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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


In human anatomy, the knee is the lower extremity joint connecting the femur and the tibia. Since in humans the knee supports nearly the entire weight of the body, it is the joint most vulnerable both to acute injury and to the development of osteoarthritis.

Function of the knee

The knee functions as a living, self-maintaining, biologic transmission, the purpose of which is to accept and transfer biomechanical loads between the femur, tibia, patella, and fibula. In this analogy the ligaments represent non-rigid adaptable sensate linkages within the biologic transmission. The articular cartilages act as bearing surfaces, and the menisci as mobile bearings. The muscles function as living cellular engines that in concentric contraction provide motive forces across the joint, and in eccentric contraction act as brakes and dampening systems, absorbing loads.

Human anatomy

Upon birth, a baby will not have a conventional knee cap, but a knee cap formed of cartilege. In Human females this turns to a normal bone knee cap by the age of 3, in males the age of 5.

The knee is a complex, compound, condyloid variety of a synovial joint which hovers. It actually comprises two separate joints.

  • The femoro-tibial joint links the femur, or thigh bone, with the tibia, the main bone of the (lower) leg. The joint is bathed in a viscous (synovial) fluid which is contained inside the "synovial" membrane, or joint capsule.

The recess behind the knee is called the popliteal fossa it can also be called a "knee pit."


Name Capsule From To Description
anterior cruciate ligament (ACL) inside lateral condyle of femur anterior intercondylar area The critically important ACL prevents the tibia from being pushed too far anterior relative to the femur.
posterior cruciate ligament (PCL) inside medial condyle of femur posterior intercondylar area Injury to this ligament is uncommon but can occur as a direct result of forced trauma to the ligament. This ligament prevents posterior displacment of the tibia relative to the femur.
capsular ligament - - - -
patellar ligament outside patella tuberosity of the tibia - 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. This very strong ligament helps give the patella its mechanical leverage and also functions as a cap for the condyles of the femur.
medial collateral ligament (MCL a.k.a. "tibial") outside medial epicondyle of the femur medial tibial condyle The MCL protects the medial side of the knee from being bent open by a stress applied to the lateral side of the knee (a valgus force).
lateral collateral ligament (LCL a.k.a. "fibular") outside lateral epicondyle of the femur head of fibula The LCL protects the lateral side from an inside bending force (a varus force).
oblique popliteal ligament outside medial condyle of tibia - Tendinous expansion of the semimembranosus muscle. Strengthens the back of the capsule
arcuate popliteal ligament outside Intercondylar area of tibia, and lateral condyle of femur, to below head of fibula Connects to the medial portion of the fibular head -This ligament strengthes the knee posterolaterally and usually when injured is in combination with a PCL and popliteus tendon injury.


These are cartilaginous elements within the knee joint which serve to protect the ends of the bones from rubbing on each other and to effectively deepen the tibial sockets into which the femur attaches. They also play a role in shock absorption. There are two menisci in each knee, the medial meniscus and the lateral meniscus. Either or both may be cracked, or torn, when the knee is forcefully rotated and/or bent.


The knee permits the following movements: flexion, extension, as well as slight medial and lateral rotation. Also, the knee has special locking and unlocking mechanisms, related to movement by the femoral condyles on the tibial plateau. The ligaments and menisci, along with the muscles which traverse the joint, prevent movement beyond the knee's intended range of motion. It is also classified as a hinge joint.

The range of movement is as follows: Flexion is permitted up to 120º when the hip is extended, 140º when the hip is flexed and 160º when the knee is flexed passively. Medial rotation is limited to 10º and lateral rotation to 30º .

Blood Supply of Knee Joint

The femoral artery and the popliteal artery help form the arterial network surrounding the knee joint (articular rete). There are 6 main branches:

1. Superior medial genicular artery
2. Superior lateral genicular artery
3. Inferior medial genicular artery
4. Inferior lateral genicular artery
5. Descending genicular artery
6. Recurrent branch of anterior tibial artery

It is important to note that the medial genicular arteries penetrate the knee joint


artificial knee

In sports that place great stress on the knees, especially with twisting forces, it is common to tear one or more ligaments or cartilages. The anterior cruciate ligament is often torn as a result of a rapid direction change while running or as a result of some other type of violent twisting motion. It can also be torn by being extended forcefully beyond its normal range, or as a result of being forced sideways. In such cases, other structures will incur damage as well. Especially debilitating is the unfortunately common "unhappy triad" of torn medial collateral and anterior cruciate ligaments and a torn medial meniscus. This typically arises from a combination of inwards forcing and twisting.

Before the advent of arthroscopy and arthroscopic surgery, patients having surgery for a torn ACL required at least nine months of rehabilitation. With current techniques, such patients may be walking without crutches in two weeks, and playing some sports in but a few months. In Australian rules football, knee injuries are among the most common, with a great deal of controversy caused in ruck contests, where the crashing of two knees during the leap has caused injuries to numerous players. This forced new rule changes in the Australian Football League for the 2005 season.

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. Techniques to minimize the risk of an ACL injury while skiing are published by Vermont Safety Research

See also

Additional images

(Images courtesy of RadsWiki)

External links

Template:Joints of lower limbs Template:Human anatomical features

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