Radiographic Changes in Retroperitoneal Corridor Dimensions During Staged Positioning for Prone Transpsoas Lateral Lumbar Interbody Fusion

Background Questions about the surgical corridor through the psoas muscle and the retroperitoneal space have persisted since the introduction of lateral lumbar interbody fusion (LLIF) in the lateral decubitus position. The prone transpsoas (PTP) approach maintains or restores lordosis and allows single-position access to the anterior and posterior columns, but the position of visceral structures during an actual PTP case has not been characterized. Methods Retrospective single-arm observational cohort of 51 consecutive adults undergoing LLIF-PTP by four fellowship-trained surgeons at three sites. The anterior-posterior (AP) distance from the anterior border of the quadratus lumborum (QL) to the posterior margin of the nearest non-fat visceral or retroperitoneal structure was measured at three stages: preoperative supine MRI (Stage 1), intraoperative prone CT after positioning but before incision (Stage 2), and intraoperative CT after retroperitoneal development and interbody placement (Stage 3). Multilevel cases were measured at the most caudal level. General linear modeling assessed change across stages, with exploratory subgroup analyses. Results Mean age was 62.8 years, 22 patients (43%) were male, mean BMI was 29.8 kg/m², and 45 cases (88%) were single-level. AP distance increased across stages (32.1 vs 37.6 vs 42.9 mm, p<0.001), a mean gain of 10.8 mm. Across stages, distance differed by sex (p=0.026); BMI category did not (p=0.053), nor did number of incisions (p=0.404). No approach-related complications occurred. Conclusions Prone positioning and staged retroperitoneal corridor development were associated with a stepwise increase in QL-to-viscera distance. The single-arm design and sample size preclude comparative or safety conclusions.