Measurement of spinopelvic sagittal alignment in the relaxed seated position rather than in the straight seated position is suitable for assessing spinopelvic mobility in patients before total hip arthroplasty.

Purpose: The relationship between spinopelvic mobility and dislocation in total hip arthroplasty (THA) has recently attracted attention. This study aimed to investigate the differences in sacral slope (SS) between two types of upright seated positions and to determine which seated position was appropriate for assessing spinopelvic mobility (change in SS from standing to sitting) before THA.

Materials and methods: This prospective cohort study included 75 hips from 75 patients who had undergone primary THA. Each patient underwent preoperative lateral spinopelvic radiography in standing (st) and two seated positions: relaxed (rs) and straight (ss). The change in SS between each position (Δ) was measured.

Results: Differences in all spinopelvic sagittal alignment parameters between the two seated positions were statistically significant (p < 0.001). The range, median, and mean values of ΔSS_ss-rs were -2.0° to 26.5°, 6.8°, and 8.3°, respectively. ΔSS_ss-rs was significantly correlated with SS, LLA, and PFA in the relaxed seated position (r = -0.52, -0.39, and 0.37; p < 0.001, p < 0.001, and p = 0.001, respectively), but was not correlated to these parameters in the straight seated position. Of the 52 patients with normal spinopelvic mobility in the relaxed seated position (ΔSS_st-rs > 10°), 24 (46%) patients were misrepresented as having a stiff spine in the straight seated position (ΔSS_st-ss < 10°).

Conclusion: The change in SS from the straight to the relaxed seated position widely varied in patients before THA. The spinopelvic radiograph in the relaxed seated position is appropriate when evaluating spinopelvic mobility for preoperative planning.