Abstract
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.
Highlights
The success of total knee arthroplasty (TKA) for returning knee functionality has contributed to its widening application for treating diseases of the knee that have failed conservative treatments [1]
A similar tendency can be seen for tibial rotation whereby the graph of TR angle plotted according to ISO 14243–3:2014 is an inverse of the graph plotted according to the modified ISO 14243–3
The most important finding of this study is that the direction of action of the AP load and TR torque from ISO 14243–1, and AP displacement and TR angle from ISO 14243–3 influences wear on the tibial insert
Summary
The success of total knee arthroplasty (TKA) for returning knee functionality has contributed to its widening application for treating diseases of the knee that have failed conservative treatments [1]. Loosening of the implant is the most common reason for requiring a second TKA [6,7,8], which is reported to be linked to malalignment of the motion axis and the generation of wear particles which can induce osteolysis around the implant [9,10]. There are two control modes for the wear test during simulated gait: displacement control according to ISO 14243–3 [11,12], and load control according to ISO 14243–1 [13,14]. The knee simulator used to perform the gait movements requires four inputs; flexion angle, axial load, AP displacement or AP load, TR angle or TR torque. The magnitude and direction of the flexion angle and axial load inputs are same across the ISO 14243 range of standards [11,12,13,14] (Fig 1).
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