Anatomical differences in patients with lumbosacral transitional vertebrae and implications for minimally invasive spine surgery

OBJECTIVE The objective of this study was to investigate the neurovascular and anatomical differences in patients with lumbosacral transitional vertebrae (LSTV) and the associated risk of neurovascular injury in minimally invasive spine surgery. METHODS The authors performed a retrospective study of CT and MR images of the lumbar spine obtained at their institution between 2010 and 2014. The following characteristics were evaluated: level of the iliac crest in relation to the L4-5 disc space, union level of the iliac veins and arteries in relation to the L4-5 disc space, distribution of the iliac veins and inferior vena cava according to the different Moro zones (A, I, II, III, IV, P) at the L4-5 disc space, and the location of the psoas muscle at the L4-5 disc space. The findings were compared with findings on images obtained in 28 age- and sex-matched patients without LSTV who underwent imaging studies during the same time period. RESULTS Twenty-eight patients (12 male, 16 female) with LSTV and the required imaging studies were identified; 28 age- and sex-matched patients who had undergone CT and MRI studies of the thoracic and lumbar spine imaging but did not have LSTV were selected for comparison (control group). The mean ages of the patients in the LSTV group and the control group were 52 and 49 years, respectively. The iliac crest was located at a mean distance of 12 mm above the L4-5 disc space in the LSTV group and 4 mm below the L4-5 disc space in the controls. The iliac vein union was located at a mean distance of 8 mm above the L4-5 disc space in the LSTV group and 2.7 mm below the L4-5 disc space in the controls. The iliac artery bifurcation was located at a mean distance of 23 mm above the L4-5 disc space in the LSTV group and 11 mm below the L4-5 disc space in controls. In patients with LSTV, the distribution of iliac vein locations was as follows: Zone A, 7.1%; Zone I only, 78.6%; Zone I encroaching into Zone II, 7.1%; and Zone II only, 7.1%. In the control group, the distribution was as follows: Zone A only, 17.9%; Zone A encroaching into Zone I, 75%; and Zone I only, 7.1%. There were no iliac vessels in Zone II in the control group. The psoas muscle was found to be rising away laterally and anteriorly from the vertebral body more often in patients with LSTV, resulting in the iliac veins being found in the "safe zone" only 14% of the time, greatly increasing the risk of vascular injury. CONCLUSIONS In patients with LSTV, the iliac crest is more likely to be above the L4-5 disc space, which increases the technical challenges of a lateral approach. The location of the psoas muscle rising away laterally and ventrally in patients with LSTV compared with controls and with the union of the iliac veins occurring more often above the L4-5 disc space increases the risk for iatrogenic vascular injury at the L4-5 level in this patient population.