An earlier study (Clarke et al JBJS(Br) 2003) suggests that smaller bearings generate less wear. In that study bearings with different metallurgy and wear properties were grouped together, a potential confounding factor. The present study does not suffer from that error and our findings do not support the view that a larger bearing diameter leads to either an increase or decrease in metal ion generation.
Dislocation rates in large headed metal-on-metal resurfacings are extremely low. However, many patients are unsuitable for resurfacing and need a replacement. In such cases, it is attractive to transfer the large-headed metal-metal bearing advantage to replacement arthroplasty in order to reduce wear and dislocation rates. Does large diameter metal-metal total hip replacement really reduce the early dislocation rate?
Age at operation ranged from 37 to 83 years. Thirty patients were 55 years or under, eighty one were 56 – 65 years and ninety five were over 65 years. There were 122 females and 67 males. Posterior approach was used in all.
The extremely low failure rate in the medium term proves the suitability of resurfacing in young active patients. However, caution needs to be exercised until long term results are available.
There is sufficient evidence in this study to prove that metal ions do cross the placenta. There is therefore a continuing need for vigilance on the possible effects on the offspring born to patients with metal-metal devices.
Old age, osteopaenia, alcohol abuse, and large cysts are risk factors for fractures. It has been suggested that performing a bilateral resurfacing puts the first side at risk of fracture from the force used in implanting the second resurfacing. Is this a true risk or a sampling error?
The low incidence of fractures (2/382, 0.5%) in this bilateral resurfacing series does not support the view that there is an increased risk of fracture from a bilateral procedure.
Bland-Altman analysis (Figure 1A) shows the limits of agreement between serum and WB are unacceptably wide (1.7 to -5.1 for chromium) suggesting poor agreement.
The dysplasia cup, which was devised as an adjunct to the Birmingham Hip Resurfacing system, has a hydroxyapatite-coated porous surface and two supplementary neutralisation screws to provide stable primary fixation, permit early weight-bearing, and allow incorporation of morcellised autograft without the need for structural bone grafting. A total of 110 consecutive dysplasia resurfacing arthroplasties in 103 patients (55 men and 48 women) performed between 1997 and 2000 was reviewed with a minimum follow-up of six years. The mean age at operation was 47.2 years (21 to 62) and 104 hips (94%) were Crowe grade II or III. During the mean follow-up of 7.8 years (6 to 9.6), three hips (2.7%) were converted to a total hip replacement at a mean of 3.9 years (2 months to 8.1 years), giving a cumulative survival of 95.2% at nine years (95% confidence interval 89 to 100). The revisions were due to a fracture of the femoral neck, a collapse of the femoral head and a deep infection. There was no aseptic loosening or osteolysis of the acetabular component associated with either of the revisions performed for failure of the femoral component. No patient is awaiting a revision. The median Oxford hip score in 98 patients with surviving hips at the final review was 13 and the 10th and the 90th percentiles were 12 and 23, respectively.
We report a retrospective review of the incidence of venous thromboembolism in 463 consecutive patients who underwent primary total hip arthroplasty (487 procedures). Treatment included both total hip replacement and hip resurfacing, and the patients were managed without anticoagulants. The thromboprophylaxis regimen included an antiplatelet agent, generally aspirin, hypotensive epidural anaesthesia, elastic compression stockings and early mobilisation. In 258 of these procedures (244 patients) performed in 2005 (cohort A) mechanical compression devices were not used, whereas in 229 (219 patients) performed during 2006 (cohort B) bilateral intermittent pneumatic calf compression was used. All operations were performed through a posterior mini-incision approach. Patients who required anticoagulation for pre-existing medical problems and those undergoing revision arthroplasty were excluded. Doppler ultrasonographic screening for deep-vein thrombosis was performed in all patients between the fourth and sixth post-operative days. All patients were reviewed at a follow-up clinic six to ten weeks after the operation. In addition, reponse to a questionnaire was obtained at the end of 12 weeks post-operatively. No symptomatic calf or above-knee deep-vein thrombosis or pulmonary embolism occurred. In 25 patients in cohort A (10.2%) and in ten patients in cohort B (4.6%) asymptomatic calf deep-vein thromboses were detected ultrasonographically. This difference was statistically significant (p = 0.03). The regimen followed by cohort B offers the prospect of a low incidence of venous thromboembolism without subjecting patients to the higher risk of bleeding associated with anticoagulant use.
Metal ions generated from joint replacements are a cause for concern. There is no consensus on the best surrogate measure of metal ion exposure. This study investigates whether serum and whole blood concentrations can be used interchangeably to report results of cobalt and chromium ion concentrations. Concentrations of serum and whole blood were analysed in 262 concurrent specimens using high resolution inductively-coupled plasma mass-spectrometry. The agreement was assessed with normalised scatterplots, mean difference and the Bland and Altman limits of agreement. The wide variability seen in the normalised scatterplots, in the Bland and Altman plots and the statistically significant mean differences between serum and whole blood concentrations suggest that they cannot be used interchangeably. A bias was demonstrated for both ions in the Bland-Altman plots. Regression analysis provided a possible conversion factor of 0.71 for cobalt and 0.48 for chromium. However, even when the correction factors were applied, the limits of agreement were greater than ±67% for cobalt and greater than ±85% for chromium, suggesting that serum and whole blood cannot be used interconvertibly. This suggests that serum metal concentrations are not useful as a surrogate measure of systemic metal ion exposure.
Metal-on-metal bearings are being increasingly used in young patients. The potential adverse effects of systemic metal ion elevation are the subject of ongoing investigation. The purpose of this study was to investigate whether cobalt and chromium ions cross the placenta of pregnant women with a metal-on-metal hip resurfacing and reach the developing fetus. Whole blood levels were estimated using high-resolution inductively-coupled plasma mass spectrometry. Our findings showed that cobalt and chromium are able to cross the placenta in the study patients with metal-on-metal hip resurfacings and in control subjects without any metal implants. In the study group the mean concentrations of cobalt and chromium in the maternal blood were 1.39 μg/l (0.55 to 2.55) and 1.28 μg/l (0.52 to 2.39), respectively. The mean umbilical cord blood concentrations of cobalt and chromium were comparatively lower, at 0.839 μg/l (0.42 to 1.75) and 0.378 μg/l (0.14 to 1.03), respectively, and this difference was significant with respect to chromium (p <
0.05). In the control group, the mean concentrations of cobalt and chromium in the maternal blood were 0.341 μg/l (0.18 to 0.54) and 0.199 μg/l (0.12 to 0.33), and in the umbilical cord blood they were 0.336 μg/l (0.17 to 0.5) and 0.194 μg/l (0.11 to 0.56), respectively. The differences between the maternal and umbilical cord blood levels in the controls were marginal, and not statistically significant (p >
0.05). The mean cord blood level of cobalt in the study patients was significantly greater than that in the control group (p <
0.01). Although the mean umbilical cord blood chromium level was nearly twice as high in the study patients (0.378 μg/l) as in the controls (0.1934 μg/l), this difference was not statistically significant. (p >
0.05) The transplacental transfer rate was in excess of 95% in the controls for both metals, but only 29% for chromium and 60% for cobalt in study patients, suggesting that the placenta exerts a modulatory effect on the rate of metal ion transfer.
This is a longitudinal study of the daily urinary output and the concentrations in whole blood of cobalt and chromium in patients with metal-on-metal resurfacings over a period of four years. Twelve-hour urine collections and whole blood specimens were collected before and periodically after a Birmingham hip resurfacing in 26 patients. All ion analyses were carried out using a high-resolution inductively-coupled plasma mass spectrometer. Clinical and radiological assessment, hip function scoring and activity level assessment revealed excellent hip function. There was a significant early increase in urinary metal output, reaching a peak at six months for cobalt and one year for chromium post-operatively. There was thereafter a steady decrease in the median urinary output of cobalt over the following three years, although the differences are not statistically significant. The mean whole blood levels of cobalt and chromium also showed a significant increase between the pre-operative and one-year post-operative periods. The blood levels then decreased to a lower level at four years, compared with the one-year levels. This late reduction was statistically significant for chromium but not for cobalt. The effects of systemic metal ion exposure in patients with metal-on-metal resurfacing arthroplasties continue to be a matter of concern. The levels in this study provide a baseline against which the
The recent resurgence in the use of metal-on-metal bearings has led to fresh concerns over metal wear and elevated systemic levels of metal ions. In order to establish if bearing diameter influences the release of metal ions, we compared the whole blood levels of cobalt and chromium (at one year) and the urinary cobalt and chromium output (at one to three and four to six years) following either a 50 mm or 54 mm Birmingham hip resurfacing or a 28 mm Metasul total hip replacement. The whole blood concentrations and daily output of cobalt and chromium in these time periods for both bearings were in the same range and without significant difference.
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.