Modelling of Dynamic Edge Loading in Total Hip Replacements with ceramic on polyethylene Bearings

The performance of total hip replacement (THR) devices can be affected by various factors such as quality of the tissues surrounding the joint, mismatch of the component centres or the cup positioning during hip replacement surgery. Experimental studies have shown that these factors can cause the separation of the two components during the 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 lifespan.
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, cup inclination angles and swing phase loads for ceramic-on-polyethylene implants.
The computational model was developed to replicate one station of the Leeds II hip simulator that mimic in vitro adverse conditions. Firstly, a computational sensitivity model was developed under standard conditions for a stable computational contact. The mechanism of separation was also added. 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, cup inclination angle and decreased swing phase load were in broad agreement with existing experimental data.
The factors that increased the separation level, also increased the permeant deformation on the cup. However, steep cup inclination angle resulted in a higher number of conditions with permanent deformation than the standard cup inclination angle. Moreover, despite the low axial load during swing phase, under some separation conditions, reduced contact area created stress value higher than those at the peak axial load.
The developed computational tool can be used to understand the effect of various factors on the separation and contact mechanics simultaneously. As separation is a multi-factorial phenomenon, this model can assist to focus on the selected factors that affect the separation experimentally. Moreover, the effect of components specifications such as materials, geometry, and the cup thickness can be investigated with this model.