A Deforming Fibrous Band From an Os Intermetatarseum as a Rare Cause of Rigid Adolescent Hallux Valgus: A Case Report.

Hallux valgus (HV) is a common forefoot deformity, and an associated os intermetatarseum is often an incidental and asymptomatic radiographic finding. The role of an os intermetatarseum as a direct etiological factor in HV, particularly when mediated by an anomalous structural tether causing deformity, is unusual. We describe the case of a 16-year-old female who presented with several years of progressive bilateral foot deformity and pain. Clinical examination revealed a rigid moderate HV deformity on the right and a flexible mild HV on the left. Plain radiographs confirmed bilateral HV and an os intermetatarseum fused to the base of each second metatarsal. Advanced imaging with CT and MRI of the right foot further identified an anomalous fibrous band extending from the os intermetatarseum to the fibular sesamoid and lateral base of the proximal phalanx of the hallux. The patient underwent surgical correction, including the excision of the os intermetatarseum and associated anomalous fibrous band, a first metatarsal Chevron osteotomy, and a proximal phalanx Akin osteotomy. Intraoperatively, this band was confirmed to exert an adduction force on the hallux. At the 12-week postoperative follow-up, the patient reported significant pain relief. Radiographs showed stable alignment with healed osteotomies, and she was cleared for a gradual return to sports.  An os intermetatarseum, when associated with an anomalous fibrous band, can act as a direct cause of HV deformity that must be addressed separately if surgical intervention is indicated. Clinicians should consider this rare anatomical variant in the diagnostic workup of unusual or rigid HV presentations, particularly in younger patients or those with a history of other congenital foot anomalies. CT and MRI can be instrumental in identifying such structures preoperatively, allowing for targeted surgical intervention to address all contributing factors for an optimal outcome.