The pisiform is a rounded carpal bone that lies over the triquetrum. Although, anatomically, the pisiform is located in the proximal carpal row, it does not participante in either the radiocarpal or the midcarpal joints. It is a sesmoid bone that's sole function appears to be to increase the moment arm (and thus the torque or rotary force) of the flexor carpi ulnaris muscle as its tendon courses over the pisiform.