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 i ...

, 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, ...

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 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 types of mus ...

. 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 cul ...

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 ...

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 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 joint in the ...

—the

(within the quadriceps 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 "h ...

—two sesamoid bones are commonly found in the

distal 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 pro ...

portions of the

first metacarpal bone The first metacarpal bone or the metacarpal bone of the thumb is the first bone proximal to the thumb. It is connected to the trapezium of the carpus at the first carpometacarpal joint and to the proximal thumb phalanx at the first metacarpophal ...

(within the tendons of adductor pollicis and

flexor pollicis brevis The flexor pollicis brevis is a muscle in the hand that flexes the thumb. It is one of three thenar muscles. It has both a superficial part and a deep part. Origin and insertion The muscle's superficial head arises from the distal edge of the fl ...

). 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 carp ...

—The

pisiform The pisiform bone ( or ), also spelled pisiforme (from the Latin ''pisifomis'', pea-shaped), is a small knobbly, sesamoid bone that is found in the wrist. It forms the ulnar border of the carpal tunnel. Structure The pisiform is a sesamoid bone, ...

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 th ...

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 Toes are the digits (fingers) of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being ''digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being ''plan ...

(both within the tendon of

flexor hallucis brevis A flexor is a muscle that flexes a joint. In anatomy, flexion (from the Latin verb ''flectere'', to bend) is a joint movement that decreases the angle between the bones that converge at the joint. For example, one’s elbow joint flexes when on ...

). 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 The metatarsal bones, or metatarsus, are a group of five long bones in the foot, located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the me ...

Common variants

Accessory and sesamoid bones of the foot - dorsoplantar projection

* One or both of the sesamoid bones under the first

metatarsophalangeal joint The metatarsophalangeal joints (MTP joints), also informally known as toe knuckles, are the joints between the metatarsal bones of the foot and the proximal bones ( proximal phalanges) of the toes. They are condyloid joints, meaning that an elli ...

(of the great toe) can be multipartite – in two or three parts (mostly bipartite – in two parts). * The

fabella 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 an accessory bone, an anatomical variation present in 39% of huma ...

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 ...

. 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 The popliteus muscle in the leg is used for unlocking the knees when walking, by laterally rotating the femur on the tibia during the closed chain portion of the gait cycle (one with the foot in contact with the ground). In open chain movements ...

. 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

of the great toe of the left foot of an adult woman.

Clinical significance

* A common foot ailment in dancers is

sesamoiditis Sesamoiditis is inflammation of the sesamoid bones. Humans Sesamoiditis occurs on the bottom of the foot, just behind the big toe. There are normally two sesamoid bones on each foot; sometimes sesamoids can be bipartite, which means they each c ...

(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. ...

(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 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 should be referred to as the

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 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 rat A cotton rat is any member of the rodent genus ''Sigmodon''. Their name derives from their damaging effects on cotton as well as other plantation crops, such as sugarcane, corn, peanut and rice. Cotton rats have small ears and dark coats, and a ...

s 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 Moles can refer to: * Moles de Xert, a mountain range in the Baix Maestrat comarca, Valencian Community, Spain *The Moles (Australian band) *The Moles, alter ego of Scottish band Simon Dupree and the Big Sound People * Abraham Moles, French engin ...

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

Footnotes

References

''Gray's Anatomy'' (1918)
(Bartleby)

External links

* {{DEFAULTSORT:Sesamoid Bone Skeletal system Horse anatomy