the ankle The dinical

examination of rotation, AP and LAT view radiography,

and surgery all should be performed from the same

per-spective, namely the perspective of the reference

frag-ment The thigh-foot axis of tibial torsion should be

measured from the knee looking toward the foot If one were to measure the torsion from the foot looking toward the knee, one would measure a different amount

of tibial torsion If computed tomographic scans are used to assess rotation deformity, they should be obtained perpendicular to the knee segment The radi-ographs should be obtained as AP and LAT views of the knee to include the tibia When operating in such a case, the knee forward position should be the reference of the leg during surgery For distal deformities, a distal refer-ence segment is preferred For proximal deformities, a proximal reference segment is preferred Fora distal tib-ial deformity, the thigh-foot axis is measured prone and

a computed tomographic scan should be obtained pendicular to the distal segment and not the proximal segment The radiographs should be AP and LAT views

per-of the ankle to include the tibia The deformity seen from the ankle is the parallactic homologue of the defor-mity seen from the knee (~ Fig l2-30e,f)

This concept is relevant to surgery irrespective of the correction method used For example, if one is using a circular external fixator, such as the Ilizarov or spatial frames, the frame must be preconstructed and applied

to the limb relative to the reference perspective used for the evaluation of the deformity If the radiographs are obtained from a proximal segment reference perspec-tive but the frame is applied from a distal reference per-spective, the magnitude and sometimes even the direc-tion of the deformity parameters will be different from those built into the frame The frame will seem not to match the deformity of the leg If using a monolateral external fixator, this problern is addressed by inserting the proximal pins relative to the reference planes of the

Fig.12-29a b

a Projectional difference in measurements caused by different

reference perspectives The two different representations of

the deformity are called parallactic homologues In the AP

view relative to the knee forward reference segment ( vertical

left), the model of the left tibia appears to have 4.75° of varus

In the AP view relative to the foot forward reference segment

( vertical right), the model of the tibia appears to have 5.5° of

valgus In the LAT view relative to the knee forward reference

segment (horizontal top), the model tibia shows 33.5° of

extension In the LAT view relative to the foot forward

refer-ence segment (horizontal bottom), the model tibia shows

33.0° of extension

b Tibial model showing clinical measurement of rotation from

foot to knee (left limb) and from knee to foot The

foot-to-knee measurement shows 21.5° of internal rotation The

knee-to-foot measurement shows 22° of internal rotation

a AP view radiograph of the lower limb obtained lar to the distal segment The magnitude of angulation mea- sures 13° valgus

perpendicu-b AP view radiograph of the tiperpendicu-bia operpendicu-btained perpendicular to the proximal segment The magnitude of angulation mea- sures 1 oo valgus

c LAT view radiograph of the tibia obtained perpendicular to the distal segment The magnitude of angulation measures 43° recurvatum

d LAT view radiograph of the tibia obtained perpendicular to the proximal segment The magnitude of angulation mea- sures 52° recurvatum

proximal segment and the distal pins relative to the erence planes of the distal segment For example, to insert the proximal pins, the knee is oriented forward and the pins are inserted in the frontal plane of the knee, either perpendicular to the tibial shaft proximal to the CORA or approximately 3° to the knee joint line For the distal pins, the ankle is oriented forward and the pins are inserted in the frontal plane of the ankle, perpendicular

ref-to the shaft of the tibia distal ref-to the CORA or parallel ref-to the ankle joint line After the osteotomy is made, the pins are brought parallel to each other This method takes into consideration the reference coordinates ofboth the proximal and distal ends

With internal fixation using closing wedge tomies, a similar approach can be used by making each bone cut perpendicular to its respective bone segment

osteo-in the same manner osteo-in which the half-posteo-ins were osteo-inserted perpendicular to their bone segment in the previous example When an oblique plane closing wedge osteo-tomy is planned, it is essential to obtain the radiographs from one perspective and to reference the oblique plane wedge with reference to the same perspective If angula-tion is eliminated by osteotomy and only axial rotation remains, no difference exists in the amount of rotation measured from the proximal or the distal end In other words, the parallactic homologues of rotation deformity

in the absence of angulation are the same as seen from

CHAPTER 12 · Six-Axis Deformity Analysis and Correction m

Fig.12-30a-f

e Thigh-foot axis viewed from the foot end measures + 10°

f Thigh-foot axis viewed from the knee end measures 0°

proximal or distal reference perspectives Therefore, with many internal and external fixation techniques, it often is easier to correct all the angulation and transla-tion deformities, leaving a bone that is straight with the exception of axial rotation deformity The axial rotation deformity is then corrected around the long axis of the straightened bone In such cases, only the angulation and translation need to be referenced to the reference segment while residual rotation is corrected at the end when no parallactic homologue error is present The same is true for length deformity correction When angulation is present, it often is difficult to accurately determine the amount of length correction required Therefore, the length assessment is deferred until the angulation is corrected The referencing is then critical

to only the angulation portion of the correction Chap 9 includes a description of a specialized inclined osteo-tomy for the simultaneaus correction of angulation and rotation The assessment of angulation and rotation with reference to one perspective must be carried over

to the execution of the inclined osteotomy relative to the same reference perspective

References

Beggs JS ( 1966) Advanced mechanism Macmillan, New York Kline M (1955) Projective geometry Sei Am, January Seide K, Wolter D, Kortmann HR ( 1999) Fracture reduction and deformity correction with the hexapod Ilizarov fixator Clin Ortbop 363:186-195

Taylor JC (2004) Correction of general deformity with the lor spatial frame fixator www.jcharlestaylor.com, November

Tay-2004

CHAPTER 13

Consequences of Malalignment

Allliving organisms are limited to a finite life span, and

humans are no exception As with any mechanical

sys-tem, the cumulative debilitating effects of time, wear,

and gravity result in an almost imperceptible gradual

degradation in performance All our tissues are

suscep-tible to these effects, although some are more resistant

than others Skin and bone are perhaps the most

resil-ient tissues, and both have an amazing capacity for

heal-ing and regeneration that is the foundation of much of

modern reconstructive surgery Although articular

car-tilage is subjected to some of the highest mechanical

demands, its capacity for repair and regeneration is

un-fortunately extremely limited and it is often among the

first tissues to manifest the effects of aging Remarkably,

this relatively fragile tissue is responsible for

transmit-ting loads exceeding several times our body weight for

an estimated billion cycles during the course of an

aver-age lifetime lt is not surprising that any disturbance of

the normal anatomic and biomechanical relationships

can result in an acceleration of this gradual degradation

characteristic of aging

Because the lower extremities are normally weight

bearing throughout our lives, axial alignment of the

low-er extremities is critical with respect to detlow-ermining the

demands to which articular cartilage is repeatedly

ex-posed during gait Alignment is therefore an important

consideration in many clinical situations, whether

con-sidering fracture reduction, total knee arthroplasty, or

deformity correction At present, there is general

agree-ment that the cause of degenerative arthropathy is

me-chanical, not infiammatory (Radin et al 1991)

Com-monly called degenerative arthritis, this expression is

inappropriate because infiammation is a secondary

re-sult and not the principle cause Arthrosis is the

pre-ferred word for describing purely degenerative

patho-logical abnormality of the joint

Unicompartmental knee arthrosis is often associated

with malalignment resulting from deformity (Barrett et

al 1990; Hernborg and Nilsson 1977; Kettelkamp et al

1988) Although the association between malalignment

and arthrosis is acknowledged, the possible pathogenic

relationship is less well documented This may represent

the response of abnormal cartilage to normal forces or

may refiect the response of normal cartilage to excessive

stress Direct clinical evidence of a cause-and-effect lationship between malalignment and arthrosis has not been possible, but substantial evidence from the ortho-paedic literature supports this hypothesis

re-Central to this hypothesis is the assumption that alignment alters stress distribution across the joints in the lower extremity, particularly the knee The concept

mal-of a weight -bearing axis is not new and is usually termed the mechanical axis (Maquet 1984; Pauwels 1980) This

is depicted as the line passing from the center of the kle to the center of the hip and represents the path of transmission of the load-bearing force relative to the lower extremity Any deformity in the coronal planethat alters the alignment of the joints of the lower extremity, resulting in malalignment, disturbs this load-bearing axis When the load-bearing axis passesmedial or later-

an-al to the center of the knee, this creates a moment arm acting to increase force transmitted across either the medial or lateral tibiofemoral compartment, respective-

ly (Kettelkamp and Chao 1972; Maquet 1984; Pauwels 1980)

Pauwels {1980) pioneered the concept of the ical axis and recognized the significance of realignment

mechan-to resmechan-torenormal force transmission across the knee He was one of the first to recognize the importance of bio-mechanics and its relationship to surgical planning for the correction of deformity by osteotomy Maquet (1984) later expanded on these ideas and elegantly showed the alteration in stress transmitted across simu-lated joints using polarized light and photoelastic mod-els (., Fig 13-1) His sturlies verified the concepts put forth by Pauwels and emphasized the importance of re-storing or correcting the mechanical axis to alter load transmission across the knee

The relationship between malalignment and quent degenerative arthropathy may seem intuitively obvious Because of the slow progression of the disease, its poor tolerance by patients, and readily available treatment alternatives, it is difficult to document the nat-ural history of the process There is ample evidence to support the contention that persistent malalignment of sufficient magnitude willlater result in degenerative ar-thropathy This includes both basic science and clinical investigations and can be most conveniently reviewed in

subse-~~~ CHAPTER 13 · ConsequencesofMalalignment

Fig.B-1

Photoelastic model in polarized light shows altered stress

dis-tribution when axial Ioad is applied eccentrically (Reprinted

with permission [Maquet 1984].)

three sections: animal models, cadaver models, and

clin-ical longitudinal studies However, before considering

malalignment, it is paramount to first establish the

Iim-its of normal alignment

Static Considerations

The normal relationship of the joints of the lower

ex-tremity has been the focus of several recent studies

(Chao et al 1994; Cooke et al 1994; Hsu et al 1990;

Moreland et al 1987; Paley et al 1994) There are two

considerations when evaluating the coronal plane axis of

the lower extremity: joint alignment and joint

orienta-tion (Paley et al 1990; Paley and Tetsworth 1992b) (see

~ Fig 1-8) Alignment refers to the collinearity of the

hip, knee, and ankle Grientation refers to the position of

each articular surface relative to the axes of the

individ-uallimb segments (tibia and femur).Alignment and

ori-entation are best judged using standing long AP view

ra-diographs of the entire lower extremity on a single

cassette Proper rotation of the limb is critical and

re-quires the patella be centered between the femoral

condyles and directed forward A standardized

tech-nique is useful to assure that the radiographs are

repro-ducible (Cooke et al 1987, 1994; Paley et al 1994}

Alignment is determined by the line extending from the center of the hip to the center of the ankle, the me-chanical axis of the limb By definition, malalignment occurs when the center of the knee does not lie close to this line The mechanical axes of the individuallimb seg-ments (tibia and femur) arealso important In the tibia, the mechanical and anatomic axes are almost the same ( Morelandet al 1987), but in the femur, they are very dif-ferent The mechanical axis of the tibia is defined by the line from the center of the knee to the center of the an-kle The mechanical axis of the femur is defined by the line from the center of the hip to the center of the knee This typically subtends a 6° angle to the anatomic axis

of the femur (Hsu et al 1990; Moreland et al 1987; Yoshioka et al 1987), which runs from the piriformis fossa to the center of the knee joint

Although normal alignment is often depicted with the mechanical axis passing through the center of the knee, a line drawn from the center of the femoral head

to the center of the ankle typically passes immediately medial to the center of the knee Morelandet al (1987) reviewed standing long AP view radiographs of both lower extremities of 25 normal male volunteers and doc-umented that the hip, knee, and ankle are nearly colin-ear Using several radiographic Iandmarks to define the center of each joint, the intersection of the femoral and tibial mechanical axes measured 1.3° varus (± 2°) Hsu et

al (1990) reviewed standing long AP view radiographs

of the lower extremities of 120 normal participants and confirmed that the mechanical axis generally passes im-mediately medial to the center of the knee In their study population, the intersection of the femoral and tibial mechanical axes measured 1.2° varus (±2.2°)

Based on these observations, the joints of the lower

extremity are considered normally aligned in a nearly

collinear fashion Any distortion of this relationship is

considered malalignment and predictably affects the

transmission ofload across the joint surfaces The hip is

approximately spherical and is best able to

accommo-date an alteration in its normal position The proximity

of the subtalar joint allows the ankle to better talerate

deformity, although subtalar stiffness is common in

posttraumatic Situationsand may be a clinically

signifi-cant factor (McMaster 1976) However, the knee is most

vulnerable to changes in the normal coronal plane

rela-tionship of the joints of the lower extremity

When coronal plane deformity results in axial

mal-alignment, the load-bearing axis passesmedial or

later-al to the center of the knee (Maquet 1984) This creates a

moment arm acting to increase force transmission

across either the medial or lateral tibiofemoral

compart-ment, and that momentarm can be depicted by

measur-ing the MAD (Paley and Tetsworth 1992a, 1992b) The

mechanical axis is drawn from hip to ankle, and a

per-pendicular segment is added, extending from the axis to

the center of the knee (see , Fig 1-8) The magnitude of

this additional segment, measured in millimeters,

re-fiects the magnitude of alteration in stress transmission

across the knee Determining MAD accounts for

defor-mity of any type, including rotation, translation, and

an-gulation It also takes into consideration the level of the

deformity The effect on the mechanical axis increases as

the apex of deformity approaches the knee ( , Fig 13-2)

(McKellop et al.1991, 1994; Puno et al.1987) This

meth-od has been useful for both preoperative planning

(Paley and Tetsworth 1992a, 1992b; Paley et al 1990,

1994) and postoperative evaluation of the results of

de-formity correction (Tetsworth and Paley 1994)

After determining the alignment of the joints of the

lower extremity, the second consideration is the

orienta-tion of the joints to the mechanical axis Each joint has a

normal inclination to the mechanical and anatomic

ax-es of both limb segments ( Chao et al 1994; Cooke et al

1994; Morelandet al.1987; Paley et al.1994) These form

reference lines and angles that are useful in preoperative

planning to determine the deformity present in each

bone segment (Paley and Tetsworth 1992a, 1992b; Paley

et al 1990, 1994) The goal of deformity correction is to

not only restore normal alignment but also maintain or

restore the normal orientation of each joint to the

me-chanical axis Cooke et al {1987, 1989, 1994) showed the

clinical significance of malorientation at the knee by

documenting an association with osteoarthritis

Theorientation of the hip on the AP view can be

char-acterized by the NSA, and the radiographic projection of

the NSA ranges from 125°-131 o In an anatomic study of

isolated cadaver femora, Yoshioka et al (1987)

deter-mined that the NSA in adult men normally measures

129°.Alternatively, Paley et al {1990) defined a line from

Effect of angulation on MAD is more profound when the apex

of the deformity is near the knee (Reprinted with permission from McKellop et al 1991)

the tip of the trochanter to the center of the femoral head, which can be used to define a joint orientation ax-

is of the proximal femur Chao et al ( 1994) measured the LPFA on the standing long radiographs of 127 normal volunteers and stratified the study group according to age and gender There was no significant change noted with age in women, and the relationship of this line to the mechanical axis of the femur measured 91.5° varus (±4.6°) in younger women and 92.7° varus {±4.9°) in older women In men, the LPFA showed an age-related tendency toward increasing varus, measuring 89.2• (±5.0°) in younger men and 94.6° (±5.SO) in older men Data from our institution (Paley et al 1994), based on a smaller group of 25 asymptomatic adults, indicate that this proximal femoral joint orientation line measures 89.9° (±5.2°) Basedonthese observations, we have ad-vocated 90° for the LPFA (Paley et al 1990, 1994; Paley and Tetsworth 1992b)

Chao et al (1994) also measured the mLDFA and stratified the data according to age and gender The aver-age mLDFA was 88.1 ± 3.2° and was independent of age and gender These results have been confirmed by our own data (Paley et al 1990), which indicate that the av-erage mLDFA is 87.8± 1.6° Cooke et al (1994) obtained standing long radiographs after positioning the patient

in a frame to enhance precision, andin 79

asymptomat-ic young adults, the distal femoral orientation line

mea-sured 86° valgus ( ± 2.1 °) Based on these data, the normal

relationship of the distal femoral joint orientation line

and the mechanical axis of the femur is considered tobe

87° (Paley et at.1990, 1994; Paley and Tetsworth 1992b)

Chao et al (1994) again stratified their data according to

age and gender for the medial proximal tibial angle and

found a significant difference when comparing older

with younger men In all groups, the MPTA measured

slightly varus relative to the mechanical axis of the tibia,

andin women, it measured 87.2° (±2.1°) Interestingly,

the subgroup of asymptomatic young men had slightly

more varus (85.5±2.9°) compared with asymptomatic

older men (87.5 ±2.6°) Perhaps some of the young men

with more varus later develop symptomatic

degenera-tive arthrosis and "drop out" of the asymptomatic group

of older men Rowever, this is largely speculative and

there currently are few data to support this conjecture

One study (Glimet et al 1979) of 50 elderly

asympto-matic French women does document that the

mechani-cal tibiofemoral angle in this select group measures

ze-ro degrees, which is consistent with this hypothesis

Cooke et al ( 1994) reviewed radiographs obtained using

a frame to position patients precisely and found the

proximal tibia in 86.7° varus (±2.3°) These results were

confirmed by our data (Paley et al 1994), with the

prox-imal tibia in 87.2° varus (± 1.9°), and by Morelandet al

(1987), who measured 87.2° varus (± 1.5°) Based on

these observations, the normal relationship of the

prox-imal tibial joint orientation line and the mechanical

ax-is of the tibia ax-is considered to be 87° varus (Paley et al

1990, 1994; Paley and Tetsworth 1992b)

The transverse axis of the knee measures

approxi-mately 3° off the perpendicular, such that the distal

fe-mur is in slight valgus and the proximal tibia is in slight

varus (Krackow 1983; Morelandet al 1987; Paley et al

1990; Paley and Tetsworth 1992b) When walking, the

feet progress along the same line, with the leg inclined to

the vertical approximately 3° This 3° varus position of

the lower limb allows the knee to maintain a parallel

orientation to the ground during gait (~ Fig 1-13 b)

(Krackow 1983) In bipedal stance with the feetapart the

width of the pelvis and the tibia perpendicular to level

ground, the knee transverse axis would be oriented in 3°

valgus relative to vertical

Morelandet al {1987) measured the ankle joint

ori-entation The lateral distal tibial angle measured 89.8±

2.7° Data from our institution (Paley et al 1994) also

showed slight valgus (LDTA=88.6±3.8°), as did data

presented byChao et al (1994) (LDTA=87.1 ±3.3°) This

relationship is variable, and up to 8° of valgus may be

normal (Moreland et al 1987) Basedonthese

measure-ments, the normal relationship of the distal tibial joint

orientation line and the mechanical axis of the tibia is

considered perpendicular (Paley et al 1990, 1994; Paley

and Tetsworth 1992b)

Although static malalignment is readily documented on standing long radiographs, this has not been a reliable means of predicting outcome after corrective osteotomy (Adriacchi 1994; Prodromos et al 1985; Wang et al 1990) The clinical Situation is far more complex, and the simple activities of daily living create dynamic loading conditions that reflect additional considerations (Adriacchi 1994; Rarrington 1983; Johnson et al 1980), including joint instability, muscle contractions, and in-dividual idiosyncrasies of gait Gait analysis is being used more frequently to assess dynamic aspects of ma-lalignment, but this technology has not been widely available and most of the Iiterature to date concerns stat-

ic assessment of malalignment

Stress transmission across the knee can be calculated using a rigid body spring model, if certain assumptions are made (Rsu et al 1990; Kettlekamp and Chao 1972) The distribution of force transmitted across the knee is normally shared unequally between the medialand lat-eral compartments (Rarrington 1983; Rsu et al 1990; Johnson et al 1980) Even in the absence of malalign-ment, calculations indicate that approximately 70% of the load across the knee in single-leg stance is transmit-ted through the medial compartment When 4°-6° of varus deformity is present, almost 90% of the knee joint force during single-leg stance passes through the medi-

al compartment (~ Fig 13-3) (Rsu et al 1990)

The dynamic loads that occur during walking and other weight-bearing activities of daily living are prob-ably more important but have been difficult to deter-mine accurately Important issues regarding the dynam-ics of knee malalignment have been reviewed in detail

by Andriacchi (1994) The normal forces that act on the lower extremity during gait produce moments tending

to flex, extend, abduct, and adduct the knee These are the primary factors influencing the distribution of me-dial and Iateralloads across the knee The ground reac-tion force acting at the foot during the stance phase of gait passesmedial to the center of the knee The perpen-dicular distance from the line of action of this force to the center of the knee is the length of the lever arm for this force The product of the magnitude of the force and the length of the lever arm results in an adduction mo-ment acting on the knee This adduction moment dur-ing gait is an externalload tending to thrust the knee in-

to varus; it is also known as lateral thrust (Prodromos et

al 1985; Wang et al 1990)

The external forces and moments acting on the lower extremity can be measured directly in a gait laboratory The internal forces acting through muscles, through Iig-aments, and on the joint surfaces are of greater interest but can only be estimated based on the external forces and moments measured (Andriacchi 1994; Rarrington

Graphkai representation of force distribution across the knee

and changes that occur as deformity is introduced With

nor-mal alignment, approximately 70 o/o of the force passes through

the medial compartment When varus malalignment of the

limb is greater than 5°, approximately 90 o/o of the force pass es

through the medial compartment (Reprinted with permission

from Hsu et al.l990)

1983; Johnson et al.1980) Mechanical equilibrium

man-datesthat external forces acting on the limb must be

bal-anced by internal forces generated by muscle and

Iiga-ments Prediction of internal forces is extremely

compli-cated because of the many combinations of muscle and

soft tissue forces that can balance the external forces and

moments acting on the limb Solving this problern

re-quires several simplifying assumptions, the most basic

being to group internal structures together Analysis of

the relationship between external loads and internal

forces under these assumptions allows estimation of the

magnitude of the joint reaction force acting across

ei-ther the medial or lateral compartment independently

The distribution of the medial and lateral joint reaction

forces shows that the adduction moment is the primary

factor producing the higher medial joint reaction force

during normal function Fora group of normal

partici-pants, the maximum joint reaction force across the knee

is approximately 3.2 tim es body weight, with 70% of this

load passing through the medial compartment The

av-erage maximum magnitude of the adduction moment

during normal gait for this population has been

calcu-lated as approximately 3.3% of the product of body

weight and height (Andriacchi 1994) This adduction

moment is greater than the moments calculated for

ei-ther fl.exion or extension of the knee in this same study

group

Some patients modify their gait, effectively reducing

the load on the medial compartment of the knee The

adaptive mechanism used reduces the adduction

mo-ment and has been related to a shorter stride length and

an increase in external rotation of the foot (toe-out

po-sition) during stance phase (Andriacchi 1994;

Prodro-meset al 1985; Wang et al 1990) The toe-out position

ol places the hindfoot closer to the midline, beneath the center of gravity This simply moves the ground reaction vector toward the center of the knee, effectively reducing the lever arm of the external ground reaction force and therefore the resulting adduction moment Patients are considered to have high adduction moments if the cal-culated moment exceeds 4% of the product of body weight and height when walking at speeds of approxi-mately 1 m/s All other patients are considered to have low adduction moments

The clinical outcome after treatment of patients with varus gonarthrosis by valgus high tibial realignment os-teotomy has been closely related to the magnitude of the adduction moment measured during preoperative gait analysis (Andriacchi 1994; Prodromos et al 1985; Wang

et al 1990) Patients in the low preoperative adduction moment group had a better clinical result initially, and this result was sustained over an average follow-up peri-

od of 6 years The valgus correction was maintained with follow-up in 79% of the low adduction moment group compared with only 20% of the high adduction moment group (Andriacchi 1994; Wang et al 1990) Load transmission across the knee can be effectively altered by adjusting the location of the center of gravity This dynamic compensation involves either use of an external support or gait modification Shifting the upper body center of mass to a position directly over the in-volved limb can decrease the medial compartment force

by 50% compared with its value when the center of ity is positioned in the midline (Hsu et al 1990) Clinical evidence has already established the importance of gait alteration and its relationship to results after corrective high tibial osteotomy (Andriacchi 1994; Prodromos et

grav-al 1985; Wang et grav-al 1990) Patients with the best clinical outcomes are able to modify their gait, externally rotat-ing the limb and developing a lower adduction moment

at the knee ( , Fig.l3-4) This is contraryto observations presented by Krackow et al (1990) regarding malrota-tion during gait During the stance phase, when load transmitted through the limb is greatest, the knee is maintained in a position of slight fl.exion Interna! rota-tion of a slightly fl.exed limb then creates apparent val-gus Externatrotation creates apparent varus and would

be expected to be associated with a poorer prognosis ter a valgus osteotomy This contradiction confirms the discrepancies that may result when attempting to corre-late static and dynamic analyses of malalignment Static considerations may not accurately refl.ect the clinical condition, and the contribution of musdes and Iigaments acting across the knee can markedly infl.uence the joint reaction forces Although the medial compart-ment may sustain higher average loads based on static analysis, recent publications suggest that the loads are more evenly balanced across the entire femorotibial articulation In a biostatic cadaver laboratory model, Inaba et al (1990) measured forces across the femorotib-

af-I!JI CHAPTER 13 · ConsequencesofMalalignment

a

Fig 13·4 a, b

Gait modifications observed dinically that alter adduction mo·

ment arm (modified from Andriacchi 1994)

a Toe-out gait by use of excessive external rotation of the

low-er limb places the ground reaction force vector doslow-er to the

center of the knee joint This reduces adductor moment arm

b Toe-in gate with internal rotation of the lower limb places

the ground reaction force vector away from the center of the

knee joint This increases adductor moment arm

ial articulation in simulated neutral and malaligned

po-sitions With the menisci intact, forcewas evenly

distrib-uted across both plateaus with a peak load of 4 MPa

Leaving the menisci intact and simulating so of varus

re-sulted in an increase in peak contact pressure to 7.3 MPa

in the medial compartment Simulating so of valgus

re-sulted in a corresponding increase of peak contact

pres-sures to 7.8 MPa in the lateral compartment

Using cadaver and magnetic resonance imaging

mea-surements, investigators at the Oxford Orthopaedic

En-gineering Center (Huss et al 2000; Lu and O'Connor

1996) developed an anatomy-based mathematical

mod-el to predict loads transmitted across the knee This model incorporates the lines of action and moment arms of the major force-bearing structures crossing the human knee joint, induding both musdes and Iiga-ments Theoretical values derived from this model rep-licate the previously published experimental measure-ments presented by Herzog and Read (1993), validating the model Induding contributions from musdes and Iigaments, both experimentally measured and theoreti-cally calculated forces across the knee are more evenly distributed than published results have suggested The difference between the static single-leg standing Simula-tions and those that factor in the surrounding musde forces is mostly attributable to the tensor fascia lata musde In a well-conditioned person, this musde coun-ters the adduction moment arm on the knee, unloading the overloaded medial side and transferringthat load to the lateral side As one gets older and naturally loses musde mass and strength, the protection afforded the medial compartment by the tensor fascia lata is dimin-ished and lost This may precipitate the progressive de-

terioration of the medial compartment that most

com-monly occurs in people older than 40 years

Joint laxity is a further confounding variable to

con-sider when determining the risk of developing

osteo-arthritis secondary to malalignment Sharma et al

( 1999) reported that ligament laxity may precede the

de-velopment of osteoarthritis Ligament laxity can result

in dynamic malalignment during gait, with associated

changes in loading patterns across the knee Collateral

ligament laxity may increase the risk of gonarthrosis

and cyclically contribute to progression of the disease

LCL laxity is typically associated with varus

malalign-ment and, when superimposed, may have a synergistic

effect The tensor fascia lata may protect the knee from

overload due to lateral collaterallaxity Again, this

pro-tection is gradually lost or overwhelmed with increasing

age, deconditioning, and deformity

Recognizing the role of limb rotation in gait

modifica-tion and its effects on load transmission, it is clear that

fixed rotational deformities can also have a potential

role in the development of degenerative arthropathy

This has been investigated by many authors with

con-flicting results, usually focusing on either the hip or knee

independently Several studies have attempted to

estab-lish a correlation between anteversion of the fern ur and

arthrosis of the hip In two published studies (Kitaoka et

al 1989; Wedge et al 1989), the attempt to correlate

in-creased anteversion with hip arthritis was unsuccessful

The relative sphericity of the hip itself may render it less

susceptible to both angular and rotational deformities

of lesser magnitude However, two other studies did

es-tablish a relationship between hip arthrosis and

abnor-mal femoral anteversion In a Scandinavian population,

Reikeras and Hoiseth (1982) showed a positive

correla-tion between increased femoral anteversion and an

increased incidence of hip osteoarthritis Conversely,

Tonnis and Heinecke (1991) later reported a positive

correlation between decreased femoral anteversion and

an increased incidence of hip arthrosis These results

suggest that there is a limit to the tolerance of the hip for

both internal and external malrotation

Investigations into the possible pathogenetic role of

rotation and arthrosis of the knee have examined either

femoral or tibial torsion independently Takai et al

(1985) reported a relationship between patellofemoral

arthropathy and increased femoral anteversion Eckhoff

et al (1994b) subsequently established a positive

corre-lation between medial compartment degenerative

ar-thritis and decreased femoral anteversion Eckhoff

(1994) suggested that the impact offernoral version

var-ies in the knee, with the patellofemoral compartment

being most affected by increased femoral anteversion

ol

and the medial compartment being most affected by decreased femoral anteversion This again suggests, as with the hip, that there is a limit to the tolerance of the knee for both internal and external femoral torsion Three published studies (Tonnis and Heinecke 1991; Turnerand Smillie 1981; Yagi and Sasaki 1986) have al-

so shown a relationship between tibial malrotation and knee arthrosis All three indicated that decreased ver-sion of the tibia results in increased incidence of arthro-pathic changes, principally in the medial compartment

An additional consideration is the rotational ment of the tibia relative to the fern ur itself, discussed in

align-at least two studies (Eckhoff et al 1994a; Takai et al 1985) as knee version This refers to a static rotation in alignment between the femur and tibia across the ex-tended knee and should not be confused with the auto-matic and dynamic rotation of the knee observed with flexion and extension, which is typically called the screw home mechanism The static external rotation of the tib-

ia relative to the femur in the fully extended knee sures greater in the arthritic knee than in the non-ar-thritic knee (Eckhoff et al 1994a)

mea-Although the Iiterature reviewed above indicates that malrotation is clinically associated with degenerative ar-thropathy, few of these studies discuss the presence or absence of coexisting axial malalignment Simultaneous axial and rotational malalignment is documented in two studies (Cooke et al.1990; Said and Hafez 1975) in which genu varum was associated with external tibial version

in patients with osteoarthritis of the knee

Eckhoff (1994) discussed many of the issues ing the effect of limb rotation on malalignment in his review article Human limbs are three-dimensional ob-jects, and any limb deformity is more likely tobe three-dimensional than two-dimensional (Eckhoff 1994; Green and Gibbs 1994) Axial malalignment is recog-nized and documented more frequently than is rotation-

regard-al mregard-alregard-alignment, but both elements of deformity can cur simultaneously Radiography is perhaps the most common method used to assess deformity; unfortunate-

oc-ly, however, this technique reduces the

three-dimension-al deformity to a two-dimensionthree-dimension-al image Restricted to two dimensions, an internally rotated limb with the knee flexed would appear as a valgus deformity in the coronal plane and the rotational component would be difficult to determine Conversely, the opposite is also true, and an externally rotated limb with the knee flexed would appear as a varus deformity in the coronal plane The common perception of limb malalignment as an iso-lated varus or valgus axial malalignment is reinforced clinically by two-dimensional radiographs that fail to accurately portray the third dimension and the coexist-ing rotational deformity Considering these limitations,

it is not surprising that rotational contributions to alignment are often inadvertently underestimated or ignored

mal

-Several experimental models have been used

successful-ly to create gradualsuccessful-ly progressive arthropathy in

labora-tory animals (Adams and Billingham 1982) Among the

first to do so were Hulth et al (1970}, who excised the

cruciate ligaments, the medial meniscus, and the

medi-al collatermedi-alligaments (MCLs) in rabbit knees The

re-sulting instability and altered joint mechanics mirnie the

response observed clinically aftermedial meniscectomy

The direct deleterious effect of abnormal contact

pres-sure on articular cartilage has been documented

repeat-edly in animal models Thompson and Bassett (1970}

investigated the morphological changes in articular

car-tilage secondary to mechanical derangement Elastic

bands were used to apply continuous compression

across an adult rabbit knee while allowing physiological

motion In addition to cartilage degradation,

hyper-trophic changes in the subchondral bone were noted,

consistent with the observations presented by Trueta

(1963) based on the study of the histology and

morphol-ogy of human osteoarthritic hip specimens

Pathologi-cal changes were observed in the deep chondral layer

and subchondral bone, presumably in response to

ab-normal mechanical demands

Springs applied across rabbit elbows were used by

Gritzka et al (1973) to provide continuous

compres-sion while allowing physiological motion The springs

exerted estimated contact pressures between 11 and

27 kg/cm2• In that range, the severity of cartilage darnage

correlated with the duration rather than the magnitude

of the compression The cartilage matrix initially

under-went fibrillation, ultimately resulting in complete

ero-sion Alternatively, a unilateral spring can be applied to

eccentrically 1oad the articular surface, indirectly

simu-lating the conditions common in malaligned limbs

Ogata et al (1977} used this method in the rabbit knee,

altering stress transmitted across the joint less

dramati-cally Steinmann pins placed in the medial femoral and

tibial condyles were connected with a spring to apply a

continuous force of 700-900 g, simulating a constant

varus stress This experimental model closely mimicked

the clinical situation and created gradually progressive

lesions Even when this small increase in varus stress

was applied, the duration of mechanical derangement

seemed to be more important than the magnitude of

the derangement in determining the severity of

carti-lage damage Although this model effectively simulates

malalignment, it does not specifically duplicate the

mechanical derangement resulting from angular

defor-mity

Reimann (1973) was one of the first to directly

docu-ment the detridocu-mental effects of malaligndocu-ment in a

labo-ratory animal model by creating a 30° valgus osteotomy

in the proximal tibia of adult rabbits She concluded that

one can induce degenerative changes in articular lage by disturbing the mechanical axis and altering the load bearing to create clear initial histological changes analogous to human osteoarthrosis

carti-Johnson and Poole (1988) successfully induced generative arthropathy in a canine model using a unilat-eral proximal tibial valgus osteotomy Wu et al ( 1990) in-vestigated the effects of malalignment in a rabbit model similar tothat used by Riemann (1973} with either a val-gus or varus proximal tibial osteotomy of 30° They found degenerative changes in the articular cartilage, increased subchondral bone thickness, and reduced tra-becular porosity, reflecting the alteration in mechanical stress transmission secondary to the malalignment pro-duced by the osteotomies

de-Repetitive impulse loading is the laboratory model that may best simulate the histological and morpholog-ical changes observed in human osteoarthrosis speci-mens (Radin 1978} Subcriticalloads applied to articu-lar cartilage in a pulsed fashion on an intermittent basis during a period of weeks leads to stiffening of the deep chondral layer ( calcified cartilage and subchondral plate) Increases in shear stress in the overlying articu-lar cartilage then create local concentrations that lead to degeneration of the cartilage base, with subsequent changes characteristic of degenerative arthrosis Radin

et al (1991) recently summarized the evidence to port the concept that altered loading affects the stiffness

sup-of the deep chondrallayer High shear in the overlying cartilage results in splitting and degeneration at the car-tilage base without disruption of the tangentiallayer at the articular surface Cartilage thickness gradually di-minishes as the tidemark and then advances into the deep chondral substance Based on observations of lab-oratory animals, increased density and stiffness in the deep chondra1layer seems to be an important compo-nent of the final common pathway for articular cartilage degradation and degenerative arthropathy resulting from malalignment

Mfld@ll!

-Pressure-sensitive film can be used to assay the ation in stress transmitted across cadaver joints under simulated clinical conditions, and this technique has been applied extensively (McKellop et al.1994; Tarr et al 1985; Ting et al 1987; Wagner et al 1984) to investigate the effect of tibial angular deformity on contact pres-sures in the ankle Laboratory studies conducted at the Kerlan Jobe Orthopaedic Clinic in Southern California (Tarr et al 1985; Wagner et al 1984) showed changes in contact area, contact shape, and contact location across the tibiotalar articulation after simulated angular mal-unions of the tibia The results suggested that changes at the tibiotalar joint were greater with distal third tibial

alter-deformities compared with alter-deformities at more

proxi-mal Ievels Contrary to conventional teaching, contact

area across the tibiotalar jointwas altered more

dramat-ically with deformities in the sagittal plane than in the

coronal plane Distal third deformities with recurvatum

or procurvatum produced a greater change than those

with varus or valgus, and those deformities in

recurva-tum produced the greatest changes in contact shape and

the most profound reduction in contact area Inman

(1976} reported that articular congruity between ankle

mortise and trochlea is best in neutral flexion and that

congruity diminishes with both plantar flexion and

dorsiflexion Simulated fracture malunians in

recurva-tum would require the foot to be positioned in plantar

flexion to achieve plantigrade contact with a Ievel

sur-face This position leaves the talar dome relatively

un-covered and potentially at greater risk for later

develop-ing degenerative arthropathy

The subtalar joint acts as a torque transmitter and

compensates for varus or valgus deformities in the tibia

(Inman 1976},but hindfoot stiffness is common in

post-traumatic conditions (McMaster 1976} The Kerlan Jobe

group later repeated the initial series of experiments

with the subtalar joint immobilized by a Steinmann pin

to account for possible compensation by the subtalar

complex (Ting et al 1987} Subtalarmotion played a

sig-nificant role, and restriction of this joint affected the

contact area for all deformities of the tibia as the

result-ant fracture angle was increased (~ Fig 13-5) When

subtalar motion was restricted, the ankle contact area

decreased significantly in allplanes of angular

deformi-ty Restrietion of the subtalar joint had a greater effect on

the ankle contact area with varus deformities than with

valgus deformities Based on these results, in the

pres-ence of concomitant hindfoot stiffness, distal third

tibi-al angular deformities in vtibi-algus and recurvatum are

potentially at greatest risk of subsequently developing

degenerative arthropathic changes

McKellop et al (1991, 1994) expanded on this

ap-proach and used a similar model to assess the effect of

tibial deformities on joint contact pressures in the knee

Using pressure-sensitive film in cadaver limbs, they were

able to show a relationship between the magnitude of

angular deformity and the level of the deformity, with a

resultant increase in contact pressure across the knee

(~ Fig.l3-6).Analogous to the results with contact

pres-sures in the ankle, a particular magnitude of angular

deformity has its greatest effect on the nearest joint; an

angular deformity in the distal tibia affects contact

pres-sure at the ankle, whereas angular deformities of the

proximal tibia have a greater effect on contact pressure

in the knee Puno et al ( 1987} had already suggested this,

based strictly on geometric analysis Rather than

con-sider angulation exclusively, they calculated

malalign-ment based on the magnitude of angulation and the

lev-el of the deformity Unfortunatlev-ely, although first to

CHAPTER 13 · ConsequencesofMalalignment ~~~

Fig 13-5

' ,-'

A

Superimposed tracing of tibiotalar contact areas Solid Une

represents tracing typical of neutral alignment Dashed line

represents diminished area of contact observed when tibia is loaded with simulated 15° valgus angular deformity of the dis- tal third and fixed subtalar joint L, Lateral; M, medial; A, axial (Reprinted with permission from Ting et al 1987)

document this important principle, they failed to

accu-rately distinguish malalignment from malorientation

This has resulted in some confusion when interpreting

the results of their subsequent clinical studies (Puno et

al 1991)

The association of malalignment with degenerative

ar-thropathy after meniscectomy is weil established (Allen

et al 1984), but the clinical course of an untreated

mal-aligned limb is not The natural history of idiopathic

de-generative arthropathy is more difficult to document

because of the protracted clinical course and the

wide-spread availability of several effective therapeutic

mea-sures There are no prospective studies to compare the

different treatment options available and few

longitudi-nal data to determine the clinical result in the absence of

intervention

Many of the natural history data were compiled in

Sweden, the earliest by Ahlback (1968) who presented a

radiographic review but failed to correlate symptoms

with radiographic appearance He did recognize the

need for weight bearing films to assess the extent of

ar-ticular cartilage erosion, but the initial radiographs that

he reviewed were obtained with the patients supine,

limiting the value of the observations Hernborg and

Nilsson (1977) reviewed 94 knees that did not undergo

surgical treatment, with a follow-up duration of 10-18

years after initial radiographs had established a

diagno-sis of osteoarthrodiagno-sis They successfully showed that the

course of the disease is generally unfavorable; the

condi-tions of half the patients deteriorated clinically, and

im-provement was rare Varus deformity, especially in

wom-en, was associated with a poor prognosis Odenbring et

al (1991) reviewed the clinical course of 189 knees with

isolated medial unicompartmental degenerative

arthro-sis that were followed for 16 years The majority (62%)

of the knees in the original study group underwent

ma-jor knee surgery, either high tibial osteotomy or total

joint arthroplasty Only 16% (31 of 189 knees) of the

ini-tial study group survived and did not undergo surgery

during the follow-up period Of these 31 knees with

me-dial arthrosis followed for the course of 16 years, 65%

had a poor result and 71% functioned on a low activity

level Of the 24 untreated knees followed with serial

ra-diographs during the follow-up period, the arthropathy

increased in severity in 83 %

An alternative means of investigating the natural

his-tory of malalignment is to consider the long-term

fol-low-up of malunited fractures (Kettelkamp et al 1988;

Kristensen et al 1989; McKellop et al 1994; Merchant

and Dietz 1989; Puno et al 1991; van der Schoot et al

1996) Although most clinicians suspect that excessive

angulation of a tibial fracture may predispose the

adja-cent knee or ankle to subsequent osteoarthrosis, there is

no general agreement regarding the acceptable Iimits for alignment after fracture reduction (Nicoll 1964; Rose-meyer and Pförringer 1979; Sarmiento et al 1984) Rec-ommendations are based largely on the clinical impres-sions and experience of various authors, taking into consideration disturbances of gait, appearance, and the potential complications of different methods of treat-ment The cause of degenerative arthropathy is un-doubtedly multifactorial, and although trauma is prob-ably the most common inciting event, there are many other associated factors Loadtransmission across joints reflects additional elements beyond mechanical align-ment and joint orientation Soft tissues, including mus-des, Iigaments, and meniscal cartilage, also participate

in joint function, and pathological conditions in these associated structures may play an important role in de-termining the ability of articular cartilage to respond adequately to increased stress imposed by malalign-ment Conversely, pathological conditions in associated structures may be weil tolerated in the normally aligned limb, yet the malaligned limb may be predisposed to premature degenerative changes These confounding variables have made it extremely difficult to obtain meaningful data from retrospective studies of posttrau-matic deformity

Consider also the effect of patient selection on the

da-ta pool The residual angulation after a fracture heals is either acceptable or unacceptable to both the patient and the treating physician When judged unacceptable,

it is corrected, either for functional or cosmetic reasons Alternatively, substantial radiographic angulation may

be compensated by adaptations of gait or a reduction in activity level If at some later point the limb becomes symptomatic, reliable forms of treatment are again readily available It would, therefore, be very unusual for

a malunited limb with significant deformity to develop degenerative arthropathy and not be corrected Any ret-rospective study involving the long-term follow-up of malunited fractures is, unfortunately, fundamentally flawed by this inherent bias in patient selection The converse is also true, and a retrospective review of a se-ries of patients treated for arthrosis developing second-ary to fracture malunion would be similarly flawed Recognizing the limitations of these studies, the re-sults nonetheless merit careful consideration Kettel-kamp et al (1988) provided clinical data suggesting a direct relationship between malalignment and subse-quent degenerative arthritis Fourteen patients with malaligned fractures of either the femur or tibia were evaluated 32 years after the initial injury Using static force analysis, they noted that an increase in the angula-tion of the knee, beyond that due to the original defor-mity, was approximately a linear function of the product

of increased force on either the medial or lateral tibial plateau and time since original injury They suggested

that the unicompartmental degeneration observed

dur-ing follow-up was a result of the increased stress and

mechanical demands arising from the fracture

angula-tion and malalignment Unfortunately, the study

popu-lation from which they drew their conclusions was

high-ly selected and the data therefore skewed

Several other groups have attempted to assess the

possible consequences of tibial malunion, and therefore

malalignment, on a consecutive series of patients in a

retrospective fashion Merchant and Dietz (1989)

re-viewed 37 patients with isolated tibial shaft fractures an

average of 29 years after original injury They found that

varus angulation greater than so was associated with

ra-diographic changes in the ankle, consistent with early

arthrosis, but were unable to document any significant

difference between fractures of the distal third

com-pared with fractures of the proximal third Theywere

al-so unable to distinguish any significant difference in the

radiographic appearance or clinical function of the

ad-jacent knee and ankle in those patients with a

combina-tion of so of angulation in the frontal plane and 10° in the

sagittal plane compared with those patients with less

substantial angulation Kristensen et al ( 1989) reviewed

92 patients an average of 28 years after they had

sus-tained an isolated fracture of the tibia Only 17 patients

had angulation exceeding 10°, but all had normal

func-tion of the ankle and no pain None of the patients in

their study group developed radiographic signs of

ar-throsis in either the ankle or the knee Patients who

re-ported mild or moderate pain had an associated

restric-tion in the range of morestric-tion of the tibiotalar or subtalar

joints, validating some of the results obtained in the

ca-daveric studies using pressure-sensitive film (Ting et al

1987) The conclusions based on these two retrospective

studies are in general agreement that limited angular

deformity is of little clinical significance

Unfortunately, both these retrospective clinical

stud-ies assessed angulation alone and failed to consider the

additional elements that contribute to malalignment

These additional elements include not only the level of

the deformity but also the presence or absence of

con-comitant translation Translation in the coronal plane

can either contribute to the overall malalignment and be

considered aggravating or may diminish the

malalign-ment and be considered compensating (Paley et al

1990) It would be most interesting to reassess the data

after measuring the extent of MAD to determine

wheth-er thwheth-ere is any correlation with clinical outcome

Puno et al (1991} retrospectively reviewed 28 tibial

fractures 6-12 years after initial injury The patients

were evaluated by compiling a clinical rating based on

pain, function, motion, and radiographic appearance

Malorientation ofboth the knee and ankle joints

result-ing from the tibial angular deformity was calculated

us-ing the mathematical method they had previously

de-scribed (Puno et al 1987} Patients were then classified

according to the degree of knee and ankle tion and not only according to the magnitude of the an-gular deformity However, again they failed to properly distinguish malalignment from malorientation, making

malorienta-it more difficult and confusing to interpret the results They were able to document a significant correlation be-tween clinical outcome and ankle malorientation but were unable to show any significant correlation between clinical outcome and knee malorientation Their results suggest that malorientation, not simply angulation, is important in determining the possibility of progression

to premature degenerative arthrosis after malunion of

an isolated tibial fracture Malorientation is a function

of the level of the apex of deformity and the magnitude

of the angular deformity

van der Schoot et al (1996} published a retrospective analysis of isolated tibial shaft fractures and attempted

to correlate angular malunion with degenerative pathy in the adjacent joints A total of 88 patients were available for follow-up an average of 1S years after sus-taining the injury They reported a significant relation-ship between tibial malalignment and subsequent devel-opment of degenerative changes in the knee and ankle, but the data are unimpressive The authors did deter-mine the true magnitude of the deformity by a geomet-ric calculation but failed to determine the extent of ma-lalignment using standing long radiographs Although

arthro-10 fractures healed with an angular deformity greater than 10°, this group was not distinguished from the oth-ers during statistical analysis The data focused instead

on the association of previous fractures and subsequent development of degenerative arthropathy This is of some interest, but it fails to address the fundamental question regarding the possible relationship between malalignment and late articular cartilage degradation Frontal plane malalignment not only affects the dis-tribution of load across the medial and lateral compart-ments of the knee but also disturbs the relationship of the patella to the trochlear groove Elahi et al (2000} in-vestigated this in depth, using radiographic methods, and showed that both varus and valgus malalignment can increase the risk of patellofemoral osteoarthrosis In

a review of 292 patients with degenerative tis, the mechanical axis was assessed using standing long radiographs The direction of the deformity correlated well with the patellar facet involved: lateral with valgus and medial with varus Although not specifically ad-dressed, superimposed malrotation could further infiu-ence this relationship Combined valgus and external ro-tation likely has the greatest risk of premature lateral patellofemoral arthrosis

osteoarthri-Malalignment alone may not be responsible for teoarthritis but is a predisposing factor Additional fac-tors must also be considered that refiect the demands placed on the joint over manyyears Sharma et al (2000}

os-confirmed the intuitive relationship between obesity,

varus malalignment, and the severity of medial

gonar-throsis Varus malalignment was only one factor that,

over time, rendered the knee more vulnerable to the

ef-fects of obesity

In this chapter, we have emphasized the importance

of proper alignment of the lower extremity to avoid

pathological loading that could lead to osteoarthritis

However, it is just as important to maintain correct

alignment in cases of total knee replacement (TKR)

(Krackow 1983) A malaligned knee prosthesis can lead

to early loosening and premature excessive wear of the

polyethylene (Ritter et al.l994) Ligamentous imbalance

after TKR leads to faulty tracking, abnormal component

contact, and excessive polyethylene wear Mont

(unpub-lished results) has described six potential malalignment

pairs in association with TKR: varus-valgus,

fiexion-ex-tension, internal-external rotation, medial-lateral

dis-placement, proximal-distal disdis-placement, and

anterior-posterior displacement Each of the three prosthetic

components (patella, tibia, femur) could theoretically be

malaligned in any combination of the above mentioned

malalignment pairs, thus rendering a potential! 08

com-binations of malalignment In addition, components can

be undersized, leading to bone overload, and oversized,

leading to Iimitation of motion and soft tissue pain

Malalignment in cases of TKR may occur in

associa-tion with preexisting bone deformities and/or

ligamen-tous laxity Bone resection and soft tissue balancing can

be performed at the time of TKR to address some but

not all of these preexisting conditions In certain more

severe cases, particularly if the bone deformity is not

ad-jacent to the knee (as may occur in femoral diaphyseal

malunion, for example), it may be necessary to treat the

bone malunion before attempting TKR In our

experi-ence, realignment of severe deformities in preparation

for TKR results in such a satisfying clinical

improve-ment that TKR is no Ionger required The technical

de-tails and considerations of bone resection and soft

tis-sue balancing have been described (Hungerford and

Mont, unpublished results) (Hungerford 1995; Krackow

1983; Laskin 1981; Wolff et al 1991) (see Chap 23)

The axial relationship of the joints of the lower ity refiects both alignment and orientation Static con-siderations are useful for preoperative planning and deformity correction, but dynamic considerations, in-cluding compensatory gait, may be morerelevant clini-cally Laboratory animal models have been developed that simulate the deleterious effect of malalignment on articular cartilage Malalignment disturbs the normal transmission of force across the knee, and altered stress distribution related to deformity has been shown in ca-daver models using pressure-sensitive film No prospec-tive data are available to document the natural history of malalignment, but several retrospective studies suggest that the clinical course is one of gradual progression resulting in degenerative arthropathy The long-term follow-up of fractures is less definitive and is difficult to interpret, considering the bias inherent in patient selec-tion Although direct clinical evidence of a cause and effect relationship between malalignment and arthrosis has not been possible, substantial evidence from the orthopaedic Iiterature supports this hypothesis

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