Capsular Mechanics After Periacetabular Osteotomy for Hip Dysplasia

Background: Hip dysplasia is characterized by insufficient acetabular coverage around the femoral head, which leads to instability, pain, and injury. Periacetabular osteotomy (PAO) aims to restore acetabular coverage and function, but its effects on capsular mechanics and joint stability are still unclear. The purpose of this study was to examine the effects of PAO on capsular mechanics and joint range of motion in dysplastic hips. Methods: Twelve cadaveric dysplastic hips (denuded to bone and capsule) were mounted onto a robotic tester and tested in multiple positions: (1) full extension, (2) neutral 0 degrees, (3) flexion of 30 degrees, (4) flexion of 60 degrees, and (5) flexion of 90 degrees. In each position, the hips underwent internal and external rotation, abduction, and adduction using 5 Nm of torque. Each hip then underwent PAO to reorient the acetabular fragment, preserving the capsular ligaments, and was retested. Results: The PAO reduced internal rotation in flexion of 90 degrees ( increment (IR) = -5 degrees; p = 0.003), and increased external rotation in flexion of 60 degrees ( increment (ER) = +7 degrees; p = 0.001) and flexion of 90 degrees ( increment (ER) = +11 degrees; p = 0.001). The PAO also reduced abduction in extension ( increment (ABD) = -10 degrees; p = 0.002), neutral 0 degrees ( increment (ABD) = -7 degrees; p = 0.001), and flexion of 30 degrees ( increment (ABD) = -8 degrees; p = 0.001), but increased adduction in neutral 0 degrees ( increment (ADD) = +9 degrees; p = 0.001), flexion of 30 degrees ( increment (ADD) = +11 degrees; p = 0.002), and flexion of 60 degrees ( increment (ADD) = +11 degrees; p = 0.003). Conclusions: PAO caused reductions in hip abduction and internal rotation but greater increases in hip adduction and external rotation. The osseous acetabular structure and capsule both play a role in the balance between joint mobility and stability after PAO.