Patterns of blood cobalt levels after metal-on-metal total hip arthroplasty, predictors of elevated levels, and associations with clinical symptoms

PURPOSE: Objectives were to assess patterns of blood cobalt levels over time in MoM THAs by head size; predictors of elevated cobalt levels; and associations between cobalt levels and clinical outcomes. METHODS: We included all primary MoM THAs performed 1998-2011 with postoperative blood cobalt concentrations. We stratified the MoM THAs in ≤ 36 mm vs. > 36 mm. RESULTS: Overall, 526 MoM THAs were included: 427 with small and 99 with large heads. Considering patients with at least two serologies before revision (n = 205), cobalt levels were higher for patients with large compared to small heads (medians: 3.5 vs. 1.3, p < 0.001). We did not find any effect of time on cobalt levels between the two consecutive serologies (p > 0.42) or when using time as a continuous variable (p > 0.55). For large heads women were more likely to show elevated cobalt levels than men (adjusted OR = 3.56, 95% CI 1.17-10.84, p = 0.02). We did not find any significant association between cobalt levels and clinical outcomes (Spearman's rs < 0.08, ps > 0.15). CONCLUSION: We observed stable cobalt levels over time in small and large MoM THAs. Repeat laboratory workups in small and large head MoM THAs seem unnecessary given that the cobalt values remained stable.