BACKGROUND: The spinopelvic axis is becoming recognized as an essential contributor to impingement and instability leading to dislocation. Computer-assisted hip surgery uses standing and relaxed-seated radiographs as a surrogate marker of pelvic tilt in all seated positions. However, the flexed-seated position is a high-risk position for dislocation, and the standing and relaxed-seated radiographs may not reflect this risk. This study aims to determine whether adding a flexed-seated radiograph affects stability assessment in 3D modelling of THR. METHODS: Ninety patients with osteoarthritis underwent computer-assisted THR and received standing, relaxed-seated, and flexed-seated radiographs. Sacral slope (SS) was measured and analysed using Pearson correlation. Key measures were degree of tilt between positions, as well as correlations between dynamic hip movements. RESULTS: Of the examined patients, 96.7% anteriorly tilted their pelvis moving from relaxed-seated to flexed-seated, and 50% of patients anteriorly tilted by >
10° SS. There was a moderate correlation between standing SS and flexed-seated SS (r = 0.33, P ≤ 0.1). There was a strong correlation between relaxed-seated SS and flexed-seated SS (r = 0.77, P ≤ 0.002)
however, there was a wide variance of flexed-seated SS for any given relaxed-seated or standing SS. CONCLUSION: The flexed-seated position poses a higher risk of anterior impingement in 96.7% of patients compared to the relaxed seated-position. The flexed-seated position cannot be predicted by existing radiographs, making it a valuable marker in surgical planning to mitigate the risk of hip instability.