D. J. W. McMinn

Metal-on-metal resurfacing of the hip in patients under the age of 55 years with osteoarthritis

The results of conventional hip replacement in young patients with osteoarthritis have not been encouraging even with improvements in the techniques of fixation and in the bearing surfaces. Modern metal-on-metal hip resurfacing was introduced as a less invasive method of joint reconstruction for this particular group. This is a series of 446 hip resurfacings (384 patients) performed by one of the authors (DJWM) using cemented femoral components and hydroxyapatite-coated uncemented acetabular components with a maximum follow-up of 8.2 years (mean 3.3). Their survival rate, Oxford hip scores and activity levels are reviewed. Six patients died due to unrelated causes. There was one revision (0.02%) out of 440 hips. The mean Oxford score of the surviving 439 hips is 13.5. None of the patients were told to change their activities at work or leisure; 31% of the men with unilateral resurfacings and 28% with bilateral resurfacings were involved in jobs that they considered heavy or moderately heavy; 92% of men with unilateral hip resurfacings and 87% of the whole group participate in leisure-time sporting activity. The extremely low rate of failure in spite of the resumption of high level occupational and leisure activities provides early evidence of the suitability of this procedure for young and active patients with arthritis.

Metal on Metal Surface Replacement of the Hip: Experience of the McMinn Prosthesis

The historical failure of surface replacement has been due to the production of wear debris with subsequent bone resorption, loosening, and failure. To avoid these problems, a surface replacement using a metal on metal bearing allowing thin components and femoral design and instrumentation to avoid varus alignment has been designed. Two hundred thirty-five joints have been resurfaced with this prosthesis in almost 5 years. There have been no femoral neck fractures and no dislocations. There have been 4 designs differing in the method of fixation. In the press fit group, 6 of 70 hips had to be revised for aseptic loosening. In the cemented group, debonding of the cup occurred in 3 of 43 cases. Six patients had hydroxyapatite coated components and have had excellent clinical outcomes. The current design uses a peripherally expanded hydroxyapatite coated cup and a cemented metal head; 116 of this design have been implanted during a 19-month period with excellent outcome. Despite short followup the authors are hopeful that the combination of a polar metal on metal bearing with appropriate fixation will yield a method of preserving bone stock in the younger patient requiring arthroplasty.

Transplacental transfer of cobalt and chromium in patients with metal-on-metal hip arthroplasty: A CONTROLLED STUDY

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 microg/l (0.55 to 2.55) and 1.28 microg/l (0.52 to 2.39), respectively. The mean umbilical cord blood concentrations of cobalt and chromium were comparatively lower, at 0.839 microg/l (0.42 to 1.75) and 0.378 microg/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 microg/l (0.18 to 0.54) and 0.199 microg/l (0.12 to 0.33), and in the umbilical cord blood they were 0.336 microg/l (0.17 to 0.5) and 0.194 microg/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 microg/l) as in the controls (0.1934 microg/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.

Blood and urine metal ion levels in young and active patients after Birmingham hip resurfacing arthroplasty FOUR-YEAR RESULTS OF A PROSPECTIVE LONGITUDINAL STUDY

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 in vivo wear performance of newer bearings can be compared.

Six-year results of a prospective study of metal ion levels in young patients with metal-on-metal hip resurfacings

We describe the findings at six years in an ongoing prospective clinicoradiological and metal ion study in a cohort of 26 consecutive male patients with unilateral Birmingham Hip Resurfacing arthroplasties with one of two femoral head sizes (50 mm and 54 mm). Their mean age was 52.9 years (29 to 67). We have previously shown an early increase in the 24-hour urinary excretion of metal ions, reaching a peak at six months (cobalt) and one year (chromium) after operation. Subsequently there is a decreasing trend in excretion of both cobalt and chromium. The levels of cobalt and chromium in whole blood also show a significant increase at one year, followed by a decreasing trend until the sixth year.

Ten-year results of a double-heat-treated metal-on-metal hip resurfacing

Second-generation metal-on-metal bearings were introduced as a response to the considerable incidence of wear-induced failures associated with conventional replacements, especially in young patients. We present the results at ten years of a consecutive series of patients treated using a metal-on-metal hip resurfacing. A distinct feature of the bearings used in our series was that they had been subjected to double-heat treatments during the post-casting phase of their manufacture. In the past these bearings had not been subjected to thermal treatments, making this a unique metal-on-metal bearing which had not been used before in clinical practice. We report the outcome of 184 consecutive hips (160 patients) treated using a hybrid-fixed metal-on-metal hip resurfacing during 1996. Patients were invited for a clinicoradiological follow-up at a minimum of ten years. The Oxford hip score and anteroposterior and lateral radiographs were obtained. The mean age at operation was 54 years (21 to 75). A series of 107 consecutive hips (99 patients) who received the same prosthesis, but subjected to a single thermal treatment after being cast, between March 1994 and December 1995, were used as a control group for comparison. In the 1994 to 1995 group seven patients (seven hips) died from unrelated causes and there were four revisions (4%) for osteolysis and aseptic loosening. In the 1996 group nine patients died at a mean of 6.9 years after operation because of unrelated causes. There were 30 revisions (16%) at a mean of 7.3 years (1.2 to 10.9), one for infection at 1.2 years and 29 for osteolysis and aseptic loosening. Furthermore, in the latter group there were radiological signs of failure in 27 (24%) of the 111 surviving hips. The magnitude of the problem of osteolysis and aseptic loosening in the 1996 cohort did not become obvious until five years after the operation. Our results indicate that double-heat treatments of metal-on-metal bearings can lead to an increased incidence of wear-induced osteolysis.

The effect of the diameter of metal-on-metal bearings on systemic exposure to cobalt and chromium.

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.

The validity of serum levels as a surrogate measure of systemic exposure to metal ions in hip replacement.

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.

Mini-incision resurfacing arthroplasty of hip through the posterior approach.

The success of metal-on-metal hip resurfacing in the medium term in young and active patients is becoming evident. The procedure now can be done using a minimal approach developed by the senior author using a single posterior incision. This mini-incision resurfacing arthroplasty of the hip has made resurfacing more attractive to surgeons and patients. But does a mini incision approach allow reliable component placement? The results of 232 consecutive Birmingham Hip Resurfacings done using this approach between January and December 2004 are presented. Mean incision length was 11.8 cm. Seventy-seven percent of the incisions were between 9 and 12 cm long. Body mass index of the patients studied ranged from 17.6 to 46.7. Comparing the traditional approach with the mini-incision resurfacing arthroplasty of hip did not show a difference between the two groups in terms of operating time. The mean hospital stay was reduced by a day in the mini-incision group. The mean inclination of the acetabular components was within the target range in the two groups. Patient feedback shows that reduced postoperative pain, faster recovery rate, and better cosmesis make the mini-incision approach very popular. Although the mini incision is indeed appealing, it has a steep learning curve. In the early phase of the learning curve, care should be taken to avoid suboptimal component placement, which has the potential to affect long-term outcome adversely. Level of Evidence: Therapeutic study, Level III (retrospective comparative study). See the Guidelines for Authors for a complete description of levels of evidence.

Stability of the Birmingham hip resurfacing arthroplasty at two years A RADIOSTEREOPHOTOGRAMMETRIC ANALYSIS STUDY

The Birmingham hip resurfacing metal-on-metal arthroplasty was introduced in 1997 and has shown promising short- to mid-term results. We used radiostereophotogrammetric analysis (RSA) to study the stability of 20 resurfacing arthroplasties over a follow-up period of 24 months. Radiological examinations for RSA were performed immediately after surgery and at two, six, 12 and 24 months after operation. Precision and detection of migration thresholds (non-zero movement) were calculated. All the results corresponded well to those found in similar experimental arrangements with standard hip prostheses. Migration of the cup and vertical and mediolateral migration of the head were calculated. The values were low at two years compared with those of earlier studies of cemented femoral components in conventional total hip replacements indicating that there was no evidence of excessive early migration or loosening of the components.