Edge Loading in Total Hip Replacements Due to Translational Surgical Mal-Positioning: In Vitro Techniques

Introduction

Stripe wear, observed on retrieved ceramic hip replacements, has only been replicated in vitro under translational mal-positioning conditions where the centres of rotation of the head and the cup are mismatched^1,2; an in vitro condition termed “microseparation”.

The aim of this study was to compare the edge loading mechanisms observed under microseparation conditions due to translational mal-positioning conditions simulated on two different hip joint simulators.

Materials and Methods

The components used in this study were zirconia-toughened-alumina ceramic-on-ceramic bearings (36 mm) inserted into titanium alloy acetabular cups (BIOLOX® delta and Pinnacle® respectively, DePuy Synthes Joint Reconstruction, Leeds, UK). Six couples were tested for two million cycles under 0.5 mm dynamic microseparation conditions on the Leeds II hip joint simulator as described by Nevelos et al² and Stewart et al³ (Figure 1). Ten bearing couples were tested for two million cycles under microseparation conditions achieved in two different ways on the ProSim pneumatic hip joint simulator (SimSol, Stockport, UK). Two conditions were tested; condition (1)- the femoral head was left to completely separate (the vertical motion was controlled at 1 mm) causing it to contact the inferior rim of the acetabular cup before edge loading on the superior rim at heel strike (n = 5) and condition (2)- springs were placed below the plate holding the femoral head to control the tilt of the head laterally towards the rim of the acetabular cup as the negative pressure was applied (n = 5; Figure 1). Wear was assessed gravimetrically every million cycles using a microbalance (Mettler AT201, UK). Three-dimensional reconstructions of the wear area on the heads were obtained using a coordinate measuring machine (Legex 322, Mitutoyo, UK) and SR3D software (Tribosol, UK).

Results

The wear rates of the 36 mm ceramic-on-ceramic bearings obtained under microseparation conditions on the ProSim, where the medial-lateral displacement was controlled (condition 2), and under microseparation conditions on the Leeds II simulator were 0.22 mm³ and 0.13 mm³/million cycles respectively, however, the difference was not statistically significant (p = 0.092). The wear rates obtained under microseparation conditions on the Leeds II and the ProSim (condition 2) were both significantly higher (p = 0.006 and p = 0.009 respectively) than the wear rate obtained under microseparation conditions on the ProSim where full distraction of the head was provided (condition 1, 0.05 mm³/million cycles). The wear stripe areas formed on the femoral head under the three simulator set-ups are shown in Figure 2.

Discussion and Conclusion

Edge loading due to translational mal-positioning with 0.5 mm dynamic medial-lateral displacement on the Leeds II simulator has been validated against retrievals and shown to replicate stripe wear mechanisms, wear rates and bimodal wear debris distribution as that observed in vivo^1,2,4,5. The edge loading due to translational mal-positioning simulated on different machines can result in different wear rates and wear mechanisms. One microseparation technique (condition 2) achieved on the ProSim simulator has demonstrated similar results to the validated Leeds II simulator.