Achieving Cup Target as Per Spinopelvic Assessment Is Associated With Improved Total Hip Arthroplasty Outcome: A Prospective, Multicentre Study

Different methods have been described to help optimizing sagittal cup orientation in total hip arthroplasty (THA) based on individual spinopelvic characteristics; including hip-spine-classification and Combined-Sagittal-Index (CSI). This study aimed to (1) assess how often CSI- and hip-spine-classification targets were achieved post-THA, without navigation or robotics, (2) compare anteversion/inclination between cups in-/outside optimal CSI-zone, and (3) determine association with outcome comparing different methods for cup orientation, including CSI, hip-spine-classification and conventional coronal cup orientation. This is a multicenter, prospective, case-cohort study of 435 primary THA for osteoarthritis (53% females; age:65±12years-old) (58% lateral-, 29% anterior-, 13% posterior-approach). No robotics or dual-mobility were used. Patients underwent pre- and post-operative standing & deep-seated spinopelvic radiographs to measure parameters including Pelvic Incidence (PI), Lumbar Lordosis (LL), Pelvic-Femoral-Angle (PFA) and Anteinclination (AI). CSIstanding was calculated as the sum of PFAstanding and AIstanding. Unbalanced spine was defined as PI-LL≥10°, stiffness as ∆LL < 2 0°. Optimal cup orientation was based on CSI-targets: 205–245° for balanced spine (n=327), or 215–235° for unbalanced spine (n=108), hip-spine-classification targets (±5°), and conventional inclination/anteversion of 40/20°±10°. Patient-reported outcome was measured using Oxford Hip Score (OHS). Optimal CSI-targets were achieved in 60% (n=261/435), whilst 44% had cup position within hip-spine-classification targets (n=125/284). Anteversion was higher among cups within optimal CSI-zone [26° (range:2°–48°) vs. 22° (range:3°–47°); p < 0 .001]. Dislocation rate was lower (0.4% vs. 1.7%; p=0.178), and post-operative OHS was better among those with optimal CSI [42pts (range:4–48) vs. 40pts (range:8–48); p=0.003] or within hip-spine-classification targets (p=0.028), but not according to conventional orientation (p=0.384). Awareness of adverse spinopelvic characteristics and using sagittal characteristics (especially CSI) can help surgeons to achieve optimal cup orientation, improving outcome and reducing dislocation risk post-THA, without the need of dual-mobility. Further studies should test whether advanced technology can improve consistency in achieving targets.