598 - Sitting and Supine Posture Changes Psoas Muscle Geometry on MRI – Impl...

General Session: MIS-1

Presented by: A.J. Buckland - View Audio/Video Presentation (Members Only)


A.J. Buckland(1), E. Isaacs(1), J.Y. Moon(1), J.M. Spivak(1)

(1) Hospital for Joint Diseases at NYU Langone Medical Center, Department of Orthopaedic Surgery, New York, NY, United States


Introduction: Oblique and trans-psoas approaches have become popular as minimally invasive lumbar fusion techniques in recent years. Lumbar plexus injury, particularly L4 nerve root injury, is a potential complication of the oblique and 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 geometry of the psoas (and therefore of the lumbar plexus) has not been previously reported.

Methods: Retrospective review of a single-spine surgeon practice over a 6-month period was performed to identify patients whom had undergone MRI of the lumbar spine for evaluation of degenerative spinal pathologies. Patients were included if aged between 18-90 years, and excluded if they had previous lumbar fusion, scoliosis, neuromuscular disease, were skeletally immature or had intrinsic abnormalities of the psoas muscle (eg. 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: 208 patients were identified with supine MRIs, and 61 patients with sitting MRI, of which 13 patients had undergone both sitting and standing MRIs (BOTH groups). For patients who underwent either standing or sitting MRI, a propensity score match (PSM) was performed to match for age, BMI, and gender to produce two groups of 43 patients. In the BOTH and PSM group, sitting MRI displayed significantly higher AP psoas:disc ratio compared with supine MRI at all intervertebral levels except L1-2. (table 1 & figure 1). A significant increase in AP psoas:disc ratio was also noted at all levels except L1-2 in patients undergoing both supine and sitting MRI. (table 1 & figure 1). The largest difference in AP psoas:disc ratio was noted at the L4-5 disc, with a 20-37% increase in sitting compared to standing.

Summary: Sitting MRI results in an increase in the anterior-posterior (AP) dimensions of the psoas muscle relative to the intervertebral disc. AP dimensional increase becomes greater at more caudal disc spaces. At the L4-5 disc, the AP psoas dimension increases by 20-37% in sitting compared with supine. These findings have implications for interpreting lumbar MRIs for surgical planning, and potentially for patient positioning in oblique and lateral approaches.

Table 1

Figure 1