External occipital protuberance revisited: morphology and classification of its anatomical variants

The external occipital protuberance (EOP) is a fundamental craniometric landmark; however, its exophytic anatomical variants (EOPVs) remain poorly defined due to a historical reliance on 2D lateral cephalometry and inconsistent nomenclature. Data on their prevalence and variability remain scarce in the pediatric population, and the literature is inconsistent. This study addresses the lack of high-resolution data on EOPV prevalence and morphology in the pediatric population.

Using thin-slice head CT scans (0.3 mm slice thickness) of 345 patients aged 0-18 years, we conducted a comprehensive morphometric analysis to validate existing classifications and introduce a more robust discriminatory framework.

Our findings demonstrate that the EOP is a compound structure comprising an anatomically stable base, at the level of the superior nuchal lines, and an exophytic variant situated at or superior to this level. We propose a novel, observer-independent EOPV definition using the data-driven EOPV angle criterion ( < 133°). We further introduce a two-tier classification system: type I (tubercle), characterized by gradual margins, and type II (wedge/V-shaped), characterized by a markedly pronounced projection and associated with external lamina delamination. The prevalence of EOPVs in the adolescent cohort (12-18 years) reached 40.3%, approximating adult estimates.

By establishing a coherent, observer-independent framework, this classification facilitates precise clinical diagnosis - relevant to cranial pain disorders and surgical planning - while providing a standardized basis for future longitudinal research.