thumb|X-ray of a normal adult right knee by anteroposterior projection. Patella is labeled in green lettering.
thumb|X-ray of a normal adult right knee by mediolateral projection. Patella is labeled in green lettering.
The patella (: patellae or patellas), 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 mice, cats, birds, humans, and dogs, but not in whales, or most reptiles.
In humans, the patella is the largest sesamoid bone (i.e., embedded within a tendon or a muscle) in the body.
Structure
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 tendon.
The front and back surfaces are joined by a thin margin and towards centre by a thicker margin. The tendon of the quadriceps femoris muscle attaches to the base of the patella, It involves the patella sliding out of its position on the knee, most often laterally, and may be associated with excruciating 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. Surgical options include reconstruction of the medial patellofemoral ligament (MPFL), realignment procedures such as osteotomy, and soft tissue adjustments around the patella.<!-- This is content from a predatory publisher (intechopen.com); recommend finding an alternative source for a comparable image. -->]]
Vertical alignment
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.
[[File:Insall-Salvati ratio of patella baja.jpg|thumb|upright=.6|Insall-Salvati ratio (A divided by B).
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), 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
An exostosis is the formation of new bone onto a bone, as a result of excess calcium formation. This can be the cause of chronic pain when formed on the patella.
Other animals
The patella is found in placental mammals and birds; most marsupials have only rudimentary, non-ossified patellae although a few species possess a bony patella. A patella is also present in the living monotremes, the platypus and the echidna. In other tetrapods, including living amphibians and most reptiles (except some lepidosaurs), the muscle tendons from the upper leg are attached directly to the tibia, and a patella is not present. In 2017 it was discovered that some frog species have a cartilaginous structure which may be kneecaps, contrary to what was previously 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 form of Latin ' or ' or paten, meaning shallow dish.
See also
- Knee pain
- Lateral release
- Lateral retinaculum
- Osteoarthritis
- Patellar reflex