The fabella is a small

sesamoid bone In anatomy, a sesamoid bone () is a bone embedded within a tendon or a muscle. 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, or can be presen ...

found in some mammals embedded in the

tendon A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...

of the lateral head of the

gastrocnemius muscle The gastrocnemius muscle (plural ''gastrocnemii'') is a superficial two-headed muscle that is in the back part of the lower leg of humans. It is located superficial to the soleus in the posterior (back) compartment of the leg. It runs from its t ...

behind the lateral

condyle A condyle (;Entry "condyle"
in

of the

femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...

. It is an

accessory bone An accessory bone or supernumerary bone is a bone that is not normally present in the body, but can be found as a anatomical variation, variant in a significant number of people. It poses a risk of being misdiagnosis, misdiagnosed as bone fracture ...

, an

anatomical variation An anatomical variation, anatomical variant, or anatomical variability is a presentation of body structure with Morphology (biology), morphological features different from those that are typically described in the majority of individuals. Anatomica ...

present in 39% of humans. Rarely, there are two or three of these bones (fabella bi- or tripartita). It can be mistaken for a loose body or

osteophyte Osteophytes are Exostosis, exostoses (bony projections) that form along joint margins. They are distinct from enthesophytes, which are bone, bony projections that form at the attachment of a tendon or ligament. Osteophytes are not always distingui ...

. The word ''fabella'' is a Latin diminutive of ''faba'' 'bean'. In humans, it is more common in men than women, older individuals compared to younger, and there is high regional variation, with fabellae being most common in people living in Asia and Oceania and least common in people living in North America and Africa. Bilateral cases (one per knee) are more common than unilateral ones (one per individual), and within individual cases, fabellae are equally likely to be present in right or left knees. Taken together, these data suggest the ability to form a fabella may be genetically controlled, but fabella

ossification 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 ...

may be environmentally controlled. Although the fabella seems to have disappeared with the

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

Hominidae The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic Family (biology), family of primates that includes eight Neontology#Extant taxa versus extinct taxa, extant species in four Genus, genera: ''Orangutan ...

, it reappeared in

humans Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

sometime after they diverged from chimpanzees. It is unknown whether it reappeared soon after this divergence, 5–7 million years ago, or more recently in human evolution. The re-emergence of the lateral fabella may be correlated with "straight-legged, bipedal locomotion" in humans. "The fabella can lead to posterolateral knee pain either due to cartilage softening (chondromalacia fabellae) or other osteoarthritic changes on its articular surface."

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Sesamoid bones Accessory bone Anatomical variations {{musculoskeletal-stub

See also

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Further reading

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