INTRODUCTION

Whilst there is a great deal of research on hip implants, few studies have looked at implant orientation and the subsequent effect upon the wear performance of a hip resurfacing. This study aimed to measure implantation angles through radiographic analysis and linear wear for retrieved acetabular cups in order to investigate possible causal links between wear and implant orientation.

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: Although clinical results for the Metal-on-Metal (MoM) devices have been excellent, recently some concerns have been raised regarding the occurrence of periprosthetic soft tissue lesions (PSTL) in some patients with MoM devices. Clinical studies and retrieval analyses have shown that devices revised due to groin pain and PSTL generally have significantly higher wear that has been attributed to edge loading of the implants.

Aim: The retrieval study was to investigate the cause of edge-loading of MoM devices in vivo.

Materials and Methods: In this study 13 retrieved Birmingham Hip Resurfacing (BHR) devices were examined. All devices were supplied with radiographs showing the in vivo position of the implant. Linear wear was assessed using a Taylor-Hobson Talyrond 290 roundness machine. Multiple roundness profiles were obtained to locate the area of maximum wear on each component. Edge loaded devices were identified when the maximum linear wear occurred at the edge of the cup. Non-edge loaded pairs showed wear area within the articulating surface of the cup.

The in vivo abduction angle and version angle of the cup were determined by superimposing the BHR models onto the radiographs (ProEngineer Wildfire 4 with ISDX II extension software) using anatomical references and specific features of the BHR.

Results: Linear wear: Among the 13 devices investigated, 11 were edge loaded with the maximum linear wear occurred at the edge of the cup. The remaining 2 pairs were non-edge loaded. The average joint linear wear rate of the edge loaded devices was 49.9 μm per year, and that for the two non-edge loaded devices was 2.4 μm per year. Edge loaded pairs had far greater linear wear than non-edge loaded components.

Cup orientation: The abduction angles of the two non-edge loaded cups were 31° and 39°, and their version angles were 12 and 16° respectively. These angles were within recommended orientation for the BHR. In contrast, the adduction angles and/or version angles of all edge loaded devices were outside the recommended orientation. Their abduction angle varied from 40° to 66° and version angle from 5° to 46°.

The edge loaded devices with higher inclination angles and/or higher version angels generally had higher linear wear. There is strong correlation between the cup orientation and the linear wear of the implant.

Conclusion: Mal-orientated devices in this study showed clear signs of edge loading which in turn resulted in significant increase in wear compared to the well orientated/non-edge loaded devices.

Introduction: Cementless cup designs in metal-on-metal (MoM) hip resurfacing devices generally depend on a good primary press-fit fixation which stabilises the components in the early post-operative period. Pressfitting the cup into the acetabulum generates non-uniform compressive stresses on the cup and consequently causes non-uniform cup deformation. That in turn may result in equatorial contact, high frictional torque and femoral head seizure. It has been reported that high frictional torque has the potential to generate micromotion between the implant and its surrounding bone and as a result adversely affect the longevity of the implant. The aim of this study was to investigate the effects of cup deformation on friction between the articulating surfaces in MoM bearings with various clearances.

Materials and methods: Six Birmingham Hip Resurfacing (BHR) devices with various clearances (80 to 306 μm) were tested in a hip friction simulator to determine the friction between the bearing surfaces. The components were tested in clotted blood which is the primary lubricant during the early post-operative period. The joints were friction tested initially in their pristine conditions and subsequently the cups were deflected by 25– 35 μm using two points pinching action before further friction tests were carried out.

Results and Discussions: It has been reported that reduced clearance results in reduced friction. However, none of the previous studies have taken cup deflection into consideration nor have they used physiologically relevant lubricant. The results presented in this study show that for the reduced clearance components, friction was significantly increased when the cups were deflected by only 30 μm. However, for the components with higher clearance the friction did not change before and after deflection. It is postulated that the larger clearances can accommodate for the amount of distortion introduced to the cups in this study.

Introduction: modern cementless joints depend on bony ingrowth for durable long term fixation. Increased friction and micromotion in the early weeks can prevent ingrowth and affect long-term success.

Most friction studies are conducted in a bovine serum- carboxymethylcellulose (bs-cmc) medium. Following implantation however, the joint is bathed in blood which contains macromolecules and cells. The effect of these on friction is not fully understood.

A progressive radiolucent line (fig 1) observed in some low clearance resurfacings raises the concern that increased friction may be affecting component fixation. The purpose of this investigation was to study the effect of clearance on friction for a given bearing diameter in the presence of blood as lubricant.

Methods: Six Birmingham Hip Resurfacing devices with a nominal diameter of 50mm each and a range of diametral clearances (80, 135, 175, 200, 243 and 306μm) were used. Frictional measurements were carried out on a Prosim Hip Friction Simulator (Simsol Simulation Solutions, Stockport, UK). The test was conducted sequentially with whole blood (viscosity 0.009Pas) and a BS-CMC mixture as the lubricants (viscosity 0.01Pas).

Results: Low clearance devices (80–175μm) generated higher friction with blood than with BS-CMC (fig 2). With blood as the lubricant, low clearance devices generated much higher friction than higher clearance devices (200–306μm).

Discussion: Ongoing research into the in vitro performance of bearings is performed in hip simulators with lubricants that are believed to simulate joint fluid in terms of viscosity. However these lubricants are unable to simulate the friction effects of macromolecules.

The results of this study suggest that reduced clearance bearings have the potential to generate higher friction when blood is the lubricant. this higher friction in the low clearance bearings may produce micromotion in the early postoperative period and hamper bony ingrowth resulting in impaired fixation with long-term implications for survival.

Hip Resurfacing has always been an attractive concept for the treatment of hip arthritis in younger patients. Introduction of modern metal-on-metal hip resurfacing in 1991 in Birmingham, UK made this concept a reality.

In the early years, resurfacings were used only by a few experienced surgeons. From 1997, Birmingham Hip Resurfacings (BHRs) are being widely used in younger and more active patients. A breakdown of the ages at operation in the regional NHS hospital in Birmingham during the period April 1999 to March 2004 show that the mean age of metal-metal resurfacings is 51 years and the mean age of total hip replacements is 70 years.

At a 3.7 to 10.8 year follow-up (mean follow-up 5.8 years), the cumulative survival rate of metal-metal resurfacing in young active patients with osteoarthritis is 99.8%. In the long term, none of these patients were constrained to change their occupational or leisure activities as a result of the procedure. The overall revision rate of BHRs in all ages and all diagnoses is also very low (19 out of 2167 [0.88%] with a maximum follow-up of 7.5 years).

Improvements in instrumentation and a minimally invasive approach developed by the senior author have made this successful device even more attractive. Although objective evidence does not support the fact that the longer approach was any more invasive than the minimal route, patient feedback shows that it is very popular with them. While minimal approach is indeed appealing, it has a steep learning curve. In the early phase of this curve, care should be taken to avoid the potential risk of suboptimal component placement which can adversely affect long-term outcome.

It is true that metal-metal bearings are associated with elevated metal ion levels. In vitro studies of BHRs show that they have a period of early run-in wear. This is not sustained in the longer term. These findings are found to hold true in vivo as well, in our studies of 24- hour cobalt output and whole blood metal ion levels. Epidemiological studies show that historic metal-metal bearings are not associated with carcinogenic effects in the long-term.

Metal ion levels in patients with BHRs are in the same range as the levels found in those with historic metal-metal total hip replacements.