Disc-level predictors of indirect decompression of posterior cord compression by the ligamentum flavum following anterior cervical discectomy and fusion

Purpose: Anterior cervical discectomy and fusion (ACDF) is widely performed to treat cervical myelopathy. However, its effectiveness in decompressing posterior elements, such as a hypertrophied ligamentum flavum, remains unclear. This study aimed to identify disc-level predictors of radiographic improvement in posterior cord compression caused by the ligamentum flavum (CCLF) after ACDF. Methods: We retrospectively analyzed 84 cervical disc levels in 51 patients who underwent ACDF or hybrid surgery (corpectomy combined with discectomy) for degenerative cervical disease with preoperative CCLF on MRI. Postoperative changes in CCLF were evaluated, and disc levels were categorized into improved and unimproved groups. Various radiographic and anatomical parameters—including preoperative disc angle, segmental lordosis, disc height, preoperative disc range of motion (ROM), cage-to-disc ratio, Hounsfield unit (HU), and facet joint degeneration—were compared. Multivariate logistic regression identified independent predictors of CCLF improvement failure. Results: Postoperative improvement in CCLF was observed in 67 of 84 disc levels (80%). The unimproved group had significantly greater preoperative disc angles (5.1°vs. 2.1°, p = 0.001) and higher HU values (365 vs. 319, p = 0.002). Multivariate analysis showed that a greater preoperative disc angle (odds ratio [OR]: 1.28, 95% confidence interval [CI]: 1.05–1.56, p = 0.015) and higher HU values (OR: 1.009, 95% CI: 1.00–1.01, p = 0.040) were independently associated with CCLF improvement failure. Conclusion: In segments with greater disc angles or higher bone density, indirect decompression via ACDF may be less effective. These findings highlight the importance of preoperative disc-level assessment in surgical planning.