Hallux pain is a common entity with a differential diagnoses including hallux valgus, hallux limitus/rigidus, and gout and specifically at the interphalangeal joint (IPJ), flexor hallucis longus (FHL) tenosynovitis, and joint arthrosis. An under-recognized source of pain is the os interphalangeus, an ossicle typically located at the plantar aspect of the hallucal interphalangeal joint. This ossicle is radiographically visible in its ossified form in 2–13% of individuals, but can also be present as an ossified or non-ossified nodule in patients. The os interphalangeus may be centrally or eccentrically located, and although originally believed to be a sesamoid bone in the FHL tendon, it is an ossicle located in the joint capsule of the IPJ and separated from the tendon by a bursa. When the ossicle is absent, the bursa is also absent and the tendon is attached to the joint capsule. Infrequently, the os may be located eccentrically under the first IPJ and reflect persistence of one of the distal phalanx. Rarely, the os interphalangeus may be dorsal to the IPJ. The os interphalangeus is best evaluated on radiographs, ultrasound, and MRI. Pain is a result of altered mechanics with arthrosis or frictional effects with bursitis, tenosynovitis, or intractable plantar keratosis (IPK). The ossicle may also displace into a dislocated IPJ, preventing reduction. The os interphalangeus may be centrally or eccentrically located, and although originally believed to be a sesamoid bone. This has been found within the plantar joint capsule of the distal hallucal interphalangeal joint and separated from the tendon by a bursa. Uncommonly, the location may be plantar eccentric and reflect persistence of one of the ossification centers of the distal phalanx. Although the ossicle can be imaged with standard AP and lateral radiographs in many cases, in those cases of unexplained pain with no radiographically visible ossicle, and the presence of friction blisters, intractable plantar keratosis (IPK), hyper-extension of the IPJ, hallux limitus/rigidus, or metatarsophalangeal joint (MTPJ) arthrodesis, an MRI or CT should be considered to identify a non-ossified fibrocartilaginous node. This is of particular concern in a patient with a history of underling diabetes mellitus or other metabolic disorders associated with diminished pedal sensation where neurotrophic changes place them most at risk for complications associated with excessive plantar pressure. Pain is a result of altered biomechanics with arthrosis, or frictional effects causing bursitis, tenosynovitis, or IPK. The ossicle may also displace into a dislocated IPJ, preventing reduction. In this article, we will describe the anatomy and imaging appearance of the common os interphalangeus variants and associated complications including frictional effects, arthrosis, and IPK and discuss conservative and surgical management of a symptomatic ossicle.
