The clinical implications of elevated blood metal ion concentrations in asymptomatic patients with MoM hip resurfacings: a cohort study

Abstract

Objective: To determine whether elevated blood cobalt (Co) concentrations are associated with early failure of metal-on-metal (MoM) hip resurfacings secondary to adverse reaction to metal debris (ARMD).

Design: Cohort study.

Setting: Single centre orthopaedic unit.

Participants: Following the identification of complications potentially related to metal wear debris, a blood metal ion screening programme was instigated at our unit in 2007 for all patients with Articular Surface Replacement (ASR) and Birmingham MoM hip resurfacings. Patients were followed annually unless symptoms presented earlier. Symptomatic patients were investigated with ultrasound scan and joint aspiration. The clinical course of all 278 patients with 'no pain' or 'slight/occasional' pain and a Harris Hip Score greater than or equal to 95 at the time of venesection were documented. A retrospective analysis was subsequently conducted using mixed effect modelling to investigate the temporal pattern of blood Co levels in the patients and survival analysis to investigate the potential role of case demographics and blood Co levels as risk factors for subsequent failure secondary to ARMD.

Results: Blood Co concentration was a positive and significant risk factor (z=8.44, p=2×10(-16)) for joint failure, as was the device, where the Birmingham Hip Resurfacing posed a significantly reduced risk for revision by 89% (z=-3.445, p=0.00005 (95% CI on risk 62 to 97)). Analysis using Cox-proportional hazards models indicated that men had a 66% lower risk of joint failure than women (z=-2.29419, p=0.0218, (95% CI on risk reduction 23 to 89)).

Conclusions: The results suggest that elevated blood metal ion concentrations are associated with early failure of MoM devices secondary to adverse reactions to metal debris. Co concentrations greater than 20 µg/l are frequently associated with metal staining of tissues and the development of osteolysis. Development of soft tissue damage appears to be more complex with females and patients with ASR devices seemingly more at risk when exposed to equivalent doses of metal debris.

Figure 1

Violin plot showing distribution blood cobalt concentrations in men and women.

Figure 2

Predicted survival curves for hip replacements for two male and two female hypothetical individuals with different levels of blood cobalt at 2 and 5 µg/l. Device is the Birmingham Hip Resurfacing. Time period is in months. Survival curves shown with 95% CI.

Figure 3

Predicted survival curves for hip replacements for two male and two female hypothetical individuals with different levels of blood cobalt of 2 and 5 µg/l. Device is the Articular Surface Replacement. Time period is in months. Survival curves shown with 95% CI.

Figure 4

Predicted survival curves for hip replacements for two male and two female hypothetical individuals with blood cobalt concentrations of 10 µg/l. Device is the Birmingham Hip Resurfacing (top two plots) and Articular Surface Replacement (bottom two plots). Time period is in months. Survival curves shown with 95% CI.

Figure 5

The incidence of osteolysis and/or moderate/severe soft tissue destruction at revision surgery. Patients have been grouped according to their preoperative blood result.

Figure 6

Operative findings of a 55-year-old patient from another country. He had minimal discomfort but was not satisfied with his surgeon's opinion that ‘there was nothing to be concerned about’. His preoperative blood cobalt concentration was 217 µg/l. Note the gross metal staining of the tissues (metallosis) and abnormal fluid. There was extensive acetabular and femoral osteolysis.

Figure 7

The femoral neck and prosthesis of a male patient in this study with a Birmingham Hip Resurfacing who was found to have blood cobalt of 155 µg/l on routine screening. He had no symptoms but elected for surgery 2 years later when he developed progressive discomfort in his hip. There were no obvious changes on plain x-rays. As can be seen on the right, there was a large cavity in the femoral neck filled with metal-stained, caseous material.