The influence of spinal morphology on the standing-to-supine pelvis mobility in preoperative planning for total hip arthroplasty. Consequences in THA planning - 02/09/26

Doi : 10.1016/j.otsr.2026.104818

Danilo Casasola, Amira Pierson, Sena Boukhelifa, Elhadi Sariali ⁎

Hôpitaux Universitaires La Pitié Salpêtrière-Charles Foix, AP-HP, Sorbonne Université, F-75013, Paris, France

^⁎ Corresponding author.

Sous presse. Épreuves corrigées par l'auteur. Disponible en ligne depuis le Wednesday 02 September 2026

Abstract

Background

Many authors have highlighted the importance of assessing spinal alignment and mobility during total hip arthroplasty (THA), particularly regarding acetabular positioning. However, few studies have investigated the impact of spinal morphology on pelvic mobility from the standing to supine positions at the time of THA, performed via a direct anterior approach. The study aims to analyse this relationship in preoperative patients and its implications for planning acetabular anteversion.

Methods

This retrospective study included 93 patients who underwent THA between January 2021 and December 2022, using 3D CT-based planning and a preoperative EOS image in the standing position. The following parameters were measured: sacral slope (SS), pelvic incidence (PI), pelvic tilt (PT), and lumbar lordosis (LL). These measurements were taken from EOS (standing) and scout view (supine). Spinal morphology was classified according to the modified Roussouly classification.

Results

The mean SS was significantly higher in the supine position compared to the standing position (46° (SD 8°) vs. 41° (SD 8°); p < 0.001). The mean difference in sacral slope (SS) between these two positions was 4.2° (–12° to 17°, SD 6.1°). The SS increased significantly in 56% of patients, remained unchanged in 35%, and decreased in 9%. An anterior pelvic tilt when moving to the supine position was observed in patients with Roussouly type 1 and type 3 spinal morphologies ( p < 0.01), whereas patients with Roussouly type 3A and type 4 morphologies showed no significant change in SS between the standing and supine positions.

Conclusion

Contrarily to the other groups who experienced an anterior pelvis tilt when moving from standing to supine, in patients with Roussouly types 3A and 4, the pelvis orientation and consequently the acetabular anteversion remained unchanged. A paradoxical posterior pelvic tilt was observed in 9% of patients (mainly DDH), resulting in increased functional acetabular anteversion and a potentially higher risk of anterior dislocation. In these patients, a reduction in cup anteversion may be considered during THA.