Cyst volume in the acetabulum and femoral head decreases after periacetabular osteotomy.

In a series of 26 consecutive patients scheduled for periacetabular osteotomy (PAO), we examined how many had acetabular or femoral head cysts, investigated whether the volume of the cysts changed after PAO, calculated the precision of the method applied and scored their hip symptoms. Magnetic resonance imaging (MRI) was performed before PAO and at 1 and 2½ years post-operatively. The number of cysts was noted and the total cyst volume in each patient was estimated with a design-based stereological method and the precision of the method was calculated.The patients filled out The Hip disability and Osteoarthritis Outcome Score (HOOS) four years after PAO. Preoperatively, 12 patients had acetabular or femoral head cysts (22 cysts), 1 year postoperative, 15 patients had cysts (23 cysts) and 2½ years postoperative, 15 patients had cysts (18 cysts). Mean total acetabular cyst volume per patient decreased significantly from 1 year (1.96 cm³, SD 3.97) to 2½ years (0.96 cm³, SD 1.70) after PAO (p= 0.04). The Limits Of Agreement for measurement of cyst volume was ± 1.73 cm³. The mean subscore for Pain was 75, Symptoms 75, ADL 83, Sport/recreation 63 and Quality Of Life 62. The number of patients having cysts did not change notably after PAO. But the mean total cyst volume/patient decreased significantly between 1 and 2½ years after PAO. The PAO patients rated their hip comparable to the scores for patients six months after total hip replacement.