In anatomy, a sesamoid bone (//) is a bone embedded within a tendon or a muscle. It is derived from the Latin word "sesamum" (sesame seed), due to the small size of most sesamoids. Often, these bones form in response to strain, or can be present as a normal variant. The kneecap 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.
Sesamoid bones at the distal end of the first metatarsal bone of the foot.
pl. ossa sesamoidea
|Anatomical terms of bone|
The sesamoid is a small nodular bone most often present embedded in tendons in the region of the thumb. Calcification of sesamoid bone is one of the important features of pubertal growth spurt, which is earlier in females than in males. Absence of sesamoid bone indicates delay in reaching puberty.
Sesamoid bones can be found on joints throughout the body, including:
- In the knee—the patella (within the quadriceps tendon). This is the largest sesamoid bone.
- 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.
- In the wrist—The pisiform of the wrist is a sesamoid bone (within the tendon of flexor carpi ulnaris). It begins to ossify in children ages 9–12.
- 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). 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.
- In the neck—Although the hyoid bone is free floating, it is not technically a sesamoid bone. All sesamoid bones form directly from the connective tissue found in tendons and ligaments. By contrast, the hyoid bone forms from a cartilaginous precursor like most other bones in the body.
- In the ear—the lenticular process of the incus is a sesamoid bone and therefore is considered the fourth ossicle of the middle ear.
- In the shoulder- the scapula
- 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). (See the X-ray photograph of the foot on the right.)
- 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.
- 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.
- 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.
- 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.
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.
Although many carnivores have radial sesamoid bones, the giant panda and red panda independently evolved to have an enlarged radial sesamoid bone. This evolution has caused the two species to diverge from other carnivores. The red panda likely originally evolved the "pseudo-thumb" in order to assist in arboreal locomotion. When the red panda later evolved to consume a bamboo diet, the enlarged bone underwent exaptation to assist in grasping bamboo. The giant panda, however, evolved the enlarged radial sesamoid bone around the same time as it evolved a bamboo diet. In the giant panda, the bone allows for a pincer-like motion and is used in grasping the bamboo. In these two panda species, DYNC2H1 gene and PCNT gene have been identified as possible causes for the pseudo-thumb development. 
Recently, the enlarged radial sesamoid bone of cotton rats has been studied. Their enlarged radial sesamoid bone and that of the giant panda have a similar morphology and size relative to the rest of the hand. The reason for this evolutionary change is still unknown however it may be to assist in grasping small objects and thin branches.
- OED 2nd edition, 1989 as /sεsəmɔɪd/.
- Entry "sesamoid" in Merriam-Webster Online Dictionary.
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- Reference list for image is located at Commons:Template:Accessory and sesamoid bones of the foot - references.
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- Hu, Yibo; Wu, Qi; Ma, Shuai; Ma, Tianxiao; Shan, Lei; Wang, Xiao; Nie, Yonggang; Ning, Zemin; Yan, Li (2017-01-31). "Comparative genomics reveals convergent evolution between the bamboo-eating giant and red pandas". Proceedings of the National Academy of Sciences. 114 (5): 1081–1086. doi:10.1073/pnas.1613870114. ISSN 0027-8424. PMC . PMID 28096377.
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- Gray's Anatomy (1918) (Bartleby)
- Media related to Sesamoid bone at Wikimedia Commons