Dynamic analysis based on functional pelvic plane of spinopelvic malalignments: Adverse pelvic mobility, flatback...

To examine the characteristics of hip dynamics caused by abnormal spinopelvic alignment in patients with spinal fusion and identify appropriate targets for acetabular component placement in such patients undergoing total hip arthroplasty.

Dynamic imaging of the chair-rising motion and computed tomography images in the supine position were obtained for 40 patients with spinal fusion and normal hips. The sagittal pelvic tilt and hip flexion angle were measured with three-dimensional dynamic analysis using the image-matching method. The hip flexion angle based on the functional pelvic plane in the supine position (supine-FPP) was calculated using pelvic tilt and hip flexion angles. The hip flexion angle based on supine-FPP was compared in participants with and without adverse pelvic mobility in extension (APM; posterior pelvic tilt >13° from supine to standing), flatback deformity (pelvic incidence-lumbar lordosis mismatch ≥10°), and stiff spinopelvic mobility (stiff-SPM; posterior pelvic tilt <10° between standing and sitting).

The flexion angle based on supine-FPP was 5.4°-7.2° lower in patients with APM than in those without APM throughout the chair-rising motion (sitting, deep-sitting, and standing), and there were no significant differences between patients with and without flatback deformity throughout the chair-rising motion and 11.1° and 9.9° higher in patients with stiff-SPM than in those without stiff-SPM in sitting and deep-sitting positions, respectively, but there were no significant differences in standing position.

The importance of adjusting cup placement according to abnormal spinopelvic alignment was demonstrated in the following cases: APM requires a decrease in cup anteversion based on supine-FPP, cup anteversion based on supine-FPP is an appropriate target for flatback deformity, and stiff-SPM requires an increase in cup anteversion.