Recently, pelvic retroversion was reported to result in decreased acetabular cover of the femoral head [12]. In patients with osteoarthritis of the hip, pelvic anteversion is thought to increase the rate of femoral head cover and decrease the load on the acetabulum. Moreover, pelvic retroversion with lumber degenerative kyphosis results in decreased acetabular cover of the femoral head and increased load on the acetabulum [5]. In the present study, models for normal, borderline dysplasia, and dysplasia hips showed the highest von Mises stress in pelvic retroversion, and the lowest in pelvic anteversion. These results demonstrate that pelvic retroversion increases the stress on the acetabulum regardless of pelvic shape.
In the present study, the body weight was placed on the superior aspect of the sacrum based on previous research [11]. By necessity, the load on the flexor muscles of the hip joint was calculated using the moment-arm equation against this body weight to keep the altitude. The body weight for all models was unified to 400 N in order to unify the load conditions across models. Young’s modulus was calculated from the Hounsfield values in the CT data on the basis of Keyak’s rule to determine the apparent density of each element in pelvis as in previous studies [10]. Furthermore, the transmission of the cartilage layer separating the two layers of the acetabular and femoral head sides was designed to closely model the living body.
In the normal hip model, the stress distribution was widely concentrated around the load-bearing area, showing a tendency to focus on the load-bearing area when the pelvis was tilted backwards. In contrast, the stress distribution was concentrated in the anterior and posterior load-bearing area in the borderline dysplasia model, and the stress distribution was strongly concentrated in the anterior load-bearing area at a pelvic tilt angle of -20°. Thus, three-dimensional acetabular dysplasia was involved in addition to the low CE angle for the borderline pelvis [13, 14, 15].
Dysplasia results in osteoarthritis of the hip due to decreased acetabular cover of the femoral head [1, 2, 16, 17]. Until now, few reports exist on the use of FEA in evaluating the effects of pelvic tilt on stress distribution in the acetabulum. In the present study, we were able to assess of the influence of pelvic tilt on stress distribution in the acetabulum. The load in the borderline dysplasia model at a pelvic tilt angle of -20° was similar to that for the dysplasia model at a pelvic tilt angle of 0°, which is a risk factor for osteoarthritis. Therefore, borderline dysplasia with a pelvic tilt angle of -20° may also be a risk factor for osteoarthritis of the hip.
The present study has several limitations to discuss. First, the number of the patients was limited due to the involvement of FEA. Second, dysplasia, borderline dysplasia, and a normal pelvis could not be compared in similar patients. Thus, models of different patients were compared. Third, the study did not examine data of a dynamic condition.