Differential Diagnosis
- Single most likely diagnosis
- Ulnar collateral ligament (UCL) avulsion fracture: This is the most likely diagnosis given the location of the avulsed fragment at the base of the proximal phalanx on the ulna aspect, which is a common site for UCL injuries, particularly in the context of a fall onto an outstretched hand or a sudden, forceful contraction of the adductor pollicis muscle.
- Other Likely diagnoses
- Volar plate avulsion fracture: Although less common than UCL injuries, avulsion fractures of the volar plate can occur at the base of the proximal phalanx and might present similarly, especially if the fragment is small and the primary complaint is pain and instability at the first metacarpophalangeal joint.
- Sesamoid bone fracture: The first metacarpophalangeal joint contains two sesamoid bones embedded within the tendons of the flexor pollicis brevis muscle. A fracture of one of these sesamoid bones could potentially present with a small avulsed fragment, though the location and clinical context would help differentiate this from a UCL avulsion.
- Do Not Miss diagnoses
- Osteochondritis dissecans: Although rare in this specific location, osteochondritis dissecans could present with a fragment of bone separated from the parent bone due to a loss of blood supply, leading to necrosis of the subchondral bone. Missing this diagnosis could lead to significant morbidity due to potential for progression to osteoarthritis if not properly addressed.
- Infection (osteomyelitis or septic arthritis): Infection could lead to bone destruction and fragmentation, and while less likely given the acute presentation of an avulsed fragment, missing an infectious process could have severe consequences, including destruction of the joint and significant morbidity.
- Rare diagnoses
- Tumor (e.g., osteochondroma, chondroma): Certain bone tumors could present with a fragment of bone that appears avulsed, though this would be uncommon and typically associated with other symptoms such as a palpable mass, chronic pain, or systemic symptoms.
- Bone cyst (e.g., unicameral bone cyst): A pathologic fracture through a bone cyst could potentially present with an avulsed fragment, though this is rare and usually associated with other radiographic findings indicative of the cyst.