In this study, patients' blood metal concentration levels peaked after one year, and remained at this level at the three and five-year measurements. The median values are well below the Norwegian threshold value of 7 μg/L, and are comparable to the levels reported in other studies of patients with a BHR prosthesis (2, 16). In a multi-centre study of 416 patients with a BHR prosthesis, a median concentration in whole blood of 1.4 μg/L for cobalt and 1.29 μg/L for chromium was found in patients with a unilateral prosthesis, and 2.1 μg/L for cobalt and 2.3 μg/L for chromium in patients with bilateral prostheses (17).
Only five patients in our study exceeded the Norwegian threshold value. All five had good clinical results assessed by hip scores, but they are considered to be at risk of developing a pseudotumour and are therefore being closely monitored.
The metal ion level in blood is used as an indicator of potential complications associated with the joint prosthesis, but sensitivity and specificity in this respect are relatively low (18), which is illustrated by our findings to some extent.
In our study, 5 of 44 patients underwent revision surgery, and in 4 of the reoperated patients, the prosthesis was replaced with no objective findings indicating a pseudotumour. All reoperated patients had metal levels below the threshold value of 7 μg/L.
The overall revision rate of resurfacing prostheses (mainly BHR prostheses) in Norway has been shown to be 5 % on average (95 % confidence interval 3.0 to 7.0 %) after five years (5). In comparison, standard total hip prostheses had a lower revision risk of 3.8 % (95 % CI 3.6 to 4.0 %) after five years (5).
There are few longitudinal studies documenting the lifespan of BHR prostheses exceeding ten years. A retrospective study of 95 people with a BHR prosthesis found an overall revision rate of 12 % at ten-year follow-up (16). The revision rate was higher in women (16 %) than in men (7 %). Australia's National Joint Replacement Registry reported a 6.6 % revision of BHR prostheses at ten-year follow-up. Other resurfacing prostheses, such as the ASR (articular surface replacement, DePuy), have had far poorer results, with up to 30 % revision surgeries after ten years, most likely due to design differences that increase the release of metal (19).
Risk factors have been described for the development of pseudotumours and other soft tissue reactions in patients with metal-on-metal hip prostheses. In addition to patients with elevated concentrations of cobalt and chromium in the blood, women and patients with smaller femoral head sizes (< 50 mm diameter) and a high cup inclination angle (> 50°) have an increased risk (20). It has been claimed that localised edge wear between the head and cup at a high angle of inclination or anteversion angle of the cup results in increased metal ion levels (8). In our study, both cup positioning and component size are within acceptable limits. Only one prosthesis had a femoral head size of less than 50 mm.
The Norwegian National Advisory Unit on Arthroplasty and Hip Fractures recommends that hospitals regularly follow up patients with all types of metal-on-metal prostheses with a head diameter of more than 32 mm, throughout the life of the prosthesis (10). The frequency of follow-up varies according to the patient's gender (women are followed more closely), the size of the femoral head (small heads are followed more closely) and the documented long-term results of the prosthesis. Standard X-rays are supplemented with measurements of metals in whole blood, and if these tests produce adverse findings or there is clinical suspicion (pain, palpable mass, reduction in function), MRI or ultrasound scans are also performed (5).
It has proved difficult to set threshold values for the concentrations of cobalt and chromium in the blood that indicate an increased risk of soft tissue reactions and revision surgery, and no consensus has been reached (17). In most European countries, including Norway, the threshold value is set at 7 μg/L (10). Some have chosen a lower threshold, all the way down to 3 μg/L (5). A lower limit will naturally result in poorer specificity. The debate on this continues.
Increased pseudotumour volume, poorer hip function scores and bilateral metal-on-metal prosthesis have also been associated with future revision surgery (5). There is therefore a need for further investigation into the relationship between metal ion levels and biological causal mechanisms for the development of tissue reactions.
One weakness of the study is that only men were included. At the start of the study, it was shown that women had poorer results with such prostheses, and the prosthesis was only used on men during the inclusion period. The strength of the study is that it was a prospective study with preoperative measurements and several postoperative follow-ups of a cohort from the same hospital. The cohort consists of approximately 10 % of all patients who have received a BHR prosthesis in Norway.