Introduction:

Lateral Lumbar Interbody Fusion(LLIF) is an established treatment method for lumbar disc pathologies. LLIF requires dissection through the psoas placing the lumbosacral plexus at risk for injury, particularly at the L4–5 level. Despite defining safe surgical corridors, the reported rate of neurologic complications in this surgical approach ranges 8%-30%. Variations in psoas anatomy, notably the recently described ‘rising psoas sign’ have been associated with an increased tendency to develop these complications. No study has been done so far to study psoas morphology in Indian population. The objectives of our study were: i) To analyse the psoas morphology in Indian population and correlate the same with age, sex and BMI ii) To correlate the psoas anatomy with respect to the safe surgical corridors defined in LLIF approach.

Methods:

A retrospective radiographic study was done in 180 patients who underwent lumbar spine MRI in a single calendar year. Exclusion criteria included age < 18 years, scoliosis, infective/traumatic pathology, prior abdominal/ spinal surgery and patients with lumbosacral transitional vertebrae. All MRI scans were independently reviewed by two spine surgeons and the following measurements were made at L1/L2, L2/L3, L3/L4, L4/L5 levels: (a) Disc AP distance, (b) Psoas-thecal sac distance (measured as the shortest distance between dorsal psoas border and ventral border of thecal sac) and (c) asymmetry between left and right psoas muscles based on cross-sectional dimensions. Each disc space was divided into Moro’s zones and the position of the dorsal psoas border was defined with respect to these zones. Correlation between psoas morphology and age/sex/BMI was done using Pearson’s coefficient. The relationship between psoas anatomy and disc AP distance was studied using Student’s t test.

Results: The AP disc distance was greater in males at all levels but no significant difference was found with respect to psoas cross-sectional dimensions. The psoas-thecal sac distance decreased with age on both right and left sides at the L4/L5 level. Significant psoas asymmetry was found at L1/L2 level with right psoas smaller than the left. Of the 180 patients, in 4 patients at L1/L2, 3 patients at L3/L4 and 26 patients at L4/L5, the dorsal border of psoas was found in Zone-IV whereas in only 4 patients it was found in Zone-III at L4/L5 level. In the rest, psoas was found in Zone-P.

Discussion: LLIF is a safe surgical technique to treat spinal pathologies at L1-L4 levels. Although, the relationship of psoas to the lumbar spine at L4/5 is variable, we find that the variation is less in Indian patients and its anatomy is more amenable to LLIF. When planning a lateral approach, particularly at L4/L5, a careful preoperative analysis of psoas and its relationship to vertebral body is important and in the event of a ‘rising psoas’ sign, an alternative approach should be considered.