19 The Importance of the ACL for the Function of the Knee: Relevance to Future Developments in Total Knee Arthroplasty
A. M. Chaudhari, C. O. Dyrby, T. P. Andriacchi
Summary
This chapter discusses the importance of the anterior cruciate ligament (ACL) for the function of the knee by examining the functional role of the cruciate ligaments in healthy knee joints and the functional results of different total knee designs. The cruciate ligaments influence the motion of the knee in multiple planes, restraining AP translations as well as axial rotations to varying degrees, based on the angle of flexion.Functional analyses of knee arthroplasty designs that retain no cruciate ligaments,the PCL only, and both ligaments show differences, with uni- compartmental knee arthroplasties retaining function that is the closest to normal function. After total knee arthroplasties, even asymptomatic patients showed gait abnormalities such as quadriceps avoidance that could lead to reduced functional ability to perform activities of daily living over time. These results and continuing positive short- and long-term follow-up of patients who have received ACL-retaining prostheses should increase enthusiasm for these procedures in the future.
Introduction
Over the past 40 years, total knee arthroplasties (TKA) have become the standard of care for end-stage knee arthritis, with excellent long-term clinical outcomes for patients. The most common TKA procedures involve re- moval of the anterior cruciate ligament (ACL), and often of the posterior cruciate ligament (PCL) as well. While these procedures do result in positive overall results for patients,they also result in changes in functional abilities, such as quadriceps avoidance [3] and changed upper- body mechanics during stair-climbing and rising from a chair [5]. Several authors have also reported postero- medial wear of the tibial component following TKA [10, 13, 15].
Alternatives to the standard ACL-sacrificing TKA are not nearly as common, but they have been shown to also
have excellent long-term results in patients. The Cloutier prosthesis, which retains both cruciate ligaments, has been shown to have good or excellent results in 97% of patients after 9-11 years, with no evidence of the postero- medial wear on the tibial component observed in ACL- sacrificing TKA [9]. Unicompartmental knee arthroplas- ties (UKA) have also shown excellent clinical results, in both the short and the long term [7]. Furthermore, com- pared with total knee arthroplasty, the cost saving from the prosthesis,length of stay,and reduced blood products make UKA an even more attractive alternative. Thus, interest in unicompartmental arthroplasty is resurging.
In most patients, UKA has potential advantages over both proximal tibial osteotomy and total knee arthro- plasty. Compared with total knee arthroplasties, UKA is a preserving procedure.First,the bone stock is preserved.
Second, with UKA the patellofemoral joint, the ACL and PCL, and the meniscus with the articular surface of the opposite compartment are preserved. Finally, if neces- sary, a UKA can be easily revised to a total knee arthro- plasty.
Differences in the patterns of motion when the cruci- ate ligaments are retained versus when they are sacrificed may also result in differences in the wear characteristics of the metal and polyethylene components. These differ- ences may affect the longevity of the prosthesis, which is especially relevant today as knee arthroplasties are considered for younger, more active populations.
Functional Role of the Cruciate Ligaments
The cruciate ligaments appear crossed when viewed in the sagittal plane and their function has often been relat- ed to the mechanics of a four-bar linkage. However, the cruciate ligaments are actually crossed in three-dimen- sional space, resulting in behavior that is more closely associated with elastic cables that provide an envelope of stability and motion. For example, at any angle of knee
flexion there is a range of possible anterior-posterior translations that are possible. It has been shown that in a neutral position the passive knee joint will move within a certain range without noticeable resistance to applied loads. This region can be considered as a region of ex- tremely low stiffness or high mobility of the knee joint.
This envelope of stiffness or flexibility varies with the angle of knee flexion. At full extension the envelope of motion is quite small and the knee remains quite stable, with the ACL completely tensed. As the knee flexes from full extension,the ACL relaxes.The PCL does not begin to tighten until the knee flexes beyond 40° of flexion. Thus, as the knee flexes from full extension the envelope (ante- rior-posterior motion) increases.At approximately 20° of flexion there is a minimum of restraint to anterior-poste- rior motion of the knee.As the knee flexes past 40° to deep flexion,the PCL tightens to reduce the envelope of motion again. This changing envelope of stability at the knee is important from a functional viewpoint.At full extension, the knee is passively stable, allowing for standing posture with a minimal amount of muscular demand. During dynamic activities such as walking (where the knee typically flexes to approximately 20° during mid-stance), the ligaments are relatively relaxed; and dynamic stabili- ty is provided by large muscle forces generated by muscle contraction.
The interaction between the orientation of the cruci- ate ligaments and the cam-like shape of the femoral condyles are the primary determinants of how the knee changes its passive stability with flexion.⊡ Figure 19-1il- lustrates the interaction between the cam-shaped geo- metry of the femoral condyles and the orientation of the cruciate ligaments.In full extension the sagittal plane ori- entation of the ACL is at its greatest angle with respect to the joint line, while the PCL is at its closest to horizontal.
As the knee flexes to 90°,the ACL becomes horizontal and the PCL becomes perpendicular to the joint line. Thus,
when the knee is flexed the posterior cruciate provides distraction stability to the knee.This function is likely im- portant for activities involving deep flexion such as squat- ting, where knee flexion can exceed 130° of flexion.
The cruciate ligaments influence stability and motion in planes other than the sagittal plane. The attachment of the ACL on the medial portion of the lateral femoral condyle relative to its tibial attachment, and the attach- ment of the PCL on the lateral portion of the medial femoral condyle relative to its tibial attachment orient the ligaments obliquely in the frontal plane (⊡ Fig. 19-2). This oblique orientation of the cruciate ligaments is compati- ble with the asymmetry in the geometry of the distal femoral condyles. As previously described (Fig. 19-1), the femur rolls posteriorly during flexion from full extension.
Since the distal curvature of the lateral femoral condyle is larger, the lateral femoral condyle will move posteriorly a greater distance than the medial femoral condyle.As a re-
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⊡ Fig. 19-1.Interaction between the cam-shaped geometry of the femoral condyles and the orientation of the cruciate ligaments
⊡ Fig. 19-2.Anterior view of the knee showing oblique orientation of anterior cruciate ligament (red) and posterior cruciate ligament (green)
sult, the femur will rotate externally with flexion relative to the tibia. With extension, the femur will rotate inter- nally relative to the tibia, a motion commonly referred to as “screw-home” motion. The cruciate ligaments act as fulcrums to further guide and constrain the motion, creating a balance between rolling and sliding motions and maintaining the normal tension in these ligaments throughout the range of motion.
When one or both of these ligaments are missing,this envelope of motion becomes disrupted. ACL-deficient patients experience reduced internal femoral rotation and reduced anterior tibial translation throughout the gait cycle during walking [1]. The geometric interaction between the anatomy of the cruciate ligaments and the geometry of the distal femoral condyles is likely an im- portant consideration in the design of knee arthroplasties where retention of the cruciate ligaments is considered.
Clearly, the interaction with the femoral condyles and the orientation of the cruciate ligament play a fundamental role in the function of the knee joint.
Functional Analyses of Knee Arthroplasty Designs
In a study of the influence of TKA design on walking and stair-climbing [3], it was found that after TKA, even asymptomatic patients with excellent clinical results had a gait abnormality. The feature of the abnormality con- sisted of shorter stride length, reduced mid-stance knee flexion, and abnormal patterns of external flexion-exten- sion moment. Furthermore, patients with less con- strained designs which retain both cruciate ligaments, such as the Cloutier and Gunston prostheses, seemed to have a more normal gait during stair-climbing than patients with more constrained cruciate-sacrificing designs.
In this analysis the Cloutier prosthesis retained the most of the native structure of the knee; both cruciate ligaments, an unconstrained articulation, and the native patellofemoral joint. This prosthesis, which anatomically most simulated the native knee functionally, performed most closely to the native knee. Furthermore, this pros- thesis most closely simulated a unicompartmental knee arthroplasty.
Chassin et al. reported on the functional analysis of patients with medial unicompartmental knee arthro- plasties [8]. The study found that 70% of the ten patients with unicompartmental knee arthroplasties maintained the normal biphasic pattern of flexion/extension mo- ments about the knee during gait. Throughout the stance phase, this normal moment pattern oscillates between an initial flexion moment, then an extension moment, and once again to a flexion moment. Two of the patients exhibited quadriceps avoidance patterns that had pre-
dominantly extensional patterns, where the moment remained extensional throughout stance phase. The other patient with an abnormal pattern exhibited a quadriceps overuse pattern. This pattern had a predom- inantly flexional moment pattern, where the moment remained flexional throughout stance phase.
The distribution of gait patterns was compared with those of a previously published group of normal control subjects [2] and a previously published group of TKA subjects [3]. In the normal group, 79% exhibited a nor- mal flexion/extension pattern. Using a Fisher’s exact test, there was no difference in distribution of flexion/ex- tension moment patterns between the unicompartmen- tal knee arthroplasties and the normal knees. However, there was a significant difference between the total knee arthroplasties and the normal knees. Finally, there was a difference between the UKA and the TKA groups. The distribution of the moment patterns for normal knees, unicompartmental knee arthroplasties, and total knee arthroplasties is summarized in ⊡ Table 19-1. Since 16%
have a quadriceps avoidance pattern and 5% have a quadriceps overuse pattern in the normal population [2], the 20% quadriceps avoidance and 10% quadriceps overuse patterns in the UKA patients appear to be a re- flection of the variability normally found in the flex- ion/extension moments about the knee.
The retention of the ACL in the unicompartmental knee arthroplasty population provides a possible expla- nation for the functional difference between the UKA and TKA populations. Sacrifice of the ACL removes a con- straint to the anterior displacement of the tibia produced by quadriceps contraction near full extension. Patients with ACL deficiency developed a quadriceps avoidance type of gait [4], suggesting that the similar quadriceps avoidance pattern of gait found in the total knee arthro- plasty patients is at least in part the result of sacrifice of the ACL during the procedure. Prostheses which retain the ACL, such as the Cloutier prosthesis or a UKA, have a low prevalence of the quadriceps avoidance pattern and therefore function more closely to the normal knee. This normal knee function affects not only the knee, but also the kinematics of the entire body.
⊡ Table 19-1. Percentage prevalence of biphasic, quadriceps avoidance, and quadriceps overuse in the normal, UKA, and TKA groups. (Reproduced from [8])
Group
Normal (n=29) UKA (n=10) TKA (n=35)
Biphasic 79 70 23
Quadriceps 16 20 46
avoidance
Quadriceps 5 10 31
overuse
Proper knee function is crucial to ascending stairs and rising from a chair. The stresses on the lower body joints during these activities are more than those that oc- cur during gait. It has been shown by other investigators that TKA patients neither ascend stairs nor rise from a chair normally [5]. Furthermore, it has been shown that for comparing lower extremity performance in contrast- ing subject groups, ascending stairs and particularly rising from a chair are superior to gait analysis in repro- ducibility and in controlling incremental alterations in mechanical demands on the subject [6].
Berger et al. reported functional performance differ- ences between seven unicompartmental knee arthro- plasties and six PCL-sparing total knee arthroplasties [5].
Twelve healthy women with no musculoskeletal com- plaints were used as a control group. Participants rose from four chair heights: 115%,100%,80%,and 65% of each participant’s knee height. The participants also ascended a standard staircase.
When rising from a chair, both knee arthroplasty groups experienced lower knee flexion moments than the normal group, although the UKA group averaged only 17% lower peak moments while the TKA group averaged 26% lower peak moments. The peak knee flexion mo- ment is attained just as the subject lifts off the chair.At this point in the motion, the knee flexion angle is greatest and the participant’s weight is furthest from the knee joint.
The mechanical demands on the body increase as chair height is diminished.Normal subjects are able to increase their knee moment as chair height is diminished.Not only do both unicompartmental and total knee arthroplasty groups develop a smaller knee moment at every chair height than normal; they also demonstrate less ability to meet increasing knee extensor demands,resulting in larg- er and larger differences from the normal group as the chair height is decreased. However, compared with TKA patients,UKA patients are able to produce a more normal knee moment at every chair height. Total knee arthro- plasty patients demonstrate a flat plateau from 80%
to 65%, indicating a limit of ability to generate a knee moment. Unicompartmental knee arthroplasty patients show less of the plateau effect, although their knee mo- ment response is not as much as the normal response [5].
Berger et al. also examined the difference in upper body velocity between groups when rising from a chair.
Upper body velocity is attained just prior to lift off and is a measure of the upper body’s momentum. This momen- tum is then harnessed to aid the lower body in rising [14].
For each successive reduction in chair height, normals produce incrementally more upper body velocity. All three groups initially attained approximately the same upper body velocity at the highest, least demanding chair height. Unlike the normal and the UKA groups, the TKA group generated significantly more upper body velocity as the mechanical demands of rising increased with de-
creasing chair height. The UKA group attained only slightly more upper body velocity than the normal group even at the highest chair height [5].Since the effects on the knee were less extreme in the UKA group, they were not required to make as extreme compensations when performing this very common and necessary activity of daily living. Making these compensations may cause other secondary problems to these patients, such as excess muscle strain in the arms or back.
Moreover, it has been reported that elderly subjects often are near the limits of their knee extensor strength when rising from a chair [12]. In this study, Hughes et al.
found that this population required 97% of their available strength when rising from the lowest chair height from which they could successfully rise. Since it is well known that in the elderly, disuse leads to muscle wasting and continued use is necessary to muscle maintenance [11], the quadriceps disuse that occurs as a result of the quadri- ceps avoidance gait observed most often in TKA patients may reduce their functional ability to perform activities of daily living such as rising from a chair and ascending stairs.
Conclusions
The cruciate ligaments perform several important func- tions in the healthy knee, from controlling the allowable envelope of anterior-posterior motion in the knee to controlling motion in other planes as well. The anterior cruciate ligament in particular affects the function of the knee extensor mechanism as an antagonist to the action of the patellar ligament at low flexion angles. The func- tional effects of sacrificing the ACL have been shown in several studies of walking, ascending stairs, and rising from a chair. While all knee arthroplasties do cause changes in the knee motion as well as in the total body response during these activities, ACL-retaining arthro- plasties such as the Cloutier prosthesis and unicompart- mental knee arthroplasties appear to retain function that is closer to normal. In addition, these knee arthroplasties offer long-term results similar to those with other total knee arthroplasty designs. Unicompartmental knee arthroplasties have the added advantages of replacing only the degenerated portion of the joint, retaining more of the bone stock,and requiring shorter hospital stays and fewer blood products. Continued positive short- and long-term follow-up of patients who have received these alternative,ACL-retaining procedures should increase en- thusiasm among clinicians in the future and improve both clinical and functional outcomes for patients even further.
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