Abstract
INTRODUCTION
The intact, healthy human knee joint is stable under anterior-posterior (AP) loading but allows for substantial internal-external (IE) laxity. In vivo clinical studies of the intact knee consistently demonstrate femoral rollback with flexion (Hill et al., 2000, Dennis et al., 2005). A tri-condylar, posterior stabilized (PS) total knee arthroplasty (TKA) with a rotating platform bearing (TKA-A) has been designed to address these characteristics of the intact knee. The third condyle is designed to guide the femoral component throughout the entire flexion arc (AP stability and femoral rollback with flexion), while the rotating platform bearing allows for IE rotation.
This study used a computer model to compare the AP and IE laxity of a new TKA-A to that of two clinically established TKAs (TKA-B: rotating PS TKA, TKA-C: fixed PS TKA) and to demonstrate improvements in AP stability, IE rotation, and femoral rollback.
METHODS
A specimen-specific, robotically calibrated computer knee model (Siggelkow et al., 2012), consisting of the femur, tibia and fibula as well as the kinetic contribution of the ligaments and capsule was virtually implanted with appropriate sizes of TKA-A, TKA-B and TKA-C adhering to the respective surgical techniques. A similar extension gap was targeted for all designs.
The following kinematic data resulting from applied loads and moments were analyzed: 1) Passive AP and IE laxity (AP load: ± 50 N, IE moment: ± 6 Nm) of the midpoint between the flexion facet centers (Iwaki et al., JBJS, 2000) under low compression (44 N), 2) AP position of the medial and lateral low points (LP) of the femoral component during a lunge motion (Varadarajan et al., 2008).
RESULTS
The TKA-A design demonstrated decreased AP laxity compared to TKA-B and TKA-C at all evaluated flexion angles beyond full extension (Figure 1, A). Both the TKA-A and TKA-B designs demonstrated greater IE laxity than TKA-C in early and mid-flexion (Figure 1, B). Considering lunge kinematics, the TKA-A design experienced the greatest rollback (change in AP position) of both condyles throughout flexion (lateral: 13 mm, medial: 7 mm) compared to TKA-B and TKA-C (lateral: 10 mm, medial: −1 mm; lateral: 9 mm medial: 6 mm), respectively. Rollback of the lateral condyle was greater than that of the medial condyle for all designs, which is consistent with the patterns reported for the intact knee (Hill et al., 2000, Dennis et al., 2005). Furthermore, the rate of rollback was more consistent for TKA-A compared to TKA-B and TKA-C.
CONCLUSION
The new tri-condylar TKA design exhibited greater AP stability at all flexion angles and greater IE laxity through mid-flexion compared to two clinically-established TKA designs. In addition, posterior femoral rollback was more continuous for this new design. We attribute these findings to the design of the third condyle and the rotating platform bearing.
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