Morphological Analysis of the Intervertebral Foramen and Feasibility of Transforaminal Full-Endoscopic Spine Surgery

Transforaminal full-endoscopic spine surgery (TF-FESS) is a minimally invasive surgical procedure for lumbar spinal disease; however, it is technically demanding in patients with foraminal stenosis because of anatomical problems. This procedure has unique surgery-related complications, such as exiting nerve root injury, causing postoperative dysesthesia or muscle weakness, resulting in a poor clinical outcome. Therefore, we have to avoid this complication. The objective of this study was to analyze the morphology of the intervertebral foramen and assess the feasibility of TF-FESS.We retrospectively reviewed the multiplanar abdominal computed tomography (CT) scans obtained for 800 consecutive patients (400 consecutive males, 400 consecutive females) at our hospitals between January 2016 and December 2018. Patients with lumbar scoliosis (Cobb angle >5 degrees), spondylolisthesis, or vertebral fracture, as well as those with a history of lumbar spinal surgery, were excluded. We evaluated the average values for disc height, position of the superior articular process (SAP), foraminal distance, foraminal area, and SAP facet contact from L1/L2 to L5/S1 on both sides in reconstructed sagittal views. A total of 8,000 intervertebral foramina were analyzed. The correlation between the disc height and these parameters was assessed at each spinal level.At the L1/L2, L2/L3, L3/L4, L4/L5, and L5/S1 levels, the mean disc height was 5.4 mm, 6.4 mm, 7.1 mm, 7.1 mm, and 5.9 mm, respectively. The SAP position, as the distance above the line of the inferior margin of the upper vertebral body, decreased from L1/L2 to L3/L4 and increased from L3/L4 to L5/S1. The respective foraminal distances were the shortest at L4/L5. The foraminal area decreased toward the lower levels. The SAP facet contact was decreasing from L1/L2 to L4/L5. There was a negative correlation between the disc height and the SAP position, and positive correlations of disc height with the foraminal area and the SAP facet contact.The results of the SAP position, the foraminal distance, and the foraminal area suggest that TF-FESS would be technically more difficult at a lower intervertebral disc level because of resection of a larger amount of the SAP. Foraminal stenosis would develop easily when a disc has collapsed. About half of the facet contact could be maintained even if the SAP is resected completely during TF-FESS, especially at a lower lumbar level.