A sesamoid bone is an actual bone that developed independently
from other bones as the body developed,unlike
accessory bones which
are separations of ordinary bones during development. The sesamoid
bones derived the term from “sesame seeds” due to the small, oval
shape in appearance. A sesamoid bone is a consistent structure,
meaning that when they form, they do so at the same position
every time. The biggest example of a sesamoid bone is the knee cap.
Sesamoids are generally embedded within capsules or tendons where they
insert or sharply change directions. They are thought to protect
excessive friction or change the direction of the pull of the tendons
for efficiency. Some sesamoids are seen on x-ray as they are ossified,
meaning that they have completely formed bone. Others are not seen as
they remain to be cartilage and unossified.

Common sesamoids of the foot are the tibial sesamoid and fibular
sesamoid under the first metatarsal head.
Sesamoiditis occurs when
these sesamoids become injured and inflammed. The Os Peroneum is the
sesamoid that occurs at the peroneus longus tendon groove on the cuboid.