Psoas Muscle Morphology

Study Design. Cross-sectional radioanatomic study Objective. To introduce a classification system using MRI to describe psoas morphology and examine the position of nearby neurovascular structures. Summary of Background Data. Oblique lumbar interbody fusion (OLIF) and lateral lumbar interbody fusion (LLIF) offer sagittal malignment correction and reduced morbidity. LLIF has a higher incidence of nerve injuries, while OLIF has a higher incidence of vascular injuries. Methods. Measurements were completed on the left psoas at the inferior L4 endplate. Class A was designated if the ventral border of the psoas muscle was >2 mm anterior; B if it was ≤ 2 mm anterior or posterior to the vertebral body, and C if >2 mm posterior to the vertebral body ventral border. Modified oblique corridor, measured as the distance between two lines, one at the medial border of the psoas muscle and the other at the lateral border of the nearest vascular structure, and a preferred LLIF trajectory was projected onto an axial image of the left psoas. If the trajectory violated the posterior third of the psoas, it was considered a dangerous approach due to potential iatrogenic nerve injury. Results. 100 patient MRIs (Class A: 44; Class B: 27; Class C: 29) were analyzed. Average modified oblique corridor was 7.49 mm. Modified oblique corridor varied amongst the three types of psoas morphologies (A: 8.99 mm vs. B: 8.10 mm vs. C: 4.66 mm, P =0.040). LLIF trajectory intersected the ‘danger zone’ in 34.1%, 3.7%, and 0.0% of patients, ( P <0.001) respectively. Conclusion. Class A psoas had the largest modified oblique corridor but highest proportion of those with a dangerous LLIF trajectory. Class C psoas had the narrowest modified oblique corridor, but no dangerous LLIF trajectories were identified.