Knee

# Overview
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-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

## 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.

## 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 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

## Injury
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