Background: 

We sought to define “at risk” loading conditions associated with rotating-platform total knee arthroplasty (TKA-RP) implants that predispose to insert subluxation and spinout and to quantify tolerances for flexion-extension gap asymmetry and laxity in order to prevent these adverse events.

Methods: 

Biomechanical testing was performed on 6 fresh-frozen cadaveric limbs with a TKA-RP implant with use of a gap-balancing technique, followed by sequential femoral component revision with variable-thickness polyethylene inserts to systematically represent 5 flexion-extension mismatch and asymmetry conditions. Each configuration was subjected to mechanical loading at 0°, 30°, and 60°. Rotational displacement of the insert on the tibial baseplate, lateral compartment separation, and insert concavity depth were measured with use of a digital caliper. Yield torque, a surrogate for ease of insert rotation and escape of the femoral component, was calculated with use of custom MATLAB code.

Results: 

Design-intended insert rotation decreased with increasing knee flexion angles in each loading configuration. Likewise, yield torque increased with increasing joint flexion and decreased with increasing joint laxity in all testing configurations. Insert instability and femoral condyle displacement were reproduced in positions of increasing knee flexion and asymmetrical flexion gap laxity. The depth of lateral polyethylene insert concavity determined femoral condylar capture and defined a narrow tolerance of <2 mm in the smallest implant sizes for flexion gap asymmetry leading to rotational insert instability.

Conclusions: 

Decreased femoral-tibial articular surface conformity with increasing knee flexion and asymmetrical flexion gap laxity enable paradoxical motion of the femoral component on the upper insert surface rather than the undersurface, as designed.

Clinical Relevance: 

Mobile-bearing TKA-RP is a technically demanding procedure requiring a snug symmetrical flexion gap. As little as 2 mm of asymmetrical lateral flexion laxity can result in decreased conformity, condyle liftoff, and insert subluxation. Flexion beyond 30° decreases bearing surface contact area and predisposes to reduced insert rotation and mechanical malfunction.

中文翻译：

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旋转平台全膝关节置换术后功能性屈曲不稳定性。

背景： 

我们试图定义与旋转平台全膝关节置换术 (TKA-RP) 植入物相关的“危险”负载条件，这些植入物容易插入半脱位和旋转，并量化屈伸间隙不对称和松弛的耐受性，以防止这些不良事件。

方法： 

使用间隙平衡技术对 6 个带有 TKA-RP 植入物的新鲜冷冻尸体肢体进行生物力学测试，然后使用可变厚度聚乙烯插入物进行连续股骨部件修正，以系统地代表 5 种屈曲-伸展不匹配和不对称情况。每种配置均承受 0°、30° 和 60° 的机械载荷。使用数字卡尺测量胫骨基板上插入物的旋转位移、侧室间隔和插入物凹入深度。屈服扭矩是股骨假体插入旋转和脱离难易程度的替代指标，是使用自定义 MATLAB 代码计算的。

结果： 

在每种负载配置中，设计预期的插入件旋转随着膝关节屈曲角度的增加而减少。同样，在所有测试配置中，屈服扭矩随着关节屈曲的增加而增加，并随着关节松弛的增加而减少。在膝关节屈曲增加和不对称屈曲间隙松弛的位置上再现了插入不稳定性和股骨髁移位​​。外侧聚乙烯插入凹面的深度决定了股骨髁的捕获，并在最小植入物尺寸中定义了 <2 mm 的窄公差，以防止屈曲间隙不对称导致旋转插入不稳定。

结论： 

股骨-胫骨关节面的顺应性随着膝关节屈曲的增加和不对称的屈曲间隙松弛而降低，使得股骨组件在上插入表面上而不是下表面上发生矛盾运动，如设计的那样。

临床相关性： 

移动轴承 TKA-RP 是一项技术要求较高的手术，需要紧密对称的屈曲间隙。只要 2 毫米的不对称侧屈松弛就会导致顺应性下降、髁突抬离和插入物半脱位。超过 30° 的弯曲会减少轴承表面接触面积，并容易导致刀片旋转减少和机械故障。