Plasma levels of cobalt and chromium ions and Metal Artefact Reduction Sequence (MARS)-MRI scans were performed on patients with 209 consecutive, unilateral, symptomatic metal-on-metal (MoM) hip arthroplasties. There was wide variation in plasma cobalt and chromium levels, and MARS-MRI scans were positive for adverse reaction to metal debris (ARMD) in 84 hips (40%). There was a significant difference in the median plasma cobalt and chromium levels between those with positive and negative MARS-MRI scans (p < 0.001). Compared with MARS-MRI as the potential reference standard for the diagnosis of ARMD, the sensitivity of metal ion analysis for cobalt or chromium with a cut-off of > 7 µg/l was 57%. The specificity was 65%, positive predictive value was 52% and the negative predictive value was 69% in symptomatic patients. A lowered threshold of > 3.5 µg/l for cobalt and chromium ion levels improved the sensitivity and negative predictive value to 86% and 74% but at the expense of specificity (27%) and positive predictive value (44%). Metal ion analysis is not recommended as a sole indirect screening test in the surveillance of symptomatic patients with a MoM arthroplasty. The investigating clinicians should have a low threshold for obtaining cross-sectional imaging in these patients, even in the presence of low plasma metal ion levels.

Oxinium™ (Smith & Nephew, Memphis, TN, US) has been used in hip arthroplasty since 2003. The surface coating is hard and provides low wear rates but if this surface coating is damaged, the soft metal core is at risk of accelerated wear. Previous reports have described accelerated wear following intra and postoperative hip dislocation. We report a case of advanced wear of an in situ Oxinium™ femoral head implant following a cracked acetabular liner. The liner had disengaged from the titanium shell, allowing the Oxinium™ head to articulate directly with the shell. The disengaged liner led to dislocation of the Oxinium™ head, with associated pronounced wear of the head and the acetabular cup. The patient had a successful revision procedure. We advise close follow-up of patients with Oxinium™ implants, especially if associated with dislocation and closed reduction.A low wear rate is a key element of long-term total hip arthroplasty survival. Ceramic femoral heads have lower wear rates than traditional cobalt-chrome when articulated with a polyethelene acetabular liner but ceramic material is brittle and fracture is a reported failure mechanism.

The insertion of distal locking screws is a difficult task in intra-medullary (IM) nailing operations of long bones and contributes a significant proportion to the overall operating time. The current technique to insert these screws uses numerous fluoroscopic images and depends on the skills and expertise of the surgeon. The Computer Assisted Orthopaedic Surgical System (CAOSS) was developed to assist orthopaedic surgeons in these operations. The laboratory based test results for insertion of distal locking screws in IM nailing procedures are presented and discussed in terms of accuracy and as part of the validation process to introduce new CAOS procedures into clinical use. This study shows that CAOSS in IM nailing is robust and reliable. Positional accuracy was shown to be within 0.3 mm and angular accuracy within 0.2 degrees with femoral IM nail. CAOSS was also shown to be very reliable and accurate at different angles of distal screws in fluoroscopic image space.

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