The problem of metal on metal, our experience in 59 cases

Objective: Recently, concerns have arisen about metal-on-metal (MoM) total hip arthroplasty (THA) due to their potential responsibility in systemic and local adverse reaction to metal debries (ARDM). Purposes of this study were to identify abnormal wear, observe the role of ions alarm levels in the diagnosis of ARDM and describe histopathology of soft tissues surrounding MoM-THAs failed. Methods: 59 primary MoM-THAs had follow-up consisting in periodical orthopedic controls using Oxford Hip Score (OHS), radiographs and testing whole blood cobalt and chromium ions. A Metal artifact reduction sequence (MARS) MRI and an eventual revision surgery was proposed to patients with anomalies in previous exams. During the surgery were collected material to exam. The prosthesis articular surface was analyzed by a contact-scanner, blood samples using Inductively coupled plasma mass spectrometry (ICP-MS), while tissue morphology was graded in light microscopy. Results: 8 prosthesis need a revision. The mean linear wear was 2,42 µm/year (range 0,41-16,54) for the femoral head and 1,27 µm/year (range 0,10-2,98) for the liner. Light microscopy revealed significant signs of corrosion at the interface head-neck in 7 of these implants. Only in 3 MoM-THA revisioned, whole blood ions levels were higher than 7 µg/l actually indicated as alarm: mean chromium 5.35 µg/l (range 0,16-11,95); mean cobalt 4,37 µg/l (range 0,35-18,7). Histology of soft tissue revealed a mixed inflammatory cell infiltrate and extensive superficial necrosis. Inflammatory cells pattern was different in each case. Conclusions: ARDM occurs also around MoM with low wear of bearing surfaces. Hypothesizing multiple sites of releasing metal particles, it could not be excluded ARDM also in others material matchings. Metal ions measurements can be used as diagnostic index of wear, but should be integrated with clinical and radiological aspects in a multilayer follow-up. The pathogenesis of ARDM is still unkown.