Review Question - QID 4905

This patient is presenting with a metal induced system hypersensitivity response in the setting of a metal-on-metal total hip arthroplasty.

A hip aspiration of a painful THR is a very useful investigation for the work up of infection, having a sensitivity of 75-85% and specificity of 85-100% for infection. Metal-on-metal THA may mimic infection as aspirate results will often show increased inflammatory infiltrate, with synovial WBC counts in the thousands. However, infected THA are more likely to produce higher percentages of PMNs (>70%) in comparison to hypersensitivity reactions/ adverse reaction to metal debris, which are more likely to produce a higher percentage of lymphocytes (>40%).

Campbell et al. looked at the histological features of pseudotumor-like tissues from metal-on-metal hips. They found that the patients with hip pain and suspected metal sensitivity had fewer metal particles but more aseptic lymphocytic vasculitis-associated lesions compared to patients with evidence of metallic wear. They concluded that pseudotumors occur more because of a hypersensitivity reaction than particle wear.

Kwon et al. examined a small cohort of patients with metal-on-metal hip arthroplasties to investigate the incidence and level of metal-induced systemic hypersensitivity. They found that lymphocyte reactivity to Co, Cr, and Ni did not significantly differ in patients with pseudotumors compared to those patients without pseudotumors. This suggests that systemic hypersensitivity type IV reactions may not be the dominant biological reaction involved in the occurrence of the soft tissue pseudotumors.

Figure A shows a patient with bilateral metal-on-metal total hip arthroplasties. There are no identifiable fractures. The position of the left acetabular cup is slightly vertical, which can increase edge loading and particle wear. Figure B shows the results from the hip aspirate.

Incorrect Answers:
Answer 1: Although WBCs > 3000, the low differential of PMNs and high lymphocytes are not consistent with a bacterial joint infection. Infected THA are more likely to produce higher percentages of PMNs (>70%).
Answer 2: Radiographs and physical exam do not suggest fracture, although CT scan or bone scan may be useful to detect subtle periprosthetic hip fractures.
Answer 3: Large-particle wear debris disease most commonly occurs with polyethylene wear. This is a metal-on-metal hip replacement. Metal surfaces are thought to give off smaller particles of debris.
Answer 5: Abductor tendon tear would present with an abnormal gait and some element of decreased abductor strength. The aspirate would also be negative.