Healio
Healio
August 01, 2007
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Effect of ACL Sacrifice, Retention, or Substitution on Kinematics After TKA

Abstract

Total knee arthroplasty (TKA) is an effective treatment for arthritic knee pain, but patients may not return to their desired recreational or functional activity level post-TKA. The difference in functional outcome between patients who underwent TKA and patients with normal (nonarthritic) knees may be related to differences in kinematics between the normal and replaced knee. Posterior cruciate-retaining TKAs are associated with paradoxical motion in which the tibia is subluxed anteriorly in extension and the femur translates anteriorly during knee flexion. However, unicompartmental and patellofemoral arthroplasties, which retain the anterior cruciate ligament (ACL), however provide relatively normal knee kinematics and favorable knee function compared with conventional TKAs, which sacrifice the ACL. Early results with a bicruciate-substituting TKA suggest that ACL function is necessary to achieve more normal kinematics after TKA.

Total knee arthroplasty (TKA) is an effective treatment for severe osteoarthritis or inflammatory arthritis of the knee. Reliable pain relief is achieved and arthroplasty survivorship at 10 years is typically >90%.1-3 However, many TKA patients are not able to participate in activities such as squatting, turning, cutting, carrying heavy objects, golfing, dancing, gardening, downhill or cross country skiing, and racquetball sports.4 Participation in most of these activities requires pain-free knee motion, joint stability, and effective quadriceps function. Although pain is effectively relieved after TKA, kinematics are markedly altered, which can result in diminished quadriceps function and joint stability.5-7

Anterior Cruciate Ligament-Sacrificing TKA

In the normal knee, the lateral condyle of the femur rolls posteriorly during knee flexion, which is associated with femoral external rotation.8,9 Posterior movement of the femur increases the moment arm of the quadriceps and femoral external rotation (or tibial internal rotation) causing relative medialization of the tibial tubercle that facilitates patellar tracking in flexion.10 Many normal anatomic structures are altered after TKA, however. The joint line is changed from 3° of varus to 0°, which is perpendicular to the mechanical axis. Both menisci are removed, the geometry of the joint surfaces is altered, and the anterior cruciate ligament (ACL) is excised. After TKA, the knee behaves as an ACL-deficient knee. Posterior cruciate-retaining TKAs typically have paradoxical motion in which the tibia is subluxed anteriorly during knee extension (Figure 1A) and the femur slides anteriorly during knee flexion (Figure 1B). Anterior subluxation of the femur leading to posterior tibiofemoral impingement can limit flexion.11 Undersizing of the femoral component or posterior cruciate ligament (PCL) release is often performed to provide more laxity in flexion to permit adequate knee flexion. However, flexion instability can develop resulting in joint swelling and pain.7 Posterior stabilized TKAs provide a cam/post mechanism to substitute for the PCL. Because the ACL is deficient, however, the tibia is typically subluxed anteriorly on the femur similar to cruciate-retaining TKAs (Figure 2A). Anterior subluxation of the tibia in extension can lead to impingement between the anterior cam and femoral posterior-stabilized box leading to wear along the anterior post.12 In flexion, the cam and post mechanism engages, which limits anterior translation of the femur (Figure 2B). In vivo flexion lateral fluoroscopic images of cruciate-retaining knees often show anterior femoral subluxation, whereas posterior-stabilized knees have more controlled rollback of the femur.6,11

The abnormal kinematics following TKA can also diminish quadriceps mechanics. Anterior femoral subluxation in flexion decreases the quadriceps moment arm. Tibial rotation relative to the femur during knee flexion is not controlled by the cam/post mechanism in a posterior-stabilized TKA or by the PCL in a cruciate-retaining TKA. As a result, the tibia may rotate externally during knee flexion which results in lateral patellar tilt and subluxation.13

Figure 1A: The tibia is subluxed anteriorly as a result of ACL deficiency

Figure 1B: The femur subluxes anteriorly in flexion (paradoxical motion)

Figure 1: Lateral diagram of a posterior cruciate-retaining TKA in extension. The tibia is subluxed anteriorly as a result of ACL deficiency (A). Lateral view of a posterior cruciate-retaining TKA. The femur subluxes anteriorly in flexion (paradoxical motion) (B).

Figure 2A: The tibia is subluxed anteriorly in extension as a result of ACL deficiency

Figure 2B: Posterior translation of the femur in flexion

Figure 2: Lateral diagram of a posterior-stabilized TKA showing a cross section view through the cam and post mechanism. The tibia is subluxed anteriorly in extension as a result of ACL deficiency (A). Lateral diagram of a posterior-stabilized TKA showing engagement of the cam and post mechanism, which causes posterior translation of the femur in flexion (B).

Figure 3A: Lateral diagram of a bicruciate-stabilized TKA

Figure 3B: Lateral diagram of a bicruciate-stabilized TKA in flexion showing engagement of the posterior cam and post resulting in femoral rollback

Figure 3: Lateral diagram of a bicruciate-stabilized TKA. The cross section view through the cam and post mechanism shows engagement of the anterior cam and post in extension, which limits anterior tibial subluxation and maintains normal orientation of the femur and tibia (A). Lateral diagram of a bicruciate-stabilized TKA in flexion showing engagement of the posterior cam and post resulting in femoral rollback (B).

ACL-retaining TKA

Currently available cruciate-retaining and posterior-stabilized knees sacrifice but do not substitute for the ACL. 1-3,6,7,11,12 However, the ACL is an important ligament constraint to provide joint stability for more vigorous activities. Early TKAs that retained both cruciate ligaments were developed, such as the Polycentric and Geomedic (Howmedica, Rutherford, NJ). These implants were essentially made of two connected medial and lateral unicondylar implants. The articular surfaces were relatively conforming, though the instrumentation to implant the prostheses was limited. The Polycentric and Geomedic were also technically difficult to implant and correcting soft tissue imbalance was not always possible, particularly with severe arthritic knee deformity.14,15 Kneeflexion was often restricted, and mechanical loosening occurred frequently.16 Despite these limitations, patients with ACL-retaining TKAs demonstrate more normal kinematics than ACL-sacrificing TKAs, and knees are reported to feel more normal.17,18

Unicompartmental and patellofemoral arthroplasties retain the ACL and provide relatively normal knee kinematics.19,20 Most unicompartmental arthroplasties are relatively nonconforming or mobilebearing and do not restrict knee flexion, unlike previous fully congruent ACL-retaining TKAs. Video fluoroscopy studies of unicompartmental or patellofemoral replacements that retain both cruciate ligaments show more normal femoral rollback in flexion.19,20 The more normal kinematics of bicruciate-retaining implants suggests that ACL function is necessary to achieve more normal function following TKA.

Bicruciate-substituting TKA

Guided-motion TKA designs depend on the geometry of the tibial and femoral articular surfaces and cam and post mechanics to provide more normal knee kinematics and tibiofemoral stability during knee flexion.21-24 Using an anterior cam and post mechanism can provide stability to anterior displacement of the tibia during extension and early flexion by contact between the anterior post and the anterior cam (Figure 3).23 As the knee flexes, the anterior cam moves proximally and no longer engages the post. However, the anterior cam and post mechanism can function effectively to substitute for ACL function up to 20° of flexion.23 Further midflexion stability can be provided by selective posterior rollback of the lateral femoral condyles while the anteroposterior (AP) position of the medial femoral condyles is relatively stationary. This permits controlled and external rotation of the femur during knee flexion similar to the kinematics of the normal knee.23 Relatively stationary AP position of the medial femoral condyle can be maintained by a highly conforming tibiofemoral articular geometry in the AP plane. Posterior rollback of the lateral femoral condyle can be achieved with a convex posteriorly sloped lateral tibial plateau in the AP plane. However, high mediolateral conformity is necessary to achieve adequate tibiofemoral contact area and minimize ultra-high molecular weight polyethylene contact stresses.23 Using a tibial insert with a relatively conformed medial plateau in the AP plane and convex lateral plateau permits guided femoral external rotation during knee flexion.

Conclusion

Early results with a bicruciate-substituting TKA having tibiofemoral geometries and a cam and post mechanism to provide more normal knee kinematics are favorable.23 Results show that patients consistently increase range of motion after TKA, and fluoroscopic analyses of in vivo kinematics demonstrate AP displacement and rotation essentially identical to the normal knee.23 These findings support the concept that anterior stability is necessary to provide more normal kinematics after TKA and that ACL function can be achieved either by retention or substitution of the ACL.

References

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Author

Dr Ries is from the Department of Orthopedic Surgery, University of California, San Francisco, Calif.

Dr Ries has received compensation from Smith & Nephew in the past 12 months.