View source for Knee

{{short description|Leg joint in primates}}
{{About|the joint}}
{{More citations needed|date=February 2022}}
{{Infobox anatomy
| Name      = Knee
| Latin     = articulatio genus
| Image     = Human knees.jpg
| Caption   = Human knees
| Width     = 150px
| Image2    = Blausen 0597 KneeAnatomy Side.png
| Caption2  = Right knee (front-right view)
| Precursor =
| System    = [[Human musculoskeletal system|Musculoskeletal system]]
| Artery    =
| Vein      =
| Nerve     = [[femoral nerve|Femoral]], [[obturator nerve|obturator]], [[sciatic nerve|sciatic]]
| Lymph     =
}}
In [[human]]s and other [[primate]]s, the '''knee''' joins the [[thigh]] with the [[leg]] and consists of two [[joint]]s: one between the [[femur]] and [[tibia]] (tibiofemoral joint), and one between the femur and [[patella]] (patellofemoral joint).<ref>{{cite book |chapter=Anatomy of Knee |chapter-url=https://books.google.com/books?id=KquYyFiVHtIC&pg=PA10 |pages=10–11 |first=Bimal |last=Chhajer |year=2006 |title=Knee Pain |publisher=Fusion Books |isbn=978-81-8419-181-3}}</ref> It is the largest joint in the human body.<ref>{{cite journal |last1=Kulowski |first1=Jacob |date=July 1932 |title=Flexion contracture of the knee |journal=The Journal of Bone and Joint Surgery |volume=14 |issue=3 |pages=618–663 |url=http://jbjs.org/content/14/3/618 |access-date=2015-05-08 |archive-url=https://archive.today/20160803181853/http://jbjs.org/content/14/3/618 |archive-date=2016-08-03 |url-status=dead }} Republished as: {{cite journal |pmid=17975372 |year=2007 |last1=Kulowski |first1=J |title=Flexion contracture of the knee: The mechanics of the muscular contracture and the turnbuckle cast method of treatment; with a review of fifty-five cases. 1932 |journal=Clinical Orthopaedics and Related Research |volume=464 |pages=4–10 |doi=10.1097/BLO.0b013e31815760ca |s2cid=9932906 |doi-access=free }}</ref> The knee is a modified [[hinge joint]], which permits [[flexion]] and [[extension (kinesiology)|extension]] as well as slight internal and external rotation. The knee is vulnerable to injury and to the development of [[osteoarthritis]].

It is often termed a ''compound joint'' having '''tibiofemoral''' and '''patellofemoral''' components.<ref name="pmid19594940">{{cite journal |doi=10.1186/1745-6673-4-19 |pmid=19594940 |pmc=2726153 |title=Occupational kneeling and radiographic tibiofemoral and patellofemoral osteoarthritis |journal=Journal of Occupational Medicine and Toxicology |volume=4 |pages=19 |year=2009 |last1=Rytter |first1=Søren |last2=Egund |first2=Niels |last3=Jensen |first3=Lilli |last4=Bonde |first4=Jens |doi-access=free }}</ref><ref name="pmid19726621">{{cite journal |doi=10.1177/0363546509341829 |pmid=19726621 |pmc=3832057 |title=Tibiofemoral and Patellofemoral Kinematics After Reconstruction of an Isolated Posterior Cruciate Ligament Injury: In Vivo Analysis During Lunge |journal=The American Journal of Sports Medicine |volume=37 |issue=12 |pages=2377–2385 |year=2009 | vauthors = Gill TJ, Van de Velde SK, Wing DW, Oh LS, Hosseini A, Li G}}</ref> (The [[fibular collateral ligament]] is often considered with tibiofemoral components.)<ref name="pmid17764523">{{cite journal |doi=10.1111/j.1469-7580.2007.00803.x |pmid=17764523 |pmc=2375777 |title=The effect of tibiofemoral loading on proximal tibiofibular joint motion |journal=Journal of Anatomy |volume=211 |issue=5 |pages=647–653 |year=2007 |last1=Scott |first1=Jacob |last2=Lee |first2=Ho |last3=Barsoum |first3=Wael |last4=Van Den Bogert |first4=Antonie J.}}</ref>
{{TOC limit|3}}

==Structure==

The knee is a modified [[hinge joint]], a type of [[synovial joint]], which is composed of three functional compartments: the patellofemoral articulation, consisting of the [[patella]], or "kneecap", and the [[patellar groove]] on the front of the [[femur]] through which it slides; and the medial and lateral tibiofemoral articulations linking the femur, or thigh bone, with the [[tibia]], the main bone of the lower leg.<ref name="Burgener">{{cite book
| url       = https://books.google.com/books?id=brsH_IqPzzoC&pg=PA390
| title     = Differential Diagnosis in Magnetic Resonance Imaging
| first1    = Francis A. | last1 = Burgener
| first2    = Steven P. | last2 = Meyers
| first3    = Raymond K. | last3 = Tan
| first4    = Wolfgang | last4 = Zaunbauer
| publisher = Thieme | year = 2002
| isbn      = 978-1588900852 |oclc= 48983197| location=Stuttgart }}, p. 390</ref> The joint is bathed in [[synovial fluid]] which is contained inside the [[synovial membrane]] called the [[Articular capsule of the knee joint|joint capsule]]. The [[posterolateral corner]] of the knee is an area that has recently been the subject of renewed scrutiny and research.<ref>{{cite journal |last1=Chahla |first1=Jorge |last2=Moatshe |first2=Gilbert|last3=Dean |first3=Chase S.|last4=LaPrade |first4=Robert F.|date=2016|title=Posterolateral Corner of the Knee: Current Concepts|journal=The Archives of Bone and Joint Surgery |volume=4 |issue=2 |pages=97–103 |pmid=27200384 |pmc=4852053 }}</ref>

The knee is the largest joint and one of the most important joints in the body. It plays an essential role in movement related to carrying the body weight in horizontal (running and walking) and vertical (jumping) directions.<ref>{{cite web |url=https://www.lecturio.com/concepts/knee-joint/|title=Knee Joint|website=The Lecturio Medical Concept Library |access-date=2021-06-22}}</ref>

At birth, the kneecap is just formed from [[cartilage]], and this will [[ossify]] (change to [[bone]]) between the ages of three and five years. Because it is the largest [[sesamoid bone]] in the human body, the ossification process takes significantly longer.<ref>{{cite web|website=HowStuffWorks.com|title=Do babies have kneecaps?|url=http://science.howstuffworks.com/life/human-biology/babies-kneecaps1.htm|url-status=live|archive-url=https://web.archive.org/web/20130315132631/http://science.howstuffworks.com/life/human-biology/babies-kneecaps1.htm|archive-date=2013-03-15|date=2009-02-19}}</ref>

{{gallery|mode=packed
|Gray351.png|Lateral aspect of right knee
|Gray352.png|Posterior aspect of right knee
|Gray246.png|Articular surfaces of femur
|Gray257.png|Articular surfaces of tibia
}}

===Articular bodies===
The main articular bodies of the femur are its [[lateral condyle of femur|lateral]] and [[Medial condyle of femur|medial]] [[Condyle (anatomy)|condyle]]s. These diverge slightly distally and posteriorly, with the lateral condyle being wider in front than at the back while the medial condyle is of more constant width.<ref name="Platzer">{{cite book |last=Platzer |first=Werner |title=Color Atlas of Human Anatomy, Vol. 1: Locomotor System |publisher=[[Thieme Medical Publishers|Thieme]] |year=2004 |isbn=978-1588901590 |edition=5th |pages=26, 192–252 |oclc=727951508}}</ref>{{rp|206}} The radius of the condyles' curvature in the [[sagittal plane]] becomes smaller toward the back. This diminishing radius produces a series of [[involute]] midpoints (i.e. located on a spiral). The resulting series of transverse axes permit the sliding and rolling motion in the flexing knee while ensuring the collateral ligaments are sufficiently lax to permit the rotation associated with the curvature of the medial condyle about a vertical axis.<ref name="Platzer" />{{rp|194–95}}

The pair of tibial condyles are separated by the intercondylar eminence<ref name="Platzer" />{{rp|206}} composed of a lateral and a medial tubercle.<ref name="Platzer" />{{rp|202}}

The patella also serves an articular body, and its posterior surface is referred to as the trochlea of the knee.<ref>{{cite book|title=The Netter collection of medical illustrations. Volume 6, Musculoskeletal system. Part II, Spine and lower limb: a compilation of paintings|author=Netter, Frank H. |date=2013|publisher=Elsevier Saunders|others=Iannotti, Joseph P., Parker, Richard D. (Orthopedist), Machado, Carlos A. G.|isbn=978-1416063827|edition=2nd|location=Philadelphia, PA|oclc=821699791}}</ref> It is inserted into the thin anterior wall of the joint capsule.<ref name="Platzer" />{{rp|206}} On its posterior surface is a lateral and a medial articular surface,<ref name="Platzer" />{{rp|194}} both of which communicate with the [[Intercondylar fossa of femur|patellar surface]] which unites the two femoral condyles on the anterior side of the bone's distal end.<ref name="Platzer" />{{rp|192}}

===Articular capsule===
{{Main|Articular capsule of the knee joint}}
The articular capsule has a [[Synovial fluid|synovial]] and a fibrous membrane separated by fatty deposits. Anteriorly, the synovial membrane is attached on the margin of the cartilage both on the femur and the tibia, but on the femur, it communicates with the suprapatellar [[bursae|bursa]] or recess and extends the joint space proximally.<ref name="Platzer" />{{rp|210}} The suprapatellar bursa is prevented from being pinched during extension by the [[articularis genus muscle]].<ref name="Reider-etal">{{cite journal |pmid=7204430 |url=http://www.jbjs.org/cgi/pmidlookup?view=long&pmid=7204430 |year=1981 |last1=Reider |first1=B |title=The anterior aspect of the knee joint |journal=The Journal of Bone and Joint Surgery. American Volume |volume=63 |issue=3 |pages=351–356 |last2=Marshall |first2=J. L. |last3=Koslin |first3=B |last4=Ring |first4=B |last5=Girgis |first5=F. G. |doi=10.2106/00004623-198163030-00004 |url-access=subscription }}{{Dead link|date=February 2020 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Behind, the synovial membrane is attached to the margins of the two femoral condyles which produces two extensions (semimembranosus bursa under medial head of the gastrocnemius and popliteal bursa under lateral head of the gastrocnemius)<ref name="Ryan 2011">{{cite book |last1=Ryan |first1=Stephanie |title=Anatomy for diagnostic imaging |date=2011 |publisher=Elsevier Ltd |isbn=978-0702029714 |oclc=
729966424 |pages=289, 292|edition=Third |chapter=Chapter 8}}</ref> similar to the suprapatellar bursa. Between these two extensions, the synovial membrane passes in front of the two cruciate ligaments at the center of the joint, thus forming a pocket direct inward.<ref name="Platzer" />{{rp|210}}

Synovium lining the capsule and its bursae. The synovium also lines infrapatellar fat pad, the fat pad that lies below the ligamentum patellae. Synovium projecting into the fat pad as two foldings.<ref name="Ryan 2011"/>

===Nerves===
From an anterior perspective, the superolateral quadrant of the knee is innervated by the nerves to the [[vastus lateralis]] and [[vastus intermedius]], the [[sciatic nerve]], and by the superior lateral genicular and [[common fibular nerve]]s; in the inferolateral quadrant, the inferior lateral genicular nerve and recurrent fibular nerves predominate; the superomedial quadrant is innervated by the nerves to the [[vastus medialis]] and vastus intermedius, the [[Obturator nerve|obturator]] and sciatic nerves, and by the superior medial genicular nerve; and the inferomedial quadrant has innervation by the inferior medial genicular nerve and the infrapatellar branch of the [[saphenous nerve]].<ref name="tran-ant">{{cite journal |vauthors=Tran J, Peng PW, Lam K, Baig E, Agur AM, Gofeld M |title=Anatomical Study of the Innervation of Anterior Knee Joint Capsule: Implication for Image-Guided Intervention |journal=Regional Anesthesia and Pain Medicine |volume=43 |issue=4 |pages=407–414 |date=May 2018 |pmid=29557887 |doi=10.1097/AAP.0000000000000778 |url=https://www.proquest.com/docview/2720408173|id={{ProQuest|2720408173}} |url-access=subscription }}</ref><ref name="fonk">{{cite journal |vauthors=Fonkoué L, Behets C, Kouassi JK, Coyette M, Detrembleur C, Thienpont E, Cornu O|display-authors=3 |title=Distribution of sensory nerves supplying the knee joint capsule and implications for genicular blockade and radiofrequency ablation: an anatomical study |journal=Surgical and Radiologic Anatomy |volume=41 |issue=12 |pages=1461–1471 |date=December 2019 |pmid=31338537 |doi=10.1007/s00276-019-02291-y |url=https://link.springer.com/article/10.1007/s00276-019-02291-y|url-access=subscription }}</ref>

The articular branches from the obturator and tibial nerves supply the posterior knee capsule, with additional supply from the common fibular nerve and sciatic nerve; the tibial nerve innervates the entire posterior capsule; the posterior division of the obturator nerve and the tibial nerve supply the superomedial aspect of the posterior capsule; the superolateral aspect of the posterior capsule is innervated by the tibial nerve, and by the common fibular and sciatic nerves.<ref name=fonk/><ref name="tran-post">{{cite journal |vauthors=Tran J, Peng PW, Gofeld M, Chan V, Agur AM |title=Anatomical study of the innervation of posterior knee joint capsule: implication for image-guided intervention |journal=Regional Anesthesia and Pain Medicine |volume=44 |issue=2 |pages=234–238 |date=February 2019 |pmid=30700618 |doi=10.1136/rapm-2018-000015 |url=https://www.proquest.com/docview/2553100556|id={{ProQuest|2553100556}} |url-access=subscription }}</ref>

===Bursae===
{{Main|Knee bursae}}
Numerous [[bursa (anatomy)|bursae]] surround the knee joint. The largest communicative bursa is the [[suprapatellar bursa]] described above. Four considerably smaller bursae are located on the back of the knee. Two non-communicative bursae are located in front of the patella and below the patellar [[tendon]], and others are sometimes present.<ref name="Platzer" />{{rp|210}}

===Cartilage===
[[Cartilage]] is a thin, elastic [[Tissue (biology)|tissue]] that protects the [[bone]] and makes certain that the [[joint]] surfaces can slide easily over each other. Cartilage ensures supple knee movement. There are two types of joint cartilage in the knees: fibrous cartilage (the [[meniscus (anatomy)|meniscus]]) and [[hyaline cartilage]]. Fibrous cartilage has tensile strength and can resist pressure.{{clarify|date=January 2024}} Hyaline cartilage covers the surface along which the joints move. Collagen fibres within the articular cartilage have been described by Benninghoff as arising from the subchondral bone in a radial manner, building so called Gothic arches. On the surface of the cartilage, these fibres appear in a tangential orientation and increase the abrasion resistance. There are no blood vessels inside of the hyaline cartilage, the alimentation is performed per diffusion. Synovial fluid and the subchondral bone marrow serve both as nutrition sources for the hyaline cartilage. Lack of at least one source induces a degeneration. Cartilage will wear over the years. Cartilage has a very limited capacity for self-restoration. The newly formed tissue will generally consist of a large part of fibrous cartilage of lesser quality than the original hyaline cartilage. As a result, new cracks and tears will form in the cartilage over time.<ref>{{cite web|url=https://www.britannica.com/science/cartilage|title=Cartilage|website=Encyclopaedia Britannica|access-date=2021-06-23}}</ref>

=== Menisci ===
{{Main|Medial meniscus|Lateral meniscus}}
The [[articular disk]]s of the knee-joint are called menisci because they only partly divide the joint space.<ref name="Platzer" />{{rp|26}} These two disks, the medial meniscus and the lateral meniscus, consist of connective tissue with extensive collagen fibers containing cartilage-like cells. Strong fibers run along the menisci from one attachment to the other, while weaker radial fibers are interlaced with the former. The menisci are flattened at the center of the knee joint, fused with the synovial membrane laterally, and can move over the tibial surface.<ref name="Platzer" />{{rp|208}}<ref>{{cite web|last=Gibbon|first=Anthony|title=Knee Anatomy|url=http://tony-gibbon.co.uk/treatment/knee-treatment/anatomy/knee-anatomy|publisher=North Yorkshire Orthopaedic Specialists|access-date=6 February 2013|url-status=live|archive-url=https://web.archive.org/web/20130423141216/http://tony-gibbon.co.uk/treatment/knee-treatment/anatomy/knee-anatomy|archive-date=23 April 2013}}</ref> The upper and lower surfaces of the menisci are free. Each meniscus has anterior and posterior horns that meet in the intercondylar area of the tibia.<ref name="Ryan 2011"/>

Medial meniscus is bigger, less curved, and thinner. Its posterior horn is thicker (14mm) than the anterior horn (6mm).<ref name="Ryan 2011"/>

The lateral meniscus is smaller, more curved (nearly circular), and has more uniform thickness than medial meniscus (10mm). The lateral meniscus is less attached to the joint capsule, because its posterolateral surface is grooved by the [[popliteus]] tendon, separating the meniscus from the capsule. The popliteus tendon is not attached to the lateral meniscus.<ref name="Ryan 2011"/>

===Ligaments===
[[File:Knee diagram.svg|thumb|Anterolateral aspect of right knee]]
[[File:Knee medial view.gif|thumb|Anteromedial aspect of right knee]]
The ligaments surrounding the knee joint offer stability by limiting movements and, together with the menisci and several bursae, protect the articular capsule.<ref>{{Citation |last1=Goldman |first1=Lee |title=Goldman's Cecil Medicine |date=2012 |work= |pages=vii |publisher=Elsevier |last2=Schafer |first2=Andrew I.|doi=10.1016/b978-1-4377-1604-7.00513-3 |doi-access=free }}</ref>

====Intracapsular====
The knee is stabilized by a pair of [[cruciate ligament]]s. These ligaments are both extrasynovial, intracapsular ligaments.<ref name="pmid20110155">{{cite journal |vauthors=Kam CK, Chee DW, Peh WC |title=Magnetic resonance imaging of cruciate ligament injuries of the knee |journal=Canadian Association of Radiologists Journal |volume=61 |issue=2 |pages=80–89 |date=April 2010 |pmid=20110155 |doi=10.1016/j.carj.2009.11.003 |s2cid=40819119 |url=}}</ref> The [[anterior cruciate ligament]] (ACL) stretches from the [[lateral condyle of femur]] to the anterior [[intercondylar area]].<ref name="Ryan 2011"/> The ACL prevents the tibia from being pushed too far anterior relative to the femur.<ref name="Ryan 2011"/> It is often torn during twisting or bending of the knee.<ref>{{Cite web |title=ACL injury - Symptoms and causes |url=https://www.mayoclinic.org/diseases-conditions/acl-injury/symptoms-causes/syc-20350738 |access-date=2022-12-30 |website=Mayo Clinic |language=en}}</ref> The [[posterior cruciate ligament]] (PCL) stretches from [[medial condyle of femur]] to the posterior intercondylar area. This ligament prevents posterior displacement of the tibia relative to the femur.<ref name="Ryan 2011"/> Injury to this ligament is uncommon but can occur as a direct result of forced trauma to the ligament.{{citation needed|date=April 2022}}

The [[Transverse ligament of knee|transverse ligament]] stretches from the lateral meniscus to the medial meniscus. It passes in front of the menisci. It is divided into several strips in 10% of cases.<ref name="Platzer" />{{rp|208}} The two menisci are attached to each other anteriorly by the ligament.<ref name="Diab">{{cite book
| title     = Lexicon of Orthopaedic Etymology
| url       = https://books.google.com/books?id=fstFQVnw8-wC&pg=PA200
| first     = Mohammad | last = Diab
| publisher = Taylor & Francis | year = 1999
| isbn      = 978-9057025976 |oclc= 43779363 }}, p. 200</ref> The [[posterior meniscofemoral ligament|posterior]] (of Wrisberg) and [[anterior meniscofemoral ligament]]s (of Humphrey) stretch from the posterior horn of the lateral meniscus to the medial femoral condyle. They pass anterior and posterior to the posterior cruciate ligament respectively.<ref name="Ryan 2011"/><ref name="Platzer" />{{rp|208}} The [[Coronary ligament of the knee|meniscotibial ligaments]] (or "coronary") stretches from inferior edges of the menisci to the periphery of the tibial plateaus.

====Extracapsular====
The [[patellar ligament]] connects the patella to the [[tuberosity of the tibia]]. It is also occasionally called the patellar tendon because there is no definite separation between the [[quadriceps tendon]] (which surrounds the patella) and the area connecting the patella to the tibia.<ref>{{cite web |title=Definition of patellar tendon |url=http://www.medterms.com/script/main/art.asp?articlekey=34200 |url-status=dead |archive-url=https://web.archive.org/web/20131105032616/http://www.medterms.com/script/main/art.asp?articlekey=34200 |archive-date=2013-11-05 |access-date=2008-12-11 |publisher=MedicineNet.com}}</ref> This very strong ligament helps give the patella its mechanical leverage<ref name="Moore">{{cite book |last1=Moore |first1=Keith L. |url=https://archive.org/details/clinicallyorient00moor_1 |title=Clinically Oriented Anatomy |last2=Dalley |first2=Arthur F. |last3=Agur |first3=Anne M. R. |publisher=[[Lippincott Williams & Wilkins]] |year=2006 |isbn=978-0781736398 |page=[https://archive.org/details/clinicallyorient00moor_1/page/594 594] |oclc=57202439 |url-access=registration}}, p. 194</ref> and also functions as a cap for the condyles of the femur. Laterally and medially to the patellar ligament, the [[lateral retinaculum|lateral and medial retinacula]] connect fibers from the [[Vastus lateralis muscle|vasti lateralis]] and [[Vastus medialis|medialis muscles]] to the tibia. Some fibers from the [[iliotibial tract]] radiate into the lateral retinaculum and the medial retinaculum receives some transverse fibers arising on the medial femoral epicondyle.<ref name="Platzer" />{{rp|206}}

The [[medial collateral ligament]] (MCL a.k.a. "tibial") stretches from the [[medial epicondyle of the femur]] to the [[medial tibial condyle]]. It is composed of three groups of fibers, one stretching between the two bones, and two fused with the medial meniscus. The MCL is partly covered by the [[Pes anserinus (leg)|pes anserinus]] and the tendon of the [[semimembranosus muscle|semimembranosus]] passes under it.<ref name="Platzer" />{{rp|206}} It protects the medial side of the knee from being bent open by a stress applied to the lateral side of the knee (a [[valgus deformity|valgus]] force).<ref name="Platzer" />{{rp|206}}

The [[fibular collateral ligament|lateral collateral ligament]] (LCL a.k.a. "fibular") stretches from the [[lateral epicondyle of the femur]] to the [[head of fibula]]. It is separate from both the joint capsule and the lateral meniscus.<ref name="Platzer" />{{rp|206}} It protects the lateral side from an inside bending force (a [[varus deformity|varus]] force). The [[anterolateral ligament]] (ALL) is situated in front of the LCL.<ref>{{Cite journal |last=Claes |first=Steven |last2=Vereecke |first2=Evie |last3=Maes |first3=Michael |last4=Victor |first4=Jan |last5=Verdonk |first5=Peter |last6=Bellemans |first6=Johan |date=2013 |title=Anatomy of the anterolateral ligament of the knee |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/joa.12087 |journal=Journal of Anatomy |language=en |volume=223 |issue=4 |pages=321–328 |doi=10.1111/joa.12087 |issn=1469-7580 |pmc=3791125 |pmid=23906341}}</ref>

Lastly, there are two ligaments on the dorsal side of the knee. The [[oblique popliteal ligament]] is a radiation of the tendon of the [[semimembranosus muscle|semimembranosus]] on the medial side, from where it is direct laterally and proximally. The [[arcuate popliteal ligament]] originates on the apex of the head of the fibula to stretch proximally, crosses the tendon of the [[popliteus muscle]], and passes into the capsule.<ref name="Platzer" />{{rp|206}}

==Muscles==
The most muscles responsible for the movement of the knee joint belong to either the [[Anterior compartment of thigh|anterior]], [[Medial compartment of thigh|medial]] or [[posterior compartment of thigh|posterior compartment]] of the thigh. The extensors generally belong to the anterior compartment and the flexors to the posterior. The two exceptions to this is gracilis, a flexor, which belongs to the medial compartment and sartorius, a flexor, in the anterior compartment. Additionally, some muscles in the [[Human leg#Structure|lower leg]] provide weak knee flexion, namely the [[gastrocnemius]], in addition to their primary function of moving the foot.

===Extensors===
{| class="wikitable" width="100%"
! width="12%" | Muscle !! width="16%" |Origin !! width="16%" | Insertion !! width="12%" | Artery !! width="12%" | Nerve !! width="20%" | Action !! width="12%" |Antagonist
|-valign="top"
| [[articularis genus muscle|Articularis genus]] || Distal end of anterior [[body of femur|femoral shaft]] || Proximal extension of the joint capsule of the knee || [[Femoral artery]] || [[Femoral nerve]] || Pulling the suprapatellar bursa during extension of the knee ||
|-valign="top"
| ''[[quadriceps femoris muscle|Quadriceps femoris]]'' || Combined [[rectus femoris]] and [[vasti|vastus]] muscles || [[Patella]] and [[tibial tuberosity]] via the [[Patellar ligament]] ||[[Femoral artery]] || [[Femoral nerve]] || [[Extension (kinesiology)|Extension]] of the knee; [[flexion]] of the hip (rectus femoris only) || [[Hamstring]]
|-valign="top"
| {{spaces|5}}[[rectus femoris muscle|Rectus femoris]] || [[Anterior inferior iliac spine]] and the exterior surface of the bony ridge which forms the iliac portion of the [[acetabulum]] || [[Patella]] and [[tibial tuberosity]] via the [[Patellar ligament]] || [[Femoral artery]] || [[Femoral nerve]] || [[Extension (kinesiology)|Extension]] of the knee; [[flexion]] of the hip || [[Hamstring]]
|-valign="top"
| {{spaces|5}}[[vastus lateralis muscle|Vastus lateralis]] || [[Greater trochanter]], [[intertrochanteric line]], and [[linea aspera]] of the [[femur]] || [[Patella]] and [[tibial tuberosity]] via the [[patellar ligament]] || [[Femoral artery]] || [[Femoral nerve]] || [[Extension (kinesiology)|Extends]] and stabilizes knee || [[Hamstring]]
|-valign="top"
| {{spaces|5}}[[vastus intermedius muscle|Vastus intermedius]] || Antero/lateral [[femur]] || [[Patella]] and [[tibial tuberosity]] via the [[patellar ligament]] || [[Femoral artery]] || [[Femoral nerve]] || [[Extension (kinesiology)|Extension]] of the knee || [[Hamstring]]
|-valign="top"
| {{spaces|5}}[[Vastus medialis]] || [[Femur]] || [[Patella]] and [[tibial tuberosity]] via the [[patellar ligament]] || [[Femoral artery]] || [[Femoral nerve]] || [[Extension (kinesiology)|Extension]] of the knee || [[Hamstring]]
|-valign="top"
|}

===Flexors===
'''Posterior compartment'''
{| class="wikitable" width="100%"
! width="12%" | Muscle !! width="16%" |Origin !! width="16%" | Insertion !! width="12%" | Artery !! width="12%" | Nerve !! width="20%" | Action !! width="12%" |Antagonist
|-valign="top"
| [[sartorius muscle|Sartorius]] || [[Anterior superior iliac spine]] || [[pes anserinus (leg)|Pes anserinus]] || [[superficial circumflex iliac artery]], lateral femoral artery, deep femoral artery, descending geniculate artery, [[femoral artery]] || [[femoral nerve]] || hip joint: flexion, lateral rotation and abduction. Knee joint: flexion and medial rotation || [[Quadriceps muscle]] (partly)
|-valign="top"
| [[biceps femoris muscle|Biceps femoris]] || Long head: [[tuberosity of the ischium]], short head: [[linea aspera]] on the femur<ref name=Gosling273>[[#Gosling|Gosling 2008, p. 273]]</ref> || The [[head of the fibula]]<ref name=Gosling273/> which articulates with the back of the [[lateral tibial condyle]] || [[Inferior gluteal artery]], [[perforating arteries]], [[popliteal artery]] || Long head: medial (tibial) part of [[sciatic nerve]], short head: lateral (common fibular) part of [[sciatic nerve]]<ref name=Gosling273/> || [[Flexion]] of knee, [[laterally rotates]] leg at knee (when knee is flexed), [[extension (kinesiology)|extends]] [[hip joint]] (long head only)<ref name=Gosling273/> || [[Quadriceps muscle]]
|-valign="top"
| [[semitendinosus muscle|Semitendinosus]] || Tuberosity of the ischium<ref name=Gosling273/> || [[pes anserinus (leg)|Pes anserinus]] || [[Inferior gluteal artery]], [[perforating arteries]] || [[sciatic nerve|Sciatic]]<ref name=Gosling273/> ([[tibial nerve|tibial]], [[lumbar spinal nerve 5|L5]], [[sacral spinal nerve 1|S1]], [[sacral spinal nerve 2|S2]]) || Flexes knee, extends hip joint, [[medially rotates]] leg at knee<ref name=Gosling273/>|| [[Quadriceps muscle]]
|-valign="top"
| [[semimembranosus muscle|Semimembranosus]] || Tuberosity of the ischium<ref name=Gosling273/> || Medial surface of [[tibia]]<ref name=Gosling273/> || [[Profunda femoris]], [[gluteal artery (disambiguation)|gluteal artery]] || [[Sciatic nerve]]<ref name=Gosling273/> || Flexes knee, extends hip joint, medially rotates leg at knee<ref name=Gosling273/> || [[Quadriceps muscle]]
|-valign="top"
| [[gastrocnemius muscle|Gastrocnemius]] || [[medial condyle of femur|Medial]] and [[Lateral condyle of femur|lateral condyle]] of the femur || [[Calcaneus]] || [[Sural arteries]] || [[Tibial nerve]] from the [[sciatic]], specifically, nerve roots [[sacral spinal nerve 1|S1]], [[sacral spinal nerve 2|S2]] || Minor [[flexion]] of knee and [[plantarflexion]]|| [[Tibialis anterior muscle]]
|-valign="top"
| [[plantaris muscle|Plantaris]] || Lateral supracondylar ridge of femur above lateral head of gastrocnemius || [[Tendo calcaneus]] (medial side, deep to [[gastrocnemius]] tendon) || [[Sural arteries]] || [[Tibial nerve]] || Flexes knee and [[plantar flexes]] [[foot]] || [[Tibialis anterior muscle]]
|-valign="top"
| [[popliteus muscle|Popliteus]] || Middle facet of the lateral surface of the [[lateral femoral condyle]] || Posterior [[tibia]] under the [[Tibial condyle (disambiguation)|tibial condyle]]s || [[Popliteal artery]] || [[Tibial nerve]] || [[Medial rotation]] and [[flexion]] of knee ||
|-
|}

'''Medial compartment:'''
{| class="wikitable" width="100%"
! width="12%" | Muscle !! width="16%" |Origin !! width="16%" | Insertion !! width="12%" | Artery !! width="12%" | Nerve !! width="20%" | Action !! width="12%" |Antagonist
|-valign="top"
| [[gracilis muscle|Gracilis]] || [[Inferior pubic ramus]]<ref name=Gosling266>[[#Gosling|Gosling et al. 2008, p. 266]]</ref> || [[pes anserinus (leg)|Pes anserinus]] || [[Obturator artery]] || [[Anterior branch of obturator nerve]]<ref name=Gosling266/> || Flexion and [[medial rotation]] of knee;<ref name=Gosling266/> adduction of hip, [[flexion]] of hip, ||
|-valign="top"
|}

===Blood supply===
[[File:Gray552.png|thumb|Arteries of the knee]]
The [[femoral artery]] and the [[popliteal artery]] help form the arterial network or [[plexus]], surrounding the knee joint. There are six main branches: two [[superior genicular arteries]], two [[inferior genicular arteries]], the [[descending genicular artery]] and the [[Anterior tibial recurrent artery|recurrent branch of anterior tibial artery]].<ref>{{Cite journal |last1=Shim |first1=S. S. |last2=Leung |first2=G. |date=July 1986 |title=Blood supply of the knee joint. A microangiographic study in children and adults |journal=Clinical Orthopaedics and Related Research |issue=208 |pages=119–125 |doi=10.1097/00003086-198607000-00025 |issn=0009-921X |pmid=3522019}}</ref>

The medial genicular arteries penetrate the knee joint.

==Function==
The knee permits [[flexion]] and [[extension (kinesiology)|extension]] about a virtual transverse axis, as well as a slight medial and lateral rotation about the axis of the lower leg in the flexed position. The knee joint is called "mobile" because the femur and lateral meniscus move<ref name="Thieme-Atlas" />{{rp|399}} over the tibia during rotation, while the femur rolls and glides over both menisci during extension-flexion.<ref name="Platzer" />{{rp|212–213}}

The center of the transverse axis of the extension/flexion movements is located where both collateral ligaments and both cruciate ligaments intersect. This center moves upward and backward during flexion, while the distance between the center and the articular surfaces of the femur changes dynamically with the decreasing curvature of the femoral condyles. The total range of motion is dependent on several parameters such as soft-tissue restraints, active insufficiency, and hamstring tightness.<ref name="Thieme-Atlas" />{{rp|398}}
{| class="wikitable" align="right"
|+ Maximum movements <ref name="Thieme-Atlas">{{cite book
| title     = Thieme Atlas of Anatomy: General Anatomy and Musculoskeletal System
| publisher = Thieme |location=New York| year = 2006 | isbn = 978-1588904195 |oclc=
474717189}}</ref>{{rp|398–399}}and muscles<ref name="Platzer" />{{rp|252}}
|-
! Extension 5–10° !! Flexion 120–150°
|-
| [[Quadriceps femoris muscle|Quadriceps]] (with<br/>some assistance from<br/>the [[Tensor fasciae latae]])
| (In order of importance)<br/>[[Semimembranosus muscle|Semimembranosus]]<br/>[[Semitendinosus muscle|Semitendinosus]]<br/>[[Biceps femoris muscle|Biceps femoris]]<br/>[[Gracilis muscle|Gracilis]]<br/>[[Sartorius muscle|Sartorius]]<br/>[[Popliteus muscle|Popliteus]]<br/>[[Gastrocnemius muscle|Gastrocnemius]]
|-
! Internal rotation* 10° !! External rotation* 30–40°
|-
| (In order of importance)<br/>Semimembranosus<br/>Semitendinosus<br/>Gracilis<br/>Sartorius<br/>Popliteus
| Biceps femoris
|-
| colspan=2 | *(knee flexed 90°)
|}

===Extended position===
With the knee extended, both the [[fibular collateral ligament|lateral]] and [[medial collateral ligament]]s, as well as the anterior part of the [[anterior cruciate ligament]], are taut. During extension, the femoral condyles glide and roll into a position which causes the complete unfolding of the [[tibial collateral ligament]]. During the last 10° of extension, an '''obligatory terminal rotation''' is triggered in which the knee is rotated medially 5°. The final rotation is produced by a lateral rotation of the tibia in the non-weight-bearing leg, and by a medial rotation of the femur in the weight-bearing leg. This terminal rotation is made possible by the shape of the medial femoral condyle, assisted by contraction of the popliteus muscle and the [[iliotibial tract]] and is caused by the stretching of the anterior cruciate ligament. Both cruciate ligaments are slightly unwound and both lateral ligaments become taut.<ref name="Platzer" />{{rp|212}}

===Flexed position===
In the flexed position, the collateral ligaments are relaxed while the cruciate ligaments are taut. Rotation is controlled by the twisted cruciate ligaments; the two ligaments get twisted around each other during medial rotation of the tibia—which reduces the amount of rotation possible—while they become unwound during lateral rotation of the tibia. Because of the oblique position of the cruciate ligaments, at least a part of one of them is always tense and these ligaments control the joint as the collateral ligaments are relaxed. Furthermore, the dorsal fibers of the tibial collateral ligament become tensed during extreme medial rotation and the ligament also reduces the lateral rotation to 45–60°.<ref name="Platzer" />{{rp|212}}

=={{Anchor|Knee injury}}Clinical significance==
[[File:918 Knee Injury.jpg|thumb|right|400px|Lateral trauma to the knee can tear the medial collateral ligament, anterior cruciate ligament, and medial meniscus]]
[[Knee pain]] is caused by trauma, misalignment, degeneration, and conditions producing [[arthritis]].<ref name=Afra/> The most common knee disorder is generally known as [[patellofemoral syndrome]].<ref name=Afra/> The majority of minor cases of knee pain can be treated at home with rest and ice, but more serious injuries do require [[surgery|surgical]] care.<ref name=Afra/>

One form of patellofemoral syndrome involves a tissue-related problem that creates pressure and irritation in the knee between the patella and the trochlea (patellar compression syndrome), which causes pain. The second major class of knee disorder involves a tear, slippage, or dislocation that impairs the structural ability of the knee to balance the leg (patellofemoral instability syndrome). Patellofemoral instability syndrome may cause either pain, a sense of poor balance, or both.<ref name=Afra>{{cite journal|vauthors=Afra R, Schepsis A|title=Addressing patellofemoral pathology: Biomechanics and classification|journal=The Journal of Musculoskeletal Medicine|date=May 28, 2008|url=http://www.rheumatologynetwork.com/articles/addressing-patellofemoral-pathology-biomechanics-and-classification|url-status=live|archive-url=https://web.archive.org/web/20150518081310/http://www.rheumatologynetwork.com/articles/addressing-patellofemoral-pathology-biomechanics-and-classification|archive-date=May 18, 2015}}</ref>

[[Prepatellar bursitis]] also known as ''housemaid's knee'' is painful inflammation of the [[prepatellar bursa]] (a frontal knee bursa) often brought about by occupational activity such as roofing.

Age also contributes to disorders of the knee. Particularly in older people, knee pain frequently arises due to osteoarthritis. In addition, weakening of tissues around the knee may contribute to the problem.<ref name=Pill>{{cite journal |vauthors=Pill SG, Khoury LD, Chin GC |title=MRI for evaluating knee pain in older patients: How useful is it? |journal=The Journal of Musculoskeletal Medicine |series=<!-- --> |date=October 29, 2008 |volume=25 |issue=11 |url=http://www.rheumatologynetwork.com/articles/mri-evaluating-knee-pain-older-patients-how-useful-it |display-authors=etal |url-status=live |archive-url=https://web.archive.org/web/20150518081407/http://www.rheumatologynetwork.com/articles/mri-evaluating-knee-pain-older-patients-how-useful-it |archive-date=May 18, 2015 }}</ref> Patellofemoral instability may relate to hip abnormalities or to tightness of surrounding ligaments.<ref name=Afra/>

[[Cartilage]] lesions can be caused by:
*Accidents (fractures)
*Injuries
*The removal of a [[meniscus (anatomy)|meniscus]]
*[[Anterior cruciate ligament]] injury
*[[Posterior cruciate ligament]] injury
*[[Posterolateral corner]] injury
*[[Medial knee injuries]]
*Considerable strain on the knee.

Any kind of work during which the knees undergo heavy stress may also be detrimental to cartilage. This is especially the case in professions in which people frequently have to walk, lift, or squat. Other causes of pain may be excessive on, and wear off, the knees, in combination with such things as [[muscle weakness]] and [[overweight]].

Common complaints:
*A painful, blocked, locked or swollen knee.
*Sufferers sometimes feel as if their knees are about to give way, or may feel uncertain about their movement.

===Overall fitness and knee injury===
Physical fitness is related integrally to the development of knee problems. The same activity such as climbing stairs may cause pain from patellofemoral compression for someone who is physically unfit, but not for someone else (or even for that person at a different time). Obesity is another major contributor to knee pain. For instance, a 30-year-old woman who weighed {{convert|120|lb|kg}} at age 18 years, before her three pregnancies, and now weighs {{convert|285|lb|kg}}, had added {{convert|660|lb|kg}} of force across her patellofemoral joint with each step.<ref>{{cite journal|author=Andrish JT|date=May 8, 2009|title=Sports injuries in weekend warriors: 20 Clinical pearls|url=http://www.psychiatrictimes.com/osteoarthritis/sports-injuries-weekend-warriors-20-clinical-pearls|url-status=dead|journal=The Journal of Musculoskeletal Medicine|volume=26|issue=5|archive-url=https://web.archive.org/web/20150518091655/http://www.psychiatrictimes.com/osteoarthritis/sports-injuries-weekend-warriors-20-clinical-pearls|archive-date=May 18, 2015}}</ref>

===Common injuries due to physical activity===
[[File:Prothese-genou-IMG 0033.jpg|thumb|Model demonstrating parts of an artificial knee]]
In sports that place great pressure on the knees, especially with twisting forces, it is common to tear one or more ligaments or cartilages. Some of the most common knee injuries are those to the medial side: [[medial knee injuries]].<ref>{{Cite book |last1=Campbell |first1=Willis C. |title=Campbell's Operative Orthopedics |last2=Canale |first2=S. T. |last3=Beaty |first3=James H. |publisher=Mosby/Elsevier |year=2008 |edition=11th |location=Philadelphia}}</ref>

===Anterior cruciate ligament injury===
{{Main|Anterior cruciate ligament injury}}
The [[anterior cruciate ligament]] is the most commonly injured ligament of the knee. The injury is common during sports. Twisting of the knee is a common cause of over-stretching or tearing the ACL. When the ACL is injured a popping sound may be heard, and the leg may suddenly give out. Besides [[swelling (medical)|swelling]] and pain, walking may be painful and the knee will feel unstable. Minor tears of the anterior cruciate ligament may heal over time, but a torn ACL requires surgery. After surgery, recovery is prolonged and low impact exercises are recommended to strengthen the joint.<ref>[http://sportsmedicine.about.com/od/kneepainandinjuries/Knee_Pain_and_Injuries.htm Knee pain and injuries] {{webarchive|url=https://web.archive.org/web/20091211214909/http://sportsmedicine.about.com/od/kneepainandinjuries/Knee_Pain_and_Injuries.htm |date=2009-12-11 }} About sports online portal. 2010-01-26</ref>

===Torn meniscus injury===
The menisci act as shock absorbers and separate the two ends of bone in the knee [[joint]]. There are two menisci in the knee, the medial (inner) and the lateral (outer). When there is torn cartilage, it means that the meniscus has been injured. Meniscus tears occur during sports often when the knee is twisted. Menisci injury may be innocuous and one may be able to walk after a tear, but soon swelling and pain set in. Sometimes the knee will lock while bending. Pain often occurs when one squats. Small [[tear of meniscus|meniscus tears]] are treated conservatively but most large tears require surgery.<ref>{{cite journal | title = Acute knee injuries: use of decision rules for selective radiograph ordering | vauthors = Tandeter HB, Shvartzman P, Stevens MA | journal = Am Fam Physician | date = 1 December 1999 | volume = 60 | issue = 9 | pages = 2599–2608 | pmid = 10605994}}</ref>

===Fractures===
{{unreferenced section|date=January 2022}} 
[[File:Xraymachine.JPG|thumb|[[Radiography]] to examine possible fractures after a knee injury]]
Knee fractures are rare but do occur, especially as a result of a [[traffic collision|road accident]]. Knee fractures include a [[patella fracture]], and a type of [[avulsion fracture]] called a [[Segond fracture]]. There is usually immediate pain and swelling, and a difficulty or inability to stand on the leg. The muscles go into [[spasm]] and even the slightest movements are painful. [[X-ray]]s can easily confirm the injury and surgery will depend on the degree of displacement and type of fracture.

===Ruptured tendon===
{{unreferenced section|date=July 2019}}
Tendons usually attach muscle to bone. In the knee the quadriceps and patellar tendon can sometimes tear. The injuries to these tendons occur when there is forceful [[muscle contraction|contraction]] of the knee. If the tendon is completely torn, bending or extending the leg is impossible. A completely torn tendon requires surgery but a partially torn tendon can be treated with leg immobilization followed by [[physical therapy]].

===Overuse===
Overuse injuries of the knee include [[tendonitis]], [[bursitis]], muscle strains, and [[iliotibial band syndrome]]. These injuries often develop slowly over weeks or months. Activities that induce pain usually delay healing. Rest, ice and compression do help in most cases. Once the swelling has diminished, [[heating pad|heat packs]] can increase blood supply and promote healing. Most overuse injuries subside with time but can flare up if the activities are quickly resumed.<ref>{{cite web | url = https://www.medlineplus.gov/kneeinjuriesanddisorders.html | title = Knee injuries and disorders | archive-url = https://web.archive.org/web/20160704221329/https://www.nlm.nih.gov/medlineplus/kneeinjuriesanddisorders.html | archive-date = 2016-07-04 | publisher = MedLine Plus | url-status = live | date = 28 June 2016 }}</ref> Individuals may reduce the chances of overuse injuries by warming up prior to exercise, by limiting high impact activities and keep their weight under control.{{citation needed|date=July 2019}}

{{Anchor|HKA}}<!--Hip-knee-ankle angle redirects here-->

===Varus or valgus deformity===
[[File:Hip-knee-ankle angle.png|thumb|80px|Hip-knee-ankle angle.]]
There are two disorders relating to an abnormal angle in the [[coronal plane]] at the level of the knee:
*''[[Genu valgum]]'' is a [[valgus deformity]] in which the tibia is turned outward in relation to the femur, resulting in a knock-kneed appearance.
*''[[Genu varum]]'' is a [[varus deformity]] in which the tibia is turned inward in relation to the femur, resulting in a bowlegged deformity.
The degree of varus or valgus deformity can be quantified by the ''hip-knee-ankle angle'',<ref name=dahl2009>{{cite journal|last1=W-Dahl|first1=Annette|last2=Toksvig-Larsen|first2=Sören|last3=Roos|first3=Ewa M|title=Association between knee alignment and knee pain in patients surgically treated for medial knee osteoarthritis by high tibial osteotomy. A one year follow-up study|journal=BMC Musculoskeletal Disorders|volume=10|issue=1|pages=154|year=2009|issn=1471-2474|doi=10.1186/1471-2474-10-154|pmid=19995425|pmc=2796991 |doi-access=free }}</ref> which is an angle between the femoral mechanical axis and the center of the [[ankle joint]].<ref name="CherianKapadia2014">{{cite journal|last1=Cherian|first1=Jeffrey J.|last2=Kapadia|first2=Bhaveen H.|last3=Banerjee|first3=Samik|last4=Jauregui|first4=Julio J.|last5=Issa|first5=Kimona|last6=Mont|first6=Michael A.|title=Mechanical, Anatomical, and Kinematic Axis in TKA: Concepts and Practical Applications|journal=Current Reviews in Musculoskeletal Medicine|volume=7|issue=2|year=2014|pages=89–95|issn=1935-973X|doi=10.1007/s12178-014-9218-y|pmid=24671469|pmc=4092202}}</ref> It is normally between 1.0° and 1.5° of varus in adults.<ref name="SheehyFelson2011">{{cite journal| vauthors = Sheehy L, Felson D, Zhang Y, Niu J, Lam YM, Segal N, Lynch J, Cooke TD | display-authors = 6 |title=Does measurement of the anatomic axis consistently predict hip-knee-ankle angle (HKA) for knee alignment studies in osteoarthritis? Analysis of long limb radiographs from the multicenter osteoarthritis (MOST) study|journal=Osteoarthritis and Cartilage|volume=19|issue=1|year=2011|pages=58–64|issn=1063-4584|doi=10.1016/j.joca.2010.09.011|pmid=20950695|pmc=3038654}}</ref> Normal ranges are different in children.<ref name=Sabharwal2009>{{cite journal|last1=Sabharwal|first1=Sanjeev|last2=Zhao|first2=Caixia|title=The Hip-Knee-Ankle Angle in Children: Reference Values Based on a Full-Length Standing Radiograph|journal=The Journal of Bone and Joint Surgery, American Volume|volume=91|issue=10|year=2009|pages=2461–2468|issn=0021-9355|doi=10.2106/JBJS.I.00015|pmid=19797583}}</ref>
<gallery widths="200">
File:Hip-knee-ankle angle by age.png|''Hip-knee-ankle angle'' by age, with 95% [[prediction interval]].<ref name=Sabharwal2009/>
</gallery>

===Chronic pain from osteoarthritis===
{{See also|Nerve block#Genicular nerve block}}
Knee [[osteoarthritis]] is a major cause of pain and disability worldwide, with prevalence estimated at about 4% of the population, particularly among the elderly.<ref name="conger">{{cite journal |vauthors=Conger A, Gililland J, Anderson L, Pelt CE, Peters C, McCormick ZL|display-authors=3 |title=Genicular Nerve Radiofrequency Ablation for the Treatment of Painful Knee Osteoarthritis: Current Evidence and Future Directions |journal=Pain Medicine|volume=22 |issue=Suppl 1 |pages=S20–S23 |date=July 2021 |pmid=34308957 |doi=10.1093/pm/pnab129 |url= https://academic.oup.com/painmedicine/article/22/Supplement_1/S20/6328220}}</ref> [[Radiofrequency_ablation#Knees|Radiofrequency ablation of certain knee nerves]] is an [[outpatient]] procedure to reduce chronic arthritic pain.<ref name=tran-ant/><ref name=fonk/><ref name=conger/> Using radiofrequency energy delivered via small electrodes positioned at target genicular nerves, the treatment achieves partial sensory denervation of the joint capsule.<ref name=conger/> Despite the extensive innervation of the knee, specifically targeting the superior lateral, superior medial, and inferior medial genicular nerves has proved to be an effective ablation method for reducing chronic knee pain.<ref name=conger/> In [[clinical research]], such treatment has been shown to produce about 50% less knee pain for up to two years after the procedure.<ref name=conger/>

===Surgical interventions===
{{unreferenced section|date=July 2019}}
Before the advent of [[arthroscopy]] and [[arthroscopic surgery]], patients having surgery for a torn ACL required at least nine months of rehabilitation, having initially spent several weeks in a full-length plaster cast. With [[anterior cruciate ligament reconstruction|current techniques]], such patients may be walking without crutches in two weeks, and playing some sports in a few months.

In addition to developing new surgical procedures, ongoing research is looking into underlying problems which may increase the likelihood of an athlete suffering a severe knee injury. These findings may lead to effective preventive measures, especially in female athletes, who have been shown to be especially vulnerable to ACL tears from relatively minor trauma.

'''[[Articular cartilage repair]] treatment''':
*[[Arthroscopic]] [[debridement]] of the knee ([[arthroscopic lavage]])
* [[Autologous chondrocyte implantation|Mosaïc-plasty]]
*Microfracture (Ice-picking)
*[[Autologous chondrocyte implantation]]
*Osteochondral [[Autograft]] and [[Allografts]]
*[[PLC Reconstruction]]

==Imaging==
===MRI===
Both anterior cruciate ligament (ACL) and posterior cruciate ligaments (PCL) are hypointense on both T1 and T2 weighted images of MRI. However, some high signal striations are often seen at the distal part of the ACL, making ACL higher intensity than PCL on MRI scans.<ref name="pmid20110155"/>
<gallery>
File:Knee MRI PD TSE FS Sagittal.jpg|Knee MRI (PD TSE FS sagittal)
File:Knee MRI T1 TSE Sagittal.jpg|Knee MRI (T1 TSE sagittal)
File:Knee MRI tse fs sag 320-R.ogg|Knee MRI (sagittal TSE FS)
File:Knee MRI t1 tse sag-R.ogg|Knee MRI (sagittal T1 TSE)
File:Knee MRI t2 tse fs cor 320.ogg|Knee MRI (coronal T2 TSE FS)
File:Knee MRI pd tse fs tra 320.ogg|Knee MRI (traverse PD TSE FS)
File:Knee MRI osteoarthritis pd tse fs sag 320.ogg|Knee MRI osteoarthritis (sagittal TSE FS)
File:Knee MRI osteoarthritis t1 tse sag R.ogg|Knee MRI osteoarthritis (sagittal T1 TSE)
File:Knee MRI osteoarthritis t2 tse fs cor 320.ogg|Knee MRI osteoarthritis (coronal T2 TSE FS)
File:Knee MRI osteoarthritis pd tse fs tra 320.ogg|Knee MRI osteoarthritis (traverse PD TSE FS)
File:Real-time MRI - Knee (central).ogv|[[Real-time MRI]]- Knee
File:Knie ct.gif|Knee MR
</gallery>

===X-ray===
<gallery>
File:Knee plain X-ray.jpg|Knee X-ray
File:Knee plain X-ray weight bearing.jpg|Knee X-ray (weight bearing)
File:Knee plain X-ray weight bearing flexion.jpg|Knee X-ray (weight bearing, flexion)
</gallery>

===Illustrations===
<gallery>
File:Legamenti crociati.jpg|Cruciate ligaments
File:Gray348.png|Left knee-joint from behind, showing interior ligaments.
File:Gray351.png|Capsule of right knee-joint (distended). Lateral aspect.
File:Knee skeleton lateral anterior views.svg|Anterior and lateral view of knee.
File:Slide2CAC.JPG|Anterior view of knee.
</gallery>

==Other animals==
{{unreferenced section|date=July 2019}}
In humans, the term "knee" refers to the joints between the femur, tibia, and patella, in the leg.

In [[quadruped]]s such as dogs, horses, and mice, the [[homology (biology)|homologous]] joints between the femur, tibia, and patella, in the [[hind leg]], are known as the [[stifle joint]]. Also in quadrupeds, particularly horses, [[ungulate]]s, and elephants, the layman's term "knee" also commonly refers to the forward-facing joint in the [[foreleg]], the [[Carpal bones|carpus]], which is homologous to the human [[wrist]].

In [[Bird feet and legs#Skeleton|birds]], the "knee" is the joint between the femur and tibiotarsus, and also the patella (when present). The layman's term "knee" may also refer to the (lower and often more visible due to not being covered by feathers) joint between the tibiotarsus and tarsometatarsus, which is homologous to the human [[ankle]].

In insects and other animals, the term knee widely refers to any [[hinge joint]].

== See also ==
{{col div|colwidth=30em}}
*[[Arthrofibrosis]]
*[[Articular cartilage repair]]
*[[Chondromalacia patellae]]
*[[Fibular collateral ligament]]
*[[Geniculum]]
*[[Genuflection]]
*[[Knee arthritis]]
*[[Knee cartilage replacement therapy]]
*[[Knee examination]]
*[[Kneecapping]]
*[[Kneeling]]
*[[Knésetja]]
*[[Medial collateral ligament]]
*Partial knee replacement [[unicompartmental knee arthroplasty]]
*[[Posterolateral corner injuries]]
*[[Reflex hammer]]
{{colend}}

==References==
{{Reflist|30em}}

==External links==
*{{Commons category-inline|Knee}}
*[https://humanbodypartsanatomy.com/knee-anatomy-parts-names-functions-diagram/ Knee Anatomy] detailed guide for medical college students and medical professionals.
{{Wiktionary|knee}}

{{Human regional anatomy}}
{{Joints of lower limbs}}

{{Authority control}}

[[Category:Knee| ]]
[[Category:Articles containing video clips]]
[[Category:Joints|*Knee]]