Abstract
Introduction
The biological response to UHMWPE particles generated by total joint replacements is one of the key causes of osteolysis, which leads to late failure of implants. Particles ranging from 0.1-1.0μm have been shown to be the most biologically active, in terms of osteolytic cytokine release from macrophages [1]. Current designs of lumbar total disc replacements (TDR) contain UHMWPE as a bearing surface and the first reports of osteolysis around TDR in vivo have appeared recently in the literature [2]. The current wear testing standard (ISO18192-1) for TDR specifies only four degrees of freedom (4DOF), i.e. axial load, flexion-extension, lateral bend and axial rotation. However, Callaghan et al. [3] described a fifth DOF, anterior-posterior (AP) shear. The aim of this study was to investigate the effect that this additional AP shear load component had on the size and morphology of the wear particles generated by ProDisc-L TDR devices over five million cycles in a spine simulator.
Methods
A six-station lumbar spine simulator (Simulation Solutions, UK) was used to test ProDisc-L TDR components (Synthes Spine, USA) under the ISO 18192-1 standard inputs and with the addition of an AP load of +175 and −140N. Wear particles were isolated at 2 and 5 mc using a modified alkaline digestion protocol [4]. Particles were collected by filtration and imaged by high resolution FEGSEM. Particle number and volume distributions were calculated as described previously [4] and were compared statistically by one way ANOVA (p<0.05).
Results
Similar particle morphologies were observed under 4DOF and 5DOF inputs, including flakes, fibrils and granules. No significant differences were observed between the size and volume distributions under 4DOF and 5DOF when comparisons were made at the same time point, although there was a trend towards larger particles being generated under 5DOF inputs. The mode of the frequency distribution was in the submicron size range with the volume distributions showing greater variability.
Discussion
This study represents the first comprehensive wear particle analysis comparing 4DOF (ISO conditions) to 5DOF inputs on a single TDR device. Vicars et al. [5] reported no significant differences in the wear volume of ProDisc-L TDR components under 4 and 5DOF inputs. The present study has shown that the particle size distributions and particle morphologies were not significantly affected by the addition of AP shear. The wear particles were similar in size and morphology to those previously reported for total hip replacements and total knee replacements [6], indicating that wear debris produced by TDR may have a similar potential for osteolysis.
