gms | German Medical Science

Objectives: This study compared patients with hip osteoarthritis (OA) to a control group without hip OA and aimed to 1) characterize the differences in contribution of movement for the hip, spinopelvic complex and lumbar spine when performing a standing-to-sitting activity and to 2) identify spinopelvic mobility (SPM) parameters, potentially being associated with a higher risk for impingement and dislocation.

Methods: A cohort of 114 patients with end-stage hip OA and a control group of 35 subjects was prospectively studied. Clinical data and lateral view radiographs in standing and sitting position were assessed and sagittal SPM was calculated as the change in positions for the lumbar lordosis angle (ΔLL), sacral slope (ΔSS), pelvic tilt (ΔPT), pelvic-femoral angle (ΔPFA) and acetabular ante-inclination (ΔAI). ΔPT allowed for patient classification based on SPM into stiff (< ±10°), normal (±10-30°), or hypermobile (>±30°). The combined sagittal index (CSI) was calculated as the sum of PFA and AI, allowing for classification of the hip in or outside of a functional safe zone in standing and sitting position (outlier: CSI standing>243°, CSI sitting<151°). A power analysis was performed, indicating an adequate sample size (1-β=99%, α=0.05).

Results and conclusion: A synergistic interaction of the hip, spinopelvic complex and lumbar spine was found in both groups (Figure 1A-C [Fig. 1]). Hip OA patients demonstrated less hip flexion (-54±16 vs. -66±14, p<0.001), an increased posterior pelvic tilt (ΔPT: 20±11 vs. 12±11, p<0.001) and more flexion of the lumbar spine (ΔLL: -20±14 vs. -14±11, p=0.01), compared to the control group when moving from standing to sitting position (Table 1 [Tab. 1]). For the study and control group, standing CSI outliers were present in 12% and 23% (p=0.08) and in 1% and 3% for the sitting position (p=0.04), respectively. Patients with stiff SPM were more likely to demonstrate a CSI value of more than 243° in the standing position (p=0.005) (Figure 1D [Fig. 1]).

The hip, spinopelvic complex and lumbar spine interacted similarly in both groups. Patients however, demonstrated decreased hip flexion due to OA, forcing the pelvis to tilt more posteriorly as a compensatory mechanism to achieve a sitting position. The lumbar spine had to flex more anteriorly in order to maintain sagittal balance. Patients with stiff spinopelvic mobility were more likely to demonstrate a CSI value of 243° in standing position, potentially being at greater risk for dislocation as reported in the literature.