Variations in pelvic tilt between relaxed-seated and flexed-seated positions affect stability assessment in 3D...

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.

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.

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 ≤ 0.001); however, there was a wide variance of flexed-seated SS for any given relaxed-seated or standing SS.

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.