Is metal allergy to blame for pseudotumor?
Large-diameter head, metal-on-metal devices for total hip arthroplasty (THA) have gained favor for providing greater range of motion and lower rates of dislocation, but their use may incur an unacceptably high rate of early revisions, according to a study conducted in Belgium and presented by Patrick Deprez, MD, at the 2010 Annual Meeting.
Dr. Deprez reported that in a series of 114 patients (120 hips) undergoing THA with the devices, about 8 percent (10 patients) required revision surgery. In two cases, the patients experienced persistent weakness and anterior thigh pain; one revision was due to stem loosening after fracture. The remaining revisions were due to the formation of a pseudotumor. This finding raises the following questions about the second-generation metal-on-metal bearings that are currently in use, according to Dr. Deprez:
- Do the metal-on-metal bearing surface and a rise in metal ions in the blood cause a systemic effect? Is there a danger for general health?
- Why does metal hypersensitivity develop in some patients?
- What are the clinical symptoms and long-term effects?
- What is the short- and intermediate-term osteolysis and loosening rate?
Same procedure, different results
The patients in the study all received the same type of titanium-coated acetabular cup and the same low-wear, cobalt-chromium alloy femoral head. The arthroplasty procedures were performed through a posterolateral, less-invasive approach. Patients were given cephalosporin intravenously preoperatively and for 24 hours after the procedure.
For the 10 revisions, the average time between the first and second procedure was 28.1 months. Seven of the eight patients who had pseudotumors were women. A postoperative infection developed in one patient who recovered, but 2 years postoperatively, a cyst developed and became infected. The cyst was removed, but the hip subsequently dislocated and was revised; otherwise there were no primary
Most of the patients with a pseudotumor reported groin pain or posterior or lateral hip pain, Dr. Deprez said. In all eight of them, swelling or fluid collection was observed around the hip joint, and three patients had visible and palpable swelling. None had a history of trauma, and in all but one, plain radiographs appeared normal. In one pseudotumor case, osteolysis was evident on plain radiographs. The fluid accumulation was discovered on sonography, and subsequently the cysts were seen with CT imaging. Laboratory tests did not detect infections that would account for the patients’ signs and symptoms.
The investigators sent the removed prosthesis and tissue samples from one patient to an independent laboratory. Wear analysis showed a low wear rate, while histologic analysis “showed the hallmark of metal sensitivity,”
Dr. Deprez said. There was a thick layer of fibrin, lost synovial surface, a necrotic fibrinoid tidemark, and dense aggregates of lymphocytes.
Dr. Deprez said that in this study, metal-on-metal prostheses had lived up to their theoretical advantage of low dislocation incidence. The revision rate, however, was unexpected and a cause for “major concern”—sufficient enough for him and his colleagues to stop using this type of appliance. They had not seen such complications with other types of bearing couples, he noted. Although the cause for these problems was not clear, evidence pointed to metal sensitivity. Most cases of metal allergy described in the literature involve hip resurfacing, he said, and more investigation is warranted to explain why this series of patients had a considerably higher rate of complications and revision related to metal sensitivity.
Coauthors of “High early revision rate due to pseudotumor formation in metal-on-metal large head diameter THA” are Luc Vanden Berghe, MD, and Marc Demuynck, MD.
Disclosure information: The authors reported no conflicts.
Terry Stanton is senior science writer for AAOS Now. He can be reached at firstname.lastname@example.org