Acetabular Morphology and Spinopelvic Characteristics: What Predominantly Determines Functional Acetabular Version?

Background: In addition to the relative size of the acetabular rim and how the pelvis is positioned in space, the plane in which the acetabular version is calculated also affects its measurement. Purpose: To determine the relative contribution of pelvic and acetabular characteristics on morphological version (measured relative to the anterior pelvic plane angle [APPA]) and functional version (measured relative to the horizontal table). Study Design: Cross-sectional study; Level of evidence, 3. Methods: Included were 50 acetabular dysplasia patients and 109 asymptomatic controls. Using image analysis software, morphological parameters of the pelvis and acetabulum were determined from 2-dimensional computed topography: pelvic incidence, pelvic tilt angle, sacral slope, APPA, morphological and functional acetabular versions, and subtended angles (measure of acetabular rim prominence relative to the femoral head center) around the acetabular clockface in 30 degrees increments. Correlation and multivariable regression analyses were performed with morphological and functional version as dependent variables and spinopelvic and acetabular parameters as independent variables. Results: Morphological version was moderately associated with differences between anterior and posterior subtended angles (R = 0.68 [P < .001] and R = 0.57 [P < .001] for differences at 165 degrees and 15 degrees and 135 degrees and 45 degrees, respectively). Functional version was moderately associated with pelvic tilt angle (R = 0.56; P <.001) and the difference in subtended angles between anterior and posterior rims (R = 0.61 [P < .001] and R = 0.50 [P < .001] for differences at 165 degrees and 15 degrees and 135 degrees and 45 degrees, respectively). Multivariate analysis revealed a good model for predicting morphological version (R (2) = 0.44; P < .01) and functional version (R (2) = 0.58; P < .01). Subtended angle difference between 165 degrees and 15 degrees (B = 0.36 [95% CI, 0.24-0.49]; P < .001) was most strongly related to morphological version, and pelvic tilt angle (B = 0.57 [95% CI, 0.46-0.68]; P < .001) was most strongly related to functional version. Conclusion: Functional acetabular version was influenced most strongly by pelvic tilt angle rather than the relative prominence of the acetabular rims. Before determining surgical management for version abnormalities, it would be prudent to assess pelvic mobility and characteristics in different functional positions. In patients with minimal pelvic tilt change dynamically, corrective osteotomy would be the treatment of choice to improve functional version.