Stand-to-Sit Kinematics of the Pelvis Is Not Always as Expected: Hip and Spine Pathologies Can Have an Impact.

Background: Stand-to-sit pelvis kinematics is commonly considered as a rotation around the bicoxofemoral axis. However, abnormal kinematics could occur for patients with musculoskeletal disorders, affecting the hip-spine complex. The aim of this study is to perform a quantitative analysis of the stand-to-sit pelvis kinematics using 3D reconstruction from biplanar x-rays.

Methods: Thirty volunteers as a control group (C), 30 patients with hip pathology (Hip), and 30 patients with spine pathology (Spine) were evaluated. All subjects underwent standing and sitting full-body biplanar x-rays. Three-dimensional reconstruction was performed in each configuration and then translated such as the middle of the line joining the center of each acetabulum corresponds to the origin. Rigid registration quantified the finite helical axis (FHA) describing the transition between standing and sitting with two specific parameters. The orientation angle (OA) is the signed 3D angle between FHA and bicoxofemoral axis, and the rotation angle (RA) represents the signed angle around FHA.

Results: The mean OA was -1.8° for the C group, 0.3° for Hip group, and -2.4° for Spine group. There was no significant difference in mean OA between groups. However, variability was higher for the Spine group with a standard deviation (SD) of 15.9° compared with 10.8° in the C group and 12.3° in the Hip group. The mean RA in the C group was 18.1° (SD, 9.0°). There was significant difference in RA between the Hip and Spine groups (21.1° [SD, 8.0°] and 16.4° [SD, 10.8°], respectively) (P = .04).

Conclusion: Hip and spine pathologies affect stand-to-sit pelvic kinematics.