• Henryk Haffer, Zhen Wang, Zhouyang Hu, Christian Hipfl, and Matthias Pumberger.
• Center for Musculoskeletal Surgery, Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität Zu Berlin, and Berlin Institute of Health, Berlin, Germany. henryk.haffer@charite.de.
• Arch Orthop Trauma Surg. 2022 Oct 1; 142 (10): 2979-2989.

IntroductionSpinopelvic mobility was identified as a contributing factor for total hip arthroplasty (THA) instability. The influence of spinopelvic function on acetabular cup positioning has not yet been sufficiently investigated in a prospective setting. Therefore, our study aimed (1) to assess cup inclination and anteversion in standing and sitting based on spinopelvic mobility, (2) to identify correlations between cup position and spinopelvic function, (3) and to determine the influence of the individual spinal segments, spinal sagittal balance, and spinopelvic characteristics on the mobility groups.Materials And MethodsA prospective study assessing 197 THA patients was conducted with stereoradiography in standing and sitting position postoperatively. Two independent investigators determined cup anteversion and inclination, C7-Sagittal vertical axis, cervical lordosis (CL), thoracic kyphosis (TK), lumbar lordosis (LL), sacral slope, pelvic tilt (PT), anteinclination (AI), and pelvic femoral angle (PFA). Spinopelvic mobility is defined based on ∆PT = PTstanding - PTsitting as ∆PT < 10° stiff, ∆PT ≥ 10-30° normal, and ∆PT > 30° hypermobile. Pearson coefficient represented correlations between the cup position and spinopelvic parameters.ResultsSignificant differences were demonstrated for cup anteversion (stiff/hypermobile 29.3°/40.1°; p < 0.000) and inclination (stiff/hypermobile 43.5°/60.2°; p < 0.000) in sitting, but not in standing position. ∆ (standing/sitting) of the cup anteversion (stiff/neutral/hypermobile 5.8°/12.4°/19.9°; p < 0.000) and inclination (stiff/neutral/hypermobile 2.3°/11.2°/18.8°; p < 0.000) revealed significant differences between the mobility groups. The acetabular cup position in sitting, was correlated with lumbar flexibility (∆LL) and spinopelvic mobility. Significant differences were detected between the mobility types and acetabular orientation (AI sit:stiff/hypermobile 47.6°/65.4°; p < 0.000) and hip motion (∆PFA:stiff/hypermobile 65.8°/37.3°; p < 0.000). Assessment of the spinal segments highlighted the role of lumbar flexibility (∆LL:stiff/hypermobile 9.9°/36.2°; p < 0.000) in the spinopelvic complex.ConclusionThe significantly different acetabular cup positions in sitting and in the ∆ between standing and sitting and the significantly altered spinopelvic characteristics in terms of stiff and hypermobile spinopelvic mobility underlined the consideration for preoperative functional radiological assessment. Identifying the patients with altered spinopelvic mechanics due to a standardized screening algorithm is necessary to provide safe acetabular cup positioning. The proximal spinal segments appeared not to be involved in the spinopelvic function.© 2021. The Author(s).

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