Bertrand W. Parcells, MD, and Alfred J. Tria Jr., MD
Authors’ Disclosure Statement: Dr. Tria reports that he receives royalties from Smith & Nephew, and is a consultant for Smith & Nephew, Medtronic, and Pacira. Dr. Parcells reports no actual or potential conflict of interest in relation to this article.
In deep flexion, beyond 130°, posterior translation continues for both condyles. The LFC experiences enough excursion to cause loss of joint congruity and partial posterior subluxation.19,20 The MFC shows little additional posterior translation, yet it too loses joint congruity through condylar lift-off. Contact between the posterior horn of the medial meniscus and the posterior femoral condyle limits further flexion.16,21
The difference in motion between the condyles leads to internal tibial rotation during flexion. The initial 10° of knee flexion produces 5° of internal rotation, and an additional 15° of internal tibial rotation occurs throughout the remainder of knee flexion.
Fluoroscopic imaging with computed tomography (CT)- or magnetic resonance (MR)-based modeling has shown the dynamic in vivo relationship of the tibiofemoral joint. Studies have confirmed significantly greater LFC posterior translation as compared to the MFC;22 however, in vivo studies have also shown notable variability in articular rotation and translation based on activity. This highlights the role of ligamentous tension and muscle contraction in kinematics.21-23
The ACL in TKA
The majority of current TKA designs sacrifice the ACL without substituting for its function. The loss of the ACL has significant effects upon the kinematics of the knee.
The ACL is composed of 2 bundles, the anteromedial and posterolateral bundles, which originate on the LFC and insert broadly onto the tibial intercondylar eminence. Its primary role is to resist anterior tibial translation, particularly from 0° to 30° of flexion, which corresponds to the peak quadriceps force that pulls the tibia anteriorly.24 ACL deficiency causes anterior tibial translation during early flexion and abnormal internal tibial rotation.25-27 ACL deficient knees demonstrate a posterior femoral position in full extension, and increased MFC translation during knee flexion.28-32
The role of the ACL in knee arthroplasty has been evaluated by comparing unicompartmental knee arthroplasty (UKA) with TKA, as a reflection of ACL preserving vs sacrificing procedures.33-35 Sagittal plane translation is similar between UKA and normal knees,33,34 while the CR TKA and PS TKA designs show anterior tibia subluxation in full extension.33-35 The difference between UKA and TKA is greatest in extension, corresponding to the ACL functional range. These findings highlight kinematic similarities between TKA designs and the ACL deficient knee.
The majority of UKAs demonstrate near-normal kinematics. A small percentage of the study group demonstrated aberrant anterior tibial motion, highlighting a concern over ACL attenuation with time. Additionally, studies that evaluate the ACL in osteoarthritic knees have questioned the baseline integrity of the ACL.36 Yet the long-term outcomes in UKA design have shown preservation of kinematics due to intact cruciates.37
The PCL in TKA
Because the majority of TKA designs sacrifice the ACL, the classic debate has focused on the utility of the native PCL. Both the CR and PS TKA are designed to offer posterior stabilization; however, kinematic studies have demonstrated notable differences.38,39
The CR TKA design relies on the PCL to resist posterior sag and to prevent the hamstring musculature from pulling the tibia posteriorly during flexion. Studies have shown paradoxical anterior translation of both femoral condyles during flexion, particularly on the medial side of the knee.40 There is also increased variability in femoral rollback. It is unclear whether the PCL can function normally in the absence of the ACL, which causes the PCL to adapt a less anatomic vertical position. The PCL may also be unable to function significantly without the ACL because of pre-existing degenerative histological changes.41
The PS TKA utilizes a cam-post mechanism for posterior stabilization. In contrast to normal knee kinematics, this mechanism creates equal MFC and LFC posterior translation, 8 mm on average at 90° flexion.40 The equivalent translation in PS designs contributes to decreased internal tibial rotation and an increased polyethylene wear at the post.
Role of Surface Geometry
The articular geometry of the knee plays an important role in normal knee kinematics. Initial TKA designs used a femoral component with a single radius of curvature for both femoral condyles.42Current TKA designs that match the femoral component to the native femoral anatomy, by differing the medial and lateral condyle geometry, have demonstrated kinematics that better resemble a native knee.43 Additional changes to the radius of curvature along the posterior facet of the femoral condyles may reduce impingement during deep flexion. These “high flex” designs have demonstrated equivalent range of motion in some studies44 and improved weight-bearing motion in others.45 Surface geometry is important but is not the entire answer to kinematics.
Advances in TKA Design
Knee motion is guided by multiple factors, including the tibiofemoral articular geometry, the surrounding soft tissue tension, and muscle tone. Bicruciate-substituting (BCS) TKA and BCR TKA are forms of evolution from the CR and PS TKA and attempt to respect the function of both cruciate ligaments and provide better kinematics.
