Blood cobalt (Co) and chromium (Cr) ion levels have been used as surveillance tools for adverse reaction to metal debris (ARMD) in metal-on-metal (MoM) articulations with varying thresholds in different countries. The aim of our study is to present the serial 12-year blood Co and Cr levels in a cohort of MoM total hip arthroplasties (THAs) with femoral head size ≥36 mm and analyze their utility in asymptomatic patients at current thresholds.
A total of 256 patients with unilateral MoM THA with femoral head size ≥36 mm were included in this study with data collected prospectively. The implants used were Birmingham hip resurfacing cup—Freeman stem (BHR-F) or an Articular Surface Resurfacing cup—Corail/S-ROM stem. Annual follow-up with blood Co and Cr measurements was done as per Medicines and Healthcare Products Regulatory Agency (MHRA) of the United Kingdom guidelines. Receiver operating characteristic curve was plotted based on the sensitivity and specificity of blood metal ion values to detect ARMD. The metal ion levels in asymptomatic patients were analyzed separately.
Receiver operating characteristic curves showed poor discriminatory ability for both Co and Cr values in predicting ARMD at 7 μg/L. The sensitivity of Co and Cr was 82.1% and 53.5%, respectively, and their positive predictive values were 43.8% and 67.6%, respectively. After 7 years, there was no significant change in Co values, and there was a decline in Cr value after 9 years in asymptomatic patients.
To the best of our knowledge, this is the first study to describe the behavior of serial blood metal ion levels in asymptomatic large-diameter MoM THA. We suggest that annual blood Co and Cr have limited discriminant capacity in diagnosing the occurrence of metallosis and their measurement beyond 7 years is of limited utility in asymptomatic patients.