Incidence and Fusion of Os Trigonum in a Healthy Pediatric Population

Background: Accessory ossicles off the talus (os trigonum) are a common radiographic finding seen within the foot with variable symptomatology in adult and pediatric patients. Although most studies have examined the incidence of these anatomic variants in a retrospective manner, few have longitudinally followed the behavior of os trigonum over time. The aim of this study was to investigate the incidence, mean age of appearance and fusion, and fusion rate of os trigonum in a large cohort of asymptomatic pediatric patients. Methods: A total of 2620 serial digitized unilateral foot and ankle radiographs from 261 healthy white children (age range, 0.25 to 7 y at enrollment) were retrospectively reviewed. Radiographs were examined to determine the incidence of os trigonum, noting age of appearance and if present, the age of fusion. Skeletal maturity was graded based on ossification pattern of the calcaneal apophysis. Results: Radiographic evidence of os trigonum was present in 18% of patients (n=16 females, n=15 males) with radiographs extending 1 SD beyond the average age of os trigonum appearance. There was no difference in incidence between females and males (P=0.27), and os trigonum was found to appear at a significantly younger age in females (9.1 +/- 1.8 y) than males (10.4 +/- 2.0 y, P=0.04). Overall, 70% (n=26) of subjects with os trigonum achieved fusion by age 17 (mean, 9.5 +/- 1.6 y in females, 11.2 +/- 1.6 in males) with no significant difference between sexes (P=0.61). Grading of skeletal maturity at the time of fusion showed that both male and female subjects underwent fusion on average between calcaneal stages 2 and 3 (P=0.5). Conclusions: This study confirms a relatively large prevalence of os trigonum in a random pediatric population, and clarifies both the average age of onset and high rate of fusion at a relatively narrow range of skeletal maturity.