Metal on metal total hip arthroplasty: Correlation between inclination of the acetabular and metal ion levels.

Due to their low friction index, hip arthroplasties with metal-metal (M-M) friction torque have been an attractive option in young patients with high functional demand, currently they have suffered a decrease in their use due to the complications of some models and physiological reactions adverse reactions related to the elevation of metal ions in the blood. Our objective is to review the patients with M-M pair operated in our center, correlating the ion level with the position of the acetabular component and with the size of the head.
Material and Methods: Retrospective review of 166 M-M hip prostheses operated between 2002 and 2011. Sixty five ruled out for different causes (death, loss of follow-up, no current ion control, no radiography or others), leaving a sample of 101 patients to be analyzed. Follow-up time, cup inclination angle, blood ion level, Harris Hip Score (HHS), and complications were recorded.
Results: One hundred and one patients (25 women and 76 men), 55years of average age (between 26 and 70), of these 8 were surface prostheses and 93 total prostheses. The mean follow-up time was 10years (between 5 and 17years). The mean head diameters was 46.25 (between 38 and 56). The mean inclination of the butts was 45.7° (between 26° and 71°). The correlation force between the verticality of the cup and the increase in ions is moderate r=0.31 for Cr and slight r=0.25 for Co. The correlation force between head size and ion increase is weak and inverse r=-0.14 for Cr and r=0.1 for Co. Five patients (4.9%) required revision (2 [1%] due to increased ions with pseudotumor). The mean time to revision was 6.5years in which the ions increased. The mean HHS was 94.01 (between 55.8 and 100). In the review of patients, we found 3 with a significant increase in ions who had not followed controls, all 3 had an HHS of 100. The angles of the acetabular components were 69°, 60° and 48° and the diameter of the head was 48.42 and 48mm, respectively.
Discussion and Conclusions: M-M prostheses have been a valid option in patients with high functional demand. A bi-annual analytical follow-up is recommended, since in our case we have detected 3patients with HHS100 who presented unacceptable elevation of cobalt >20μm/L (according to SECCA) of the ions and 4 with very abnormal elevation of cobalt ≥10μm/L (according to SECCA), all of them with cup orientation angles >50°. With our review we can conclude that there is a moderate correlation between the verticality of the acetabular component and the increase in blood ions and that the follow-up of this patient with angles >50° is essential.
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