Sesamoid

Sesamoids, also known as sesamoid bones, are focal areas of ossification within tendons as they pass over joints ¹. They can also occur in ligaments and usually measure a few millimeters in diameter. Their function is purported to be to alter the direction of the tendon and modify pressure, thereby reducing friction ². 

Some sesamoids form part of normal human anatomy (e.g. patella), while others are anatomical variants (e.g. fabella). 

Sesamoid bones of the lower limb include:

• patella
• fabella
• cyamella
• os peroneum
• hallux sesamoid

See the article on ossicles of the lower limb.

Sesamoid bones of the upper limb are usually found in the hand at the following joints ³:

• 1^st metacarpophalangeal joint sesamoid (two)
  • reported incidence of 100% ⁴
• 1^st interphalangeal joint sesamoid
• 2^nd metacarpophalangeal joint sesamoid
• 5^th metacarpophalangeal joint sesamoid

Less commonly, they may also occur at the 3^rd and 4^th metacarpophalangeal joints ⁴.

Sesamoid ossicles can also form in the nuchal ligament. 

History and etymology

Sesamoids get their name because the smallest bones resemble sesame seeds in size and morphology ^1,3.