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 muscles, 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 fluid which is contained inside the "synovial" membrane, or "joint capsule". The recess behind the knee is called the "popliteal fossa".
Ligaments
The human knee is associated with the following ligaments:
anterior cruciate ligament (ACL). The critically important ACL prevents the tibia from being pushed too far anterior relative to the femur.
medial collateral ligament (MCL a.k.a. tibial collateral ligament). 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 collateral ligament). The LCL protects the lateral side from an inside bending force (a varus force).
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. 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, locking, unlocking, and slight rotation. The ligaments and menisci, along with the muscles which traverse the joint, prevent movement beyond the knee's intended range of motion.
Injury
In sports such as wrestling, basketball, competitive swimming, American football, Australian rules, skiing and soccer, or other sports that involve great stress to the knees, it is common to tear one or more ligaments or cartilages. Especially debilitating is the unfortunately common triad of torn medial collateral and anterior cruciate ligaments and a torn medial meniscus. Before the advent of arthroscopy and arthroscopic surgery, patients having surgery for this injury 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 AFL 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]
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.
