Muscle reorientation as a novel cause of motor weakness after spinal realignment surgery: a hypothesis based on psoas muscle overstretching.

Abstract

The aim of this study was to investigate whether postoperative reorientation of the psoas muscle is associated with hip flexion weakness in patients who underwent posterior-only spinal realignment surgery (SRS). It was hypothesized that changes in spinal alignment might indirectly contribute to hip flexion weakness by altering the anatomical orientation and function of the psoas muscle. A total of 101 patients who underwent posterior-only SRS between 2012 and 2022 were retrospectively reviewed. Patients without evidence of intraoperative nerve injury, as confirmed by intraoperative neurophysiological monitoring, and without postoperative nerve injury, as confirmed by electromyography or nerve conduction velocity studies were included. Radiographic parameters were measured to assess psoas muscle orientation using two novel reference lines: the vertical line (VL) and the diagonal line (DL). Cross-sectional areas (CSAs) of the psoas muscle were evaluated on axial MRI at the L4-5 disc level. Group comparisons were conducted to assess differences in radiological parameters and CSAs between patients with and without hip flexion weakness. Hip flexion weakness was observed in 14 patients (13.9%) and recovered in all cases with a median recovery time of 55.2 days. Postoperatively, VL and DL measurements increased significantly, and the psoas muscle CSAs decreased. Changes in VL, DL, and CSAs were significantly greater in the group with hip flexion weakness. Correlation analysis revealed significant negative associations between VL/DL changes and CSA reduction. In multivariate regression, only VL changes remained an independent predictor of postoperative CSA reduction. This study demonstrates that postoperative hip flexion weakness can occur after SRS, even without direct nerve or muscle injury. The findings suggest that spinal alignment changes alone might mechanically rearrange the psoas muscle, contributing to transient hip flexion weakness.