| Patella | |
|---|---|
 Right knee | |
| Details | |
| Pronunciation | /pəˈtɛlə/ |
| Origins | Present at the joint of femur and tibia fibula |
| Identifiers | |
| Latin | patella |
| MeSH | D010329 |
| TA98 | A02.5.05.001 |
| TA2 | 1390 |
| FMA | 24485 |
| Anatomical terms of bone | |
Thepatella (pl.:patellae orpatellas), also known as thekneecap, is a flat, rounded triangularbone which articulates with thefemur (thigh bone) and covers and protects the anterior articular surface of theknee joint. The patella is found in many tetrapods, such asmice,cats,birds anddogs, but not inwhales, or mostreptiles.
In humans, the patella is the largestsesamoid bone (i.e., embedded within a tendon or a muscle) in the body. Babies are born with a patella of softcartilage which begins toossify into bone at about four years of age.
The patella is asesamoid bone roughly triangular in shape, with the apex of the patella facing downwards. The apex is the mostinferior (lowest) part of the patella. It is pointed in shape, and gives attachment to thepatellar ligament.
The front and back surfaces are joined by a thin margin and towards centre by a thicker margin.[1] Thetendon of thequadriceps femoris muscle attaches to the base of the patella.,[1] with thevastus intermedius muscle attaching to the base itself, and thevastus lateralis andvastus medialis 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 hasexostoses. The middle third has numerousvascularcanaliculi. The lower third culminates in the apex which serves as the origin of thepatellar ligament.[1] The posterior surface is divided into two parts.[1]
The upper three-quarters of the patellaarticulates 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 12 cm2 (1.9 sq in) and covered bycartilage, which can reach a maximal thickness of 6 mm (0.24 in) 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, theinfrapatellar fat pad.
Emarginations (i.e.patella emarginata, a "missing piece") are common laterally on the proximal edge.[1]Bipartite patellas are the result of an ossification of a secondcartilaginous 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.[citation needed] 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:
In the patella anossification centre develops at the age of 3–6 years.[1] The patella originates from two centres of ossification which unite when fully formed.[citation needed]
The primary functional role of the patella is knee extension. The patella increases theleverage that thequadriceps tendon can exert on the femur by increasing the angle at which it acts.
The patella is attached to thetendon of thequadriceps femoris muscle, which contracts to extend/straighten theknee. The patella is stabilized by the insertion of the horizontal fibres ofvastus medialis and by the prominence of thelateral femoral condyle, which discourages lateral dislocation during flexion. The retinacular fibres of the patella also stabilize it during exercise.
Patellar dislocations occur with significant regularity, particularly in young female athletes.[2] 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.[3] 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.[3]
Apatella alta is a high-riding (superiorly aligned) patella. Anattenuated patella alta is an unusually small patella that develops out of and above the joint.
Apatella baja is a low-riding patella. A long-standing patella baja may result in extensor dysfunction.[5]
TheInsall-Salvati ratio helps to indicate patella baja on lateralX-rays, and is calculated as the patellar tendon length divided by the patellar bone length. An Insall-Salvati ratio of< 0.8 indicates patella baja.[6]
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), 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.[7]
Anexostosis is the formation of new bone onto a bone, as a result of excesscalcium formation. This can be the cause of chronic pain when formed on the patella.
The patella is found inplacental mammals andbirds; mostmarsupials have only rudimentary, non-ossified patellae although a few species possess a bony patella.[8] A patella is also present in the livingmonotremes, theplatypus and theechidna. In other tetrapods, including livingamphibians and mostreptiles (except somelepidosaurs), the muscle tendons from the upper leg are attached directly to thetibia, and a patella is not present.[9] In 2017 it was discovered thatfrogs 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.[10][11]
The wordpatella originated in the late 17th century from thediminutive form ofLatinpatina orpatena orpaten, meaning shallow dish.[12][13]
