CUP ORIENTATION WHEN STANDING IS SIGNIFICANTLY DIFFERENT TO WHEN SUPINE: IMPLICATIONS FOR PRE-OPERATIVE PLANNING

Arthroplasty surgeons traditionally assess cup orientation on supine radiographs, which fail to account for individual postural changes when standing. Standing assessments have been proposed for pre-operative THA planning to account spinopelvic characteristics. To-date no large cohort data exists on standing cup orientation post-THA and how this differs to supine assessment. This study aims to 1. Characterize cup orientation supine and standing; 2. Determine cup orientation differences between postures; 3. Identify factors associated with magnitude of orientation difference. This is a two-center, multi-surgeon, prospective, consecutive cohort study. 462 patients (500 hips) that underwent primary THA for hip arthritis were included (58% females; age: 65±11; BMI: 29±5). Most patients underwent anterior approach (65%), followed by posterior (29%) and lateral (6%). All had an uncemented cup. All patients underwent supine and standing AP pelvic and lateral spinopelvic radiographs at 1-year post-THA. Cup orientation was measured using validated software. Target cup orientation was defined as inclination/anteversion of 40/20°±10°. A significant change in orientation (ΔInclination/ΔAnteversion) between postures was defined as >5° Two of the 500 THAs sustained a dislocation. Inclination increased from 41°±7 supine, to 43°±7 standing (p 5°) was seen in 48% of cases. ΔAnteversion was dependent on cup anteversion (ρ=0.5) and standing pelvic tilt (ρ=0.2). Standing cup orientation is associated with significant increase in inclination/anteversion, and many cups might be erroneously considered sub-optimally orientated from standing assessments. If cup orientation targets are defined from standing spinopelvic characteristics the resultant cup inclination/anteversion when supine (in post-op X-Rays) should be lower than the defined standing target. Individual assessment of pelvic tilt can help predict change in cup orientation.