Knee

In humans and other primates, the knee is the joint that connects the thigh to the leg and plays a central role in locomotion and weight-bearing. It is the largest joint in the human body and one of the most complex, owing to its need to combine stability with mobility. Anatomically, the knee consists of two principal articulations: the tibiofemoral joint between the femur and tibia, and the patellofemoral joint between the femur and the patella. Because it integrates multiple articulations within a single joint capsule, the knee is often described as a compound joint.
Functionally, the knee is classified as a modified hinge synovial joint. While its primary movements are flexion and extension, it also permits a limited degree of internal and external rotation, particularly when the knee is flexed. This combination of movements is essential for walking, running, jumping, and maintaining balance, but it also makes the knee especially vulnerable to injury and degenerative conditions such as osteoarthritis.

General Structure and Joint Compartments

The knee joint is composed of three functional compartments. The patellofemoral compartment involves the articulation between the patella (kneecap) and the patellar groove on the anterior surface of the femur. The other two compartments are the medial tibiofemoral and lateral tibiofemoral articulations, which link the femur with the tibia, the main bone of the lower leg.
As a synovial joint, the knee contains a joint cavity filled with synovial fluid, enclosed by an articular capsule. This arrangement allows smooth movement while reducing friction and distributing mechanical loads generated during activity.

Bones and Articular Surfaces

The principal bones forming the knee are the femur, tibia, and patella. The femur contributes two rounded articular surfaces, the medial and lateral femoral condyles, which articulate with corresponding surfaces on the tibia. These condyles diverge slightly towards the back, with the lateral condyle being broader anteriorly and the medial condyle maintaining a more uniform width.
The curvature of the femoral condyles decreases posteriorly, creating a changing radius of curvature that allows a combination of rolling and sliding movements during knee flexion. This geometry ensures efficient movement while maintaining ligamentous stability.
On the tibial side, the medial and lateral tibial condyles are separated by the intercondylar eminence, which consists of medial and lateral tubercles. This region serves as an important attachment area for ligaments and menisci.
The patella is the largest sesamoid bone in the human body. It develops within the tendon of the quadriceps femoris muscle and begins as cartilage at birth, ossifying between the ages of three and five years. Its posterior surface contains medial and lateral articular facets that articulate with the femur, improving the mechanical efficiency of the quadriceps muscle during knee extension.

Articular Capsule and Synovial Membrane

The articular capsule of the knee consists of an outer fibrous layer and an inner synovial membrane, separated in places by fatty deposits. The synovial membrane lines the joint cavity and produces synovial fluid, which lubricates the joint and nourishes the articular cartilage.
Anteriorly, the synovial membrane extends upwards to form the suprapatellar bursa, which communicates with the knee joint cavity and allows free movement of the patella during extension and flexion. Posteriorly, the membrane forms additional bursae beneath the heads of the gastrocnemius muscle, including the semimembranosus bursa and popliteal bursa.
The synovium also lines the infrapatellar fat pad, a cushioning structure located below the patella that helps accommodate changes in joint shape during movement.

Bursae of the Knee

Numerous bursae surround the knee joint, reducing friction between tendons, ligaments, bones, and skin. The largest is the suprapatellar bursa, which communicates directly with the joint cavity. Smaller bursae are found posteriorly around the knee, while non-communicating bursae lie anterior to the patella and below the patellar tendon. Inflammation of these bursae can lead to conditions such as prepatellar or infrapatellar bursitis.

Cartilage

Two main types of cartilage are present in the knee. Hyaline cartilage covers the articular surfaces of the femur, tibia, and patella, providing a smooth, low-friction surface for movement. Fibrocartilage forms the menisci, which enhance joint stability and load distribution.
Hyaline cartilage lacks blood vessels and receives nutrition through diffusion from synovial fluid and the underlying subchondral bone. Its limited capacity for repair means that damage often leads to degeneration over time. Newly formed cartilage following injury is usually fibrocartilage, which is mechanically inferior to the original hyaline cartilage and more prone to wear.

Menisci

The knee contains two crescent-shaped fibrocartilaginous structures known as the menisci: the medial meniscus and the lateral meniscus. They partially divide the joint cavity and function to distribute load, absorb shock, and enhance joint congruency.
The medial meniscus is larger, less curved, and more firmly attached to the joint capsule, making it more susceptible to injury. Its posterior horn is thicker than its anterior horn. The lateral meniscus is smaller, more circular, and more mobile due to its looser attachment, partly because of the interposition of the popliteus tendon.
Each meniscus has anterior and posterior horns that attach to the intercondylar area of the tibia. Strong circumferential collagen fibres provide tensile strength, while radial fibres help maintain structural integrity.

Ligaments of the Knee

The knee is stabilised by a complex system of ligaments that limit excessive movement while permitting functional mobility.
The most important intracapsular ligaments are the cruciate ligaments, which lie within the joint capsule but outside the synovial cavity. The anterior cruciate ligament (ACL) extends from the lateral femoral condyle to the anterior intercondylar area of the tibia and prevents excessive anterior displacement of the tibia relative to the femur. It is commonly injured during twisting or sudden deceleration movements.
The posterior cruciate ligament (PCL) runs from the medial femoral condyle to the posterior intercondylar area of the tibia and prevents posterior displacement of the tibia. Injuries to the PCL are less common and usually result from direct trauma.
The transverse ligament of the knee connects the anterior horns of the medial and lateral menisci and helps coordinate their movement during knee motion.
Collateral ligaments, including the medial (tibial) collateral ligament and lateral (fibular) collateral ligament, provide additional stability by resisting valgus and varus forces.

Nerve Supply

The knee joint receives a rich and complex nerve supply, reflecting its functional importance. Innervation is derived from branches of the femoral, obturator, tibial, and common fibular nerves. These nerves supply different quadrants of the joint, with anterior regions innervated mainly by branches to the quadriceps muscles and posterior regions by the tibial and obturator nerves. This extensive innervation contributes to proprioception and protective reflexes.