anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having its ...

, a sesamoid bone () is a

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...

embedded within a

tendon A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...

or a

muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other ...

. Its name is derived from the Arabic word for '

sesame Sesame ( or ; ''Sesamum indicum'') is a flowering plant in the genus '' Sesamum'', also called benne. Numerous wild relatives occur in Africa and a smaller number in India. It is widely naturalized in tropical regions around the world and is c ...

seed', indicating the small size of most sesamoids. Often, these bones form in response to strain, or can be present as a normal variant. The

patella The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as m ...

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.

Structure

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

knee In humans and other primates, the knee joins the thigh with the human leg, leg and consists of two joints: one between the femur and tibia (tibiofemoral joint), and one between the femur and patella (patellofemoral joint). It is the largest join ...

—the

(within the

quadriceps The quadriceps femoris muscle (, also called the quadriceps extensor, quadriceps or quads) is a large muscle group that includes the four prevailing muscles on the front of the thigh. It is the sole extensor muscle of the knee, forming a large ...

tendon). This is the largest sesamoid bone. * In the

hand A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala (which has two opposable thumbs on each " ...

—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 In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carpal ...

—The pisiform of the

is a sesamoid bone (within the tendon of flexor carpi ulnaris). It begins to

ossify Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

in children ages 9–12. * In the

foot The foot ( : feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg mad ...

—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.

Common variants

Accessory and sesamoid bones of the foot - dorsoplantar projection

* 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). * The fabella is a small sesamoid bone found in some mammals embedded in the

of the lateral head of the gastrocnemius muscle behind the lateral condyle of the

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...

. 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. File:Accessory and sesamoid bones of the foot - lateral projection.jpg, Lateral view. File: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 significance

* 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 Avascular necrosis (AVN), also called osteonecrosis or bone infarction, is death of bone tissue due to interruption of the blood supply. Early on, there may be no symptoms. Gradually joint pain may develop which may limit the ability to move. C ...

(bone death from lack of blood supply), which is very difficult to treat.

Other animals

In equine anatomy, the term sesamoid bone usually refers to the two sesamoid bones found at the back of the

fetlock Fetlock is the common name in horses, large animals, and sometimes dogs for the metacarpophalangeal and metatarsophalangeal joints (MCPJ and MTPJ). Although it somewhat resembles the human ankle in appearance, the joint is homologous to the ...

or metacarpophalangeal and metatarsophalangeal joints in both hindlimbs and forelimbs. Strictly these should be termed the

proximal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position ...

sesamoid bones whereas the

navicular bone The navicular bone is a small bone found in the feet of most mammals. Human anatomy The navicular bone in humans is one of the tarsal bones, found in the foot. Its name derives from the human bone's resemblance to a small boat, caused by the ...

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 The giant panda (''Ailuropoda melanoleuca''), also known as the panda bear (or simply the panda), is a bear species endemic to China. It is characterised by its bold black-and-white coat and rotund body. The name "giant panda" is sometimes u ...

and

red panda The red panda (''Ailurus fulgens''), also known as the lesser panda, is a small mammal native to the eastern Himalayas and southwestern China. It has dense reddish-brown fur with a black belly and legs, white-lined ears, a mostly white muzz ...

independently evolved Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

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 Exaptation and the related term co-option describe a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are commo ...

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.

Elephants Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantidae ...

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

Structure

Common variants

Clinical significance

Other animals

See also

Footnotes

References

External links