Articular Capsule of the Knee Joint: Definition & Function

Instructor: Dan Washmuth

Dan has taught college Nutrition, Anatomy, Physiology, and Sports Nutrition courses and has a master's degree in Dietetics & Nutrition.

The articular capsule of the knee joint surrounds the knee joint and consists of various structures and tissues. In this lesson, learn about the definition and function of the articular capsule of the knee joint.

Articular Capsule of the Knee Joint

The knee is an amazing joint. The knee can bend, flex, and rotate which allows a person to stand up, walk their dog, play sports with friends, and sit down. The knee is able to perform all these functions due in large part to the articular capsule of the knee joint.

In order to truly understand the articular capsule of the knee joint, a person must first learn about the knee joint itself. The knee joint is formed by the connection of the lower part of the femur (thigh bone) and the upper part of the tibia (shin bone). The patella (knee cap) is a round bone that moves along the front of the knee and it is attached to the tibia by the patellar tendon.

The bones that make up the knee joint are the tibia, femur, and patella.
knee joint

Articular Capsule: Definition

The articular capsule of the knee joint surrounds the knee and consists of two main layers, an outer and inner layer. The outer layer is made up of a tough, fibrous membrane that is made up of ligament tissue. The inner layer is made up of a synovial membrane, which secretes, a clear, yellowish fluid called synovial fluid. The articular capsule of the knee also contains bursae, which are fluid filled sacs, and a fat pad located behind the patella.

The articular capsule of the knee also contains bursae, synovial fluid, and a fat pad.

Articular Capsule: Function

All of these structures of the articular capsule of the joint (outer layer, inner layer, synovial fluid, bursae, fat pad) all have specific functions in the knee.

Outer Layer

The main function of the fibrous membrane of the outer layers is to provide stability to the joint by holding the bones of the knee (femur, tibia, patella) in their correct positions in the knee.

Inner Layer

As it was mentioned before, the synovial membrane of the inner layer secretes a viscous material called synovial fluid. This synovial fluid functions to lubricate the joint, helping to reduce friction and irritation of the bones, ligaments, and tendons when the knee joint moves.

Imagine running and diving head-first onto a slip and slide that was dry. This would most likely cause a person to burn and scrape their stomach and chest, leading to a lot of pain. Slip and slides definitely work best when they are wet or lubricated. This same concept applies to the knee joint. Without proper lubrication provided by the synovial fluid, the knee joint would be very painful any time a person moved the knee.

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