Fact-Checking Hand Anatomy: Are There 27 Bones, or Do Sesamoids Change the Count?

Are fact-Checking Hand Anatomy: Are There 27 Bones, or Do Sesamoids Change the Count a topic you are researching? Discover key explanations right here.

Count the bones on a standard medical poster and you will stop at 27. Examine an adult radiograph, however, and small pebble-shaped nodules appear near the thumb joint. These are sesamoid bones, calcified structures embedded inside tendon tissue rather than linked to neighboring bones via conventional ligaments.

The human hand regularly develops two consistent sesamoids within the adductor pollicis and flexor pollicis brevis tendons, sitting right over the palmar surface of the first metacarpophalangeal joint. In a 2024 anatomical survey examining high-resolution hand computed tomography (CT) scans across 450 patients, radiologists observed both palmar thumb sesamoids in 98.2% of adult hands. Variable sesamoids also appear at the second and fifth metacarpal joints in roughly 35% to 42% of individuals.

Why do anatomists leave these out of the textbook 27? In classical taxonomy, sesamoids are classified as developmental leverage aids rather than structural framework units. The pisiform bone in the wrist is itself a sesamoid embedded within the flexor carpi ulnaris tendon. Yet anatomists grandfathered the pisiform into the standard 8 carpals while classifying the thumb sesamoids as optional accessories. Functionally, these thumb sesamoids absorb tremendous mechanical friction, acting as pulleys that amplify the muscle torque required to pinch and grasp objects.