The continued improvement of ceramic materials for total hip arthroplasty led to the development of Zirconia and Zirconia toughened Alumina materials such as BIOLOX® Twenty-six retrieved BIOLOX® Raman spectra were collected using a confocal Raman spectrometer (WITec Gmbh, Germany) a laser wavelength of 532nm, a 50× objective and a 100μm pinhole. Twenty-five measurements were made on each retrieval at random locations outside any visible wear scars and inside visible wear scars. The average monoclinic content was calculated based on the method of Clarke and Adar [1]. For comparison 5 new BIOLOX® Introduction
Methods
Osteoarthritis (OA) can be artificially simulated ex vivo on healthy articular cartilage (AC) samples by use of proteolytic enzymes. In this article we will present preliminary analyses of the physical degradation of AC when subjected to alternating mechanical stresses. Since AC damage due to OA is believed to be mechanically induced, the first step towards the realisation of an improved understanding of degenerative behaviour of AC under physiological loading conditions is to perform ex vivo tests which mimic such conditions at best. Porcine AC was subjected to biochemical stimulation or left as native AC. Biochemical degradation was performed using combinations of trypsin and Matrix Metalloproteinases (MMPs) to induce the loss of proteoglycan and collagen. A comparison of the biochemical and mechanical properties, topography and difference in response to mechanical damage between the digested AC and healthy AC was made using White Light Interferometry (WLI), Atomic Force Microscopy (AFM) and mechanical testing. The mechanical damage was induced by subjecting AC to shear under physiological and non physiological conditions. The AC was mechanically tested in a Phosphate Buffered Saline (PBS) bath. After mechanical testing, biochemical analysis of the collagen and aggrecan content of the tissue and PBS present in the bath during the mechanical test was performed. Collagen content was determined by measurement of the amount of hydroxyproline (HPRO), and aggrecan content by the amount of glycosaminoglycans (GAG). The mechanical test was either performed on healthy (native) AC or on AC which had first been digested.INTRODUCTION
METHODS
We report a prospective study of clinical data collected pre, intra and post operation to remove both cup and head components of 118 failed, current generation metal on metal (MOM) hips. Whilst component position was important, the majority were unexplained failures and of these the majority (63%) had cup inclination angles of less than 55 degrees. Poor biocompatibility of the wear debris may explain many of the failures. Morlock et al reported a retrospective analysis of 267 MOM hips but only 34 head and cup couples (ie most were femoral neck fractures) and without data necessary to define cause of failure. The commonest cause of failure in the National Joint Registry (NJR) is unexplained.SUMMARY
BACKGROUND
Wear measurements of 100 explanted hips have been carried out on a Taylor Hobson 365 Roundness Machine using the LIRC Wear Protocol. It was found that 50% of explanted cups were wearing less than 5 μm/year and 60% of components were wearing less than 10 μm/year. Wear tests on hip joint simulators predict wear rates between 2 and 8 μm/year. However, 6% of cups are wearing faster than 100 μm/year, with 16% of cups have wear patches deeper than 100 μm and that 4% have a wear patch deeper than 300 μm.
Cup position is an important factor, all of the high wearing components are outside the Lewinick’s Box, however it is shown that mal position is does not always lead to extreme wear. Further analysis is taking place to calculate the size of the contact patch between head and cup (based on patient data and biomechanics) and the proximity of the contact patch to the edge of the cup.
Data on retrieval analysis of current generation metal on metal hip replacements is scarce. Such analysis may help to reduce the incidence of failure and revision procedures. Our aim was to investigate the wear characteristics of explanted (ie failed) metal on metal (MOM) acetabular components in terms of; 1) wear rate; and 2) distribution of the wear (specifically edge loading). 30 hips were collected from 20 centres. The types of prostheses were: 15 BHR; 10 Cormet and 5 ASR. Wear of the acetabular components of the prostheses was measured using an out of roundness (Rondcom 60A) machine. We recorded the implantation and removal date of each hip. The median linear wear rate was 7.32μm/year; this is at least 3 times greater than steady state wear rates reported for similar components worn in hip simulator studies. For 24 out of 30 cups, the greatest linear wear was recorded at the cup edge. Failed metal-on-metal acetabular components were associated with higher than expected wear rates. The highest wear was seen closest to the cup edge in the majority of patients suggesting edge loading had occurred and probably explained the high wear rates. Accurate cup placement (to avoid edge loading) may reduce the failure of MOM hips.
linear wear rate (depth of the femoral head and acetabular socket wear patch/time from operation); the diagnosis and severity of ALVAL from histological sections of periprosthetic tissue (Wilhert grading system); pre-revision whole blood cobalt, and chromium levels using Inductively Coupled Plasma Mass Spectrometry. All implants and tissue samples were analysed against control samples from patients undergoing revision of MOM hips for fractured femoral neck or impingement.
