Predictive factors for metal ion levels in metal-on-metal total hip arthroplasty

Introduction

Although metal-on-metal (MoM) total hip arthroplasty (THA) and hip resurfacings (HR) have similar bearing surfaces and comparable wear rates, metal ion levels and risk of failure are higher for MoM–THA. The mechanism behind the increased metal ion levels in large head MoM–THA is not completely understood. The current study aims to identify predictive factors for increased metal ion levels in unilateral and bilateral large head MoM–THA.

Materials and methods

99 Birmingham modular MoM–THA in 87 patients with metal ion levels at least 36 months after index procedure were analyzed. Mean follow-up time was 61.3 months (range 37–108) and the relationship of the following variables (gender, age, BMI, follow-up time, UCLA Activity Score, cup inclination, femoral head size, bilateral surgery) on metal ion levels were analyzed with multivariate regression models.

Results

Multivariate regression analysis revealed that bilateral MoM–THA surgery (p < 0.001) had a positive predictive effect on cobalt serum levels, while BMI had a negative (p = 0.018). Female gender (p = 0.012), activity (p = 0.001) and bilateral MoM–THA (p = 0.004) were positively correlated with chromium levels. Positive independent predictors for the cobalt–chromium ratio in the multivariate analysis were overall follow-up time (p = 0.004), bilateral MoM–THA (p < 0.001) and femoral head size (p = 0.007).

Conclusions

The data of the current study suggest that bilateral MoM–THA, increased patient activity levels and female gender are associated with increased chromium levels. Patients with larger component size, longer follow-up time and bilateral MoM–THAs have an increased cobalt–chromium ratio. These patients might be at increased risk for adverse local soft tissue reactions secondary to corrosion. Continuous close monitoring is recommended and bearing-surface change should be discussed if local tissue reactions occur.