Biomechanical Analysis of the Dysplastic Hip: Pre- and Post-Pelvic and Femoral Osteotomy

Biomechanical analysis of the hip joint, using the rigid body spring model (RBSM) was performed to investigate the mechanism responsible for the progression of coxarthrosis and to obtain guidelines for treatment. As dysplastic change progressed, the abductor muscle strength required for one leg standing increased, as did pelvic displacement. After Chiari’s operation, pelvic displacement decreased remarkably, while more abductor muscle strength was required, due to the verticalization of the abductor muscle and the shortening of its lever arm. Varus osteotomy appeared to preserve the efficiency of abductor muscle strength by lengthening its lever arm. Clinically, in all patients, pain was either relieved or disappeared as a result of the muscle strengthening exercises having been done. When the practical abductor muscle strength was reduced to 50%–70% of the theoretical value calculated with the RBSM, hip pain score was about 20 points. Thus, by assessing the muscle strength both practically and theoretically, in addition to making an X-ray diagnosis, we were able to select a suitable treatment plan for patients with dysplastic hips.