DISLOCATION OF LARGE DIAMETER METAL ON METAL BEARINGS: AN INDICATOR OF METAL SENSITIVITY? A REPORT OF FOUR CASES

Abstract

Introduction: The advantages of metal on metal bearings (MoM) include improved wear characteristics and lower dislocation rate. Metal sensitivity and Aseptic Lymphocyte Dominated Vasculitis Associated Lesion (ALVAL) reaction are becoming increasingly recognised. The BOA has recently issued a statement regarding MoM bearings. They suggested that any revisions for symptoms of pain and soft tissue reaction should be reported to the MHRA and histological specimens forwarded to the Stanmore Retrieval Centre.

Methods and results: We report four patients (all females), who presented with late dislocation of a large diameter MoM bearing (three following total hip replacements and one following resurfacing). They all made good initial recovery with complete relief of pain. They developed pain around groin between one and two years following the surgery. Inflammatory markers were normal. MRI/Ultrasound scanning showed the presence of cystic lesions around the hip. They all presented with relatively late dislocation (26 months to six years). At exploration there was a large joint effusion with necrosis and detachment of the abductors. A characteristic finding was an avascular bare trochanter stripped free of any soft tissues. Histology showed a predominantly histiocytic response in keeping with the findings of Willert and Pandit.

Discussion: In the absence of any obvious causes for dislocation, one should have a high index of suspicion for the possibility of metal hypersensitivity causing joint effusion, muscle necrosis and thereby leading to instability due to the progressive periosteal and soft tissue erosion. These patients should be considered for early revision of the bearing surface to prevent further damage to the muscle and bone. If the abductors are completely detached a more constrained type of bearing should be used to improve the stability. To the best of our knowledge this is the first report of this unusual complication following MoM bearings.