Unexpected dislocation following accurate total hip arthroplasty caused by excessive hip joint laxity during myasthenic crisis: a case report

BACKGROUND: Dislocation following total hip arthroplasty is mainly caused by malposition. However, the coexistence of neuromuscular disorders is also considered a risk for dislocation due to excessive hip joint laxity. To minimize risk of dislocation, preoperative planning using combined anteversion has been widely used. The recommended combined anteversion angle (the total of cup and stem anteversion angles) is 50 ± 10°. CASE PRESENTATION: A 33-year-old Japanese woman underwent elective total hip arthroplasty due to osteonecrosis of the femoral head associated with corticosteroid pulse therapy for myasthenia gravis. Intraoperatively, no tendency of dislocation was found when simulating an evoking position under general anesthesia. In postoperative X-ray and computed tomography scans, cup inclination, cup anteversion, and stem anteversion angles were 37°, 13°, and 35° respectively. The resulting combined anteversion was 48°, which was set as the target along with accurate placement. Her postoperative course was normal and she was discharged without adverse events. Three months postoperatively, due to worsening of myasthenic weakness in her lower extremities while resting, she tended to raise her left limb up using both hands for sitting up. An anterior dislocation occurred when her legs were in a figure-of-four position. She was brought to an emergency department, and reduction of dislocation was performed. It was inferred that myasthenic crisis in the affected limb enabled excessive passive motion due to joint hyperlaxity. At the end of 2016, elective total hip arthroplasty on the contralateral side was performed. Cup anteversion, stem anteversion, and the combined anteversion angles were 27°, 24°, and 51° respectively. We instructed her to exercise care during passive leg movement, which may worsen her myasthenic condition. She returned to a normal life and was able to walk long distances without a cane. No recurrence of dislocation was seen at final follow-up. CONCLUSIONS: Even if accurate component orientation is attained in total hip arthroplasty, patients with neuromuscular disorders such as myasthenia gravis have a potential risk of muscle weakness in the affected limb. Therefore, physicians’ instructions and patients’ careful attention are required to prevent dislocation due to excessive hip joint laxity under conditions of motor weakness.