Abstract
Introduction
The number of young and more active patients requiring total knee replacement (TKR) is increasing. Preclinical evaluation and understanding the long-term failure of TKR is therefore important. Preclinical wear simulation of TKR is usually performed according to the International Standards Organization (ISO) recommendations. Two international standards for preclinical wear simulation of TKRs have been developed so that the anterior-posterior (AP) translation and internal-external (IE) rotation can be driven in either force or displacement control. However, the effects of using different control regimes on the kinematics and wear of the same TKR have not been investigated. The current study investigated the kinematics, contact mechanics and wear performance of a TKR when running under ISO force and displacement control standards using an experimentally validated computational model.
Materials/Methods
Three different ISO control standards were investigated using a size C Sigma curved TKR (DePuy, UK), with moderately cross-linked UHMWPE curved inserts; ISO-14243-3-2004, ISO-14243-3-2014 and ISO- 14243-1-2009. Axial force and flexion-extension angle are common for the three standards. AP and IE motions are displacement controlled in ISO-14243-3-2004 and ISO-14243-3-2014, with the only difference being a reversal of AP polarity between the two standards, and are force controlled in ISO-14243-1-2009. The test setup and soft tissue constraints were defined in accordance with ISO recommendations. The wear model was based on the modification of Archard's law where the wear volume is defined as a function of contact area, sliding distance, cross-shear and contact stress. The simulation framework has been independently validated against experimental wear rates under three different standard and highly demanding daily activities (Abdelgaied et al. 2018).
Results
Reversing AP in the displacement control ISO-2014, compared to ISO-2004, resulted in high contact stresses of more than 70 MPa in the posterior direction. The predicted AP and IE from the force control ISO-2009 were in different directions and magnitudes to ISO-2014 AP and IE. The predicted wear rates were 1.8, 2.0, and 5.5 [mm3/mc] for ISO-14243-3-2004, ISO-14243-3-2014 and ISO-14243-1-2009 respectively.
Discussion
Reversing AP in the displacement control ISO-2014, without revising the femoral centre of rotation, resulted in high stress edge loading in the posterior direction, due to femoral rollback, and more than 10% increase in wear rate compared to ISO-2004. The predicted AP and IE from the force control ISO-2009 had different polarities and magnitudes to the corresponding displacement control ISO-2014 AP and IE. In addition, the predicted wear rate under the force control ISO-2009 was more than double that measured under displacement control standards due to the increased AP and IE motions predicted under the force control standard. In addition to the previous validation of the model, the predicted wear rate under the force control ISO-2009 of 5.5 mm3/mc was within the 95% confidence limits of the reported experimental wear rate for the same TKR of 4.71±1.29 mm3/mc (Johnston et al. 2018) which gives more confidence in the model.
Conclusion
The study showed significant differences between ISO force and displacement control standards and between ISO displacement standards with different AP polarities. These differences should therefore be considered when choosing a control regime for preclinical simulation of TKR.
