Is the Hallux Interphalangeal Ossicle Clinically Relevant? A Cross-Sectional Study on Its Prevalence and Biomechanical Implications

Background: The hallux interphalangeal ossicle (HIO) is commonly considered as an incidental anatomical variant; however, its biomechanical role remains poorly understood. This study aimed to investigate the influence of HIO on hallux joint biomechanics. Methods: A cross-sectional correlational study was conducted, including 419 feet (218 individuals). The presence of HIO was assessed using ultrasound imaging. Range of motion (ROM) of the metatarsophalangeal joint (MTPJ) and interphalangeal joint (IPJ) were evaluated under both open kinetic chain and dynamic conditions. Statistical comparisons between HIO and non-HIO groups were made, and receiver operating characteristic (ROC) curve analysis was used to assess the discriminative capacity of IPJ ROM. Results: HIO was present in 48% of cases and was bilateral in all participants. Individuals with HIO exhibited significantly greater IPJ extension under both open kinetic chain and dynamic conditions (p < 0.05). No significant differences were observed in MTPJ ROM between groups. A positive, albeit variable, relationship was found between ossicle size and IPJ extension. ROC analysis demonstrated moderate discriminative ability of IPJ ROM for detecting HIO (sensitivity 63.2%, specificity 54.6%). Conclusions: The presence of HIO is associated with increased IPJ extension, suggesting a measurable influence on hallux biomechanics. These findings support the notion that the HIO is a biomechanically relevant structure rather than purely incidental an asymptomatic anatomical variant. Increased IPJ extension may represent an early functional adaptation with potential clinical implications.