Radiographic factors associated with ossification of the nuchal ligament in degenerative cervical myelopathy patients

Background: Ossification of the nuchal ligament (ONL) is a radiographic finding frequently observed in patients with degenerative cervical myelopathy (DCM). Although its etiology remains unclear, posterior cervical anatomical factors, including paraspinal muscle degeneration and spinous process length parameters, may alter the local mechanical environment of the nuchal ligament and contribute to ONL formation. This study aimed to evaluate whether cervical paraspinal muscle fatty infiltration (FI) and C6 spinous process length are associated with ONL presence, exploratory early-stage ONL, and ONL length in patients with DCM. Methods: This single-center retrospective cohort study included 152 patients who underwent surgery for DCM between 2022 and 2024. Patients were classified into ONL and non-ONL groups using sagittal computed tomography images. ONL length was analyzed as a continuous radiographic variable. Because no validated ONL grading system is currently available, the ONL-positive cohort was additionally divided by median ONL length into shorter- and longer-length subgroups; the shorter-length subgroup was considered an exploratory morphological surrogate for early-stage ONL rather than a validated clinical stage. Paraspinal muscle FI was assessed using the Goutallier grading system on T2-weighted magnetic resonance imaging at C4–C5, C5–C6, and C6–C7 levels. Spinous process length parameters included absolute C5, C6, and C7 spinous process lengths and C5:C6 and C6:C7 ratios. Multivariable regression, variance inflation factor analysis, and receiver operating characteristic (ROC) analysis were performed. Results: Among 152 patients, 96 had ONL and 56 did not. Male sex and smoking status were more common in the ONL group. The ONL group showed higher C4–C5 and C5–C6 multifidus FI grades, greater absolute C5, C6, and C7 spinous process lengths, and a higher C6:C7 ratio. Multivariable logistic regression showed that the C6:C7 ratio was independently associated with ONL presence ( p = 0.012, OR = 1.051, 95% CI = 1.011–1.092). In the exploratory early-stage ONL model, C4–C5 multifidus FI and the C6:C7 ratio were associated with shorter-length ONL. ROC analyses showed modest discriminatory performance, with area under the curve values ranging from 0.658 to 0.682. Exploratory continuous ONL length analysis showed associations with age, C4–C5 multifidus FI, and the C6:C7 ratio in the adjusted model. Conclusions: Relative C6 spinous process length was associated with ONL presence, while C4–C5 multifidus FI and relative C6 spinous process length were associated with exploratory early-stage ONL in patients with DCM. These parameters may serve as auxiliary radiographic markers and provide a hypothesis-generating framework for understanding the anatomical, biomechanical, and mechanobiological background of ONL formation.