Sesamoid bone

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In anatomy, a sesamoid bone (Template:IPAc-en)<ref>OED 2nd edition, 1989 as {{#invoke:IPA|main}}</ref><ref>Entry "sesamoid" in Merriam-Webster Online Dictionary.</ref> is a bone embedded within a tendon or a muscle.<ref name="aofas.org">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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,<ref name="auto">Template:Cite book</ref> 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.<ref name="aofas.org" />

StructureEdit

File:Sesamoid bones of the fingers.jpg

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Sesamoid bones can be found on joints throughout the human body, including:

In the knee—the patella (within the quadriceps tendon). This is the largest sesamoid bone.<ref name="auto"/>
In the hand—two sesamoid bones are commonly found in the distal portions of the first metacarpal bone (within the tendons of adductor pollicis and flexor pollicis brevis). There is also commonly a sesamoid bone in distal portions of the second metacarpal bone and fifth metacarpal bone.<ref>{{#invoke:citation/CS1|citation

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In the wrist—The pisiform of the wrist is a sesamoid bone (within the tendon of flexor carpi ulnaris).<ref>Template:Cite book</ref> It begins to ossify in children ages 9–12.<ref>Template:Cite book</ref>
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).<ref>Template:Cite book</ref> 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 variantsEdit

File:Accessory and sesamoid bones of the foot - dorsoplantar projection.jpg

X-ray of the foot by dorsoplantar view, with most common accessory and sesamoid bones.<ref name=Commons-foot>Reference list for image is located at Commons:Template:Accessory and sesamoid bones of the foot - references.</ref>

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).<ref>{{#invoke:citation/CS1|citation

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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.<ref name="Radiopaedia">Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref>

Accessory and sesamoid bones of the foot - lateral projection.jpg

Lateral view.<ref name=Commons-foot/>
Pie metatarso-falanges.JPG

Bipartite medial sesamoid bone under the first metatarsophalangeal joint of the great toe of the left foot of an adult woman.

Clinical significanceEdit

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.<ref name="aofas.org"/>
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.<ref>{{#invoke:citation/CS1|citation

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Other animalsEdit

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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,<ref name=":0">Template:Cite journal</ref> the giant panda and red panda independently evolved to have an enlarged radial sesamoid bone.<ref name=":0" /><ref name=":1">Template:Cite journal</ref> This evolution has caused the two species to diverge from other carnivores.<ref name=":0" /> The red panda likely originally evolved the "pseudo-thumb" in order to assist in arboreal locomotion.<ref name=":1" /><ref name=":0" /> When the red panda later evolved to consume a bamboo diet, the enlarged bone underwent exaptation to assist in grasping bamboo.<ref>Template:Cite journal</ref><ref name=":0" /><ref>Template:Cite journal</ref><ref name=":1" /> The giant panda, however, evolved the enlarged radial sesamoid bone around the same time as it evolved a bamboo diet.<ref name=":1" /> In the giant panda, the bone allows for a pincer-like motion and is used in grasping the bamboo.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> In these two panda species, DYNC2H1 gene and PCNT gene have been identified as possible causes for the pseudo-thumb development.<ref>Template:Cite journal</ref>

Recently, the enlarged radial sesamoid bone of cotton rats has been studied.<ref name=":2">Template:Cite journal</ref> Their enlarged radial sesamoid bone and that of the giant panda have a similar morphology and size relative to the rest of the hand.<ref name=":2" /> The reason for this evolutionary change is still unknown; however, it may be to assist in grasping small objects and thin branches.<ref name=":2" />

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 three to seven 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.<ref>Template:Cite journal</ref>

The forepaws of moles also possess a prepollex consisting of an enlarged, sickle-shaped sesamoid.