Stress Fracture of the Inferior Pubic Ramus after Periacetabular Osteotomy ~ Biomechanical Study Using a Finite Element Model

INTRODUCTION Periacetabular osteotomy (PAO) is a useful treatment for patients with dysplastic hips and has been performed for the young patients with or without osteoarthritis of the hips. We have occasionally experienced inferior pubic ramus fractures in patients several months after PAO (Figure 1). Stress fractures of the pubis are known to occur in athletes or patients who have undergone total hip or knee arthroplasty. Although the causes of such fractures are reported to be tensile stress from the adductor muscles or leg length discrepancy, no patients in this series had high activity after PAO or obvious leg length discrepancies. We supposed that the superior pubic ramus and the ischium were shifted from the weight bearing axis after osteotomy, thus causing a load concentration at the inferior ramus. A computed tomography (CT) based finite element (FE) model was used to ascertain the stress load on the pubis before and after PAO. CTbased FE models are known to be able to make accurate predictions on fracture loads on femur or vertebra. Several recent studies of FE models have verified that the predictive strength of the bones significantly correlated with the actual measured bone strength. We studied the load stress on the pelvis before and after PAO using a CT-based FE model to investigate the cause of inferior pubic fracture.