One-year prospective comparative study of three large-diameter metal-on-metal total hip prostheses: serum metal ion levels and clinical outcomes.

INTRODUCTION: The good clinical outcomes and low wear obtained with 28-mm metal-on-metal implants for total hip replacement prompted the development of large-diameter heads that more closely replicated the normal hip anatomy, with the goal of improving prosthesis stability. However, the blood release of metal ions due to wear at the bearing surfaces and the high rate of groin pain seen with large-diameter implants are causing concern. To determine whether these events are related to the geometry and metal composition of the prosthesis components, we conducted a prospective study of clinical outcomes and serum chromium and cobalt levels 1 year after implantation of three different acetabular cups. HYPOTHESIS: Serum levels of metal ions are comparable with different types of large-diameter metal-on-metal total hip prostheses. PATIENTS AND METHODS: We compared 24 Durom™ cups (D), 23 M2a Magnum™ cups (M2a), and 20 Conserve Total™ (C) cups regarding serum chromium and cobalt levels, Postel-Merle d'Aubigné (PMA) scores and Oxford Hip Scores (OHS), as well as radiographic cup orientation and position at 1-year follow-up. Mean age was 66 years (45-85 years), mean body mass index was 28 Kg/m(2) (18-45), patients were almost equally divided between males and females, and the reason for hip replacement was primary hip osteoarthritis in 65 patients and avascular necrosis in two. Metal ions were assayed in serum from blood drawn through non-metallic catheters, using mass spectrometry.
RESULTS: Dislocation occurred in two patients (one D and one M2a) and revision to change the bearing couple was required in two patients in the D group. Serum cobalt levels in the C group were significantly higher (P=0. 0003) than in the two other groups (7.5 μg/L versus 2. 7 μg/L with D and 2. 2 μg/L with M2a). Clinical outcomes were better in the M2a group (PMA, 17.7 [16-18]; and OHS, 15.2 [12-30]; P<0.05). The PMA score and OHS were 17.5 (16-18) and 18.2 (12-42), respectively, with D; and 16.75 (10-18) and 22. 2 (12-42), respectively, with C cups. When all three cup models were pooled, serum ion levels were higher in patients with pain than without pain (chromium, 7.1 μg/L versus 2.1 μg/L [P=0.002], and cobalt, 8 μg/L versus 2.6 μg/L [P=0.0004]). DISCUSSION: Serum chrome and cobalt levels increased after metal-on-metal total hip replacement, and the increase was greater with large-diameter implants than previously reported with 28-mm implants. Persistent pain was significantly associated with higher metal ion levels, with a probable cobalt cut-off of about 8 μg/L. Differences in modular head-neck concepts may explain the observed variations.