A theoretically attrac-tive concept compared with fixed-bearing knee arthroplasty, MBKA was described as an evolutionary advance in total knee design.3 All MBKA devices can be characteri
Trang 1Mobile-bearing knee arthroplasty
(MBKA) was introduced in the late
1970s for two main purposes.1,2 The
first was to increase contact area so
as to reduce long-term wear, and the
second was to recreate normal knee
kinematics A theoretically
attrac-tive concept compared with
fixed-bearing knee arthroplasty, MBKA
was described as an evolutionary
advance in total knee design.3 All
MBKA devices can be characterized
as involving a moving polyethylene
bearing separating the femoral
condyle from the tibial tray Most
orthopaedic total joint
manufactur-ers have developed or are currently
developing an MBKA prosthesis
Theoretically, the increased
gruity and decreased axial
con-straint of an MBKA prosthesis
should lead to less penetrative wear
of the polyethylene and reduced loosening torque at the prosthesis-bone interface.4 Other stated bene-fits are improved patellofemoral and tibiofemoral biomechanics with increased maximal flexion Al-though the reported results with current total knee arthroplasty (TKA) designs have been excellent,5-7 most surgeons consider youth a rela-tive contraindication Low-wear MBKA has been suggested as the next evolution in TKA design to ex-pand its indications to include the young, active patient.8 It is important for the surgeon who is considering using these prostheses to understand the elements of MBKA design, the kinematics of mobile-bearing TKA devices, the constraint-conformity conflict, reduction of wear, the po-tential for bearing dislocation and
breakage, stability, clinical results, and indications
Types of MBKA Devices
The term MBKA describes a variety
of dissimilar knee prostheses that fea-ture a mobile polyethylene bearing that articulates with a metallic fem-oral condyle and a metallic tibial tray Walker and Sathasivam9have classi-fied current designs into four types
on the basis of bearing mobility: (1) The “internal-external rotation only” design (Fig 1) allows the knee
to locate to a preferred rotational
ori-Dr Vertullo is Fellow, Division of Orthopaedic Surgery, Duke University Medical Center, Durham, NC Dr Easley is Assistant Profes-sor of Orthopaedic Surgery, Duke University Medical Center, Durham Dr Scott is Direc-tor, Insall Scott Kelly Institute for Orthopae-dics and Sports Medicine, New York, NY Dr Insall is Director, Insall Scott Kelly Institute for Orthopaedics and Sports Medicine One or more of the authors or the departments with which they are affiliated have received something of value from a commercial or other party related directly or indirectly to the sub-ject of this article.
Reprint requests: Dr Easley, Box 2950, Division of Orthopaedics, Duke University Medical Center, Durham, NC 27710 Copyright 2001 by the American Academy of Orthopaedic Surgeons.
Abstract
Mobile-bearing knee arthroplasty (MBKA) has potential advantages compared
with conventional fixed-bearing total knee arthroplasty (TKA) By allowing
unconstrained axial rotation, MBKA can offer greater articular conformity
with-out an increased probability of loosening due to increased axial torque Increased
articular conformity minimizes polyethylene contact stresses, thereby reducing
linear wear and subsurface fatigue failure Axial rotation of the platform also
enables self-correction of tibial component malrotation Despite these
advan-tages, the long-term clinical results obtained with current MBKA devices are
similar to those obtained with well-designed fixed-bearing TKA prostheses, with
no data suggesting their superiority The disadvantages of MBKA include
bear-ing dislocation and breakage, soft-tissue impbear-ingement, a steep technique learnbear-ing
curve, and concerns about volumetric wear Hypothetically, longer-term
follow-up of MBKA results may reveal a significant difference from fixed-bearing TKA
results as the fatigue failure threshold of incongruent polyethylene is exceeded.
J Am Acad Orthop Surg 2001;9:355-364
Mobile Bearings in Primary Knee Arthroplasty
Christopher J Vertullo, MBBS, FRACS(Ortho), Mark E Easley, MD,
W Norman Scott, MD, and John N Insall, MD
Trang 2entation Backward motion of one
condyle is accompanied by forward
movement of the other.10
(2) The “internal-external
rota-tion about a medial axis” design
(Fig 2) better simulates anatomic
motion as the normal knee rotates
through a longitudinal axis on the
medial tibial plateau.11,12
(3) The third type allows internal-external rotation and anteroposterior (AP) translation so that the knee can locate at a preferred rotational and translational orientation (Fig 3)
This design relies on ligamentous structures for stability and kinemat-ics This type can be posterior cru-ciate ligament (PCL)–retaining or PCL-sacrificing and includes menis-cal-bearing designs.10
(4) In the “guided motion” type, internal-external rotation is allowed, and AP translation is guided by intercondylar cams or guide sur-faces in an attempt to reproduce the
AP motion of the natural knee (i.e., back with flexion and roll-forward with extension) This type includes the posteriorly stabilized MBKA devices and designs that have an intercondylar saddle-shaped cam.13 Posteriorly stabilized designs produce roll-back only with flexion Saddle designs produce roll-back and roll-forward with flex-ion and extensflex-ion, respectively The guided roll-back achieved with these designs in high flexion is preferable to that achieved with rotation-only designs; however, it requires a partially conforming femoral-tibial bearing surface Some prostheses have two separate fully congruent bearing surfaces on each condyle, one for 0 to 8 degrees of flex-ion and one for 8 degrees to maxi-mum flexion
Mobile-bearing designs also dif-fer in the types of constraint mecha-nisms used to prevent bearing dislo-cation These include a cone-in-cone articulation of the polyethylene insert with a tray recess, longitudi-nal curved sliding tracks, or a tibial tray post that articulates with a poly-ethylene recess Some designs also include stops to limit excessive AP translation and/or rotation; how-ever, there are concerns about the generation of polyethylene wear particles Designs incorporating meniscal bearings on curved tracks
to allow axial rotation have been
criticized for not allowing AP trans-lation without approaching or re-ceding from each other, decreasing the meniscofemoral contact area.1
An MBKA prosthesis can be par-tially congruent or fully congruent The contact area differs between designs as a result of variations in the sagittal radius of the femoral condyle Partially congruent, or gait-congruent, MBKA devices have large contact areas in the first 20 degrees of flexion that decrease with flexion due to a decreasing sagittal radius (Fig 4) Gait-congruent pros-theses were designed so as to maxi-mize the contact area in the more important low end of the flexion range while decreasing the sagittal radius, to improve flexion range Fully congruent MBKA devices have
a constant sagittal femoral radius, allowing much larger contact areas However, fully congruent MBKA prostheses have a theoretical limit of
120 degrees of flexion due to poste-rior impingement of the tibial com-ponent.10
Some MBKA designs are offered
as part of a knee arthroplasty system with the ability to change from fixed-bearing to mobile-bearing intraoperatively once the knee disor-der has been better assessed Other MBKA systems are stand-alone, with only a mobile-bearing arthro-plasty possible (Table 1) Currently, the only devices approved by the US Food and Drug Administration (FDA) for noninvestigational use are the Low Contact Stress Rotating Platform and the Low Contact Stress Meniscal Bearing Knee (DePuy, Warsaw, Ind)
Kinematics of MBKA Devices
With progressive flexion, the nor-mal knee undergoes posterior dis-placement (roll-back) of the femur
on the tibia and internal rotation of the tibia This passive motion can
Figure 1 With the “internal-external
rota-tion only” design, there is a cone-in-cone
constraint mechanism.
Figure 2 With the “internal-external
rota-tion about a medial axis” design, there is
rotation about a longitudinal axis through
the medial tibial plateau.
Trang 3be described as femoral rotation
about an axis through the femoral
epicondyles coupled with tibial
rotation about an axis parallel with
medial to the long axis of the
tib-ia.11,12 Roll-back occurs primarily as
a function of the PCL, optimizing
the quadriceps lever arm in flexion
and allowing clearance of posterior
structures, thereby increasing the
range of flexion The increased
quadriceps lever arm is an
advan-tage during downhill walking and
stair descent
Although one of the original
aims of MBKA was to recreate
nor-mal knee kinematics,1,2this remains
theoretical The current literature
on MBKA kinematic behavior
sug-gests that no current design closely
mimics the normal knee, but rather
shows numerous kinematic
abnor-malities observed with fixed-bearing
designs.10,14,15 Abnormalities
in-clude paradoxical anterior femoral
translation, reverse axial rotational
patterns, and femoral condylar
lift-off As would be expected,
cruciate-sacrificing and posteriorly stabilized
rotating platforms show less AP
translation in gait and less
variabil-ity between individuals than
fixed-bearing designs, due to increased
sagittal femorotibial conformity.10
An in vivo fluoroscopic weight-bearing kinematic analysis of fem-oral roll-back in a cruciate-retaining mobile-bearing device suggested that it behaves unpredictably with abnormal kinematic function, simi-lar to other cruciate-retaining fixed-bearing designs.16 Roll-back oc-curred at up to 90 degrees of flexion, but anterior translation was ob-served with flexion greater than 90 degrees
An in vitro comparison of rotating-bearing and fixed-rotating-bearing knees with either cruciate substitution or cruciate retention demonstrated that all four designs underwent some degree of roll-back, but that both of the cruciate-retaining de-signs showed greater roll-back.14 However, the cruciate-retaining fixed-bearing knee in that study was less conforming than the cruciate-substituting fixed-bearing and rotating-bearing knees, and there-fore allowed greater AP translation
Normal internal tibial rotation with flexion is reduced in both fixed-bearing and rotating-only mobile-bearing total knees.10 PCL-substituting and PCL-sacrificing rotating-only designs showed tibial internal rotation with deep knee bends However, in gait the
cruciate-sacrificing rotating-only design underwent paradoxical tibial exter-nal rotation with gait
Rotating-platform and meniscal-bearing MBKA designs have similar
or less flexion than fixed-bearing designs.10,15,17-19 The least amount
of flexion occurs with the cruciate-sacrificing rotating-platform de-signs, which often exhibit anterior femoral roll-back in deep flexion An
in vitro study comparing the maximal flexion of multidirectional MBKA, rotating MBKA, AP-translating MBKA, and fixed-bearing TKA de-vices showed no differences.20 All articulating surfaces had identical
Figure 3 With this design, the polyethylene may rotate, translate, or rotate and translate
on the tibial baseplate The polyethylene is in a neutral position relative to the baseplate in
extension (A), but with flexion, it rotates (B) and translates (C).
Figure 4 Flexion may be enhanced by reducing the posterior sagittal radius of the femoral component relative to its radius in extension.
Trang 4geometry, and the PCL was
re-tained in all knees It has been
sug-gested that maximal flexion can be
obtained with a mobile-bearing
de-sign with a decreasing sagittal
con-dyle radius that is posteriorly
stabi-lized to achieve predictable femoral roll-back.10
Theoretically, the self-alignment ability of MBKA tibial platforms can improve patellofemoral mechanics
However, currently there is little
kinematic data to support this hy-pothesis Authors have reported minimal anterior knee pain and no patellar subluxation or dislocation
in large series,10,21but this may be attributable to excellent surgical
Table 1
Design Features of Various MBKA Devices *
(DePuy, Warsaw, Ind)
Meniscal Bearing
(DePuy)
(DePuy)
posteriorly stabilized Genesis II Mobile Bearing Gait-congruent Multidirectional or Tibial tray post PCL-retaining,
options
* Abbreviations: ACL = anterior cruciate ligament; AP = anteroposterior; PCL = posterior cruciate ligament.
Trang 5technique rather than prosthetic
design
Wear, Conformity, and
Contact Stress
Reducing the generation of
poly-ethylene wear particles improves
TKA survivorship by decreasing
loosening secondary to aseptic
osteolysis.22,23 Knee wear is a
com-plex, multifactorial process affected
by a wide range of variables,
in-cluding polyethylene quality and
processing, sterilization techniques,
articular kinematics, lubrication,
applied load, and articular
topogra-phy.24 Wear occurs at the superior
femur–polyethylene surface and
the inferior tibia–polyethylene
sur-face (backside wear) in both
fixed-bearing and mobile-fixed-bearing TKA
prostheses.25 The complexity of the
in vivo wear process in TKA is
demonstrated by the lack of a
coherent theoretical model and
conflicting in vitro wear data from
studies with nonuniform testing
conditions Researchers have
sug-gested that it is unlikely that any in
vitro test can become a primary
standard for wear measurements.24,26
Two types of wear patterns most
commonly occur in TKA: abrasive
wear and fatigue failure, which
pro-duces pitting and delamination.23
Fatigue failure commonly
pro-duces delamination and surface
pit-ting due to subsurface stress that
exceeds the polyethylene failure
threshold of 9 MPa.27 Round-on-flat
TKA designs with extremely high
contact stresses are an example of
nonconforming articulations that
are subject to accelerated
delamina-tion and pitting.28 By increasing the
articular conformity, subsurface
stresses can remain below the
stress-yield threshold, preventing fatigue
failure of polyethylene.27
Decreasing abrasive wear is a
more complex issue, with the
mini-mal contact stress value that
initi-ates in vivo wear affected by the polyethylene quality, material pro-cessing, sterilization techniques, lubrication, load, and articular kine-matics.25 In vitro evidence suggests that wear rates are less dependent
on contact stress below 6.9 MPa, a level above which wear rates accel-erate substantially.26
A recent finite-element analysis examined the effects of different conformity ratios and loads on polyethylene stress levels in total knee prostheses.29 A ratio of 0 rep-resented a flat-on-round design, and
a ratio of 0.99 represented a fully conforming design Polyethylene stresses were more sensitive to changes in conformity than to load changes Doubling the load from 3,000 to 6,000 N resulted in less stress increase than changing the conformity ratio from 0.99 (fully conforming) to 0.95 The effect of increasing conformity ratio on the reduction in stress was more pro-nounced for ratios above 0.8 The deleterious effect of a load increase for a flat tibial tray was double that for one with full congruity
By increasing articular conformity and decreasing contact stresses to less than 6.9 MPa, the most com-mon modes of TKA wear can be decreased However, the disadvan-tages of highly conforming fixed-bearing TKA devices are decreased freedom of motion and resultant increased transmission of torque forces to the bone-prosthesis inter-face.1,10,30 Mobile-bearing knee ar-throplasty has been proposed as a method to overcome this kinematic conflict of low-stress articulations with free rotation.1,31
Finite-element analysis suggests that MBKA devices generally achieve lower contact stresses and subsur-face stresses than fixed-bearing TKA devices at heel-strike.3,25,27,32 Predic-tions of low linear wear for highly congruent mobile-bearing knees derived from in vitro analysis have been matched by retrieval studies
of the fully congruent mobile-bearing Oxford Knee Replacement (Biomet, Bridgend, UK).33,34 The unicom-partmental Oxford device had a mean wear of 0.036 mm per year, and the bicompartmental device had a mean wear of 0.043 mm per year, both of which compare favor-ably with wear rates in highly con-forming total hip replacement Failure due to wear or osteolysis has been reported at very low rates
in clinical series of MBKA Sorrells35 reported the results with a partially congruent rotating platform at a follow-up interval of 1 to 11 years The failure rate due to wear was 0.2%; the single failure in that study was ascribed to poor-quality poly-ethylene He reported no aseptic loosening Callaghan et al21reported the results with 119 MBKA devices
of the same design There was no periprosthetic loosening or failure due to wear at a follow-up interval of
9 to 12 years At intermediate
follow-up of fully congruent multidirec-tional platforms used in 172 knees, Kaper et al36 reported that only 2 (1.2%) required revision due to wear However, these results are no better than those obtained with well-designed fixed-bearing TKA de-vices.6,37 In the longer term, a clear difference in in vivo wear may be-come apparent as the fatigue limit of fixed-bearing TKA prostheses is reached.38
An unanswered question remains about excessive volumetric wear in MBKA designs compared with fixed-bearing TKA designs.39 Dual-articulation MBKA devices typically have a much larger articulating-surface contact area than fixed-bearing TKA devices, especially fully congruent designs It is un-known whether, despite low linear wear rates, a greater or lesser vol-ume of particles is produced in vivo due to the larger contact area Conflicting in vitro evidence exists concerning volumetric wear
in MBKA Finite-element modeling
Trang 6based on current theories of
poly-mer failure suggests that low
con-tact stress will result in minimal
generation of abrasive wear
parti-cles.25 In an in vitro experimental
model, Jones et al39 suggested that
multidirectional polyethylene
plat-forms have wear rates nine times
greater than unidirectional
plat-forms In a 10-million-cycle knee
simulator study comparing a fully
congruent multidirectional
plat-form with a posteriorly stabilized
fixed-bearing device, the
multidi-rectional platform exhibited less
linear wear than the fixed-bearing
knee, but approximately 30% more
volumetric wear.15 Although
negli-gible wear occurred at the
femur-polyethylene surface, substantial
abrasive and adhesive wear
oc-curred at the tibia-polyethylene
articulation No delamination was
observed at either surface Further
in vivo investigation is necessary in
this area, especially regarding the
role of multidirectional platforms
and undersurface wear
It is unclear whether the optimal
articulation geometry for MBKA is
the fully congruent design with
proximal contact stresses of less
than 5 MPa over a surface area
greater than 1,000 mm2or a
gait-congruent MBKA design with
prox-imal contact stresses of 5 to 8 MPa
over a surface area of 500 mm2
Both designs can avoid the stress
thresholds associated with
delami-nation and pitting; however, the
issue of minimizing linear wear
ver-sus volumetric wear remains
unre-solved
It has been suggested that
MBKA is one solution to
wear-particle generation from the
under-surface of the polyethylene insert
in TKA This undersurface wear
has been related to poor locking
mechanisms in fixed-bearing TKA
devices that allow micromotion
against a rough tibial tray.40 In
the-ory, the easily manufactured,
pol-ished bearing surface of an MBKA
tibial tray can avoid this excessive backside wear.9 However, no evi-dence exists that MBKA devices dis-play less backside wear than fixed-bearing TKA prostheses (some of which are also available with a pol-ished tibial tray)
Two other design features of MBKA prosthesis may minimize undersurface wear First, baseplate stiffness in MBKA devices typically exceeds that in conventional fixed-bearing devices Mobile-fixed-bearing tibial plates constructed of cobalt-chrome alloys rather than titanium exhibit less deflection in load test-ing This increased stiffness allows the baseplate to maintain even load distribution for the polyethylene.15 Second, titanium tibial trays have poor wear characteristics that pre-clude their use in mobile-bearing knees in an untreated form.36 Most current MBKA designs use cobalt-chrome alloys
Wear-particle generation from MBKA constraint mechanisms and from the tibial tray or polyethylene insert has been raised as a potential concern for mobile-bearing knees.41 The results of recent experiments in which 1.5 million cycles of 400- to 800-N shear stress was applied to a multidirectional MBKA suggest that no plastic deformation of the polyethylene occurred due to the presence of tibial-baseplate stop mechanisms designed to limit poly-ethylene rotation and/or transla-tion.15 However, malalignment and poor ligament balancing in MBKA could produce cyclic poly-ethylene impingement on con-straint stops in gait, producing excessive polyethylene particle generation
Rotating-platform devices toler-ate modertoler-ate amounts of tibial component malrotation compared with fixed-bearing TKA prosthe-ses.36 An in vitro cadaver analysis showed lower femur–polyethylene surface peak stress in MBKA de-vices with 15 degrees of tibial
malro-tation compared with fixed-bearing designs.42 This advantage, however, does not obviate good surgical tech-nique With severe malrotation (45 degrees), edge loading occurs, with the polyethylene bearing overhang-ing the tibial tray, markedly in-creasing undersurface stress The argument that less wear debris is generated with an MBKA device than with a well-designed fixed-bearing TKA prosthesis has not been proved.10 It must be re-membered that some fixed-bearing TKA knees have lower contact stresses in flexion than existing MBKA knees.10,42
Reduction in Torque Forces
An ideal TKA design maximizes articular conformity while minimiz-ing axial constraint Constraint is the resistance to a particular degree
of freedom, such as AP translation
or axial rotation.9 Conformity is a geometric measure of the closeness
of fit of the knee articulation In a flat-tibia TKA design, constraint is nominally zero, except for friction at the articulating surfaces; in a hinged TKA, the constraint is infinite.9,30 Hinged knees and early highly con-strained TKA designs transferred ex-cessive axial torque or varus-valgus forces to the prosthesis-bone inter-face and tended to fail early.35,43,44 Fixed-bearing TKA cannot be fully conforming without being exceed-ingly constrained to axial rotation, transferring large rotational stresses
to the prosthesis-bone interface Mobile-bearing devices can over-come this conformity–axial con-straint conflict by allowing uncon-strained axial rotation with fully conforming articulations, reducing the axial stress to the prosthesis-bone interface.10,45
Unanswered questions remain about modern fixed-bearing de-signs that are more conforming
Trang 7than earlier round-on-flat TKA
prostheses By increasing
confor-mity to decrease wear, theoretically
more axial torque is applied to the
prosthesis-bone interface.30,44 The
threshold of axial torque stress at
which prosthesis loosening occurs
is unknown; however, minimizing
this stress appears to be
advanta-geous Improvements in
polyethyl-ene wear properties and
quantifica-tion of the acceptable degree of
constraint may allow fixed-bearing
prostheses to outperform MBKA
devices
Stability
Ligamentous competence has a
greater role in MBKA than in
fixed-bearing TKA Bicompartmental
meniscal-bearing knees can retain
either both cruciate ligaments or just
the PCL Higher failure rates for
meniscal-bearing MBKA with an
incompetent anterior cruciate
liga-ment (ACL) or ACL sacrifice have
been reported by a number of
authors.4,10,43,46,47 In one study,4 a
bearing fracture was related to ACL
sacrifice that allowed posterior
sub-luxation However, other clinical
studies have reported excellent
results with an ACL-sacrificing
mobile-bearing knee.10
The production of anterior
soft-tissue impingement by excessive
anterior-platform translation of a
multidirectional MBKA device has
been reported In one study of 16
cruciate-retaining multidirectional
MBKA devices,48 9 demonstrated
unrestricted anterior translation of
the platform with resultant
im-pingement on the patellar tendon
The design lacked a stop
mecha-nism for excessive translation All 9
multidirectional platforms that
al-lowed impingement had to be
con-verted to a rotating platform The
authors theorized that PCL
incom-petence in a cruciate-retaining
de-sign results in excessive anterior
translation Cruciate-retaining mul-tidirectional MBKA devices are more reliant on PCL competence than similar fixed-bearing TKA devices, making successful recess-ing of a tight PCL imperative.48 Stop mechanisms may decrease or prevent excessive AP translation in
a multidirectional MBKA prosthesis
in a patient with an incompetent PCL.15
Matsuda et al,20in an in vitro cadaver study, compared stability
in multidirectional MBKA, rotating MBKA, AP-translating MBKA, and fixed-bearing TKA prostheses and
in normal knees The multidirec-tional and AP-translating devices showed increased AP laxity in the absence of the ACL compared with normal knees and rotating MBKA and fixed-bearing TKA devices
The rotational stability of multidi-rectional and rotating MBKA de-vices was similar to that of normal knees; however, the fixed-bearing TKA showed decreased rotational deflection, which is evidence of rotational stress being transmitted
to the prosthesis-bone interface
Hence, rotating and multidirectional MBKA devices have an advantage over fixed-bearing TKA prostheses
in maintaining rotational stability while decreasing axial stress load
The authors also noted that malrota-tion of tibial components in fixed-bearing TKA results in a flexion contracture due to poor articular-surface mating
Dislocation and Breakage
In addition to the accepted complica-tions associated with fixed-bearing TKA devices, the complexity of a mobile-bearing platform or menis-cus increases the chance of bearing subluxation or dislocation, bearing breakage, and soft-tissue impinge-ment Bearing dislocations have been reported with a variety of MBKA designs; however, it must be
noted that some large series have reported no bearing dislocations with either multidirectional36 or rotating platforms.21 Dislocation of rotating-only platforms was reported
in 4 (9%) of 43 patients in one series.43 However, the rate was only 0.15% in
a much larger series (665 patients) involving the same MBKA design.35 Reported rates of dislocation or subluxation with bicompartmen-tal meniscal-bearing devices have ranged between 2.2%49 and 7.6%.44 Dislocation rates of unicompartmen-tal meniscal bearings show a wide disparity between surgeons, hospi-tals, and countries.10,50 From the lit-erature, it is evident a steep learning curve is associated with patient se-lection and surgical technique.35 Smaller series have a much higher rate of complications Many authors have emphasized good surgical technique to avoid bearing disloca-tion, especially balancing of flexion and extension gaps.15,36,43 Many designs incorporate platform stop mechanisms that may reduce the risk of dislocation
Broken bearings appear to be more common with meniscal-bearing MBKA prostheses that utilize curved tracks without stops Of the 16 pa-tients in one series,44 (25%) had bro-ken lateral meniscal bearings, com-pared with 7 (1.5%) in a larger se-ries of 473 patients.49 The authors theorized that the bearing breakage was due to posterior subluxation of the bearing and entrapment be-tween the femoral component and the posterior edge of the bearing track
Clinical Results
The results of MBKA at intermedi-ate to long-term follow-up are equal
to the best results reported for fixed-bearing TKA Results at follow-up intervals in excess of 5 years are available for four prostheses: the Low Contact Stress Rotating
Trang 8Plat-form (DePuy), the Low Contact
Stress Meniscal Bearing (DePuy),
the Self Aligning Knee (Sulzer,
Austin, Tex), and the Oxford
Bi-compartmental Knee Replacement
(Biomet)
Callaghan et al21reported the
9-to 12-year follow-up data on 119
consecutive cruciate-sacrificing,
cemented Low Contact Stress
rotat-ing platform meniscal-bearrotat-ing
knees No periprosthetic osteolysis
or loosening was observed, and no
revisions were required The
aver-age Hospital for Special Surgery
scores improved from a
preopera-tive value of 57 to 84 at follow-up
Average clinical and functional
Knee Society scores improved from
30 and 44 preoperatively to 90 and
75 at follow-up
In another investigation,49 the
Kaplan-Meier survival estimate at 8
years for 473 consecutive
cement-less cruciate-retaining Low Contact
Stress Meniscal Bearing Knees was
94.6% This investigation included
a small percentage of bilaterally
cruciate-preserving prostheses
Buechel and Pappas51have
sup-ported the findings of Callaghan et
al.21 In their study of a rotating
plat-form device, the 10-year survival
rate for cemented prostheses was
97.5%, and the 6-year survival rate
for cementless designs was 98.1%.51
Sorrells35also reported favorable
results with the Low Contact Stress
Rotating Platform prosthesis In a
series of consecutive noncemented,
PCL-sacrificing Low Contact Stress
Rotating Platform implants, the rate
of a good or excellent outcome at 1
to 11 years was greater than 98%,
and the survivorship at 11 years
was 95%
Results with the Oxford
Bicom-partmental meniscal-bearing knee
have been similar, provided the
ACL remains intact.46,52 At the
6-year follow-up, the success rate
was 93% for patients with intact
ACL function, compared with 73%
when the ACL was compromised
Kaper et al36reported the results with 172 PCL-retaining multidirec-tional Self Aligning Knee prostheses
The mean follow-up interval was 5.6 years (range, 5 to 8 years) Of the 141 patients, 132 (94%) described their sults as good or very good Two re-vision procedures were necessitated
by polyethylene wear, but none of the remaining knees showed evi-dence of wear Kaplan-Meier sur-vival curves showed the probability
of survival to be 91.7% with revision surgery for any reason as an end point and 98.8% for revision surgery because of polyethylene wear as an end point
Short-term follow-up for other mobile-bearing TKA devices appears promising,15 but longer follow-up for some of these designs remains unpublished Furthermore, these studies involved older patients (aged
64 to 70 years); no results are avail-able for mobile-bearing TKA per-formed in cohorts limited to younger, more active patients
The results of MBKA have been similar to those obtained with well-designed fixed-bearing TKA devices
Investigators have reported 95%
good-to-excellent results and implant survival rates consistently greater than 94% with conventional fixed-bearing TKA devices at a follow-up interval of 10 to 15 years.5-7,37 Re-ports of fixed-bearing TKA in pa-tients less than 55 years are en-couraging53-55; however, the average follow-up interval was less than 10 years
Indications and Limitations
The literature on the indications for MBKA is scant Theoretically, MBKA may offer improved kine-matics and reduced wear to the rela-tively young patient; however, no data exist to support this hypothesis
It is clear that not all patients are candidates for MBKA Obviously,
severe malalignment and ligamen-tous incompetence necessitate the use of constrained fixed-bearing knees, especially in the elderly.56 However, the degree of deformity that can be treated with MBKA is unclear Marked fixed flexion defor-mities make accurate soft-tissue bal-ancing of the knee while retaining the PCL technically difficult In this situation, multidirectional platforms that rely on the PCL to avoid exces-sive anterior translation would be relatively contraindicated
With MBKA-only systems, the surgeon must decide preoperatively which knee system to use or accept the added cost of having both a fixed-bearing and a mobile-bearing system available intraoperatively Total knee arthroplasty systems that allow the surgeon intraoperative choice in the type of bearing to use are more flexible; however, none of these systems has FDA approval, nor are there intermediate clinical results available
No clear indications yet exist as
to the appropriate clinical applica-tions for the large variety of MBKA permutations currently available Rotating platforms offer greater
AP stability and less reliance on PCL function compared with PCL-retaining multidirectional plat-forms, while being rotationally unconstrained and self-aligning Multidirectional platforms with stop mechanisms appear to over-come this concern
Summary
The MBKA designs have theoreti-cal advantages over fixed-bearing designs due to their ability to de-crease axial constraint while pro-viding less linear wear by virtue
of increased conformity and self-correction of tibial component mal-rotation However, MBKA devices have disadvantages related to bear-ing dislocation and breakage,
Trang 9soft-tissue impingement, technique
learn-ing curve, and concerns about
volu-metric wear Current MBKA designs
have kinematic abnormalities similar
to those that fixed-bearing knees
exhibit
The long-term clinical results
with both fixed-bearing and
par-tially congruent rotating-platform
MBKA devices are excellent,
al-though the theoretical benefits of
MBKA have not yet improved the
clinical results compared with
fixed-bearing TKA Hypothetically,
longer-term follow-up may reveal a
statistically significant difference as
the fatigue failure threshold of
fixed-bearing knees is exceeded,37but
there are as yet no clinical data
Surgeons willing to accept the
disad-vantages of MBKA can continue to
do so, knowing that the current clin-ical results of MBKA match those obtained with the best fixed-bearing TKA devices, with the chance of improved survival past 20 years
Mobile-bearing designs offer a confusing array of options, includ-ing degree of conformity, constraint mechanisms, directional mobility of the bearing, and PCL management
Results at follow-up intervals longer than 10 years are available only for partially congruent, PCL-sacrificing rotating platforms and PCL-retaining meniscal-bearing designs The in-termediate results for
multidirection-al MBKA are promising; however, longer-term results are needed to overcome concerns about AP instabil-ity and volumetric wear Evidence-based, stepwise introduction of new
orthopaedic devices allows safe and controlled implementation of new technologies while exposing as few patients as possible to the risk of fail-ure.57 When used as part of a TKA system, MBKA appears to allow re-duction in inventory and offer greater intraoperative flexibility
A mobile bearing will not com-pensate for poor surgical technique
or basic TKA design flaws Al-though platforms are rotationally self-aligning, this does not allow decreased attention to surgical tech-nique Avoidance of bearing dislo-cation and breakage is dependent on balanced flexion and extension gaps
In summary, MBKA is theoretically attractive, but there are as yet no data indicating that it is superior to fixed-bearing TKA
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