Fig. 1

Wrist anatomy and hypotheses of human pisiform ossification. a Carpal configuration of the human wrist (palmar view). b–d Ulnar view showing pisiform shape, projection, and articulations in human (b), chimpanzee (c), and macaque (d). Palmar is up, dorsal is down. The pisiform articulates only with the triquetral in humans and chimpanzees while it articulates with both the triquetral and ulnar styloid process in macaques (dashed line shows ulna articular surface of the pisiform). The human pisiform is pea-shaped with minimal projection beyond the hamate, while both chimpanzees and macaques have a rod-shaped pisiform with palmar projection beyond the hamate. Abbreviations: metacarpals (numbered I–V), capitate (C), hamate (H), hamate hamulus (H*), lunate (L), pisiform (P), radius (R), scaphoid (S), trapezoid (Td), trapezium (Tm), triquetral (Tq), ulna (U), ulnar styloid process (U*). Scale bars = 1 cm. e Hypotheses for possible mechanisms underlying human pisiform reduction. Cartilage is gray and bone is black. The ancestral condition represents a primary ossification center with one secondary ossification center on the palmar side of the bone with a growth plate between. Four possible mechanisms for human pisiform ossification are: (i) early fusion of two ossification centers; however, regular development of two ossification centers has not been documented in humans, (ii) failure to form a secondary ossification center within the cartilaginous epiphysis, but maintenance of a growth plate and subchondral surface, (iii) loss of the secondary ossification center with direct invasion from the primary center toward the palmar end of the bone, or (iv) loss of the primary ossification center with direct invasion from the epiphysis toward the dorsal end of the bone. Arrows within pisiform cartilage indicate an advancing ossification front