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{{other uses}}{{redirect|Kneecap|other uses|NECAP}}

| Latin = patella| Image = Knee diagram.svg | Caption = Right knee| Origins =present at the joint of femur and tibia fibula| Insertions =| Articulations =}}The patella, also known as the kneecap, is a flat, circular-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 mice, cats and birds, but not in whales, or most reptiles.In humans, the patella is the largest sesamoid bone in the body. Babies are born with a patella of soft cartilage which begins to ossify into bone at about four years of age.


The patella is a sesamoid bone 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 front and back surfaces are joined by a thin margin and towards centre by a thicker margin.WERNER > LAST = PLATZER, Color Atlas of Human Anatomy, Vol. 1: Locomotor SystemThieme Medical Publishers>Thieme, 3-13-533305-1 edition = 5th, 194, The tendon of the quadriceps femoris muscle attaches to the base of the patella., with the vastus intermedius muscle attaching to the base itself, and the vastus lateralis and vastus 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 has exostoses. The middle third has numerous vascular canaliculi. The lower third culminates in the apex which serves as the origin of the patellar ligament. The posterior surface is divided into two parts.Patella ant.jpg|Human left patella from the frontPatella post.jpg|Human left patella from behindKnee Biomechanics.JPG|Flexion and extension of kneeThe 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 {{convert|12|cm2|sqin|abbr=on}} and covered by cartilage, which can reach a maximal thickness of {{convert|6|mm|in|abbr=on}} in the centre at about 30 years of age. Due 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.


File:Patella bipartita.jpg|thumbnail|In this X-ray, an anatomical variation of the patella can be seen – the bipartate patella, in which the patella is split into two parts. ]]Emarginations (i.e. patella emarginata, a "missing piece") are common laterally on the proximal edge. Bipartite patellas are the result of an ossification of a second cartilaginous 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|date=July 2015}} 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:
  1. Most commonly the medial articular surface is smaller than the lateral.
  2. Sometimes both articular surfaces are virtually equal in size.
  3. Occasionally, the medial surface is hypoplastic or
  4. the central ledge is only indicated.


In the patella an ossification centre develops at the age of 3–6 years. The patella originates from two centres of ossification which unite when fully formed.{{citation needed|date=July 2015}}


The primary functional role of the patella is knee extension. The patella increases the leverage that the quadriceps tendon can exert on the femur by increasing the angle at which it acts.The patella is attached to the tendon of the quadriceps femoris muscle, which contracts to extend/straighten the knee. The patella is stabilized by the insertion of the horizontal fibres of vastus medialis and by the prominence of the lateral femoral condyle, which discourages lateral dislocation during flexion. The retinacular fibres of the patella also stabilize it during exercise.

Clinical significance


Patellar dislocations occur with significant regularity, particularly in young female athletes.JOURNAL, Palmu, S., Kallio, P.E., Donell, S.T., Helenius, I., Nietosvaara, Y., 2008, Acute patellar dislocation in children and adolescents: A randomized clinical trial, Journal of Bone and Joint Surgery, 90, 3, 463–470, 10.2106/JBJS.G.00072, 18310694, 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.JOURNAL, Dath, R., Chakravarthy, J., Porter, K.M., 2006, Patella Dislocations, Trauma, 8, 5–11, 10.1191/1460408606ta353ra, 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.

{{anchor|baja}}Vertical alignment

(File:Patella baja.jpg|thumb|Patella baja.JOURNAL, Melloni, Pietro, Veintemillas, Maite, Marin, Anna, Valls, Rafael, Imaging Patellar Complications After Knee Arthroplasty, 2013, 10.5772/53666, (CC-BY-3.0))A patella alta is a high-riding (superiorly aligned) patella. An attenuated patella alta 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.WEB,weblink Patella baja, Yuranga Weerakkody and Frank Gaillard, Radiopaedia, 2018-01-16, (File:Insall-Salvati ratio of patella baja.jpg|thumb|left|150px|Insall-Salvati ratio (A divided by B).)The Insall-Salvati ratio helps to indicate patella baja on lateral X-rays, and is calculated as the patellar tendon length divided by the patellar bone length. An Insall-Salvati ratio of Less than|

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