Clinical experience has shown that an isolated ruptured medial collateral ligament MCL of the knee can heal with nonoperative care, but that a midsubstance tear of the anterior cruciate
Trang 1The incidence of knee ligament
injuries resulting from sports
activi-ties has stimulated interest in
liga-ment healing Clinical experience
has shown that an isolated ruptured
medial collateral ligament (MCL) of
the knee can heal with nonoperative
care, but that a midsubstance tear of
the anterior cruciate ligament (ACL)
usually does not An ACL deficiency
can result in chronic knee instability
with a 2-degree injury to other
soft-tissue structures and eventually
progressive osteoarthritis Primary
surgical repair of the ACL has not
been successful; as a result,
recon-struction with the use of
replace-ment grafts is usually performed to
restore knee stability
Studies involving animal models
have also demonstrated that isolated
MCL injuries can heal
spontane-ously, with excellent knee function
These results have supported the clinical decision that grade III MCL injuries should continue to be treated nonoperatively The ability of the MCL to heal primarily offers an op-portunity for the examination of the mechanism of ligament healing
These experiments have provided useful information on the outcome
of isolated MCL injuries as well as the effect of intrinsic and extrinsic factors on that outcome The asso-ciation of an ACL disruption with an MCL tear completely changes the prognosis, however Tissue engi-neering, including the use of growth factors, cell therapy, and gene transfer techniques, has shown some potential for enhancing the healing process Although most of the experimentation has dealt with
the ACL and MCL in the knee, the principles elucidated and the knowl-edge gained can serve as the basis for further studies to improve un-derstanding of the healing of other ligaments and tendons as well
Healing of Grade III MCL Tears
Although methods of treating liga-mentous injuries have seen sub-stantial improvements in recent years, there remain many questions about enhancing the rate, quality, and completeness of extra-articular ligament healing To adequately address the intricacies of this process and improve on techniques used in clinical practice, it is essen-tial to increase our knowledge
Dr Woo is Ferguson Professor of Orthopaedic Surgery and Director, Musculoskeletal Research Center, University of Pittsburgh.
Ms Vogrin is Research Engineer, Musculo-skeletal Research Center Mr Abramowitch is Graduate Research Assistant, Musculoskeletal Research Center.
One or more of the authors or the department 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 Woo, Musculoskeletal Research Center, PO Box 71199, Pittsburgh,
PA 15213.
Copyright 2000 by the American Academy of Orthopaedic Surgeons.
Abstract
Although methods of treating ligamentous injuries have continually improved,
many questions remain about enhancing the rate, quality, and completeness of
ligament healing It is known that the ability of a torn ligament to heal depends
on a variety of factors, including anatomic location, presence of associated
injuries, and selected treatment modality A grade III injury of the medial
collat-eral ligament (MCL) of the knee usually heals spontaneously Surgical repair
followed by immobilization of an isolated MCL tear does not enhance the healing
process In contrast, tears of the anterior cruciate ligament (ACL) and the
poste-rior cruciate ligament often require surgical reconstruction The MCL
compo-nent of a combined ACL-MCL injury has a worse prognosis than an isolated
MCL injury The results of animal studies suggest that nonoperative treatment
of an MCL injury is effective if combined with operative reconstruction of the
ACL Experimentation using animal models has helped to define the effects of
ligament location, associated injuries, intrinsic factors, surgical repair,
recon-struction, and exercise on ligament healing New techniques utilizing growth
factors and cell and gene therapies may offer the potential to enhance the rate and
quality of healing of ligaments of the knee, as well as other ligaments in the body.
J Am Acad Orthop Surg 2000;8:364-372
Savio L.-Y Woo, PhD, DSc, Tracy M Vogrin, MS, and Steven D Abramowitch
Trang 2about the basic science of ligament
healing
Due to the accessibility, frequency
of injury, and healing properties of the
MCL, it has become a primary model
for scientific research, the results of
which can be transferred to many of
the other ligaments in the body The
healing process in an isolated MCL
tear is affected by various systemic
and local factors and is somewhat
similar to that in vascular tissues.1
Clinically, grade I and grade II MCL
injuries heal well within 11 to 20 days
after injury.2 In contrast, healing of a
grade III MCL tear may continue for
years after the initial injury
The healing process can be
roughly divided into four
overlap-ping phases: hemorrhage,
inflam-mation, repair, and remodeling Its
description can be divided into
his-tologic, biochemical, and
biome-chanical events
Histology
After a midsubstance tear of the
MCL (which is characterized by the
mop-end appearance of its torn
ends), the hemorrhage phase begins
with blood flowing into the gap
cre-ated by the retracting ligament to
form a hematoma In response to
the increased vascular and cellular
reactions resulting from the injury,
inflammatory and monocytic cells
migrate into the injury site, convert
the clot into granulation tissue, and
phagocytize the necrotic tissue
This marks the beginning of the
in-flammatory stage Within
approxi-mately 2 weeks, a continuous
net-work of immature, parallel collagen
fibers replaces the granulation
tis-sue The inflammatory phase
con-cludes with the formation of
extra-cellular matrix in a random pattern
in the central region of the ligament
The formation of extracellular
matrix by fibroblasts also marks the
beginning of the reparative phase,
in which the ligament superficially
resembles its preinjury appearance
The torn ends of the ligament are
no longer visible, and the granula-tion tissue has been replaced by immature, parallel collagen fibers
New blood vessels begin to form, and fibroblasts continue to actively produce extracellular matrix This phase begins within 5 to 7 days after injury and concludes after sev-eral weeks
Overlapping with the reparative phase, the remodeling phase be-gins several weeks after injury and continues for months or even years
In this phase, collagen fibers con-tinue to align along the long axis of the ligament, resulting in increased maturation of collagen matrix The conversion from a random pattern
to one demonstrating alignment of the fibers has been shown to corre-late directly with an improvement
in the biomechanical properties of the ligament
Long-term animal studies have demonstrated that the histologic and morphologic appearance of healed ligaments is different from that of injured ligaments When tis-sue is viewed by using transmission electron microscopy after 2 years of healing, the number of collagen fibrils is increased compared with the noninjured ligament, but their diameters and masses are actually smaller.3 Additionally, “crimping”
patterns within the healing liga-ment remain abnormal for up to 1 year, and collagen fiber alignment remains poor.1,4
Biochemistry
Studies conducted on animal models have shown that the extra-cellular matrix of the healing liga-ment exhibits a number of important changes, particularly in glycosami-noglycans, elastin, and other glyco-proteins Early in the process, there are changes in collagen fiber type and distribution, with a greater pro-portion of type III fibers than in nor-mal ligaments This ratio returns to normal after approximately 1 year.1 Although the healing ligament
shows increases in the number of collagen fibers, the number of ma-ture collagen cross-links is only 45%
of the normal value after 1 year.5 If the joint is immobilized during the healing process, significant changes
in collagen synthesis and degrada-tion can occur There is a direct rela-tionship between the decrease in the biomechanical properties of the healing MCL and the number of col-lagen cross-links, as well as a de-crease in the mass and diameter of the collagen fibers.5 The collagen types at the bone-MCL interface have also been studied; types II, IX,
X, and XIV have been identified.6 Further studies are needed to assess whether ligament injury and healing affect the presence and amount of these types of collagen
Biomechanics
Biomechanical characterization
of a healing ligament is based on two elements Functional testing involves determination of the con-tribution of the ligament to knee kinematics as well as the in situ forces in the ligament in response
to external loading conditions Tensile testing provides an assess-ment of the structural properties of the bone-ligament-bone complex and the mechanical properties of the ligament substance.4
To analyze the functional capa-bilities of the healing MCL, the effect of surgical repair and immo-bilization on the varus-valgus laxity
of the knee was studied Canine MCLs with grade III injuries that were treated nonoperatively with early mobilization and full weight bearing demonstrated restoration of normal stability by 48 weeks post-operatively Ligaments that were surgically repaired and then immo-bilized tended to have more valgus laxity than control ligaments It is important to note, however, that the number of degrees of freedom al-lowed during testing of the knee can have a considerable impact on
Trang 3the results obtained.4,7 After
sec-tioning of the MCL, only small
in-creases in valgus laxity (21%) were
observed if knee motion was
al-lowed in all directions However,
when anterior-posterior translation
and internal-external rotation were
constrained, valgus laxity increased
significantly (171% [P<0.05]) These
results suggest that with normal
knee joint motion, other structures,
especially the ACL, compensate for
the absence of the MCL during
val-gus rotation.4
A robotic universal force-moment
sensor testing system provides a
method for making multiple
determi-nations of knee kinematics as well as
the in situ force or tension in
liga-ments in response to external loading
conditions.8 With this technology,
the ability exists to evaluate various
knee conditions and reconstruction
techniques with comparison to the
intact knee, thus minimizing
intra-specimen variability The potential
also exists to characterize knee
kine-matics in vivo and to determine the
in situ forces in knee ligaments for in
vivo loading conditions In one study
using this testing system in a goat
model,9the healing MCL showed
increased valgus rotation compared
with control ligaments at both 6 and
12 weeks However, knee stability
did not improve from 6 to 12 weeks
Future studies using this system will
examine the effects of ACL
recon-struction in a combined ACL-MCL
injury model
Tensile testing can provide
valu-able information on the strength and
quality of healing tissue and can
allow comparison with the intact
lig-ament Two sets of data can be
obtained from a uniaxial tensile test:
the load-elongation curve illustrates
the structural properties of the
bone-ligament-bone complex, and the
stress-strain curve demonstrates the
mechanical properties of the
liga-ment substance Figure 1 shows a
typical load-elongation curve The
curve is nonlinear and consists of a
toe region, a linear region (where the slope reflects the stiffness of the femur-MCL-tibia complex), and a failure region The variables repre-senting the structural properties of the femur-MCL-tibia complex in-clude linear stiffness, ultimate load, and energy absorbed at failure
Figure 2 is a typical stress-strain curve of the MCL midsubstance By normalizing for the cross-sectional area of the tissue, the stress (defined
as force per unit area) can be calcu-lated The strain is calculated as the change in length of the tissue under the tensile load, divided by its origi-nal length A nonlinear stress-strain relationship can therefore be illus-trated, with toe, linear, and failure regions similar to those of the load-elongation curve Properties that can be obtained from this graph include modulus of elasticity (the proportional constant between stress and strain), tensile strength, ultimate strain, and strain energy density
In tensile testing studies using animal models, Weiss et al10 dem-onstrated that the biomechanical properties of the healing MCL remain inferior to those of the intact ligament for as long as 1 year after
injury After 52 weeks of healing, only the stiffness of the femur-MCL-tibia complex returned to near-normal levels, while the ulti-mate load was still significantly
(P<0.05) lower than the control
value Furthermore, the mechanical properties of the midsubstance of healing MCLs remained significantly
(P<0.05) inferior, even though the
cross-sectional area of the healing MCL was much larger than that of the intact MCL Thus, the healing pro-cess involves a larger quantity of lesser-quality ligamentous tissue
Factors Influencing Ligament Healing
There are numerous factors that af-fect the healing response of an in-jured ligament These include the site and severity of the injury, vari-ous intrinsic factors (e.g., circulation and infection), the type of treat-ment, and the degree of mobiliza-tion after injury
Isolated Ligament Injuries
In laboratory studies using ani-mal models, the MCL has been
Ultimate load Linear
stiffness
Ultimate elongation
Elongation, mm
400
300
200
100
0
Energy absorbed
at failure
Toe Region
Linear Region
Failure Region
Figure 1 Typical load-elongation curve of the bone-MCL-bone complex.
Trang 4shown to heal well compared with
the ACL and the posterior cruciate
ligament (PCL) These findings
have been supported by clinical
ob-servations.11,12 Variations in healing
ability may be attributable to
differ-ences in the blood supply and in the
articular environment (i.e.,
intra-articular or extra-intra-articular) The
more “hostile” synovial
environ-ment surrounding the ACL may
also be a factor; however, some
studies have suggested positive
effects of synovial fluid on ligament
healing.13 Structural differences
(e.g., fiber orientation and crimp
pattern) and cellular differences
(e.g., fibroblast shape) may
con-tribute to these variations.14
Biomechanical factors may also
play a role in the ability to heal For
example, the ACL contributes to
knee stability in multiple directions
(i.e., anterior, internal, and valgus),
while the MCL primarily restrains
valgus knee rotation Thus, a
rup-tured MCL receives some protection
from other structures, such as the
ACL and the joint capsule, and is not
subject to the same forces that may
impede healing In contrast, the soft-tissue structures surrounding the ACL may not be able to accommo-date the multidirectional demands
so as to allow healing.15 Therefore, information about the in vivo loads
in ligaments is crucial to determina-tion of the optimal load and amount
of stretch that will optimize ligament healing
Combined Ligamentous Injuries
Some knee injuries involve multi-ple ligaments The prognosis for these combined injuries is generally worse regardless of which type of treatment is selected Some authors have reported satisfactory results with nonoperative treatment of com-bined ACL-MCL injuries.16 Others advocate surgical reconstruction of the ACL with repair of the MCL to adequately restore knee function.17 Still others choose to surgically re-construct the ACL without address-ing the MCL.11 Clinical studies have generally been inconclusive
Hillard-Sembell et al18 reported on
66 patients treated with MCL repair and ACL reconstruction (n = 11),
ACL reconstruction only (n = 33), or nonoperative treatment for both lig-aments (n = 22) No differences in valgus instability or in knee func-tion during activities were observed between the three groups studied Further clinical studies are needed
to determine the optimal treatment for these combined injuries, but some evidence may be obtained from animal models
The effects of ACL deficiency on the healing of the injured MCL have been studied by using both rabbit and canine models.15 Biomechanical evaluation indicated that knees with untreated combined ACL-MCL
in-juries showed significantly (P<0.05)
increased valgus laxity (Fig 3) and a reduction in tissue quality of the healed MCL Considerable degener-ation of the joint was also observed Laboratory studies in a rabbit model have suggested that nonoper-ative treatment with full weight bearing and mobilization of the injured MCL with reconstruction of the ACL can result in successful healing of the MCL.19 Although other animal studies also suggested that MCL repair combined with ACL reconstruction reduced valgus laxity and improved the structural properties of the femur-MCL-tibia complex better in the short term (12 weeks), after 52 weeks no differ-ences in biomechanical or biochemi-cal properties were observed
Intrinsic Factors
A number of intrinsic factors may also contribute to the healing response of the injured ligament Any disease that affects endocrine
or metabolic homeostasis may affect ligament healing In a study on the healing MCL of hypophysectomized rats, interstitial cell–stimulating hor-mone and testosterone replacement markedly affected the ultimate load
of the repaired ligament and the rates of collagen and glycosamino-glycan synthesis or degradation.20 Diabetes mellitus results in
circula-Figure 2 Typical stress-strain curve describing the mechanical properties of the MCL
midsubstance.
Tensile strength Modulus of
elasticity
Ultimate strain
Strain, %
80
60
40
20
0
Strain energy density
Toe
Region
Linear Region
Failure Region
Trang 5tory abnormalities, and insulin
defi-ciency alters collagen synthesis and
cross-linking; therefore, both
condi-tions may negatively affect ligament
healing Ligament healing can also
be affected by local conditions, such
as poor circulation and infection,
that hinder the proliferation of cells,
thereby prolonging the inflammatory
phase of healing
Type of Treatment
Studies have shown that
treat-ment selection can also have an
impact on the process of ligament
healing Several animal studies of
isolated MCL injuries have shown
better results with nonoperative
treatment than with surgical repair
followed by immobilization In one
study,4nonoperative treatment
with-out immobilization was compared
with surgical repair and 6 weeks of
immobilization in a transected MCL
canine model Histologic sections
revealed that the alignment of the
fibroblasts was more longitudinal in
the repaired ligaments at 12 weeks,
but at 48 weeks both the repaired
and the nonrepaired tissues were
similar but neither resembled the normal MCL Biomechanical data indicated that valgus rotation, stiff-ness, and ultimate load of the femur-MCL-tibia complex for the nonre-paired group were closer to control values throughout the 48-week study than those for the repaired and immobilized group However, the quality of the healed tissue of both the repaired and nonrepaired MCLs
was significantly (P<0.05) different
from control values
Similar results were obtained in
a study using a rabbit model.10 A
“mop-end” tear of the MCL sub-stance was created by placing a stainless steel rod beneath the MCL and pulling it medially, rupturing the MCL in tension and causing a midsubstance tear and damage to the insertion sites Treatment was either nonoperative with no immo-bilization or surgical repair No sta-tistically significant differences could be demonstrated between the repaired and nonrepaired groups at
6 or 12 weeks for any biomechani-cal property, including structural properties of the femur-MCL-tibia
complex (Fig 4), the mechanical properties of the MCL midsub-stance (Fig 5), and varus-valgus knee rotation These findings are in agreement with clinical reports of positive outcomes with nonopera-tive treatment followed by early motion and functional rehabilita-tion.21 This is now generally con-sidered to be the preferred method
of treatment for isolated grade III injuries of the MCL.11
Surgical repair of midsubstance tears of the ACL and PCL has been inadequate and appears to fail over time.12 As a result, direct repair is generally not performed, and liga-ment reconstruction is the preferred treatment for most cruciate liga-ment injuries For the ACL, autolo-gous tendons of the knee, particu-larly the medial hamstring tendon and the central third of the patellar tendon–bone complex, are the most commonly used graft sources The reported biomechanical prop-erties of cadaveric grafts are some-what variable due to differences in testing methods and graft sizes A 14-mm-wide patellar tendon graft is much less stiff than the ACL of a young adult (27.4 ± 3.0 N/mm vs
242 ± 28 N/mm) but is stronger (2,900 ± 260 N vs 2,160 ± 157 N).22,23 However, a recent study has demon-strated that a 10-mm patellar tendon graft, which is most often used for ACL reconstruction surgery, has a stiffness of 210 ± 66 N/mm and an ultimate load of 1,784 ± 580 N.24 The corresponding values for a braided quadrupled semitendinosus-gracilis graft were 238 ± 71 N/mm and 2,421
± 538 N, respectively.24 Bone–patellar tendon–bone grafts offer the advantage of bone-to-bone healing, which may occur more quickly than tendon-to-bone heal-ing Patellar tendon grafts have the disadvantages of increased graft-site morbidity, a decrease in quadriceps strength, and a greater prevalence of anterior knee pain Hamstring ten-don grafts reduce graft-site
morbid .6
-.4 -.2
.2 4 6
Valgus Varus
Intact
Varus-valgus moment, N-m
MCL transected
MCL + ACL transected
Figure 3 A typical plot demonstrating the nonlinear relationship between varus-valgus
rotation of the knee (horizontal axis) and applied varus-valgus moment (vertical axis) for a
time-zero specimen The graph also illustrates the increase in rotation of the knee after
transection of the MCL and after transection of both the MCL and the ACL (Reproduced
with permission from Woo SLY, Young EP, Ohland KJ, Marcin JP, Horibe S, Lin HC: The
effects of transection of the anterior cruciate ligament on healing of the medial collateral
lig-ament: A biomechanical study of the knee in dogs J Bone Joint Surg Am 1990;72:382-392.)
Trang 6ity, but their disadvantages include
slower tendon-to-bone healing and
diminished hamstring function
Clinically, the choice of graft
ma-terial remains a subject of debate
However, no single reconstructive
technique has proved superior in
terms of functional stability,
preven-tion of osteoarthritis, or
complica-tion rate Allografts are also a valid
alternative to autografts, particularly
for revision ACL reconstructions
The optimal method of graft fixa-tion has been a subject of recent research, especially because of recent clinical reports of bone-tunnel en-largement after ACL reconstruction
It has been suggested that the con-struct should be stiff enough that there is minimal motion between the graft and the bone tunnel, so as to facilitate healing between the tunnel and the graft A strong, stiff con-struct has also been considered
important to withstand the stresses during early rehabilitation Interfer-ence screws made of either titanium
or a biodegradable material are the fixation devices of choice for patellar tendon grafts However, a wide va-riety of devices for hamstring fixation have been used, including staples, washers, suture and post, titanium buttons, cross-pins, and interference screws.24 The ultimate load and stiff-ness of numerous fixation devices
250
300
350
150
100
50
200
Sham
6 weeks
12 weeks
Nonrepaired
Elongation, mm
0
250 300 350
150 100 50
200
Sham
6 weeks
12 weeks
Repaired
Elongation, mm
0
Figure 4 A, Load-elongation curves representing the structural properties of the femur-MCL-tibia complex for sham-operated and
nonre-paired groups B, Load-elongation curves representing the structural properties of the femur-MCL-tibia complex for sham-operated and
surgically repaired groups (Reproduced with permission from Weiss JA, Woo SLY, Ohland KJ, Horibe S, Newton PO: Evaluation of a new
injury model to study medial collateral ligament healing: Primary repair versus nonoperative treatment J Orthop Res 1991;9:516-528.)
30
20
10
Nonrepaired
Strain, %
Sham
6 weeks
12 weeks
0
30
20
10
Repaired
Strain, %
Sham
6 weeks
12 weeks
0
Figure 5 A, Stress-strain curves representing the mechanical properties of the MCL substance for sham-operated controls and
nonre-paired groups B, Stress-strain curves representing the mechanical properties of the MCL substance for sham-operated controls and
surgi-cally repaired groups (Reproduced with permission from Weiss JA, Woo SLY, Ohland KJ, Horibe S, Newton PO: Evaluation of a new
injury model to study medial collateral ligament healing: Primary repair versus nonoperative treatment J Orthop Res 1991;9:516-528.)
Trang 7have been quantified, but additional
biomechanical studies are needed to
assess the effect of cyclic loading
(used to simulate the low-intensity,
repetitive loading of rehabilitation)
on their stability
In some circumstances, patients
with an isolated PCL injury do well
with nonoperative treatment and
early mobilization Reconstruction
of the PCL is generally performed
only on patients with multiple
liga-mentous injuries and those who are
high-performance athletes.25
Be-cause patients with grade III PCL
tears are known to develop medial
compartment and patellofemoral
chondrosis, many surgeons opt to
treat only the larger and stronger
anterolateral bundle of the PCL,
using an autologous patellar
ten-don graft However, it has been
shown in laboratory testing that
Achilles tendon allograft offers a
large amount of collagen, which
can fill the bone tunnels completely,
allowing bone fixation on the
fem-oral side with a calcaneal bone plug
and on the tibial side with a
soft-tissue washer.26
Clinically, no one graft has been
proved superior to the other
op-tions, and residual knee laxity and
early arthritis have been reported
after PCL reconstruction surgery
A recently proposed double-bundle
PCL reconstruction appears to have
some biomechanical benefits in
ca-daveric knees.27
Immobilization Versus
Controlled Motion and Exercise
In the past, immobilization after
ligament injury was believed to be
necessary to protect the healing
lig-ament from stress However, it has
been shown in the laboratory that
immobilization results in
disorgani-zation of collagen fibrils, decreases
in the structural properties of the
bone-ligament-bone complex,
re-sorption of bone at ligament
inser-tion sites, and other detrimental
effects on the knee joint.28 In
con-trast, controlled motion has been shown to be beneficial to the heal-ing ligament Intermittent passive motion has been reported to im-prove the longitudinal alignment
of cells and collagen at 6 weeks, as well as matrix organization and collagen concentration, and also to increase the ultimate load of the femur-MCL-tibia complex by as much as four times.28 Tipton et al29 reported that in a canine model ex-ercise positively affected the heal-ing MCL as evidenced by the in-creased ultimate load of the healing tissue Follow-up studies in a rat model revealed that exercise en-hanced ligament healing, as evi-denced by a more rapid return of DNA and collagen synthesis
There are also some clinical data that demonstrate the advantages of motion after ligament injury Reider
et al21have reported favorable clini-cal results 5 years after treatment of isolated MCL injuries with early motion and functional rehabilitation
Current clinical recommendations after MCL injuries include early con-trolled range-of-motion exercises as soon as pain subsides.11,21 However,
in an unstable joint, motion too early
or applied too aggressively may be detrimental to the healing process
The effects of motion after treat-ment of ACL and PCL injuries are hotly debated, although it is gener-ally agreed that rehabilitation is criti-cal to prevent arthrofibrosis and re-store knee function The appropriate timing of rehabilitation has not yet been established Early accelerated rehabilitation has been advocated by some to minimize knee stiffness and ensure complete knee extension.30 However, others have supported less aggressive rehabilitation to allow vascularization and incorporation of the graft Additional laboratory studies evaluating the in situ forces
in the ligament and joint kinematics during rehabilitation are needed in conjunction with randomized, pro-spective clinical studies
Tissue Engineering and Ligament Healing
Because the biomechanical and bio-chemical properties of the healing MCL fail to return to normal, and the quantity of healing tissue appar-ently increases to make up for the deficiency, researchers are now exploring other modalities that can improve the quality of healing tis-sues, as well as accelerate the rate of healing Advances in the fields of molecular biology and biochemistry may have applications in the liga-ment healing process Although the results are still preliminary, new techniques utilizing growth factors and gene transfer and cell therapies may prove useful in accomplishing these goals in the ligaments of the knee, as well as other ligaments throughout the body
Growth Factors
Growth factors are small polypep-tides that bind to specific receptors
on the surfaces of cells, activating pathways for complex intracellular signal transduction By modulating cellular behavior, growth factors have shown the ability to affect cell proliferation and migration, matrix synthesis, and the secretion of addi-tional growth factors Because ex-pressions of various growth factors and their receptors have been dem-onstrated during various phases of the healing process, it is essential to understand their significance Before growth factors can be used in vivo, their effects on fibro-blast proliferation, matrix synthesis, and cell migration must be exten-sively evaluated in an in vitro set-ting To date, studies using various animal models have shown that while transforming growth factor-β
is a good promoter of matrix syn-thesis, platelet-derived growth fac-tor, basic fibroblast growth facfac-tor, and epidermal growth factor are positive mitogens on fibroblasts of the ACL and MCL
Trang 8In vivo studies based on the
re-sults from in vitro experiments have
shown that high doses of
platelet-derived growth factor applied with
fibrin sealant can have positive
effects on the injured ligaments of
rabbits, causing significant (P<0.05)
increases in the structural
proper-ties of the femur-MCL-tibia
com-plex.31 However, studies in other
animal models, with varying dosage
and method and timing of
applica-tion, have also shown dramatic
results when growth factors are
ad-ministered independently
Collec-tively, the results of these studies
show variations that may be species-,
dosage-, and treatment-specific,
thus demonstrating the complex
nature of ligament healing
Addi-tional studies should be directed at
defining the variables that affect the
specificity of growth factors
Gene Transfer Technology
Growth factors have half-life
periods of a few days in vivo
Therefore, their use as a treatment
modality necessitates repetitive
applications in order to maintain
potency For this reason, focus has
been placed on designing and
en-hancing delivery vehicles for
growth factors As gene transfer
technology develops, the ability to
control the expression and
regula-tion of proteins in a host cell will
enable researchers and clinicians to
administer treatment over extended
periods without the need for
repet-itive applications
Although using gene transfer as
a method for treating ligament
in-juries is still in its infancy, this
tech-nology has shown some promise
Indirect methods of gene transfer involving the transplantation of genetically altered tissues via a ret-roviral vector have resulted in the
expression of lacZ marker gene for
as long as 6 weeks in the ACLs and MCLs of animals.32 Methods of direct gene transfer using HVJ-liposome viruses and adenoviral vectors have shown similar poten-tial.32,33 Studies of gene transfer as
a therapeutic method for manipu-lating ligament healing have also shown positive effects on collagen fibril diameter and distribution, as
well as significant (P<0.05) increases
in the mechanical properties.33
Cell Therapy
Another area of research with potential applications in ligament healing is cell therapy The concept
is that implantation of genetically manipulated cells can enhance the repair of ligaments as those cells become constituents of the healing tissue In vitro and in vivo studies with mesenchymal stem cells have shown their ability to differentiate into various cell types involved in many of the phases of ligament healing In one study involving the transplantation of nucleated cells, including mesenchymal stem cells, from bone marrow into a pocket around the transected MCL of in-bred rats, donor cells could be identified in the midsubstance of the ligament after 7 days, demon-strating the potential for migration
of transplanted cells This study highlights the possibility that cell therapy using nucleated cells may lead to new methods of treatment for ligament injuries.34
Summary
A great deal of progress has been made in elucidating the biology and biomechanics of ligament healing, which has influenced the clinical management of ligament injuries Studying the effects of stress and motion on healing ligaments is criti-cal to understanding the healing process and the role played by mol-ecules and cells This knowledge will be furthered by studying load-ing conditions that closely simulate the stresses and motions that occur
in vivo
The future contributions of basic science research appear to lie in the engineering of ligament healing so
as to accelerate the rate of healing and improve the quality of healing tissue Potential applications in-clude the use of growth factors and the development of vehicles for their delivery The use of scaffolds and biomatrices on which cells can be seeded may provide a better means
of replacing injured soft tissues Al-though much of the current focus has been on the ligaments of the knee, particularly the MCL and ACL, the knowledge gained may be applicable to other ligaments Mul-tidisciplinary collaboration between molecular biologists, morphologists, bioengineers, and clinicians will en-hance the understanding of ligament healing and ultimately improve clin-ical outcomes
Acknowledgments: The assistance of
Nobuyoshi Watanabe, MD, and the sup-port of the Musculoskeletal Research Center and NIH Grant #AR41820 are grate-fully acknowledged.
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