Os trigonum (OT) is one of the most common accessory ossicles in the foot and ankle region. It is believed that OT was first described by Rosenmuller in 1804, but von Bardeleben is the one who coined the nomenclature for this anatomical variation in 1883. The literature indicates that OT is extremely variable in prevalence, with reported rates ranging from 1.7% to 32.5%. In clinical practice, OT is a considerable factor that may predispose patients to posterior ankle impingement syndrome (PAIS). The aim of this study was to provide comprehensive data on the prevalence and anatomical characteristics of OT. We conducted a comprehensive search of the main electronic databases to find the relevant research. Information concerning geographical origin, prevalence, gender distribution, imaging modality, and morphometry of OT was extracted. The PRISMA guidelines were strictly followed throughout the study. The reliability of the included studies was assessed using the AQUA tool. A total of 41 studies (n = 36,612 feet) were included in this meta-analysis. This meta-analysis showed that OT was present in 9.0% (95% CI: 7.4–10.8) of the feet, and was present bilaterally in 32.7% (95% CI: 23.3–43.7) of cases. Regarding the types of imaging modality used in the evaluation of OT by the included studies, prevalence assessed by X-rays was 8.2% (95% CI: 6.8–9.9), MRI was 24.2% (95% CI: 14.6–37.3), CT was 21.0% (95% CI: 12.8–32.5), and cadaveric dissection was 5.0% (95% CI: 3.4–7.4). OT was most commonly found in East Asia (11.0%, 95% CI: 7.1–16.8). Two studies provided enough data to calculate the odds ratio linking PAIS with OT. The analysis showed that patients with PAIS are almost 16 times more likely to have OT compared to those without PAIS (OR = 15.98, 95% CI = 0.255–1002.8). OT is a very common accessory ossicle and is present in nearly one in 10 feet. The highest prevalence is noted in the East Asian population. MRI and CT show the highest prevalence of OT. It is important to keep in mind the possible presence of OT when a patient is diagnosed with PAIS.
