Anatomic Variation with Supine to Prone Positioning: Implications for Prone Transpsoas Single-Position Lumbar Fusion

The prone transpsoas (PTP) approach is a novel, single-position lumbar fusion technique that enables lateral lumbar interbody fusion (LLIF) entirely in the prone position, allowing simultaneous access to both the anterior and posterior spinal columns. While it offers advantages such as achieving circumferential fusion without repositioning the patient, it also presents challenges, including hemodynamic shifts, pressure-related complications, and technical difficulties in navigating complex anatomical structures. This study describes anatomical shifts relative to the lumbar spine when transitioning from the supine to the prone position.

This retrospective review included patients who underwent posterior lumbar fusion between 2018 and 2024 and had both preoperative magnetic resonance imaging (MRI) and intraoperative prone computed tomography-guided imaging. Patients with deformity, infection, trauma, prior fusion, or malignancy were excluded. Anteroposterior (AP) and mediolateral (ML) distances (in mm) were measured on axial slices using reference lines aligned to vertebral endplates at each lumbar level. Measurements included AP and ML distances to the abdominal great vessels, psoas major, and intervertebral discs. Dependent samples t-tests and analysis of variance were used to assess anatomical shifts from supine to prone and to compare segmental differences.

Among the 74 patients (47% female), the mean age was 62.7±12.2 years, and the mean body mass index was 29.8±5.8 kg/m². Significant AP translation was observed at L2-L3 and L3-L4 for the inferior vena cava (p<0.001) and aorta (p<0.01), and at L4-L5 for the common iliac arteries (p<0.001) and right iliac vein (p<0.05). Symmetric AP excursion of the psoas major muscle was noted at L2-L3 and L4-L5 in the prone position (p<0.05). No significant differences in mean translation were found across lumbar levels.

Prone positioning induces measurable anterior translation of both the psoas major muscle and great vessels, potentially altering the operative corridor utilized in the PTP approach. These discrepancies between supine MRI and prone intraoperative anatomy emphasize the need to account for positional anatomical changes to minimize neurovascular risk during PTP LLIF.