The patella (: patellae or patellas), also known as the kneecap, is a flat, rounded triangular

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, ...

which articulates with 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 ...

(thigh bone) and covers and protects the anterior articular surface of 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 hu ...

joint. The patella is found in many tetrapods, such as

mice A mouse (: mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...

cat The cat (''Felis catus''), also referred to as the domestic cat or house cat, is a small domesticated carnivorous mammal. It is the only domesticated species of the family Felidae. Advances in archaeology and genetics have shown that the ...

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s, and

dog The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the gray wolf. Also called the domestic dog, it was selectively bred from a population of wolves during the Late Pleistocene by hunter-gatherers. ...

s, but not in

whale Whales are a widely distributed and diverse group of fully Aquatic animal, aquatic placental mammal, placental marine mammals. As an informal and Colloquialism, colloquial grouping, they correspond to large members of the infraorder Cetacea ...

s, or most

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

s. In humans, the patella is the largest

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 ...

(i.e., embedded within a tendon or a muscle) in the body. Babies are born with a patella of soft

cartilage Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...

which 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 the ...

into bone at about four years of age.

Structure

The patella is a

roughly triangular in shape, with the apex of the patella facing downwards. The apex is the most inferior (lowest) part of the patella. It is pointed in shape, and gives attachment to the

patellar ligament The patellar tendon is the distal portion of the common tendon of the quadriceps femoris, which is continued from the patella to the tibial tuberosity. It is also sometimes called the patellar ligament as it forms a bone to bone connection when ...

. The front and back surfaces are joined by a thin margin and towards centre by a thicker margin. 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

quadriceps femoris muscle 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 ...

attaches to the base of the patella., with the

vastus intermedius The vastus intermedius () (Cruraeus) arises from the front and lateral surfaces of the body of the femur in its upper two-thirds, sitting under the rectus femoris muscle and from the lower part of the lateral intermuscular septum. Its fibers end ...

muscle attaching to the base itself, and the

vastus lateralis The vastus lateralis (), also called the vastus externus, is the largest and most powerful part of the quadriceps femoris, a muscle in the thigh. Together with other muscles of the quadriceps group, it serves to extend the knee joint, moving the ...

and

vastus medialis The vastus medialis (vastus internus or teardrop muscle) is an extensor muscle located medially in the thigh that extends the knee. The vastus medialis is part of the quadriceps muscle group. Structure The vastus medialis is a muscle presen ...

are attached to outer lateral and medial borders of patella respectively. The upper third of the front of the patella is coarse, flattened, and rough, and serves for the attachment of the tendon of the quadriceps and often has

exostoses An exostosis, also known as a osteochondroma, is a benign chondrogenic lesions derived from aberrant cartilage from the perichondral ring. Exostoses can cause chronic pain ranging from mild to moderate, but rarely severe, depending on the shape, s ...

. The middle third has numerous

vascular Vascular can refer to: * blood vessels, the vascular system in animals * vascular tissue Vascular tissue is a complex transporting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue ...

canaliculi. The lower third culminates in the apex which serves as the origin of the

. The posterior surface is divided into two parts. Patella ant.jpg, Human left patella from the front Patella post.jpg, Human left patella from behind Knee Biomechanics.JPG, Flexion and extension of knee The upper three-quarters of the patella articulates with the femur and is subdivided into a medial and a lateral facet by a vertical ledge which varies in shape. In the adult the articular surface is about and covered by

, which can reach a maximal thickness of in the centre at about 30 years of age. Owing to the great stress on the patellofemoral joint during resisted knee flexion, the articular cartilage of the patella is among the thickest in the human body. The lower part of the posterior surface has vascular canaliculi filled and is filled by fatty tissue, the

infrapatellar fat pad The infrapatellar fat pad (Hoffa's fat pad) is a cylindrical piece of fat that is situated inferior and posterior to the patella bone within the knee, intervening between the patellar ligament and synovial fold of the knee joint. Clinical signifi ...

Variation

Emarginations (i.e. ''patella emarginata'', a "missing piece") are common laterally on the proximal edge.

Bipartite patella Bipartite patella is a condition where the patella, or kneecap, is composed of two separate bone, bones. Instead of ossification, fusing together as normally Patella#development, occurs in early childhood, the bones of the patella remain separated ...

s are the result of an ossification of a second

cartilaginous Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...

layer at the location of an emargination. Previously, bipartite patellas were explained as the failure of several ossification centres to fuse, but this idea has been rejected. Partite patellas occur almost exclusively in men. Tripartite and even multipartite patellas occur. The upper three-quarters of the patella articulates with the femur and is subdivided into a medial and a lateral facet by a vertical ledge which varies in shape. Four main types of articular surface can be distinguished: # Most commonly the medial articular surface is smaller than the lateral. # Sometimes both articular surfaces are virtually equal in size. # Occasionally, the medial surface is

hypoplastic Hypoplasia (; adjective form ''hypoplastic'') is underdevelopment or incomplete development of a Tissue (biology), tissue or Organ (biology), organ.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 ...

centre develops at the age of 3–6 years. The patella originates from two centres of ossification which unite when fully formed.

Function

The primary functional role of the patella is knee extension. The patella increases the

lever A lever is a simple machine consisting of a beam (structure), beam or rigid rod pivoted at a fixed hinge, or '':wikt:fulcrum, fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, l ...

age that the

quadriceps tendon In human anatomy Human anatomy (gr. ἀνατομία, "dissection", from ἀνά, "up", and τέμνειν, "cut") is primarily the scientific study of the morphology of the human body. Anatomy is subdivided into gross anatomy and micros ...

can exert on the femur by increasing the angle at which it acts. The patella is attached to the

of the

, which contracts to extend/straighten the

. The patella is stabilized by the insertion of the horizontal fibres of

and by the prominence of the

lateral femoral condyle The lateral condyle is one of the two projections on the lower extremity of the femur. The other one is the medial condyle. The lateral condyle is the more prominent and is broader both in its front-to-back and transverse diameters. Clinical sig ...

, which discourages lateral dislocation during flexion. The retinacular fibres of the patella also stabilize it during exercise.

Clinical significance

Dislocation

Patellar dislocations occur with significant regularity, particularly in young female athletes. It involves the patella sliding out of its position on the knee, most often laterally, and may be associated with extremely intense pain and swelling. The patella can be tracked back into the groove with an extension of the knee, and therefore sometimes returns into the proper position on its own.

Treatment

For recurrent patellar dislocations or cases with chronic instability, surgical intervention may be necessary. Patella stabilization surgery aims to restore proper kneecap tracking and reduce the risk of future dislocations. Surgical options include reconstruction of the

medial patellofemoral ligament The medial patellofemoral ligament (MPFL) is one of several ligaments on the medial aspect of the knee. It originates in the superomedial aspect of the patella and inserts in the space between the adductor tubercle and the medial femoral epicond ...

(MPFL), realignment procedures such as

osteotomy An osteotomy is a surgical operation whereby a bone is cut to shorten or lengthen it or to change its alignment. It is sometimes performed to correct a hallux valgus, or to straighten a bone that has healed crookedly following a fracture. It i ...

, and soft tissue adjustments around the patella. MPFL reconstruction typically involves harvesting a hamstring tendon, commonly the gracilis, to create a new ligament that is positioned anatomically and secured with suture anchors. Recovery from patella stabilization surgery typically requires six to eight weeks, during which patients may need to use crutches initially and follow a structured physical therapy program to regain range of motion and strengthen the surrounding musculature. Patella baja

Vertical alignment

A ''patella alta'' is a high-riding (superiorly aligned) patella. An

attenuated patella alta Attenuated patella alta is an extremely rare condition affecting mobility and leg strength. It is characterized by an unusually small knee cap (patella) that develops out of and above the joint. Typically, as the knee cap sits in the joint, it ...

is an unusually small patella that develops out of and above the joint. A ''patella baja'' is a low-riding patella. A long-standing patella baja may result in extensor dysfunction. Insall-Salvati ratio of patella baja

The ''Insall-Salvati ratio'' helps to indicate patella baja on lateral

X-rays An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

, and is calculated as the patellar tendon length divided by the patellar bone length. An Insall-Salvati ratio of < 0.8 indicates patella baja.

Fracture

The kneecap is prone to injury because of its particularly exposed location, and fractures of the patella commonly occur as a consequence of direct trauma onto the knee. These fractures usually cause swelling and pain in the region, bleeding into the joint (

hemarthrosis Hemarthrosis is a bleeding into joint spaces. It is a common feature of hemophilia. Causes It usually follows injury but occurs mainly in patients with a predisposition to hemorrhage such as those being treated with warfarin (or other anticoagu ...

), and an inability to extend the knee. Patella fractures are usually treated with surgery, unless the damage is minimal and the extensor mechanism is intact.

Exostosis

exostosis An exostosis, also known as a osteochondroma, is a benign chondrogenic lesions derived from aberrant cartilage from the perichondral ring. Exostoses can cause chronic pain ranging from mild to moderate, but rarely severe, depending on the shape, s ...

is the formation of new bone onto a bone, as a result of excess

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

formation. This can be the cause of chronic pain when formed on the patella.

Other animals

The patella is found in

placental mammals Placental mammals ( infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguish ...

and

s; most

marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...

s have only rudimentary, non-ossified patellae although a few species possess a bony patella. A patella is also present in the living

monotreme Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

s, the

platypus The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or monotypi ...

and the

echidna Echidnas (), sometimes known as spiny anteaters, are quill-covered monotremes (egg-laying mammals) belonging to the Family (biology), family Tachyglossidae , living in Australia and New Guinea. The four Extant taxon, extant species of echidnas ...

. In other tetrapods, including living

amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...

s and most

s (except some

lepidosaurs The Lepidosauria (, from Greek meaning ''scaled lizards'') is a Order (biology), superorder or Class (biology), subclass of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata also includes Lizard, lizards and Snake, snakes. Sq ...

), the muscle tendons from the upper leg are attached directly to the

tibia The tibia (; : tibiae or tibias), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two Leg bones, bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outsi ...

, and a patella is not present. In 2017 it was discovered that

frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...

s have kneecaps, contrary to what was thought. This raises the possibility that the kneecap arose 350 million years ago when tetrapods first appeared, but that it disappeared in some animals.

Etymology

The word ''patella'' originated in the late 17th century from the

diminutive A diminutive is a word obtained by modifying a root word to convey a slighter degree of its root meaning, either to convey the smallness of the object or quality named, or to convey a sense of intimacy or endearment, and sometimes to belittle s ...

form of

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

' or ' or ''paten'', meaning shallow dish.

References

External links

{{Authority control Knee Sesamoid bones Bones of the lower limb