INTRODUCTION

Hip wear simulator test results could be affected by many non-bearing related factors such as fixation surface conditions, equipment calibration and component set-up. In an effort to improve the accuracy, reliability and repeatability of hip simulator test, a quality management system has been established at the IDC hip tribology laboratory, which has been accredited by UKAS (United Kingdom Accreditation Service) in accordance with the recognised international standard ISO17025. This study demonstrates that under well-controlled laboratory and testing conditions, satisfactory repeatability can be achieved during hip simulator studies.

Introduction

Ion analysis has been used as one of the key indicators to assess the performance of MoM devices in patients. Modular devices, in particular having larger overall surface area (the stem and sleeve), and locking interfaces (head – bore, sleeve- taper and sleeve-bore, stem-taper surfaces) than other MoM devices are expected to release greater number of ions. Concerns have been expressed that the ion release at the taper junction might be a potential cause leading to the failure of the implant [Garbuz et al, 2010].

The aim of this study was to look into the wear and the associated ion release from the taper junction and the articulating surface of modular devices.

Metal-on-Metal devices generate significantly lower volumetric wear than conventional total hip replacements. However, clinically some patients may suffer some form of laxity in their joints leading to subluxation of the joint, which in turn may cause edge loading of an implant thereby increasing the chances of failure due to higher than expected wear.

In this study, the effect of subluxation on MoM implant wear was investigated on a hip joint simulator.

INTRODUCTION

Analysis of retrieved ceramic components have shown areas of localized ‘stripe wear’, which have been attributed to joint laxity and/or impingement resulting in subluxation of the head, causing wear on the edge of the cup. Studies have been conducted into the effects of mild subluxation, however few in vitro tests have looked at severe subluxation. The aim of this study was to develop a more clinically relevant subluxation protocol.

Introduction

Hip implant research has been carried out for decades using hip simulators to reflect situations in vivo. With regards to metal on metal (MoM) implant testing, it has been reported that there is no significant difference between the wear generated by various cobalt chromium (CoCr) microstructures. On the contrary, higher wear, metal ion levels and subsequent failures have been reported in heat treated (high carbon, low carbide) devices compared to as cast (high carbon, high carbide) devices in vivo. During testing, the bearing surfaces may be masked from the effect of microstructure on wear under fast and continuous cycles, while in vivo, the extensive range of kinetics and kinematics, stop/start motion, varying walking frequencies could break down the fluid film, resulting in a less favourable lubrication regime. The aims of this study were to develop a more physiologically relevant hip simulator test protocol, and investigate the effect of microstructure on wear.

Introduction: In vitro studies have shown that low clearance bearings have the potential to generate low wear. However, cementless acetabular cups are designed to be press fitted into the acetabulum, which could generate compressive stresses and non-uniform cup deformation during implantation. Deformation of the low clearance acetabular cups could also potentially lead to clamping or seizure of the joints and high frictional torque leading to implant failure. To obtain the benefit of low clearance and low wear, without compromising the tribological performance of the cup, a deflection compensation (DefCom) cup was designed. DefCom offers the benefits of low wear associated with low clearance components whilst reducing the risk of component seizure and high frictional torque due to component deformation.

Aim: The study was conducted in order to evaluate the tribological performance of a DefCom acetabular cup.

Materials and Methods: 50 mm diameter metal-on-metal DefCom hip resurfacing cups were used in this study. The components had an average clearance of 105±3 μm at the articulating sphere. Three of the cups were deformed plastically, along the ilial-ischeal column of the acetabulum. The degree of deformation was measured using the coordinate measuring machine, measuring the change in diameter of the cup in the direction of deformation. The cups were deformed on average by 65μm. The devices were tested in a ProSim hip wear Simulator for 5 million cycles. The lubricant was new born calf serum with 0.2% sodium azide diluted with de-ionised water to achieve protein concentration of 20 mg/ml. The flexion/extension was 30° and 15° with an internal/external rotation of ±10°. The force was Paul-type stance phase loading with a maximum load of 3 kN and a swing phase load of 0.3 kN, conducted at 1 Hz.

Results: The DefCom and deformed DefCom components showed a similar bi-phasic wear pattern to that of the BHR devices. Showing a period of ‘running in’ wear up to 1 Mc and then a reduced wear rate during the steady state phase from 1 Mc onwards. The DefCom devices produced a wear rate of 0.24 mm3/Mc, whilst the deformed DefCom joints produced a wear rate of 0.48 mm3/Mc for the running-in phase. Steady state wear was achieved for all joints after 1 Mc. The average steady state wear (1.0–5.0 Mc) rate for the DefCom joints was 0.12 mm3/Mc, with 0.14 mm3/Mc for the deformed joints joint. The wear rate for the non-deformed DefCom device is lower than that generated by the BHR, which were 0.72 mm3/Mc and 0.18 mm3/Mc for the running-in and steady state wear, respectively.

Conclusion: The study has shown that the DefCom acetabular cup has the potential to reduce the initial running-in wear by reducing the clearance at the contact area between the head and cup, whilst compensating for deformation that may occur during cup implantation.

Introduction: The accepted method of assessing wear following a hip simulator test has been to use a precision balance. As the MoM devices produce significantly less weight loss than hard-on-soft bearings, the measurements of MoM devices are now almost at the detection limit of many balances. There is a need for a method that can be used in conjunction with gravimetric analysis that will provide an accurate assessment of ion concentration levels that will support the gravimetric measurements.

Aim: To develop a method to assess wear using metal ion analysis in order to support gravimetric measurements of metal on metal devices.

Materials and methods: Hip simulator test: Three pairs of 50 mm diameter as cast CoCr MoM devices were tested in a ProSim hip wear simulator (SimSol Stockport/UK) under physiologically relevant conditions. The lubricant was new born calf serum with 0.2 % sodium azide concentration diluted with de-ionised water for protein concentration of 20 g/l. Stop-start motion was implemented every 100 cycles. Lubricant changed every 125 k cycles. The frequency was 0.5 Hz. Wear was assessed gravimetrically at every 0.5 million cycles (Mc) interval.

Ion analysis: Serum was collected from test station and allowed to settle for 12 hours. An aliquot of 20 ml from lubricant was collected. Each sample was centrifuged at 2500 g-force for 10 minutes. A 10 ml aliquot was collected from each sample and was further centrifuged at 2500 g-force for 10 minutes. 1.5 ml aliquot was collected and stored at −20 °C. A high resolution inductively-coupled plasma mass spectrometry instrument (ELEMENT, ThermoFinnigan MAT, Bremen/Germany) was then used for the analysis of metal ions.

Results and Discussion: The average cumulative metal ion levels at 0.5, 1 and 1.5 Mc showed similar trends in wear to that of the average cumulative weight loss assessed gravimetrically. There were similar biphasic wear trends in both metal ion levels and gravimetric weight losses. Other studies have also shown similar correlation between volume loss and ion concentration levels. The percentage distribution of Co, Cr and Mo in the metal ion samples are in close agreement with nominal chemical composition of the material tested.

Conclusion: This study showed that metal ion measurements can help to confirm gravimetrically measured material loss.

Introduction: To develop a more physiologically relevant hip simulator test protocol and study the effect of microstructure on the wear performance of as-cast (AC) and double heat treated (DHT) devices under the new protocol.

Methods: Three pairs of AC and four pairs of DHT 50 mm CoCr metal-on-metal (MoM) devices were tested. The lubricant used was bovine serum. Stop-start motion was implemented between the two sets of kinetics and kinematics that alternated every 100 cycles throughout the test. Condition one: The flexion/extension was 30° and 15° respectively. The internal/external rotation was ±10°. The force was Paul type stance phase loading with a maximum load of 3 kN and a standard ISO swing phase load of 0.3 kN. Condition two: The flexion/extension was ±22°. The internal/external rotation was ±8°. The force was a maximum stance phase load of 2.2 kN and a swing phase load of 0.24 kN at 0.5 Hz frequency. Wear was assessed gravimetrically.

Result: The masking effect of 1 Hz speed and uninter-rupted motion, in providing exaggerated lubrication regime, was exposed under more physiologically relevant test conditions. The AC devices have significantly reduced wear when compared to the DHT devices. It can also be seen that from 0.5 to 2 Mc the divergence in wear has increased.

Conclusion: A more physiologically relevant hip simulator test protocol was successfully developed and implemented, in showing the effect of microstructure on wear as seen in vivo, where high wear of DHT devices has been observed. 295

Hip simulators have been used for ten years to determine the tribological performance of large-head metal-on-metal devices using traditional test conditions. However, the hip simulator protocols were originally developed to test metal-on-polyethylene devices. We have used patient activity data to develop a more physiologically relevant test protocol for metal-on-metal devices. This includes stop/start motion, a more appropriate walking frequency, and alternating kinetic and kinematic profiles.

There has been considerable discussion about the effect of heat treatments on the wear of metal-on-metal cobalt chromium molybdenum (CoCrMo) devices. Clinical studies have shown a higher rate of wear, levels of metal ions and rates of failure for the heat-treated metal compared to the as-cast metal CoCrMo devices. However, hip simulator studies in vitro under traditional testing conditions have thus far not been able to demonstrate a difference between the wear performance of these implants.

Using a physiologically relevant test protocol, we have shown that heat treatment of metal-on-metal CoCrMo devices adversely affects their wear performance and generates significantly higher wear rates and levels of metal ions than in as-cast metal implants.