Knee

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. 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. Articular bodies The main articular bodies of the femur are its lateral and medial condyles. 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. 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. It is inserted into the thin anterior wall of the joint capsule. 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) 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. 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. Bursae Numerous 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. Menisci The articular disks of the knee-joint are called menisci because they only partly divide the joint space. 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. Intracapsular The knee is stabilized by a pair of cruciate ligaments. These ligaments are both extrasynovial, intracapsular ligaments. The anterior cruciate ligament (ACL) stretches from the lateral condyle of femur to the anterior intercondylar area. 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. This very strong ligament helps give the patella its mechanical leverage and also functions as a cap for the condyles of the femur. Laterally and medially to the patellar ligament, the lateral and medial retinacula connect fibers from the vasti lateralis and 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. 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 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. ==Muscles==

The most muscles responsible for the movement of the knee joint belong to either the anterior, medial or 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 lower leg provide weak knee flexion, namely the gastrocnemius, in addition to their primary function of moving the foot. Extensors Flexors Posterior compartment Medial compartment: Blood supply 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 recurrent branch of anterior tibial artery. The medial genicular arteries penetrate the knee joint. ==Function==

The knee permits flexion and 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 over the tibia during rotation, while the femur rolls and glides over both menisci during extension-flexion. 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. Extended position With the knee extended, both the lateral and medial collateral ligaments, 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. 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°. ==Clinical significance==

Knee pain is caused by trauma, misalignment, degeneration, and conditions producing arthritis. 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. Patellofemoral instability may relate to hip abnormalities or to tightness of surrounding ligaments. Common injuries due to physical activity 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. 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 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. 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 meniscus tears are treated conservatively but most large tears require surgery. Fractures to examine possible fractures after a knee injury Knee fractures are rare but do occur, especially as a result of a 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-rays can easily confirm the injury and surgery will depend on the degree of displacement and type of fracture. Ruptured tendon 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 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, 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. 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. Varus or valgus deformity 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, which is an angle between the femoral mechanical axis and the center of the ankle joint. It is normally between 1.0° and 1.5° of varus in adults. Normal ranges are different in children. File:Hip-knee-ankle angle by age.png|Hip-knee-ankle angle by age, with 95% prediction interval. Radiofrequency ablation of certain knee nerves is an outpatient procedure to reduce chronic arthritic pain. Using radiofrequency energy delivered via small electrodes positioned at target genicular nerves, the treatment achieves partial sensory denervation of the joint capsule. 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. In clinical research, such treatment has been shown to produce about 50% less knee pain for up to two years after the procedure. Surgical interventions 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 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) • 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. 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 X-ray 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) Illustrations 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. ==Other animals==

In humans, the term "knee" refers to the joints between the femur, tibia, and patella, in the leg. In quadrupeds such as dogs, horses, and mice, the homologous joints between the femur, tibia, and patella, in the hind leg, are known as the stifle joint. Also in quadrupeds, particularly horses, ungulates, and elephants, the layman's term "knee" also commonly refers to the forward-facing joint in the foreleg, the carpus, which is homologous to the human wrist. In 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 ==