How Does Spinopelvic Mobility and Sagittal Functional Cup Orientation Affect Patient-Reported Outcome 1 Year after THA?-A Prospective Diagnostic Cohort Study.

Abstract Background This prospective cohort study aimed to characterize how spinopelvic characteristics change post-THA and determine how patient-reported outcome measures (PROMs) are associated with 1) individual spinopelvic mobility and 2) functional sagittal cup orientation post-THA. Methods One hundred consecutive patients that received unilateral THAs for end-stage hip OA, without spinal pathology were studied. Pre- and post-operatively, patients underwent clinical and radiographic evaluations. Patient reported outcomes were collected using the HOOS-PS. Radiographic parameters measured from standing and relaxed-seated radiographs, included the lumbar-lordosis-angle, pelvic tilt (PT), pelvic femoral-angle and cup orientation in the coronal (inclination/anteversion) and sagittal (ante-inclination) planes. Spinopelvic mobility was characterized (ΔPT: ‘stiff’ ( 30°)). Results Pre-operative spinopelvic characteristics were not associated with HOOS-PS. Post-THA, the spinopelvic characteristics changed, with less patients having spinopelvic hypermobility (7%) compared to pre-op (14%). Postoperatively, patients with spinopelvic hypermobility showed significantly worse HOOS-PS scores (21±17 vs. 21±22 vs. 41±23; ANOVA p=0.037). Sagittal but not coronal cup orientation was associated with post-operative spinopelvic characteristics. Cup ante-inclination was less in the patients with post-operative spinopelvic hypermobility (27±7° vs. 36±8° vs. 36±10°; ANOVA: p=0.035). Conclusion We hypothesize that spinopelvic hypermobility is secondary to impingement and reduced hip flexion; in order to achieve a seated position, impinging hips require more posterior pelvic tilt. Patients with spinopelvic hypermobility are likely impinging secondary to the low cup ante-inclination (sagittal mal-orientation despite optimum coronal orientation) and thus have lower HOOS-PS compared. Sagittal assessments are thus important to adequately study hip mechanics. Level of Evidence Level II, diagnostic study.