The knee is the lower extremity joint connecting the femur, patella, 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 the development of osteoarthritis.

Human anatomy

Upon birth, a baby will not have a conventional knee cap, but a growth formed of cartilage. 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-patellar joint consists of the patella, or "kneecap", a so-called sesamoid bone which sits within the tendon of the anterior thigh muscle (m. quadriceps femoris), and the patellar groove on the front of the femur through which it slides.
• 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."

Ligaments

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. It is often torn during twisting or bending of the knee.
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 displacement 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.

Menisci

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.

Synovial Fluid

Synovial fluid is a thick, stringy fluid found in the cavities of synovial joints. With its egg-like consistency (synovial comes from Latin for "egg"), synovial fluid reduces friction between the articular cartilage and other tissues in joints to lubricate and cushion them during movement.

Movements

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

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

The medial genicular arteries penetrate the knee joint

Injury

In sports that place great pressure on the knees, especially with twisting forces, it is common to tear one or more ligaments or cartilages. An increasingly common victim to injury is the anterior cruciate ligament, often torn as a result of a rapid direction change while running or some other, violent twisting motion. It can also be torn by extending the knee forcefully beyond its normal range. In some such cases, other structures 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, especially in ruck contests, involving the crashing of two knees during the leap. These injuries forced new rule changes 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, especially in female athletes, who have been shown to be especially vulnerable to ACL tears from relatively minor trauma. Techniques to minimize the risk of an ACL injury while skiing are published by Vermont Safety Research

Diagnosis

Several diagnostic maneuvers help clinicians diagnose an injured ACL. In the anterior drawer test, the examiner applies an anterior force on the proximal tibia with the knee in 90 degrees of flexion. The Lachman test is similar, but performed with the knee in only about twenty degrees of flexion, while the pivot-shift test adds a valgus (outside-in) force to the knee while it is moved from flexion to extension. Any abnormal motion in these maneuvers suggests a tear.

The diagnosis is usually confirmed by MRI, the availability of which has greatly lessened the number of purely diagnostic arthroscopies performed.

Animal anatomy

In humans the knee refers to the joints between the femur, tibia and patella. In quadrupeds, particularly horses and ungulates the term is commonly used to refer to the carpus, probably because of its similar hinge or ginglymus action. The joints between the femur, tibia and patella are known as the stifle in quadrupeds. In insects and other animals the term knee is used widely to refer to any ginglymus joint.

See also

• Cartilage
• Knee Cartilage Replacement Therapy
• Knee examination
• Kneebar
• Reflex hammer
• Partial knee replacement

Additional images

References