Three-Dimensional Hip Joint Congruity Evaluation of the Borderline Dysplasia: Zonal-acetabular Radius of Curvature.

In theory, a hemispherical acetabulum provides the ideal hip congruity in any hip position. However, it remains unknown how the three-dimensional acetabular morphology of borderline dysplastic and frank dysplastic hips compare to normal hips. This study inquires if borderline dysplastic zonal-acetabular curvatures in the anterior, superior, and posterior zones are different from normal or dysplastic hips three-dimensionally. One-hundred and fifteen hips, grouped as control (25 degrees <= LCEA <40 degrees), 36 hips; borderline (20 degrees <= LCEA <25 degrees), 32 hips; dysplasia (LCEA <= 20 degrees), 47 hips were analyzed. The radii of acetabular curvature for the anterior, superior, and posterior zones were calculated as the zonal-acetabular radius of curvature (ZARC). The mean acetabular roof obliquity of the borderline (10.6 +/- 4.3 [SD]degrees) was significantly larger than the control (3.0 degrees +/- 5.4 degrees; P < .001) and smaller than the dysplasia (19.3 degrees +/- 5.7 degrees; P < .001). Although the mean acetabular anteversion angle of the borderline (21.3 degrees +/- 3.7 degrees) was significantly larger than control (17.9 +/- 3.5 degrees; P = .001), that of the borderline was not different from the dysplasia (23.3 degrees +/- 4.0 degrees; P = .053). The mean anterior ZARC in the borderline (29.8 +/- 2.6 mm) was significantly larger than the control (28.0 +/- 2.2 mm; P = .011) and smaller than the dysplasia (31.5 +/- 2.7 mm; P = .009). The mean superior ZARC in the borderline (25.7 +/- 3.0 mm) was not different from the control (25.9 +/- 2.2 mm; P = .934) or the dysplasia (25.8 +/- 2.5 mm; P = .991). Although the mean posterior ZARC in the borderline (27.2 +/- 2.5 mm) was not different from the control (26.4 +/- 1.9 mm; P = .455), that of the borderline group was significantly smaller than the dysplasia (30.4 +/- 3.3 mm; P < .001); that is, the severity of lateral under-coverage affects the anterior and/or posterior zonal-acetabular curvature.