A morphological adaptation? The prevalence of enlarged external occipital protuberance in young adults

Enthesophytes are bony projections that arise from the sites of ligament, tendon or joint capsule attachment to a bone. They are seen rarely in radiographic findings in young adults, as these bony adaptations are assumed to develop slowly over time. However, in recent years, the presence of an enlarged external occipital protuberance (EEOP) has been observed frequently in radiographs of relatively young patients at the clinic of the lead author. Accordingly, the aim of this project was to assess the prevalence of an EEOP in a young adult population. Analysis involved a retrospective analysis of 218 lateral cervical radiographic studies of 18-30-year-old participants. Group A (n = 108; males = 45, females = 63) consisted of asymptomatic university students, while Group B (n = 110; males = 50, females = 60) were an age-matched mildly symptomatic, non-student population. The external occipital protuberance (EOP) size was defined as the distance from the most superior point of the EOP (origin) to a point on the EOP that is most distal from the skull. To avoid ambiguity, the threshold for recording the size of an EOP was set at 5 mm, and an EOP was classified as enlarged if it exceeded 10 mm. Reliability testing was also undertaken. Results indicated that an EEOP was present in 41% of the total population, with 10% of all participants presenting with an EOP ≥ 20 mm. An EEOP was significantly more common in males (67.4%) than in females (20.3%), with the mean EEOP size for the combined male population (15 ± 7 mm) being significantly larger (P < 0.001) than for females (10 ± 4 mm). The longest EEOP in the male population was 35.7 mm, while in the female population it was 25.5 mm. Additionally, the mean EEOP size for Group A (14 ± 7 mm) was also significantly greater (P = 0.006) than that recorded for Group B (12 ± 6 mm). This study identified that an EEOP is a condition that is prevalent in the populations tested. The age of the populations, and the prevalence of EEOP, suggest that biomechanical drivers for this phenomenon may be the main reason for this condition in these populations.