CONTRIBUTION OF SERUM COBALT KINETICS TO THE FOLLOW-UP OF METAL-ON-METAL TOTAL HIP ARTHROPLASTY

Abstract

Purpose: A significant increase in serum cobalt level has been reported after metal-on-metal total hip arthroplasty with wide individual variability related to activity level, mechanical conditions of the implant, and urinary elimination of cobalt. We studied serum cobalt levels over time to further analyse these factors.

Material and methods: The Metazul® prosthesis was implanted in 119 patients (72 men and 47 women, 12 bilateral implantations) (131 implants). We selected 50 patients (27 men and 23 women, mean age 53 years) who had two blood samples after the procedure allowing an assessment of the serum cobalt kinetics. Other chromium-cobalt implants, vitamin B12 intake, renal failure, or haematological disorders were recorded. An activity questionnaire was filled out by the patients at the time of the blood sample. Samples were drawn with a special kit to avoid metal contamination. The detection limit was 1 nmol/L (0.06 μg/L) with direct electrothermic atomic spectrometric absorption.

Results: In the overall series, serum cobalt level was 44 nmol/L for a physiological level in a control population of 4.28 nmol/L. The difference was significant (p < 0.0001) between the levels observed before surgery and after 18 months implantation. There was no significant correlation with the indication for arthroplasty, presence of dislocation or subdislocation, functional outcome or radiographic findings. Activity level the week before sampling did not influence the results. For the 50 cases evaluated longitudinally, four groups of patients could be identified. The first group (29 patients) had a serum cobalt level below 50 nmol/L over the entire study period. The second group (nine patients) had a level greater than 50 nmol/L followed by a decline ending with a final level below 50 nmol/L. In the third group (six patients) serum cobalt was greater than 50 nmol/L with no trend to a decline. In the fourth group (six patients) the cobalt levels were very high (greater than 150 nmol/L).

Discussion: The six patients in the fourth group were very particular. There were three patients with secondary bilateral implants with a late peak in serum cobalt, one with an impingement on the acetabular rim, one with renal failure, and one who had a very high level of physical activity. The first group had what appears to be a favourable course, similar to the second group where a stabilisation phenomenon could be operating. An explanation in the third group is difficult but could involve a third segment abrasion phenomenon.

Conclusion: Longitudinal analysis of serum cobalt levels provides more information than point measures in patients with metal-on-metal arthroplasties. Intercurrent mechanical phenomena can be detected; unexpected behaviour of the metal-on-metal junction can be suspected in certain patients.