Pegg Elise C, Murray David W, Pandit Hemant G, O'Connor John J, Gill Harinderjit S
Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, Nuffield Orthopaedic Centre, University of Oxford, Oxford, UK.
Proc Inst Mech Eng H. 2013 Nov;227(11):1213-23. doi: 10.1177/0954411913494326. Epub 2013 Aug 12.
Cases of fractured mobile unicompartmental knee bearings have recently been reported. The purpose of this study was to understand the mechanics behind these fractures and to examine the influence of different design modifications. A parametric finite element model was used to examine the influence of different geometrical factors on the stresses within the bearing. Crack initiation occurred clinically in the centre of the bearing; this correlated with the position of the maximum von Mises stress. Tensile stresses, thought to propagate the fatigue crack, were maximal at the medial-lateral sides of the bearing, and the tensile vectors were normal to the fracture direction observed clinically. Fully congruent femoral articulation on the bearing, use of a thicker bearing size, and minimising wear of the component reduced the risk of fracture. For example, an unworn 6.5-mm-thick bearing (no clinical fractures reported) had 21.6% lower medial-lateral tensile stress compared to an unworn 3.5 mm bearing (five clinical fractures reported). In turn, an unworn 3.5 mm bearing had 34.3% lower tensile stress compared to a 3.5 mm bearing after 1.9 mm wear (average linear wear reported for clinically fractured bearings). The fracture risk was also reduced when the radio-opaque marker wire was positioned further from the centre of the bearing, and when marker balls were used instead of marker wires (19% reduction in tensile stress in some regions). These results indicate the importance of minimising component wear; the data also support the current component design which uses posterior marker balls instead of marker wires, and the continuing use of a congruous femoral component.
最近有报道称出现了可移动单髁膝关节轴承骨折的病例。本研究的目的是了解这些骨折背后的力学原理，并研究不同设计改进的影响。使用参数化有限元模型来研究不同几何因素对轴承内应力的影响。临床上裂纹起始于轴承中心；这与最大冯·米塞斯应力的位置相关。被认为会使疲劳裂纹扩展的拉应力在轴承的内侧 - 外侧最大，并且拉应力向量与临床观察到的骨折方向垂直。轴承上股骨关节完全贴合、使用更厚的轴承尺寸以及尽量减少部件磨损可降低骨折风险。例如，一个未磨损的6.5毫米厚的轴承（未报告临床骨折）与一个未磨损的3.5毫米轴承（报告了5例临床骨折）相比，其内侧 - 外侧拉应力降低了21.6%。反过来，一个未磨损的3.5毫米轴承与磨损1.9毫米后的3.5毫米轴承（临床骨折轴承报告的平均线性磨损）相比，拉应力降低了34.3%。当不透射线的标记线距离轴承中心更远时，以及当使用标记球而不是标记线时，骨折风险也会降低（某些区域拉应力降低19%）。这些结果表明尽量减少部件磨损的重要性；数据还支持当前使用后部标记球而非标记线的部件设计，以及继续使用贴合的股骨部件。
