The spontaneous restoration of the course of psoas muscles after corrective surgery for adult spinal deformity

Background: A previous study demonstrated that spinopelvic alignment and morphology influence a deviation in the course of psoas muscle (PM). However, it is unknown whether such deviations might be caused by a decrease in lumbar lordosis (LL) or the lateral deviation of the lumbar spine following scoliosis. The purpose of this study was to elucidate the close relationship between the coronal and sagittal lumbar alignment and a deviation in the course of PM. Methods: We investigated the preoperative and postoperative spinopelvic parameters and the morphology of PM at L4/5 level in 30 patients treated with corrective surgery for adult spinal deformity who were diagnosed with "rising psoas sign" before surgery. Spinopelvic parameters were measured on X-ray films. Investigation of the morphology of PM and the morphological measurements were performed using computed tomography (CT) images. The "rising psoas sign" was classified as bilateral- or unilateral-type as defined in the previous study. Results: Among 18 patients who had bilateral-type rising psoas sign before surgery, 11 patients remained bilateral-type after surgery despite an increase in LL. The pelvic incidence of these 11 patients was significantly larger than that of the other 7 patients (53.5 +/- 10.2 degrees vs 43.2 +/- 5.8 degrees) (p = 0.037). The magnitude of postoperative increase in LL positively correlated with that of the posterior shift of PM (r = 0.41, p = 0.025). The degree of restoration of the lumbar spine following scoliosis was positively correlated with that of the medial shift of PM (r = 0.66, p = 0.025). Conclusion: The decrease in LL and the lateral deviation of lumbar spine following scoliosis caused a deviated course of the PM, which was spontaneously corrected by the restoration of lumbar alignment. However, the bilateral-type deviation in patients with higher pelvic incidence was considered to be within the range of normal variation. (C) 2019 The Japanese Orthopaedic Association. Published by Elsevier B.V. All rights reserved.