The impact of variations in input directions according to ISO 14243 on wearing of knee prostheses

ISO 14243 is the governing standard for wear testing of knee prostheses, but there is controversy over the correct direction of anterior-posterior (AP) displacement and loading and the correct direction of tibial rotation (TR) angles and torque. This study aimed to analyze how altering the direction of AP and TR affected wear on the tibial insert. Modifications to the conditions specified in ISO 14243–1 and ISO 14243–3 were also proposed. As such, five loading conditions were applied to FEA models of a knee prosthesis: (1) Modified ISO 14243–3 with positive AP displacement and TR angle, (2) ISO 14243–3:2004 with negative AP displacement and positive TR angle, (3) ISO 14243–3:2014 with positive AP displacement and negative TR angle, (4) Modified ISO 14243–1 with positive AP load and TR torque, and (5) ISO 14243–1:2009 with negative AP load and positive TR torque. This study found that changing the input directions for AP and TR according to ISO 14243–1 and 14243–3 had an influence on the wear rate and wear contours on the tibial insert model. However, the extent of wear varies depending on the design features of the tibial insert and shape of the input curves. For displacement control according to ISO 14243–3, changing the direction of AP displacement had a marked influence on the wear rate (272.77%), but changing the direction of TR angle had a much lower impact (2.17%). For load control according to ISO 14243–1, reversing the AP load (ISO 14243–1:2009) only increased the wear rate by 6.73% in comparison to the modified ISO 14243–1 conditions. The clinical relevance of this study is that the results demonstrate that tibial wear is affected by the direction of application of AP and TR. Incorrect application of the loading conditions during the design stage may lead to an ineffective preclinical evaluation and could subsequently influence implant longevity in clinical use.ISO 14243是膝关节假体磨损测试的主导标准，但目前在前-后（anterior-posterior, AP）位移与载荷的正确方向，以及胫骨旋转（tibial rotation, TR）角度与扭矩的正确方向上存在争议。本研究旨在分析改变AP与TR方向会如何影响胫骨衬垫的磨损情况，并提出了对ISO 14243–1和ISO 14243–3中规定的试验条件的修改方案。为此，本研究对某膝关节假体的有限元分析（Finite Element Analysis, FEA）模型施加了五种加载工况：（1）采用正AP位移与正TR角度的改良版ISO 14243–3工况；（2）采用负AP位移与正TR角度的ISO 14243–3:2004工况；（3）采用正AP位移与负TR角度的ISO 14243–3:2014工况；（4）采用正AP载荷与正TR扭矩的改良版ISO 14243–1工况；（5）采用负AP载荷与正TR扭矩的ISO 14243–1:2009工况。本研究发现，按照ISO 14243–1与ISO 14243–3的要求改变AP与TR的输入方向，会对胫骨衬垫模型的磨损率与磨损轮廓产生影响。不过，磨损程度会因胫骨衬垫的设计特征与输入曲线的形状而异。针对ISO 14243–3规定的位移控制工况，改变AP位移方向会对磨损率产生显著影响（增幅达272.77%），而改变TR角度方向的影响则相对微弱（仅为2.17%）。针对ISO 14243–1规定的载荷控制工况，与改良版ISO 14243–1工况相比，反转AP载荷方向（即ISO 14243–1:2009工况）仅使磨损率提升了6.73%。本研究的临床意义在于，研究结果表明胫骨磨损会受到AP与TR施加载荷方向的影响。若在设计阶段错误应用加载工况，可能会导致临床前评估失效，进而影响植入物在临床使用中的使用寿命。