Increased Femoral Head Offset is Associated With Elevated Metal Ions in Asymptomatic Patients With Metal-on-Polyethylene Total Hip Arthroplasty

Predisposing factors for trunnionosis and elevated metal ion levels in metal-on-polyethylene (MOP) total hip arthroplasty (THA) are currently unknown.

This retrospective cohort study enrolled 80 consecutive patients (43 males) with an asymptomatic MOP THA at 2- to 5-year follow-up and no other metal implants. Serum cobalt (Co) and chromium (Cr) levels were collected at the time of enrollment, and retrospective review was performed regarding demographic, implant, and surgical characteristics. Mean age at the time of surgery was 65.7 years (range 35.6-85.9 years), and mean postoperative follow-up was 28.7 months (range 24.4-58.9 months).

Femoral head offset was the only evaluated factor shown to increase serum Co ion levels above baseline within the cohort. Mean difference in Co level for high and low offset implants was 0.58 ppb (95% confidence interval [CI] = 0.05-1.11 ppb; P = .03). Mean difference in Cr level for high and low offset implants was 0.19 ppb (95% CI = −0.23 to 0.60 ppb; P = .37). Mean difference in Co level for small and large femoral heads was 0.20 ppb (95% CI = −0.41 to 0.81 ppb; P = .59). Mean difference in Cr level for small and large femoral heads was 0.28 ppb (95% CI = −0.18 to 0.74 ppb; P = .06). Age, gender, Harris Hip Score, and implant duration were not associated with changes in metal ion levels.

Femoral head offset appears to be an important source of elevated metal ion levels in MOP THA. Further studies will be needed to understand if increasing femoral head offset is associated with subsequent adverse local tissue reactions.