Gender-specific patterns of external occipital protuberance hyperplasia: associations with nuchal ligament ossification and cervical sagittal imbalance in myelopathy patients

Retrospective cross-sectional analysis of 187 consecutive patients undergoing surgical treatment for cervical spondylotic myelopathy (CSM). To investigate sexual dimorphism in external occipital protuberance (EOP) hyperplasia morphology and elucidate its clinical correlation with ossification of the nuchal ligament (ONL) and cervical sagittal imbalance. Emerging evidence implicates EOP hyperplasia as potential biomarker of chronic neck strain, yet its relationship with ONL and cervical sagittal imbalance remains underexplored in surgical CSM cohorts. Cervical radiographs were analyzed. EOP hyperplasia was classified into three subtypes with standardized length measurements. Variables encompassed demographics, ONL-related indices, and sagittal parameters. Subtype comparisons and multivariate regression analyses (with EOP length as dependent variable) were conducted. Analysis of 187 CSM patients (64.2% male) identified gender-specific patterns: males exhibited greater EOP length (9.4±6.8 mm vs. 4.6±3.4 mm, p<0.001). Type III EOP demonstrated male predominance (82.4% vs. type I 31.8%, type II 51.4%; p<0.001), with associated longer hyperplasia length (11.6±6.6 mm vs. type II 5.1±1.9 mm, p<0.001). Type III EOP was associated with higher ONL prevalence (type III 64.8% vs. type I 45.5%, type II 41.9%; p=0.010) and longer ONL osteophyte length (type III 18.8±9.8 mm vs. type I 14.2±8.1 mm, type II 14.2±9.4 mm; p=0.046). Multivariate regression confirmed male sex (β=-3.82, p=0.009), ONL osteophyte length (β=0.16, p=0.017), T1 slope (β=0.27, p=0.041), and spino-cranial angle (β=-0.19, p=0.009) as factors independently associated with EOP length (adjusted R²=0.382). Severe EOP hyperplasia exhibits a male-predominant distribution pattern and demonstrates significant radiological associations with ONL and cervical sagittal imbalance in CSM patients. These findings advocate for EOP evaluation in clinical evaluations to identify high-risk biomechanical profiles.