FP6.10 Pelvic incidence is not correlated with pelvic mobility in non-arthritic patients with hip pain and positive impingement test

Introduction: Pelvic mobility is calculated as the change (∆) in either pelvic tilt (PT) or sacral slope (SS) between extreme positions, and can be considered insufficient (stiff) if ∆SS or ∆PT 30° in the arthritic population. Furthermore, it is postulated that pelvic mobility is correlated to pelvic incidence (PI) in non-arthritic patients with femoroacetabular impingement (FAI). Moreover, it is suggested that symptoms of FAI depend on pelvic mobility, as the same cam lesion could be symptomatic in hip users (PI<40°), but asymptomatic in spine users (PI≥40°). This study aims to assess pelvic mobility’s repeatability (∆SS and ∆PT) and its association with PI in non-arthritic hip pain patients with a positive impingement test.

Methods: The cohort comprised 82 patients aged 31.8±7.4, with hip pain and positive impingement test. Stereo- radiographic images were acquired in three positions (neutral standing, neutral sitting, and flexed- forward sitting). PI, pelvic tilt (PT), and sacral slope (SS) were measured. Repeatability was evaluated. Pelvic mobility was calculated as ΔPT and ΔSS from i) standing to sitting, ii) neutral to flexed-forward- sitting, and iii) maximum to minimum values. Correlations of PI with PT, SS, ΔPT, and ΔSS were assessed.

Results: Repeatability was excellent for all pelvic mobility measurements (intraclass correlation coefficients, ICC >0.97). ΔPT was 25.9±8.3º from standing to sitting, 14.4±11.2º from standing to flexed-forward-sitting, and 37.8±13.7º from maximum to minimum values. ΔSS was 24.0±7.6º from standing to sitting,14.2±11.6º from standing to flexed-forward-sitting, and 35.9±13.7º from maximum to minimum values. PI was strongly correlated with PT in standing (r=0.7) and SS in standing (r=0.7), and moderately correlated with PT in sitting (r=0.6) and SS in sitting (r=0.5), but was not correlated with neither ΔPT nor ΔSS (r <0.3).

Conclusions: Pelvic mobility, calculated as ΔPT and ΔSS, has excellent repeatability, and is not correlated with PI in non-arthritic patients with hip pain. Therefore, PI should not be considered for diagnosis and treatment of painful hips with positive impingement test, nor to distinguish hip users from spine users.