New Tests For The Investigation Of Patients With Painful Metal-on-metal Resurfacing Arthroplasties

Main Category: Bones / Orthopaedics
Article Date: 05 Jun 2009 - 0:00 PDT

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This paper investigates 26 patients with painful metal-on-metal resurfacing arthroplasties and describes three tests for the investigation of the phenomenon. The tests are metal artefact-reduction MRI, 3-D CT measurement of the position of the component and inductively-coupled plasma mass spectrometry.

Current techniques for assessing patients with painful MOM hips are the same as for painful non-MOM hips, however, this research demonstrates that hip resurfacing appears to have specific modes of failure which are not detectable using these methods. Therefore, the authors investigated the use of metal artefact-reduction MRI, 3-D CT measurement of the position on the component and inductively-coupled plasma mass spectrometry analysis of cobalt and chromium levels in the whole blood.

As an aseptic lymphocyte-dominated vasculitis-lesion can be a possible cause of unexplained pain in a MOM hip the 26 patients were tested with MRI and 16 hips showed periprosthetic lesions, with 14 collections of fluid and two soft tissue masses. The diagnosis of aseptic lymphocyte-dominated vasculitis-lesion can only be made from tissue samples, usually collected during revision. Therefore, because the lesions were seen in both men and women and in symptomatic and asymptomatic hips the significance of this finding is unclear.

It has also been suggested that unexplained hip pain is associated with high levels of metal ions in the blood so the levels of cobalt and chromium may be biomarkers of early failure. In the patients tested, the levels of blood metal ions tended to be higher in painful compared to well-functioning metal-on-metal hips. Specifically the level of cobalt were significantly greater, suggesting that painful hip resurfacings have greater wear rates. However, the levels of chromium were barely raised in patients with unilateral MOM hips and therefore analysis of blood metals ions as a screening test for painful MOM is likely to be sensitive but not specific.

The 3-D CT scans showed that the median inclination of the acetabular component was 55° and its positioning was outside the Lewinnk safe zone in 13 of 16 cases. The position of the component may influence the risk of failure of a MOM hip because suboptimal acetabular version may cause impingement, and inclination angles of greater than 50° may cause high rates of wear.

The authors protocol for the painful MOM hip demonstrated a high probability of periprosthetic masses on metal artefact reduction sequences, a higher blood metal level than in the well-functioning MOM hip and a position of the component outside the Lewinnek safe zone. Taken together, these observations may help to determine causes of failure, indicate the most appropriate treatment and aid the choice of prosthesis if replacing the contralateral hip.

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Source
The Journal of Bone and Joint Surgery