In
anatomy, a sesamoid bone (/ˈsɛsəmɔɪd/)[1][2] is a
bone embedded within a
tendon or a
muscle.[3] Its name is derived from the Greek word for '
sesame seed', indicating the small size of most sesamoids. Often, these bones form in response to strain,[4] or can be present as a
normal variant. The
patella is the largest sesamoid bone in the body. Sesamoids act like
pulleys, providing a smooth surface for tendons to slide over, increasing the tendon's ability to transmit
muscular forces.[3]
Structure
Sesamoid bones can be found on joints throughout the human body, including:
In the
foot—the
first metatarsal bone usually has two sesamoid bones at its connection to the
big toe (both within the tendon of
flexor hallucis brevis).[10] One is found on the lateral side of the first metatarsal while the other is found on the medial side. In some people, only a single sesamoid is found on the first
metatarsal bone.
Common variants
One or both of the sesamoid bones under the first
metatarsophalangeal joint (of the
great toe) can be multipartite – in two or three parts (mostly bipartite – in two parts).[12]
The
fabella is a small sesamoid bone found in some mammals embedded in the
tendon of the lateral head of the
gastrocnemius muscle behind the lateral
condyle of the
femur. It is
a variant of normal anatomy and present in humans in 10% to 30% of individuals. The fabella can also be mutipartite or bipartite.[13]
The
cyamella is a small sesamoid bone embedded in the tendon of the
popliteus muscle. It is a variant of normal anatomy. It is rarely seen in humans, but has been described more often in other primates and certain other animals.[14]
A common foot ailment in dancers is
sesamoiditis (an inflammation of the sesamoid bones under the first metatarsophalangeal joint of the big toe). This is a form of tendinitis which results from the tendons surrounding the sesamoid becoming inflamed or irritated.[3]
Sesamoid bones generally have a very limited blood supply, rendering them prone to
avascular necrosis (bone death from lack of blood supply), which is very difficult to treat.[15]
In
equine anatomy, the term sesamoid bone usually refers to the two sesamoid bones found at the back of the
fetlock or metacarpophalangeal and 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.
Although many carnivores have radial sesamoid bones,[16] the
giant panda and
red pandaindependently evolved to have an enlarged radial sesamoid bone.[16][17] This evolution has caused the two species to diverge from other carnivores.[16] The red panda likely originally evolved the "pseudo-thumb" in order to assist in arboreal locomotion.[17][16] When the red panda later evolved to consume a bamboo diet, the enlarged bone underwent
exaptation to assist in grasping bamboo.[18][16][19][17] The giant panda, however, evolved the enlarged radial sesamoid bone around the same time as it evolved a bamboo diet.[17] In the giant panda, the bone allows for a pincer-like motion and is used in grasping the bamboo.[20][21] In these two panda species, DYNC2H1 gene and PCNT gene have been identified as possible causes for the pseudo-thumb development.[22]
Recently, the enlarged radial sesamoid bone of
cotton rats has been studied.[23] Their enlarged radial sesamoid bone and that of the giant panda have a similar morphology and size relative to the rest of the hand.[23] The reason for this evolutionary change is still unknown; however, it may be to assist in grasping small objects and thin branches.[23]
Elephants have similarly enlarged sesamoid bones in both their forelimbs and hindlimbs, referred to as the prepollex and prehallux, respectively. These sesamoids function as "sixth toes", helping to distribute the animals' weight. In contrast to other sesamoids in elephants, which ossify at 3-7 years of age, the ossification of the prepollex and prehallux is delayed and is known to not have yet occurred in animals in excess of 20 years of age. The prehallux is further divided into two elements; the more proximal of these is fixed, whilst the more distal is mobile. Evidence of these "predigits" has also been found in certain fossil
proboscideans.[24]
The forepaws of
moles also possess a prepollex consisting of an enlarged, sickle-shaped sesamoid.
^Endo, Hideki; Sasaki, Motoki; Kogiku, Hiroyuki; Yamamoto, Masako; Arishima, Kazuyoshi (2001). "Radial sesamoid bone as a part of the manipulation system in the lesser panda (Ailurus fulgens)". Annals of Anatomy - Anatomischer Anzeiger. 183 (2): 181–184.
doi:
10.1016/s0940-9602(01)80045-5.
PMID11325067.