Abstract
Introduction
The performance of total hip replacement (THR) devices can be affected by the quality of the tissues surrounding the joint or the mismatch of the component centres during hip replacement surgery. Experimental studies have shown that these factors can cause the separation of the two components during walking cycle (dynamic separation) and the contact of the femoral head with the rim of the acetabular liner (edge loading), which can lead to increased wear and shortened implant lifespan1. There is a need for flexible pre-clinical testing tools which allow THR devices to be assessed under these adverse conditions. In this work, a novel dynamic finite element model was developed that is able to generate dynamic separation as it occurs during the gait cycle. In addition, the ability to interrogate contact mechanics and material strain under separation conditions provides a unique means of assessing the severity of edge loading. This study demonstrates these model capabilities for a range of simulated surgical translational mismatch values, for ceramic-on-polyethylene implants.
Methodology
The components of the THR were aligned and constrained as illustrated in Figure 1. CAD models of commercially available implant geometries were used (DePuy Synthes, Leeds, UK) modified for model simplicity by removing anti-rotation features.
The polyethylene cup liner was given elastic-plastic behaviour. An axial load following the Paul cycle pattern (5 repetitive cycles) with maximum of 3KN and swing phase load of 0.3KN, was applied through the cup holder. The effect of translational mismatch was implemented by using a spring element connected to the cup unit on the lateral side. The spring was compressed by a fixed amount to replicate a degree of medial-lateral mismatch of the components. The instantaneous resultant force vector dictated the dynamic sliding behaviour of the cup against the head. In this study, translational medial-lateral mismatch values of 1, 2, 3 and 4mm were used and the medial-lateral dynamic separation, contact pressure maps and plastic strain were recorded.
Results
The highest level of dynamic separation is achieved when the minimum axial load (during swing phase) is applied. The dynamic separation increases as the surgical translation mismatch increases (figure 2), with values over 0.5mm (radial clearance) representing cases where the head is in contact with the rim of the cup. Maximum separation occurred towards the end of the swing phase. Plots of the shape of the contact pressure at that point can be seen in Figure 3. Only the 4mm mismatch created substantial plastic deformation.
Conclusion
The finite element model was able to predict medial-lateral separation as it occurred dynamically in the gait cycle, including cases where the femoral head was in contact with the rim of the cup. The increase in medial-lateral separation with increased translational mismatch was in broad agreement with existing experimental data2. Substantial plastic deformation was only seen in cases where the translational mismatch caused the femoral head to be in contact with the rim of the polyethylene cup.
