Anatomy of the dysplastic hip and consequences for total hip arthroplasty.

Total hip arthroplasty in dysplastic hips is challenging because of the modified anatomy of the proximal femur and acetabulum. We studied three-dimensional anatomic parameters in 247 dysplastic hips from 218 adult patients using radiographs and computed tomography, and analyzed the consequences for total hip arthroplasty. A cohort of 310 primary osteoarthritic hips was used as a control group. According to the classification of Crowe et al, 78 of the dislocated hips were graded Class I, 26 as Class II, 20 as Class III or IV, and 169 dysplastic hips had no subluxation. Compared with primary osteoarthritis, the intramedullary femoral canal had reduced mediolateral and anteroposterior dimensions. With high grade subluxations the femoral neck shaft angle decreased but with low grades, especially in Class II, the neck shaft angle increased. The proximal femur had more anteversion with individual variations ranging from 1 degrees to 80 degrees. The true acetabulum had a reduced anteroposterior diameter. The large individual morphologic variability across all levels of dysplastic hips suggests the femoral prosthesis cannot be chosen on the basis of the severity of the subluxation alone.