Spontaneous reduction in both dislocated head and dissociation of head-neck in a modular total hip arthroplasty.

A year after the right total hip replacement for primary osteoarthritis of the hip, a patient presented to the hospital with dislocation of femoral head from the acetabulum and dissociation of the head-neck junction after a trivial rotational movement of the lower limb. While waiting for the surgery, spontaneous reduction in dislocated femoral head into the acetabulum and spontaneous relocation of the femoral neck occurred. This rare phenomenon and the possible mechanism of such dissociation and relocation are discussed in this report. Such relocations provide dramatic pain relief to the patient but they are not therapeutic. Revision surgery was required to correct the underlying problem.