Morphometric analysis of safe working zones for minimally invasive lateral extrapleural/retroperitoneal approach via transdiaphragmatic and transpsoas to the thoracolumbar and lumbar spine.

backgroundThe thoracolumbar and lumbar spine is mainly involved in T11 to L5, which is the transition from the relatively fixed thoracic vertebrae to the more mobile lumbar vertebrae, where the main stress of the trunk is concentrated.This study aims to perform a detailed morphometric analysis of the thoracolumbar and lumbar region using high-resolution CT scans to define safe working zones for the transdiaphragmatic and transpsoas approaches.

methodsPatients who underwent CT examination from February 2024 through September 2024 were from our database. Measurements were performed from T10-T11,T11-T12,T12-L1,L1-L2,L2-L3,L3-L4 and L4-L5 disc levels and were determined using the PACS software computer digitizer and SYNGO System. Using this data, one surgeon carried out measurements of the lower vertebral endplate at each level (sagittal and transversal), position of muscle attachment area, position of the nerve roots, position of the retroperitoneal vessels.The safe zone is defined as the region between retroperitoneal vessels (including arteries and veins) and nerve roots. Its superior boundary is demarcated by the horizontal plane of the most inferior vessel border, while the inferior boundary corresponds to the horizontal plane of the most superior nerve root border.

resultsIn all subjects, the transverse diameters demonstrated a progressive increase across the T10–T11 to L4–L5 spinal segments. Sagittal diameters exhibited a similar ascending trend from T10–T11 to L2–L3, followed by a gradual reduction from L2–L3 to L4–L5. Notably, the smallest sagittal diameter was recorded at T10–T11, while the maximum value peaked at L2–L3.The projection of nerve roots is identical on both sides. The nerve-vertebral overlap height progressively decreased from T10/T11 to L1/L2, then progressively increased from L1/L2 to L4/L5, with the maximal overlap observed at L4/L5 and the minimal at L1/L2.The vascular-vertebral overlap height progressively decreased from T10/T11 to L1/L2, then progressively increased from L1/L2 to L4/L5, with the maximal overlap observed at T10/T11 and the minimal at L1/L2.No diaphragmatic attachments were observed bilaterally at the T10-T11 level. Partial bilateral diaphragmatic attachments with craniocaudal elevation were identified at T11/T12 in 28.3% of specimens(17/60), while the majority exhibited diaphragmatic attachments at T12/L1 in 98.3% of specimens(59/60).A progressive increase in the safety zone ratio was observed sequentially across the T10–T11, T11–T12, T12–L1, and L1–L2 spinal levels, reaching its maximum at L1–L2. Conversely, a gradual decrease occurred from L1–L2 to L2–L3, L3–L4, and L4–L5, with the L1–L2 level demonstrating the highest safety zone ratio.The safe working zone was 51.8% of the lower endplate of the vertebral body sagittal diameter at T10-T11, 63.9% at T11-T12, 73.5% at T12-L1,79.6% at L1–L2, 67.6% at L2–L3, 60.7% at L3–L4 and 48.6% at L4–L5 levels.

conclusionThree-dimensional CT reconstruction enables clear visualization of vascular structures, vertebral bodies, muscle attachment sites, nerve roots, and their spatial relationships. This modality further allows precise preoperative delineation of discectomy and corpectomy boundaries via transdiaphragmatic and psoas muscle approaches, while quantitatively assessing safety working zones for surgical planning.