, a sesamoid bone
is a bone
embedded within a tendon
. They usually resemble sesame
seed, hence the name.
Sesamoid bones are typically found in locations where a tendon passes over a joint, such as the hand, knee, and foot. Functionally, they act to protect the tendon and to increase its mechanical effect. The presence of the sesamoid bone holds the tendon slightly farther away from the center of the joint and thus increases its moment arm. Sesamoid bones also prevent the tendon from flattening into the joint as tension increases and therefore also maintain a more consistent moment arm through a variety of possible tendon loads. This differs from menisci, which are made of cartilage and rather act to disperse the weight of the body on joints and reduce friction during movement.
Sesamoid bones can be found on joints throughout the body, including:
Injuries of the sesamoid bones
- A common foot ailment in dancers is sesamoiditis.
- A bi-partite sesamoid bone is when the sesamoids are in 2 separate entities - usually congenital, but may be related to a history of trauma.
In equine anatomy
, the term sesamoid bone usually refers to the two sesamoid bones found at the back of the fetlock
or metacarpophalangeal/metatarsophalangeal joints in both hindlimbs and forelimbs
. Strictly these should be termed the proximal
sesamoid bones whereas the navicular bone
should be referred to as the distal
sesamoid bone. The patella is also a form of sesamoid bone in the horse.
In both the giant panda
and the red panda
, the radial sesamoid has evolved
to be larger than the same bone in counterparts such as bears
. It is primarily a bony support for the pad above it, allowing the panda's other digits to grasp bamboo
while eating it. The panda's thumb is often cited as a classical example of exaptation
, where a trait evolved for one purpose is commandeered for another.