Influence of spinopelvic alignment and morphology on deviation in the course of the psoas major muscle.

BACKGROUND

In some people, the psoas major rises laterally or anteriorly at the L4/5 disc level and detaches from the most posterior aspect of the disc despite the absence of transitional vertebrae; this is called the "rising psoas sign." There are no reports of the relationship between spinopelvic parameters and rising psoas sign. The objective of this study was to investigate the relationship between spinopelvic parameters and deviations in the location and shape of psoas major muscle at the L4/5 disc level.

METHODS

We investigated the preoperative location and shape of both psoas major muscles in 64 patients treated with lateral lumbar interbody fusion. Spinopelvic parameters were measured on X-ray films. The morphology of psoas major at the L4/5 disc level was investigated with magnetic resonance images. The morphological measurements were normalized by the anteroposterior diameter of the center of the L4 vertebral body, which was measured by computed tomography. The rising psoas sign was classified into 2 types: bilateral or unilateral.

RESULTS

The pelvic incidence (PI) was significantly larger for the bilateral type than the others (normal and unilateral types) (60.2 ± 11.0 vs. 46.7 ± 8.7, p < 0.001). The PI correlated significantly with the normalized anteroposterior diameter of the pelvis (R = 0.66, p < 0.001). The receiver-operator characteristic curve showed an optimal cutoff value of PI = 54, with 75% sensitivity and 78.5% specificity. The coronal L1-4 Cobb angle was significantly larger in the unilateral type than the others (normal and bilateral types) (p < 0.0001). In the unilateral type, the Cobb angle in the recumbent position correlated significantly with the normalized distance of the lateral deviation of psoas major (R = 0.60, p = 0.0085).

CONCLUSION

The rising psoas sign was related to a higher PI and lumbar scoliosis. It was firstly elucidated that the spinopelvic alignment and morphology influence the deviation of the course of the psoas major muscle.

STUDY DESIGN

A retrospective morphological study.