Hamate and pisiform coalition: a case report and introduction to the carpal C-sign on lateral radiograph.

Hamate-pisiform coalition is an exceptional form of carpal coalition. Case reports are essential to gain a better understanding of this variant. We report a case of congenital bilateral hamate-pisiform coalition in a 20-year-old male discovered in the context of a right wrist trauma. Radiographs also revealed a bilateral scapholunate diastasis. Clinical examination and radiological findings suggested that the right wrist scapholunate diastasis was related to scapholunate instability. Left wrist scapholunate diastasis could be related to (1) a pathological feature or (2) a normal variant associated with hamate-pisiform coalition. Lateral radiographs showed a volar C-shaped osseous bridge corresponding to the coalition. We associated it with a new sign: the "carpal C-sign". Computed tomography with three-dimensional reconstruction provides helpful information about the type of coalition (osseous versus non-osseous) and excludes potential fracture. We discuss the specific embryologic features of the hamate-pisiform coalition, as well as its prevalence, radiographic classification, clinical significance, and treatment.