Reducing the Technical Difficulty of Transforaminal Endoscopic Lumbar Discectomy for Upward-Migrated Lumbar Disc Herniation Involving the High Axillar Space: Key Surgical Steps and Proposed Coronal MRI-Based Modified Migration Grading

Background/Objectives: Upward-migrated lumbar disc herniation (LDH) presents technical challenges during the transforaminal endoscopic approach, particularly when fragments migrate cranially and medially beyond the usual working corridor. This study was based on the hypothesis that surgical difficulty is influenced by both vertical migration height and medial location. We aimed to describe key surgical steps for accessing these difficult lesions and to propose a coronal MRI-based modification of existing migration grading to support preoperative localization and surgical planning. Methods: This retrospective observational cohort study included 66 patients who underwent transforaminal endoscopic lumbar discectomy for upward-migrated LDH. Sagittal MRI-based migration grading was modified on coronal MRI using two reference lines: the inferior pedicle line for vertical grades 1–3 and the inner pedicle line for horizontal grades L and M. Cases with grade 2 or 3 plus grade M were defined as involving the high axillar space (HAS). Patients were divided into non-HAS and HAS groups. Key surgical steps included superior articular process flattening, detachment of the superior extraforaminal ligament, and cranial rotation with oblique tilting of the endoscope. Results: Thirty-five cases (53.0%) involved the HAS. Grade 2 was the most common vertical grade (65.2%), and grade M accounted for 56.1% of cases. Despite more demanding anatomical characteristics, the HAS group achieved early clinical outcomes comparable to those of the non-HAS group, with excellent or good outcomes in 85.7% and 93.6% of patients. Conclusions: Standardized key surgical steps, including SAP flattening, detachment of the superior extraforaminal ligament, and cranial rotation with oblique tilting of the endoscope, may facilitate access to upward-migrated LDH involving the HAS while maintaining favorable early clinical outcomes. The proposed coronal MRI-based modified migration grading may help assess the vertical and medial extent of upward-migrated LDH and support preoperative planning for these technically demanding cases.