FP1.5 Variation Between Supine and Standing Pelvic Tilt in Patients Undergoing Periacetabular Osteotomy (PAO)

Abstract ICLB01 - The Instability Masterclass: Correcting Bone, Biology, and Bad Outcomes, Session Room 6 (BMCC Level 3), October 8, 2026, 10:30 - 12:30 Introduction Periacetabular osteotomy (PAO) is an effective treatment modality for hip dysplasia and acetabular retroversion. Pre-operative pelvic radiographs are a mainstay in assessing acetabular morphology and guiding pre-operative planning for correction. Previous studies have demonstrated how functional sagittal rotation of the pelvis can vary between supine and standing positions, altering the projected radiographic acetabular morphology. Interpretation of this may significantly influence pre-operative planning, however, limited studies exist investigating the magnitude and pattern of this variation in functional pelvic tilt (PT) in hip preservation patients. This study aimed to evaluate the variation in PT between supine and standing radiographs in patients undergoing PAO. Methods 101 patients (mean age 28.1±6.8) who underwent PAO surgery for either hip dysplasia or acetabular retroversion at a tertiary referral centre for hip preservation were retrospectively analysed. Measurements of PT using the sacro-femoral-pubic (SFP) angle and pubic symphysis-sacroiliac (PS-SI) index were obtained from routine pre-operative supine and standing pelvic radiographs. Correlation between measurements was assessed using Pearson correlation, along with intra- and inter-observer reliability using intraclass correlation coefficient (ICC). The paired t-test was used to compare supine vs standing measurements. Statistical significance was set at p<0.05. Results Intra-observer reliability was excellent for SFP (ICC=0.968) and good for PS-SI (ICC=0.887). Inter-observer reliability was good for both SFP (ICC=0.870) and PS-SI (ICC=0.851). Strong correlations were observed between SFP and PS-SI in both supine (r=0.766, p<0.001) and standing (r=0.821, p<0.001) positions. Mean SFP angle decreased from 70.4±5.0° supine to 65.3±5.5° standing (p<0.001), corresponding to an increase in PT from 4.6±5.0° to 9.7±5.5° (p<0.001). PS-SI index decreased from 107±14 mm supine to 92±15 mm standing (p<0.001). Conclusion PT significantly increased between supine and standing AP radiographs in patients undergoing PAO. Although the overall cohort demonstrates a predictable directional change, variation within and between individuals is wide and may not be reliably predictable. This variation in functional PT may have a significant impact on analysis of acetabular deformity, as well as the optimisation of planned corrections in PAO surgery. We recommend that surgeons consider multiple functional pelvic views in order to optimise decision making, as well as individualised fragment position and outcomes.