Femoral morphologic differences in subtypes of high developmental dislocation of the hip.

BACKGROUND Previous studies show the shape of the femur in developmental dislocation of the hip (DDH) becomes more abnormal with increasing subluxation. Two kinds of high dislocations associated with DDH have been observed in clinical practice, one with (Type C1) and one without (Type C2) a false acetabulum. The presence or absence of a false acetabulum in high dislocated hips is associated with different loading patterns and could influence the development and shape of the proximal femur. QUESTIONS/PURPOSES We therefore determined whether (1) the proximal femoral shape and dimension in Type C1 and Type C2 hips differ from each other, and (2) the femur dislocated with the same height in Types C1 and C2 hips. PATIENTS AND METHODS We examined the following variables on 54 proximal femurs from 54 patients with high DDH (28 Type C1 hips and 26 Type C2 hips) on AP and lateral radiographs; the ML widths of the cortical and medullary canals, height of the femoral head, height of dislocation, and height of the greater trochanter. Reproducibility of the measurements was tested by two researchers with high interobserver and intraobserver agreement. RESULTS The proximal femur in Type C2 hips was narrower and stovepipe shaped, with a smaller flare index (2.7 ± 0.6), compared with Type C1 hips (3.5 ± 1.2). The proximal femur migrated an average of 18 mm more superiorly in Type C2 than in Type C1 hips. CONCLUSIONS Our data confirm distinctions in the shape of the proximal femur in the presence and absence of a false acetabulum. CLINICAL RELEVANCE Owing to the abnormal shapes, special implants of different geometries or modular stems may be needed for reconstruction Type C2 high dislocations.