CORR Insights®: Does Acetabular Coverage Vary Between the Supine and Standing Positions in Patients with Hip Dysplasia?

Our understanding of hip dysplasia has greatly evolved since 1939, when Wiberg’s monograph described the lateral center-edge angle (LCEA) [12]. While his writings concentrated on lateral acetabular coverage, numerous papers have since examined the comprehensive evaluation of acetabular coverage [5, 13]. A comprehensive radiographic evaluation of acetabular coverage now includes LCEA, Tönnis angle, anterior and posterior wall index on the AP pelvis radiograph, and anterior centeredge angle (ACEA) on the false-profile radiograph [5]. Three-dimensional (3D) imaging is also more commonly ordered in the young adult with hip pain, where measurements like acetabular version at 1, 2, and 3 o’clock, coronal and sagittal center-edge angles, and femoral version can be obtained to further understand the 3-D anatomy. Some researchers have even quantified the cartilage surface area of the acetabulum, which is important in order to understand the true weightbearing surface that makes up the lunate cartilage, and ultimately, whether the socket is deficient or not [6]. This allows us to compare the degree of dysplasia in patients with anterior versus posterior acetabular deficiency or a large acetabular fossa. Improved radiographic evaluation and advanced imaging has led us to better understand anterior, posterior, and lateral undercoverage of the acetabulum. In the current study, Tachibana and colleagues [7] add sector angles to quantify geometric coverage. The sector angles used in this study and the correlation to radiographic measures give us a powerful new tool to evaluate 3-D acetabular coverage on CT, and validates our radiographic measures of anterior and posterior coverage, the anterior and posterior wall indicies. Regarding the differing morphologies of hip dysplasia [5, 13], one study found that women more commonly presented with anterolateral undercoverage while men presented more commonly with posterior undercoverage [5]. Tachibana and colleagues build off of this by measuring sector angles on CT scans to examine femoral head coverage of the acetabulum in multiple planes. In addition, they found differences in both CT and radiographic measures from the supine to standing position and found an increased posterior pelvic tilt in the standing position, which increases the functional acetabular anteversion compared to the supine position. This is illustrated by their decreased anterior and anterior-superior sector angles on CT imaging and decreased anterior wall index on radiographs. While there are small differences in LCEA and Tönnis angle, these are likely not noteworthy changes. This is similar to prior reports of minimal differences in LCEA and Tönnis angle in different degrees of pelvic tilt [8].