Background

Hemiarthroplasty of the hip involves the replacement of the femoral side of the joint with a metal prosthesis, resulting in metal-on-cartilage articulation. The two most common types of hemiarthroplasty used are the Austin Moore and the Thomson, both of which are available in either Titanium (Ti) or cobalt chromium (CoCr). Hemiarthroplasty may be more cost effective in elderly patients who have lower life expectancy and are less active.

Metal-on-metal hip resurfacing prostheses are a relatively recent intervention for relieving the symptoms of common musculoskeletal diseases such as osteoarthritis. While some short term clinical studies have offered positive results, in a minority of cases there is a recognised issue of femoral fracture, which commonly occurs in the first few months following the operation. This problem has been explained by a surgeon's learning curve and notching of the femur but, to date, studies of explanted early fracture components have been limited.

Tribological analysis was carried out on fourteen retrieved femoral components of which twelve were revised after femoral fracture and two for avascular necrosis (AVN). Eight samples were Durom (Zimmer, Indiana, USA) devices and six were Articular Surface Replacements (ASR, DePuy, Leeds, United Kingdom). One AVN retrieval was a Durom, the other an ASR. The mean time to fracture was 3.4 months. The AVNs were retrieved after 16 months (Durom) and 38 months (ASR).

Volumetric wear rates were determined using a Mitutoyo Legex 322 co-ordinate measuring machine (scanning accuracy within 1 micron) and a bespoke computer program. The method was validated against gravimetric calculations for volumetric wear using a sample femoral head that was artificially worn in vitro. At 5mm³, 10mm³, and 15mm³ of material removal, the method was accurate to within 0.5mm³. Surface roughness data was collected using a Zygo NewView500 interferometer (resolution 1nm).

Mean wear rates of 17.74mm³/year were measured from the fracture components. Wear rates for the AVN retrievals were 0.43mm³/year and 3.45mm³/year. Mean roughness values of the fracture retrievals (PV = 0.754, RMS = 0.027) were similar to the AVNs (PV = 0.621, RMS = 0.030), though the AVNs had been in vivo for significantly longer.

Theoretical lubrication calculations were carried out which found that in both AVN retrievals and in seven of the twelve cases of femoral fracture the roughening was sufficient to change the lubrication regime from fluid film to mixed. Three of these surfaces were bordering on the boundary lubrication regime. The results show that even before the femoral fracture, wear rates and roughness values were high and the implants were performing poorly.

Introduction

We conducted independent wear analysis of retrieved metal on metal (MoM) hip components from around the world. All patients with resurfaced hips who developed adverse reactions to metal debris (ARMD) were found to have increased wear of the bearing surfaces. This was untrue in patients with large diameter (?36mm) MoM total hip replacements. This led us to search for other factors leading to ARMD.

Introduction

Total hip prostheses which use a ceramic head within a metal liner are a relatively recent introduction. As such, survivorship rates from independent centres alongside explant analysis are rare. The early experience with this novel ceramic-on-metal (CoM) bearing couple is reported.

Background

The worldwide withdrawal of the DePuy Articular Surface Replacement (ASR) device in both its resurfacing and total hip replacement (THR) form on 26 August 2010, after 93,000 were implanted worldwide, has had major implications. The 2010 National Joint Registry for England and Wales quoted figures of 12-13% failure at five years; however these figures may be an underestimate.

Finger arthroplasty lacks the success seen with hip and knee joint replacements. The Van Straten Leuwen Poeschmann Metal (LPM) prosthesis was intended for the proximal interphalangeal (PIP) joints. However revision rates of 30% after 19 months were reported alongside massive osteolysis. Three failed LPM titanium niobium (TiNb) coated cobalt chrome (CoCr) components were obtained- two distal and one proximal.

All three components were analysed using an environmental scanning electron microscope (ESEM). This gave the chemical composition of the surface to determine if the TiNb surface coating was still intact. The distal components were analysed using a ZYGO non-contact profilometer (1nm resolution) with the proximal component unable to be analysed due to its shape. ZYGO analysis gave the roughness average (Ra) of the surface and determined the presence of scratches, pitting and other damage.

Images obtained from both the ZYGO and the ESEM indicated that the surfaces of all components were heavily worn. On the articulating surfaces of both distal components unidirectional scratching was dominant, while the non-articulating surface showed multidirectional scratching. The presence of unidirectional scratching suggested two-body wear, whilst the multidirectional scratching on the non-articulating surface of the distal component suggested that trapped debris may have caused three-body wear.

The ESEM chemical analysis showed that in some regions on the distal component the TiNb coating had been removed completely and in other areas it had been scratched or penetrated. On the proximal component the TiNb coating had been almost completely removed from the articulating surfaces and was only present in small amounts on the non-articulating surfaces. There was little evidence of bone attachment to the titanium coating which was intended to help provide fixation.

ESEM images showed the coating had been removed in some sections where there was minimal scratching, suggesting this scratching did not impact significantly in the coating removal. Therefore here the main cause of coating removal may have been corrosion, although scratching may have also have played a part.

The osteolysis reported clinically may have been linked to the wear debris from the failed coating.

Metal-on-metal hip resurfacing prostheses are a relatively recent intervention for relieving the symptoms of common musculoskeletal diseases such as osteoarthritis. While some short term clinical studies have offered positive results, in a minority of cases there is a recognised issue of femoral fracture, which commonly occurs in the first few months following the operation. This problem has been explained by a surgeon's learning curve and notching of the femur but, to date, studies of explanted early fracture components have been limited.

Tribological analysis was carried out on fourteen retrieved femoral components of which twelve were revised after femoral fracture and two for avascular necrosis (AVN). Eight samples were Durom (Zimmer, Indiana, USA) devices and six were Articular Surface Replacements (ASR, DePuy, Leeds, United Kingdom). One AVN retrieval was a Durom, the other an ASR. The mean time to fracture was 3.4 months. The AVNs were retrieved after 16 months (Durom) and 38 months (ASR).

Volumetric wear rates were determined using a Mitutoyo Legex 322 co-ordinate measuring machine (scanning accuracy within 1 micron) and a bespoke computer program. The method was validated against gravimetric calculations for volumetric wear using a sample femoral head that was artificially worn in vitro. At 5mm3, 10mm3, and 15mm3 of material removal, the method was accurate to within 0.5mm3. Surface roughness data was collected using a Zygo NewView500 interferometer (resolution 1nm).

Mean wear rates of 17.74mm3/year were measured from the fracture components. Wear rates for the AVN retrievals were 0.43mm3/year and 3.45mm3/year. Mean roughness values of the fracture retrievals (PV = 0.754nm, RMS = 0.027nm) were similar to the AVNs (PV = 0.621nm, RMS = 0.030nm), though the AVNs had been in vivo for significantly longer.

Theoretical lubrication calculations were carried out which found that in both AVN retrievals and in seven of the twelve cases of femoral fracture the roughening was sufficient to change the lubrication regime from fluid film to mixed. Three of these surfaces were bordering on the boundary lubrication regime. The results show that even before the femoral fracture, wear rates and roughness values were high and the implants were performing poorly.

Wear debris induced osteolysis is a recognized complication in conventional metal-on-polyethylene hip arthroplasty. One method of achieving wear reduction is through the use of metal-on-metal articulations. One of the latest manifestations of this biomaterial combination is in designs of hip resurfacing which are aimed at younger, more active patients. But, do these metal-on-metal hip resurfacings show low wear when implanted into patients?

Using a Mitutoyo Legex 322 co-ordinate measuring machine (scanning accuracy less than 1 micron) and a bespoke computer program, volumetric wear measurements for retrieved Articular Surface Replacements (ASR, DePuy) metal-on-metal hip resurfacings were undertaken. Measurements were validated against gravimetric calculations for volumetric wear using a sample femoral head that was artificially worn in vitro. At 5mm3, 10mm3, and 15mm3 of material removal, the method was shown to be accurate to within 0.5mm3.

Thirty-two femoral heads and twenty-two acetabular cups were measured. Acetabular cups exhibited mean volumetric wear of 29.00mm3 (range 1.35 - 109.72mm3) and a wear rate of 11.02mm3/year (range 0.30 - 63.59mm3/year). Femoral heads exhibited mean wear of 22.41mm3 (range 0.72 - 134.22mm3) and a wear rate of 8.72mm3/year (range 0.21 - 31.91mm3/year). In the 22 cases where both head and cup from the same prosthesis were available, mean total wear rates of 21.66mm3/year (range 0.51 - 95.50mm3/year) were observed.

Revision was necessitated by one of five effects; early femoral neck fracture (4 heads), avascular necrosis (AVN) (2 heads, 1 cup), infection (1 head, 1 cup), adverse reaction to metal debris (ARMD) (19 heads, 18 cups) or ARMD fracture (6 heads, 2 cups). Mean paired wear rates for the AVN and infection retrievals were 0.51mm3/year and 3.98mm3/year respectively. In vitro tests typically offer wear rates for metal-on-metal devices in the region of 2-4mm3.

Mean paired wear rates for ARMD and ARMD fracture were 17.64mm3/year and 68.5mm3/year respectively, significantly greater than those expected from in vitro tests. In the 4 cases of early fracture, only the heads were revised so a combined wear rate calculation was not possible. The heads exhibited mean wear rate of 8.26mm3/year. These high wear rates are of concern.

Although bovine serum is the lubricant recommended by several international standards for the wear testing of orthopedic biomaterials there are issues over its use. The inherent batch variation in protein content means that two bovine serum lubricants can give different wear rates. Due to degradation, the lubricant needs to be changed regularly, so that any third body wear particles are removed, thus potentially influencing wear regimes. There are also cost and safety issues with the use of bovine serum. For these reasons, alternative lubricants were investigated.

A 50-station wear test rig was used, which applied multi-directional motion to each ultra-high molecular weight polyethylene (UHMWPE) test pin. Each pin articulated against a cobalt chrome plate polished to better than 0.05 microns Ra. The following lubricants were used: 50% dilute bovine serum; soy protein; olive oil; wheatgerm oil; soya oil; albumin and globulin (AG) mix; albumin, globulin and chondroitin sulphate (AGC) mix; whole milk; Channel Island milk; 11 mg/ml protein egg white; 20 mg/ml egg white; and 40 mg/ml egg white. A minimum of 6 UHMWPE pins per lubricant were wear tested and the tests ran to 2.5 million cycles. Gravimetric measurements were taken throughout the test to determine the volume of wear and at the end of the test the samples were examined using a SEM.

The lubricants giving the closest results to bovine serum were 20 and 40 mg/ml egg white, with mean UHMWPE total wear volumes of 17.4 mm3 and 17.8 mm3 compared to bovine serum which gave 20.7 mm3. Surface topographies showed similar features too. The 11 mg/ml egg white lubricant and the AG and AGC lubricants were next closest in terms of wear. An UV absorbance assay found that all the protein based lubricants suffered from a high degradation rate, and the rate increased with increasing protein content.

Egg white may offer a less expensive alternative to dilute bovine serum as a test lubricant although it is likely that it too would need to be changed as regularly as bovine serum.

Bearing diameter and acetabular component orientation have been shown to be important variables effecting blood metal ion levels following hip resurfacing arthroplasty. So far no studies on bilateral hip resurfacings have taken into account these variables.

We examined the serum ion results of patients under the care of two experienced hip resurfacing surgeons who carry out ion analysis as part of routine post operative care. Surgeon 1: Patients were implanted exclusively with a “third generation” resurfacing device. Surgeon 2: Patients were implanted with the same “third generation” device and also a low clearance “fourth generation” resurfacing device. Only ion results from patients who were 12 months post surgery were included. Bilateral patients were matched to unilateral patients according to the surgeon performing the operation; the resurfacing system implanted and cup inclination and anteversion angles. The ion data from each bilateral group was tested against the corresponding unilateral groups using the Mann Whitney U test for non parametric data. Significance was drawn at p<0.05.

Surgeon 1: There were 310 patients with unilateral joints and 50 with bilateral joints. There were no significant differences with regard to time to follow up, activity levels, joint sizes or cup orientations. Serum chromium (Cr) and cobalt (Co) concentrations were significantly greater in the bilateral group (p<0.001). Median ion levels were greater in the bilateral group by a factor of >2 in the smallest joint sizes and <2 in the largest joints. Surgeon 2: There were 11 patients with bilateral third generation resurfacing joints and 50 with unilateral joints of the same design. The same relationship as described above was identified. There was a notable difference in the fourth generation implant group (n=13 bilateral, n=100 unilateral). Median ion levels for patients receiving bilateral joints of sizes <47mm were ten times greater than in the corresponding unilateral group.

Bearing diameter and component design are critical factors in determining metal ion levels following bilateral hip resurfacings. Surgeons must consider the potential implications of gross increases in metal ion levels prior to performing bilateral hip resurfacings in smaller patients.

Background: There is a paucity of published data with regard to the wear of failed metal on metal (MoM) resurfacing devices.

Materials and Methods: MoM components retrieved from patients from two independent centres experiencing failure secondary to ARMD were analysed using a Mitutoyo Legex 322 coordinate measuring machine (CMM) which has an accuracy of 0.8 microns. Between 4000 – 6000 points were taken on each explant, dependent on the size of the bearing surface. Maximum wear depths and total volumetric wear were calculated. These values were compared to those from control samples retrieved following uncomplicated fractures/femoral collapse secondary to avascular necrosis (after calculating equivalent yearly wear rates).

Results: 58 ARMD components were analysed. This included 22 36mm MoM THRs (DePuy Pinnacle), 28 DePuy ASRs and 8 Zimmer Duroms. There were 30 resurfacing fracture/avascular necrosis controls. Volumetric wear rates and maximum wear depths of ARMD resurfacing components were significantly greater than the resurfacing control group for both the ASR and Duroms (p< 0.05) however 2 ARMD components exhibited similar amounts of wear compared to controls. Wear rates of the ARMD THR group were significantly lower than the ARMD resurfacing group (p< 0.05).

Conclusions: Increased articular wear is associated with an increased incidence of local adverse effects including tissue necrosis, joint effusions and fractures. However, there are a minority of patients who can develop tissue necrosis in the absence of accelerated wear, implying a spectrum of sensitivity This is reflected in the incidence of ARMD in the patient groups at the main study centre: > 5% in the ASR group and approx 1% in the THR group. We believe this indicates a failure of adequate lubrication and the resultant negative effects in larger bearing devices.

Background: Blood metal ion levels have been shown to correlate with wear of retrieved components. Increased articular wear is associated with early failure. It is not clear what the management of patients with raised metal ions who remain asymptomatic should be.

Methods: A prospective study of the DePuy ASR resurfacing device was commenced in 2004 at our independent centre. Blood and serum metal ion analysis is carried out routinely. We assessed the clinical outcome of all patients with cobalt (Co) concentrations > 5μg/L (n=25). We consider this value to be a clear indicator of a poorly performing bearing surface. Patients who remained asymptomatic were brought back for repeat ions testing and clinical assessment. A parallel study was carried out to examine the relationship between the wear depth of retrieved explants (assessed using a coordinate measuring) and blood ion levels

Results: Increases in chromium(Cr) and Co levels from the second blood sample correlated with the first sample (p< 0.05). The four patients with the highest Cr Co levels went on to suffer spontaneous femoral neck fractures between 3 and 4 years post op (pseudotumours in two of these cases). Of the remaining 25, 5 were revised for worsening pain (large effusions and areas of tissue necrosis were seen at revision) and 10 were under investigation for new onset of pain. Wear depths correlated well with blood ion concentrations (p< 0.05) substantiating the results of another centre.

Conclusion: 60% of patients with raised metal ion levels went on to develop complications within two years of the blood test. This suggests patients may only have temporary tolerances to excessive concentrations of metal wear debris. One novel association appears to be spontaneous delayed femoral fracture. Results suggest increased ion concentrations associated with small joint size/sub optimal cup orientation will continue to rise and these patients should be followed up closely.

In our independent centre, from 2002 to 2009, 155 BHRs (mean F/U 60 months) have been implanted as well as 420 ASR resurfacings and 75 THRs using ASR XL heads on SROM stems (mean F/U 35)

During this period we have experienced a number of failures with patients complaining of worsening groin pain at varying lengths of time post operatively. Aspiration of the hip joints yielded a large sterile effusion on each occasion. At revision, there were copious amounts of green grey fluid with varying degrees of necrosis. There were 17 failures of this nature in patients with ASR implants (12 females) and 0 in the BHR group. This amounts to a failure of 3.5% in the ASR group.

Tissue specimens from revision surgery showed varying degrees of “ALVAL” as well as consistently high numbers of histiocytes. Particulate metal debris was also a common finding.

The mean femoral size and acetabular anteversion and inclination angles of the ARMeD group/all asymptomatic patients was 45/49mm (p< 0.001), 27/20°(p< 0.001) and 53/48°(p< 0.08). Median blood chromium(Cr) and cobalt(Co) was 29 and 69 μg/L respectively in the ARMeD group versus 3.9 and 2.7 μg/L in the asymptomatic patients (n=160 with ion levels). Explant analysis confirmed greater rates of wear than expected.

Lymphocyte proliferation studies involving ARMeD patients showed no hyper reactivity to Cr and Co in vitro implying that these adverse clinical developments are mediated by a toxic reaction or a localised immune response.

Our overall results suggest that the reduced arc of cover of the fourth generation ASR cup has led to an increased failure rate secondary to the increased generation of metal debris. This failure rate is 7% in ASR devices with femoral components _47mm.

This report documents the clinical and histological findings, the operative management and the explant analyses of patients with early aseptic failure of large metal-on-metal (MonM) bearing joints.

Three hundred and fifty patients have been implanted with the ASR bearing surface (resurfacing or a modular THR) by a single surgeon at an independent centre since 2004. Six patients (all female) have been revised secondary to aseptic failure. All complained of severe groin pain exacerbated by straight leg raise and routine investigations were unable to establish a cause. Large amounts of sterile, highly viscous green fluid were aspirated from the hip joints in each case. Gross swelling of the pseudocapsule and a similar green fluid surrounding the implants were found at revision surgery. Histological examination of periprosthetic tissue samples showed changes consistent with ALVAL/metallosis, and analysis of the fluid revealed dense numbers of inflammatory cells. Symptoms in patients revised to ceramic-on-ceramic bearings improved post operatively. This was not the case with those reimplanted with MoM joints. Data from a subset of 76 patients (all unilateral resurfacings) showed that malaligned cups (anteverted > 20° +/− inclination angle > 45°) were associated with significantly higher whole blood metal ion levels than cups positioned within this range. All the patients with early aseptic failure had malaligned cups. Independent explant analysis revealed significant increases in the surface roughness values of the articular surfaces.

Our results suggest that some patients develop a significant inflammatory reaction to metal implants. This may be due to high levels of metal debris around the implant as a result of accelerated wear. Poor component alignment was found in all our patients with early aseptic failure.

Metal ion concentrations following metal on metal hip resurfacing arthroplasty remain a concern. Variables associated with increased metal ion concentrations need to be established. This study provides metal ion data from a consecutive cohort of the first 76 patients implanted with a fourth generation hip resurfacing prosthesis.

All patients agreed to post-operative blood metal ion sampling at a minimum of one year. Post-operative radiographic measurements of cup inclination and anteversion were obtained using the EBRA software. Mean whole blood chromium (Cr) and cobalt (Co) concentrations in patients receiving the smallest femoral implants (Ł51mm) were greater than in the patients implanted with the largest prostheses (ł53mm) by a factor of 3 and 9 respectively. Ion concentrations in the small femoral group were significantly related to acetabular inclination (R=0.439, P< 0.001 for Cr, R=0.372, P=0.004 for Co) and anteversion (R=0.330, P=0.010 for Cr, R=0.338, P=0.008 for Co). This relationship was not significant in the large implant group. Mean Cr and Co concentrations in patients with accurately orientated cups (inclination < 45°, anteversion < 20°) were 3.7μg/l and 1.8 μg/l respectively, compared to 9.1μg/l and 17.5μg/l in malaligned cups.

A reduced surface contact area caused by cup malalignment may increase contact stresses, resulting in a high wear rate if fluid film lubrication is inadequate. Improved fluid film lubrication has previously been found in larger heads in vitro. Accurate acetabular component positioning is essential in order to reduce metal ion concentrations following hip resurfacing.

Background: There are no large comparative metal ion studies of commercially available hip resurfacing devices which have taken into account the effects of femoral size and cup inclination and anteversion.

Patients and methods: Metal ion analysis is carried out routinely at our independent centre. We present the metal ion results of 95 unilateral ASR patients and 70 unilateral BHR patients. For all patients, acetabular cup orientation was assessed using EBRA software. Patients with other metallic implants and those within 12 months of surgery were excluded.

Results: Whole blood/serum chromium (Cr) and cobalt (Co) concentrations were inversely related to femoral component size in both the ASR and BHR group (p< 0.05). Cr and Co levels were only seen to increase in the BHR group when the cup was implanted with an inclination greater than 55°. A significant relationship was identifed between the anteversion of the BHR cup and Cr and Co (p< 0.05 for Co, Spearman Rank correlation), with an increase in ions observed at anteversion angles > 17°. Cr and Co were more strongly influenced by cup position in the case of the ASR, with an increase in metal ions observed at inclinations greater than 45° and anteversion angles of < 10° and > 20°.

Discussion: The increased tolerance of the BHR cup to inclinations between 45–55° is likely due to the larger BHR cup providing greater protection against edge loading. When the cohort was divided by gender, the median Cr concentrations of the male ASR patients were significantly lower than those of the BHR males (p< 0.001). This suggests that in larger components positioned at more satisfactory angles of inclination and anteversion, the lower clearance of the ASR proves more significant than the extra coverage provided by the BHR cup. The BHR appears to be more sensitive to changes in anteversion than inclination.

Background: There is a paucity of published literature with regard to metal ion levels following bilateral hip resurfacings.

Method: Recent research has identified femoral component size and acetabular cup anteversion and inclination as important variables involved in metal ion release into the blood. We compared 13 patients with bilateral joints resurfaced using the ASR and 11 patients with bilateral BHRs to those with unilateral joints of similar size and cup inclinations/anteversions. Patients were excluded if the last procedure was within 12 months of blood sampling and if they had other metallic implants. Analysis of standing radiographs to determine cup orientation was carried out using EBRA software. Blood samples were analysed using ICPMS.

Results: Median whole blood Cr and Co values were higher in the BHR bilateral group when compared to the unilateral BHR group by a factor of 1.12 and 1.24 respectively. (5.17 vs 4.59 for Cr, 2.40 vs 1.93 for Co). The difference was significant for Co (p=0.030) but not for Cr (p=0.136). For the large ASR joints (53mm), median Cr and Co values were higher in the bilateral group by a factor of 1.5 and 1.85 respectively. (4.55 vs 2.97 (Cr) 2.83 vs 1.53 (Co)). The difference was significant for both Cr (p=0.001) and Co (p=0.022). For the small ASR joints (< 53mm), median Cr Co values were higher in the bilateral group by a factor of 1.95 and 2.30 respectively. (8.29 vs 4.25 (Cr), 6.78 vs 2.94 (Co)) (p=0.019 Co) (p=0.007 Co).

Conclusion: Metal ion concentrations are significantly greater in patients with bilateral resurfacings. The Cr Co concentrations observed in our patients with bilateral small ASR joints are double those in the published literature on bilateral 28mm metal on metal(MoM) joints implying that the lubrication achieved by small ASRs is sub optimal.

In our independent centre, in the period from January 2003 to august 2008, over 1100 36mm MoM THRs have been implanted as well as 155 Birmingham Hip Resurfacing procedures, 402 ASR resurfacings and 75 THRs using ASR XL heads on SROM stems.

During this period we have experienced a number of failures with patients complaining of worsening groin pain at varying lengths of time post operatively. Aspiration of the hip joints yielded a large sterile effusion on each occasion. At revision, there were copious amounts of green grey fluid with varying degrees of necrosis. There were 11 failures of this nature in patients with ASR implants (10 females) and 2 in the 36 MoM THR group (one male one female).

Tissue specimens from revision surgery showed varying degrees of ‘ALVAL’ as well as consistently high numbers of histiocytes. Metal debris was also a common finding.

A fuller examination of our ASR cohort as a whole has shown that smaller components placed with inclinations > 45° and anteversions < 10 or > 20° are associated with increased metal ion levels. The 11 ASR failed joints were all sub optimally positioned (by the above definition), small components.

Explant analysis using a coordinate measuring machine and out of roundness device confirmed greater than expected wear of each component. The lower number of failures in the 36mm MoM group, as well as the equal sex incidence, suggests that the majority of these failures are due to the instigation of an immune reaction by large amounts of wear debris rather than adverse reactions to well functioning joints. It is likely that small malpositioned ASRs function in mixed to boundary lubrication, and this, combined with the larger radius of these joints compared to the 36mm MoM joints, results in more rapid wear.

Resurfacing metal-on-metal hip arthroplasty is currently showing promising clinical results. However there are concerns related to such implants, including the elevated levels of metal ions typically seen in patients. Valuable data can be obtained from explanted prostheses but due to their recent introduction few retrieval studies on resurfacing hip prostheses have been published.

Five ASR hip resurfacing prostheses were revised due to pain. From two patients, head and cup were available for independent explant analysis. In the other three cases only femoral components were available. All were removed from female patients and all were revised to ceramic-on-ceramic hip prostheses. Post-operative radiographic measurements of cup inclination and ante-version were obtained using the EBRA software. The surface roughness values of the articulating surfaces of the explants were measured using a non-contacting profilometer. A co-ordinate measuring machine was used to measure the diameter of the head and the cup and thus the diametral clearance. The same measurements were then taken from a new unused ASR prosthesis and compared. Using elastohydrodynamic theory the minimum effective film thickness of the implant was calculated. In turn this allowed the lubrication regime to be determined.

The average roughness values of the head and the cup of one implant were found to be 0.135microns and 0.058microns respectively, with a diametral clearance of 110microns. These results indicated that, at the time of removal, the prosthesis would have operated in the boundary lubrication regime. Other explants showed evidence of localised contact between the head and the rim of the acetabular cup, and these showed articulating surfaces with typical roughness values of between 0.025microns and 0.050microns. The new ASR had head and cup surface roughness values of 0.010microns and 0.012microns respectively and a diametral clearance of 87microns, implying that a new implant would operate under fluid film lubrication. All cups five were implanted with inclination angles over 45 degrees and anteversion over 25 degrees.

These results suggest that components with high inclination and anteversion angles display greater than expected wear and may operate in boundary rather than fluid film lubrication which may eventually lead to early failure.

Injections of hyaluronic acid solutions, often known as visco-supplements, into the joints of patients suffering from osteoarthritis are an accepted therapy. In most replacement joints, wear of the biomaterials used in them is a critical concern. For in vitro wear tests of such materials the recommended lubricant is one based on bovine serum. However, unlike synovial fluid, bovine serum does not contain hyaluronic acid. The aim of the work reported here was to take a clinically used hyaluronic acid solution, Ostenil, and to investigate its influence on the wear of two orthopaedic biopolymers

Ultra high molecular weight polyethylene (UHMWPE) and poly tetra fluoro ethylene (PTFE) were tested in turn using a four-station, multi-directional, pin-on-plate wear test rig which had previously been shown to reproduce clinical wear factors for UHMWPE, PTFE and polyacetal. For each biopolymer three lubricants were employed: 33% bovine serum (2 stations); 33% bovine serum + Ostenil (1 station); and distilled water + Ostenil (1 station). Polymeric test pins were subject to a load of 40N and articulated against polished stainless steel plates. Wear factors were determined by dividing the volume lost by the product of the load and the sliding distance (units × 10-6mm3/Nm).

The UHMWPE wear tests ran to 66.3km sliding distance. The addition of Ostenil to dilute bovine serum resulted in a wear factor of 1.4 × 10-6mm3/Nm for UHMWPE. The wear factor was 1.6 × 10-6mm3/Nm when dilute bovine serum alone was used as the lubricant. This shows good agreement with a wear factor of 2.1 × 10-6mm3/Nm reported for failed UHMWPE acetabular cups. PTFE provides an accelerated wear test with clinical validity. In the presence of 33% bovine serum a mean wear factor for PTFE of 40 × 10-6mm3/Nm was measured. The wear factor was 59 × 10-6mm3/Nm for dilute bovine serum plus Ostenil. For explanted PTFE acetabular cups a wear factor of 37 × 10-6mm3/Nm has been calculated. For both polymers wear was least when the lubricant was distilled water plus Ostenil. However a transfer film was found and such films are not clinically valid.