A New Method for Lumbosacral Transitional Vertebra and Anatomic Orientation of Sacrum that could be applied as a Peroperative Indicator for Lumbosacral Surgery:Anterior Transitional Arch (ATA1) - Anterior Transitional Angle (ATA2)

Purpose: Sacral slope (SS), Lumbar lordosis (LL), Pelvic tilt (PT), Pelvic incidence (PI), Sagittal vertical axis (SVA) measurements regarding anatomical structure and orientation of the lumbosacral region are very important for surgical planning and postoperative follow-up of patients. The lumbosacral transitional vertebra, which is determinant in the formation of these measurements, is effective the sacral vertebra shape and can show various anatomical variations, referred to as "sacralized L5" or "lumbarized S1". Methods: These variations can significantly alter the sacral surface longitudinally concave on the ventral side and convexity on the dorsal side, which are observed under normal conditions. Therefore, during surgical planning in the spine, the sacrum should be determined radiologically and the vertebrae should be counted correctly. The aim of this study is to demonstrate a practical use for determining the orientation of the sacrum for sagittal balance calculation, and to recognize the sacralized L5 or lumbarized S1 segment as LSTV to avoid incorrect level surgery. Results: Metrologically, a new index was created using the "Anterior Translational Arch (ATA1)" distance and the "Anterior Translational Angle (ATA2)" angle. 527 cases (83.3%) with anatomically normal sacrum, 35 cases (5.5%) with lumbalization and 71 cases (11.2%) with sacralization were evaluated using this index. Conclusion: As a conclusion, the index calculated in terms of “Anterior Translational Arch (ATA1)” distance and “Anterior Translational Angle (ATA2)” provides a simple measurement to determine the transitional vertebra.