This is the first six-year report of a prospective longitudinal study of daily output of cobalt and chromium in urine and their levels in blood following hybrid metal-on-metal surface arthroplasty. Urine and whole blood specimens were analyzed before and periodically after hip resurfacing in 26 male patients after informed consent (mean age 52.9 years, mean BMI 27.9). Two of these patients have undergone contralateral hip resurfacings for progressive pain from end-stage arthritis and had to be excluded. All patients were found to have well-functioning resurfacings at 6-year follow-up. No patient complained of persistent pain or disability. The median 6-year Oxford hip score was 12. Urine chromium and cobalt at six years were 3.9 and 7.8 μg/24 hrs and blood levels were 1.11 and 1.17μg/l respectively. Both urine and blood levels show a statistically significant early increase reaching a peak six months to one year postoperatively followed by a steady decrease over the following five years, although the individual reductions are not statistically significant, except for blood chromium where the 4 and 6 year levels were significantly lower than the 1-year level. Elevated systemic metal exposure following MM bearing arthroplasty continues to cause concern. Our results show that metal release in these bearings shows a reducing trend after an initial peak dispelling the fear that a steady build-up of in vivo metal occurs with progressively increasing blood levels. However, as long as the significance of these elevated levels remains unknown, the need for continued vigilance persists.
It has been suggested that metal ion level elevations in certain bilateral MM bearing arthroplasties were overwhelming the renal threshold for metal excretion leading to systemic build-up of metal ions above the expected levels. In order to investigate this we studied renal concentrating efficacy through concurrent specimens of urine and whole blood over a range of metal levels. Concurrent specimens from 305 unselected patients were obtained. They include preoperative patients (76) and those with unilateral and bilateral hip arthroplasties (229) through to ten years after operation. 39 pre-operative specimens and 4 follow-up specimens had blood levels at or below the detection limit for cobalt or chromium and were excluded. The ratio of urine to whole blood concentration was 0.78 in the pre-operative patients. In patients with MM arthroplasties the different ratios in patients increased (from 3.1 to 9.2) with increasing urine concentrations. The ratio of urine cobalt concentration to blood cobalt concentration is a measure of renal concentrating efficacy. Amongst pre-operative controls, this ratio is 0.78, indicating that there is renal conservation of cobalt. In terms of cobalt, these patients’ urine was dilute in comparison to whole blood. In patients with MM bearings, the ratio went up to 4.8 indicating that the kidneys were now actively excreting against a concentration gradient in an attempt to maintain internal milieu. If renal threshold was being breached at higher levels, then the ratio should progressively fall at higher concentrations. The trend in this experiment shows quite the contrary effect and the ratio reached 9.2 in those with urine cobalt >
15 μg/l, demonstrating that renal clearance efficiency holds up even against this steep gradient and that the threshold is not breached within clinically relevant levels.
The mean difference in cord chromium between the study and control groups is not statistically significant (p >
0.05), although cord cobalt in the study group is significantly higher (p <
0.01) than that in controls. Whilst there is a four-fold elevation of maternal cobalt in the study patients and an almost 7-fold increase in maternal chromium levels as compared to the control group, the elevation in the cord cobalt and chromium in the study group are smaller.
Some authors have suggested that metal wear in patients with well-functioning MM bearings occurs only during the run-in wear phase and that continued corrosion of metal particles released during that period is responsible for metal level elevation later on. However the reducing trend in the later phase following revision in this study suggests that metal ion elevation from corrosion is not sustained indefinitely and therefore cannot by itself account for the persistent elevation of systemic metal levels throughout. Bearing wear continues to occur throughout bearing life.
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.
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.
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.
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.
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.
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.
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.