Introduction

Increased wear rates [1, 2] and acetabular rim fracture [3] of hip replacement bearings reported clinically have been associated with edge loading, which could occur due to rotational and/or translational mal-positioning [4]. Surgical mal-positioning can lead to dynamic microseparation mechanisms resulting in edge loading conditions. In vitro microseparation conditions have replicated stripe wear and the bi-modal wear debris distribution observed clinically [5, 6]. The aim of this study was to investigate the effect of steep cup inclination, representing rotational mal-positioning, on the magnitude of dynamic microseparation, severity of edge loading, and the resulting wear rate of a ceramic-on-ceramic bearing, under surgical translational mal-positioning conditions.

Summary Statement

The frictional torque of ceramic-on-ceramic bearings tended to increase with increasing the bearings size (32, 48, 56mm). However, the frictional torque was significantly lower than that measured on metal-on-metal bearings under well positioned and well lubricated conditions.

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.

INTRODUCTION

Ceramic-on-ceramic hip replacements have generated great interest in recent years due to substantial improvements in manufacturing techniques and material properties¹. Microseparation conditions that could occur due to several clinical factors such as head offset deficiency, medialised cup combined with laxity of soft tissue resulting in a translation malalignment, have been shown to cause edge loading, replicate clinically relevant wear mechanisms^2,3 and increase the wear of ceramic-on-ceramic bearings^3,4. The aim of this study was to investigate the influence of increasing the femoral head size on the wear of ceramic-on-ceramic bearings under several clinically relevant simulator conditions.

INTRODUCTION

Retrieval and clinical studies of metal-on-metal (MoM) bearings have associated increased wear¹ and elevated patient ion levels² with steep cup inclination angles and edge loading conditions. The University of Leeds have previously developed a hip simulator method that has been validated against retrievals and shown to replicate clinically relevant wear rates and wear mechanisms^3,4. This method involves introducing lateral microseparation to represent adverse joint laxity and offset deficiency. This study aimed to investigate the effect of microseparation representing translational malpostion, and increased cup inclination angle, representing rotational malposition, in isolation and combined on the wear of different sizes (28 and 36mm) MoM bearing in total hip replacement (THRs).

In vitro the introduction of microseparation and edge loading to hip simulator gait cycle has replicated clinically relevant wear rates and wear mechanisms in ceramic-on-ceramic bearings^[1], and elevated the wear rates of MoM surface replacements (SR) to levels similar to those observed in retrievals^[2]. The aim was to assess the wear of two different sized MoM total hip replacement bearings under steep cup inclination angles and adverse microseparation and edge loading conditions.

Two tests were performed on the Leeds II hip joint simulator using two different size bearings (28mm and 36mm). Cups were mounted to provide inclination angles of 45 degrees (n=3) and 65 degrees (n=3). The first three million cycles were under standard gait conditions. Microseparation and edge loading conditions as described by Nevelos et al^[1] were introduced to the gait cycle for the subsequent three million cycles. The lubricant was 25% new born calf serum. The mean wear rates and 95% confidence limits were determined and statistical analysis was performed using One Way ANOVA.

Under standard gait conditions, when the cup inclination angle increased from 45 degrees to 65 degrees, the wear of size 28mm bearing significantly (p=0.004) increased by 2.7-fold, however, the larger bearings did not show any increase in wear (p=0.9). The introduction of microseparation conditions resulted in a significant (p=0.0001) increase in wear rates for both bearing sizes under both cup inclination angle conditions. Under microseparation conditions, the increase in cup inclination angle had no influence on the wear rate for both bearing sizes (Figure 1).

With larger bearings, head-rim contact occurs at a steeper cup inclination angle providing an advantage over smaller bearings. The introduction of edge loading and microseparation conditions resulted in a significant increase in wear rates for both bearing sizes. The wear rates obtained in this study under combined increased cup inclination angle and microseparation were half of those obtained when SR MoM bearings were tested under similar adverse conditions^[2]. This study shows the importance of prosthesis design and accurate surgical positioning of the head and acetabular cup in MoM THRs.

Ceramic-on-ceramic total hip replacements (THRs) have shown low wear volumes in standard gait hip simulator studies1. However clinical reports have indicated a variation in wear rates and formation of stripe wear on the ceramic femoral heads2. The aim of this study was to investigate the influence of different clinical conditions such as cup inclination angle and microseparation (head offset deficiency) on the wear of ceramic-on-ceramic THRs. The six station Leeds II hip joint simulator was used to investigate the wear of size 28mm ceramic-on-ceramic bearing couples. The alumina matrix composite ceramic material (AMC, Biolox Delta, CeramTec AG, Germany) was used in this study. The lubricant used was 25% bovine serum. The study was carried out for a total of five million cycles; the first two million cycles under standard gait conditions and a further three million cycles under microseparation conditions. During microseparation, a lateral movement of 0.5mm was applied to the cup relative to the head during the swing phase of the gait cycle3. Three of the cups were mounted to provide a clinical angle of 55°, which is referred to as the ‘standard’ condition; and the other three cups were mounted to provide a clinical angle of 65°, which is referred to as the ‘steep angle’ condition. These combinations provided four different testing conditions: standard, steep cup angle, microseparation, and combination of steep cup angle and microseparation conditions. Volumetric wear was determined gravimetrically and statistical analysis was performed using One Way ANOVA (significance at p< 0.05). Increasing the cup inclination angle from 55° to 65° had no significant effect on the wear rate in Biolox Delta ceramic-on-ceramic THRs under both standard (p> 0.42) and microseparation (p> 0.55) conditions. Under standard gait conditions, the mean wear rate for both cup inclination angles was very low at 0.05 mm3/million cycles. The introduction of microseparation to the standard gait cycle significantly increased the mean wear rates (p< 0.01) to 0.13 mm3/ million cycles for the ‘standard’ cup inclination angle of 55° and 0.11 mm3/million cycles for the ‘steep’ cup inclination angle of 65°. A stripe of wear on the head also formed, with corresponding superior rim wear on the cup. For comparison, the steady state wear rate of HIPed third generation alumina ceramic (Biolox Forte) under microseparation conditions was 1.3 mm3/million cycles [4]. In conclusion, increasing the cup inclination angle by 10° had no influence on the wear rate of Biolox Delta ceramic-on-ceramic bearings. The introduction of microseparation conditions significantly increased the wear rate and resulted in stripe-like wear on the femoral head, which has previously been observed on retrieved ceramic prosthesis. However, these wear rates were still low, and were ten times lower than those previously reported for Biolox Forte.

Introduction: There is increasing interest in the use of ceramic on ceramic bearings for hip replacement, due to recognition of their extremely low wear and biocompatibility of the wear debris [1].

The aim of this study was to investigate the influence of cup inclination angle and head position on the wear of ceramic-on-ceramic total hip replacements.

Methods: The wear of Biolox Delta alumina matrix composite ceramic (CeramTec AG, Germany) was investigated using the six station Leeds II Physiological Anatomical hip joint simulator, using 25% bovine serum as a lubricant. Three ceramic-on-ceramic bearings were mounted with the cup providing a clinical angle of 55o (representing the standard condition) and three were mounted to provide a clinical angle of 65o (representing the steep cup angle condition). Simulator studies were carried out under standard gait conditions for 2 million cycles, and under micro-separation conditions for a further 3 million cycles. Micro-separation and dynamic lateralisation of the position of the head replicate head/cup rim contact at heel strike and simulate stripe wear on a ceramic femoral head as found on ceramic-on-ceramic retrievals [2]. Volumetric wear was determined gravimetrically and statistical analysis was performed using One Way ANOVA.

Results: There was no difference in the wear rates under standard gait conditions for the standard and steep cup angles, with a wear rate of 0.05 mm3/million cycles. Under micro-separation conditions the wear rates increased significantly to 0.13 and 0.11 mm3/million cycles for the standard and steep cup angles respectively. However, there was no significant difference between the standard and steep cup angle groups.

Discussion: Micro-separation and dynamic lateralisation of the position of the head during gait simulation significantly increased wear. However, the inclination of the cup in ceramic-on-ceramic THRs did not have a significant effect on the wear under either standard gait or micro-separation conditions.