Knee (joint)

The knee is an articulation connecting the femur (thighbone) and the tibia (larger bone of the lower leg), with the patella (kneecap) forming part of the structure. It is one of the body's largest joints and bears substantial weight when a person stands, walks, or runs.

The primary movements of the knee are flexion and extension; when the knee flexes, limited inward and outward rotation becomes possible. Knee stability depends on ligaments, the femoral muscles, and the joint capsule, since the bone structure alone does not provide complete stability.

Anatomy

The lower end of the femur has two rounded projections (condyles) that rest on the upper surface of the tibia. Between them lie two wedge-shaped menisci (cartilage structures) that distribute weight and act as shock absorbers. The bone surfaces are covered by smooth cartilage that reduces friction. The joint capsule contains synovial fluid that lubricates the articulation.

Key ligaments

Four ligaments are commonly described as the most important: two cruciate ligaments inside the joint (anterior and posterior) that control forward and backward movement of the tibia, and two collateral ligaments on the sides (medial and lateral) that prevent sideways motion. The quadriceps muscle of the front thigh ends in a tendon attached to the patella, which is used to extend the knee.

Injury and disease

Common injuries include rupture of the anterior cruciate ligament, meniscus tears, and damage to the joint capsule from sudden acceleration or deceleration—often in sports. Long-term conditions mainly include cartilage degeneration (osteoarthritis), related to age, weight-bearing load, and previous injuries. Treatment ranges from rest and physical rehabilitation to surgery, including joint replacement.