Abstract
INTRODUCTION
Clinical studies have shown that the knee tends to experience laterally higher AP motion (posterior directed) than medially (Asano at al., 2001; Dennis et al., 2005; Hill et al., 2000; Moro Oka et al., 2007). Traditional posterior stabilized (PS) total knee arthroplasty (TKA) designs allow deep flexion stability and femoral rollback once cam/spine engagement occurs, however mechanical stability provided by tibial bearing conformity during early to mid-flexion is highly variable. In this study a computer knee model is used to compare AP kinematics in PS TKA designs while evaluating multiple sagittal tibia bearing conformities. We hypothesized that highly conforming designs would be necessary to promote AP stability prior to cam/spine engagement.
METHOD
A specimen specific computer model consisting of the femur, tibia and fibula, as well as the contribution of the ligaments and capsule was virtually implanted with TKA designs of the appropriate size at 5° tibia slope with the posterior cruciate ligament sacrificed. A single PS femoral component was evaluated with five PS tibia bearing designs with variable sagittal conformity ratios ranging from 1.05:1 to 2.2:1 (conformity ratio = tibia bearing sagittal radius / femur sagittal condylar radius). Designs were fully conforming frontally, with cam/spine engagement beyond 90° flexion. In all designs, lateral conformity ratios were increased relative to medial conformity ratios to facilitate lateral femoral rollback. Resultant AP kinematic predictions were obtained for femoral Low Points (LP) during 1) envelope of motion during internal external (IE) laxity evaluation and 2) knee bend functional activity.
RESULTS
Designs with increased conformity resulted in improved AP stability in early to mid-flexion. In the envelope of laxity evaluations (Figure 1), the Medial LP laxity was similar for the 1.05:1 and 1.15:1 designs (approximately 2–3mm from extension to flexion), but then increased for the 1.35:1, 1.55:1, and 2.20:1 designs. Lateral LP laxity was more similar in all designs, however the designs with lower medial conformity ratio tended to have a greater Lateral LP laxity. During the Knee Bend activity (Figure 2), minimal (<1mm) Medial LP AP motion occurred from extension to cam/spine engagement in both the 1.05:1 and 1.15:1 designs, and greater AP motion occurred in the remaining designs.
DISCUSSION
This study demonstrates that similar knee kinematics in PS TKA can be achieved over a small range of sagittal conformities, when designs approach 1:1 conformity. More conforming medial geometries can lead to greater lateral LP motion, suggesting greater medial conformity is necessary to replicate normal knee kinematics. Further work could evaluate impact of other factors such as tibia slope, further refinement in conformity, and additional specimen sizes.
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