Does Replication of Native Acetabular Anatomy Satisfy Recommended Hip-Spine Targets for Cup Orientation?

Background: The aim of this study was to assess whether replication of native acetabular anatomy would satisfy cup orientation targets using 3 commonly described hip-spine algorithms. Whether spinopelvic characteristics influence ability to achieve cup orientation targets when replicating native anatomy and the agreement between algorithm recommendations was tested. Methods: A prospective database was queried to identify patients with adverse (n = 70) spinopelvic characteristics. These were matched for age and sex with patients without adverse characteristics (n = 70). Spinopelvic characteristics were obtained from radiographs and computed tomography (CT) scans. CT scans were segmented to determine native acetabular anatomy, particularly anteversion. Three hip-spine planning algorithms were evaluated for each patient (Optimized Positioning System [OPS], Combined-Sagittal Index [CSI], Hip-Spine Classification). Differences between target orientations and native anatomy were determined. Agreement between algorithms was tested. Results: OPS plan had significantly reduced inclination compared with native (39° vs. 52°, p < 0.001). No significant difference between OPS and native anteversions was seen (18° vs. 18°, p = 0.1) for the adverse group. OPS-planned anteversion was greater than native (23° vs. 16°, p < 0.001) in the nonadverse group. Most native orientations met published CSI targets (90% nonadverse, 59% adverse). Most native acetabular orientations (61% adverse and 58% nonadverse) failed to meet Hip-Spine Classification targets. Overall, in 88% of cases, replication of native acetabular version and 40° of inclination satisfied at least one suggested target. Agreement of all 3 algorithms was 31%; greater agreement was seen between Hip-Spine Classification and OPS (64%). Conclusions: Native acetabular anteversion and radiographic inclination of 40° are reliable targets, satisfying at least one hip-spine algorithm and thus justifying such practice, when advanced hip-spine analysis is not performed. The discrepancy between suggested orientations by the various published techniques, despite their validated low dislocation rates, emphasizes that although achieving a target cup orientation is important, the nature of hip stability is multifactorial and merits a holistic approach.