The knee joint is the largest joint in the human body and is of elementary importance for the upright gait of humans. Because of this prominent position, it is prone to wear and injury and is one of the most common reasons for seeing a doctor in an orthopedic office.
What is the knee joint?
Schematic diagram showing the anatomy and structure of the knee joint. Click to enlarge. The knee joint is actually a compound joint made up of 3 bones: the thigh bone (femur), the shin bone (tibia) as well as the kneecap (patella). Anatomically, the joint between the tibia and fibula is also part of the knee, but it does not participate in the actual movements of the knee joint. Movement in the knee joint is basically a hinge movement between extension and flexion, plus a slight rotation.
Anatomy and structure
In addition to the bones involved, the anatomy describes ligaments, joint capsule, and structures that run along them, such as blood vessels and nerves. Here, the bony joint surfaces are covered with cartilage and encased by a joint capsule, which uses synovial fluid to provide the lowest possible friction contact between the joint surfaces. Two large rollers at the end of the thigh bone, the so-called femoral condyles, articulate with the rather flat joint surfaces of the tibia. The tibial surfaces are framed on the inside and outside by the two menisci. They form a sliding bearing for the joint rollers of the femur, frame the middle and outer joint like two sockets and guarantee the rotation of the knee joint through their mobility. The cruciate ligaments, which connect the femur with the tibia and cross each other in their course, are located in the middle between the inner and outer femoral rolls. The anterior cruciate ligament runs from above-outside-back to below-inside-front; the posterior cruciate ligament from above-inside-front to below-outside-back. They primarily limit rotation. On each side of the knee joint there is a lateral ligament, which prevents the knee joint from folding up sideways. At the front of the knee joint is the patella, which is embedded in the tissue (fat body) via a tendon connection between the anterior thigh musculature and the anterior edge of the sham bone, and whose posterior surface is in contact with the femur and slides along it. The major blood vessels and nerves all pass through the popliteal fossa. Here, the popliteal pulse can be palpated and the structures necessary to supply the lower legs and feet are optimally protected from injury. A very susceptible spot for nerve pressure damage is formed by the course of the so-called fibular nerve, which runs very superficially at the head of the fibula, i.e. on the outside laterally below the knee joint.
Function and tasks
The knee joint is a wheel-angle joint, a combination of a wheel and hinge joint. Four main movements are possible about two main axes:
Extension and flexion are the main directions; in addition, external as well as internal rotation is possible with slight flexion. When the knee joint is extended, the maximally tensioned outer ligaments prevent this rotation. Hyperextension is only possible with special training or ligament slackness. The lower leg can be brought up to a flexion angle of 160 degrees to the back of the thigh. At the end, it is not the ligamentous apparatus of the joint but the soft tissues of the upper and lower leg that prevent further flexion. Recording the degrees of motion and ligament integrity and function with specific tests are the basis of every trauma surgery and orthopedic examination of the knee joint.
Diseases and complaints
In young people, injuries are the primary concern: ligaments are most commonly torn during sports, especially skiing and playing soccer. The anterior cruciate ligament is a highly vulnerable structure in this regard, especially during rotational movements (skis tilted, hole in the ground on the soccer field, etc.). A combination injury of several ligaments is common, e.g. the so-called “Unhappy triad” of anterior cruciate ligament tear, internal meniscus injury and rupture of the internal collateral ligament. However, the menisci can also be destroyed as part of general degeneration (osteoarthritis).Since a knee joint injury often involves the tearing of small blood vessels, there is often joint effusion, which makes targeted physical examination difficult for the physician (“every movement hurts”). X-rays, MRI scans or diagnostic knee arthroscopy are then often necessary to determine the exact extent of the injury. In older people, knee joint arthrosis is one of the most common joint complaints. Initially with pain only on exertion (“start-up pain”), it can become permanent pain via increasing inflammation within a short or long period of time, which severely impairs the quality of life. If painkillers such as aspirin or ibuprofen help at first, knee joint lavage and finally joint replacement with a prosthesis often become necessary. This represents a definitive healing option and can make the pain forgotten just a few days after the operation, but should only be at the end of the therapy strategy.