Degeneration in dysplastic hips. A computer tomography study

Hip dysplasia is considered pre-osteoarthritic, causing degeneration in young individuals.

To determine the pattern of degenerative change in moderate to severely dysplastic hips in young patients.

One hundred and ninety-three consecutively-referred younger patients with hip pain believed to be caused by hip dysplasia constituted the study cohort. The average age was 35.5 years (range, 15-61 years). They were examined by close-cut transverse pelvic and knee computed tomography and antero-posterior radiographs (CT). We identified 197 hips with moderate to severe dysplasia, and 78 hips with normal morphology in the study cohort, whilst 111 hip joints were borderline dysplastic according to preset definitions. Comparative analyses of anatomy and distribution of degeneration between dysplastic and normal hips in the study cohort were performed.

In dysplastic hips the anterior acetabular sector angle was significantly and inversely associated to femoral anteversion (p < 0.001). The center-edge (CE) angle, the acetabular angle (AA), and the acetabular depth ratio (ADR) were significantly interrelated (p < 0.001; correlation coefficients ranging from -0.8 to 0.7). Fifty-one hips were subluxated (24R/27L). There were no cases of complete dislocation. The formation of subchondral cysts or osteophytes in dysplastic hips was significantly associated with reduced minimum joint space width (p ranging from 0.005 to 0.02). However, in 67 hips with acetabular cysts, only 6 hips had minimum joint space widths = 2.0 mm (8.9%) in the coronal plane. In 96 cases with acetabular cysts found in the sagittal plane, 43 cases had minimum joint space widths = 2.0 mm (44.7%). Bony rim detachment at the site of labral insertion was recorded in 30 hips. Twenty-three of these were dysplastic (p = 0.01).

Degeneration was most often found in the anterolateral part of the dysplastic hip joints. Most cysts were located above the transition zone between the bony and the fibrocartilaginous acetabulum, and we found a significantly- increased number of cases with avulsed bony fragments at the antero-lateral labral insertion in dysplastic hips compared to normal hips. It seems likely that the early degenerative process in dysplastic hips originates at the watershed zone between the acetabular labrum and the acetabular cartilage in response to subluxation and femoroacetabular impingement.