How psoas morphology differs between a supine and a sitting MRI of the lumbar spine and its implications for lateral lumbar interbody fusion

Abstract

BACKGROUND: The psoas major is an important muscle that is part of the iliopsoas complex, which is also known as the hip flexor and contains a major web of nerves called the lumbar plexus. The location of the lumbar plexus within the psoas muscle has been studied on cadaveric dissections previously, particularly with respect to the location of the L4 nerve root but the effect of posture on psoas morphology has not previously been studied. Hip flexion along with the potential changes in spinal alignment while in an upright sitting position may cause significant changes in the positioning and geometry of the psoas and may also change the orientation of the lumbar plexus within the muscle. Current controversy exists in determining patient suitability for Lateral Lumbar Interbody Fusion (LLIF) based on psoas morphology. Oblique and trans-psoas approaches have become a popular minimally invasive lumbar fusion technique in recent years. Lumbar plexus injury, particularly L4 nerve root injury, is a known potential complication of the oblique and trans-psoas approach and may be minimized by careful assessment of the psoas anatomy preoperatively. Quadriceps weakness as a result of L4 nerve root injury is a known potential complication of the trans-psoas approach and may be minimized by careful assessment of the psoas anatomy preoperatively. Patients may present with a sitting MRI rather than supine MRI, however, the effect of posture on the geometry of the psoas muscle, and therefore of the lumbar plexus, has not been previously reported. METHODS: We conducted a retrospective review of a single-spine surgeon practice over a 6-month period to identify patients who had undergone MRI of the lumbar spine for evaluation of degenerative spinal pathologies. Male and female patients were included if aged between 18-90 years presenting with degenerative lumbar spinal pathology between 2015-2016, and excluded if they had previous lumbar fusion, scoliosis, diagnosed with neuromuscular disease, were skeletally immature or had intrinsic abnormalities of the psoas muscles (e.g. tumor, infection or trauma). The anteroposterior (AP) dimension of the psoas muscle was measured at each disc space from L1 to L5 and compared to the AP dimension of the intervertebral disc, as measured at the inferior vertebral endplate. The AP psoas:disc ratio was then calculated and compared between patients undergoing sitting and/or supine MRIs. RESULTS: With a total of 269 patients, 113 of them were male and 157 were female. 209 patients were identified with supine-, and 60 patients with sitting- MRIs, of which 13 patients had undergone both sitting and supine MRIs (BOTH group). A propensity score match (PSM) was performed for patients undergoing either a supine or sitting MRI to match for age, BMI and gender to produce two groups of 43 patients. In the BOTH and PSM group, the sitting MRIs displayed significantly higher AP psoas:disc ratio compared with the supine MRIs at all intervertebral levels except L1-L2. The largest difference observed was a mean 32-37% increase in sitting AP psoas:disc ratio at the L4-L5 disc in sitting MRIs compared to supine MRIs in the BOTH group (range 0-137%). CONCLUSIONS: The psoas muscle and the lumbar plexus became anteriorly displace in sitting MRIs, with a greater effect noted at caudal intervertebral discs. This may have implication in selection suitability for LLIF and intra-operative patient positioning.