The pisiform is a sesamoid bone. It is located in the flexor carpi ulnaris (FCU) wrist tendon. It protects this tendon by supporting and bearing its forces as it moves across the triquetrum during wrist movement. The triquetrum is a proximal carpal bone located between the pisiform and lunate bones. The pisiform is located opposite the wrist's carpal base plate and communicates with the abductor digiti minimi of the hand. Specifically, it is located where the carpus joins the ulna, which is the inner forearm bone. Chronic or acute pain is common in the pisiform because it is where tendinopathy of the FCU occurs at insertion. Osteoarthritis, mechanical overuse, and bony fractures can also affect the pisiform. Pain in the pisiform is usually examined by a sonographic evaluation. Fluid collection and the thickening of soft tissue are common in the pisiform bone. The pisiform is sphere-shaped, like a pea. In fact, its name means "pea-shaped." The pisiform has four types of surfaces: dorsal, palmar, lateral, and medial. The latter three surfaces are rough, allowing the pisiform to attach to the carpal ligament. However, the dorsal surface is smooth, allowing for the bone's articulation with the triquetrum.