Hamate-pisiform coalition: morphology, clinical significance, and a simplified classification scheme for carpal coalition.

Hamate-pisiform coalition is characterized by the abnormal union of the pisiform bone and hamulus of the hamate. Because most reported cases are isolated, and literature on the subject is sparse, relatively little is known about this condition and its clinical significance. The purpose of this report is to discuss the occurrence, morphology, and frequency of hamate-pisiform coalition identified in a skeletal sample of native South Africans, and to conduct a metaanalysis of all known cases in order to clarify the sex distribution, laterality, form, and clinical significance of this condition. Five new cases (three male, two female) of hamate-pisiform coalition were identified in 527 native South Africans. Results indicate that hamate-pisiform coalition is infrequent (0.76%) but may be more likely encountered in individuals of African ancestry. Morphologically, non-osseous examples ranged in appearance from minor expressions involving pitting of an expanded hamulus base, to a variably pitted articulation between an elongated pisiform and hamulus. Osseous union between the two bones tends to extend beyond the hamulus base to adjacent areas of the hamate. Cases involving osseous union appear predisposed to fracture while ulnar neuropathy is significantly more frequent in individuals exhibiting non-osseous coalition. As both non-osseous and osseous cases can have clinical significance, awareness of the variable manifestations of this condition is necessary for hand specialists. A simplified classification system is suggested to more consistently characterize carpal coalitions.