Synovial joint

The Synovial Joint and Synovial Fluid 

What is a synovial joint?

Most of the joints in the body are synovial joints. Let's look at the knee joint in more detail as it can be classed as a typical synovial joint.

Inside the knee joint there are two bones joined together: the femur and tibia. The ends of the bones are covered in a very smooth layer of a tough, rubbery substance known as cartilage. The joint is enclosed in a stiff and elastic capsule made of strong, fibrous tissue. The joint capsule completely encloses the space around the joint surfaces and is lined by a synovial membrane. The joint capsule contains a thick, slippery liquid called synovial fluid.

Other types of synovial joints are the ball-and-socket joint in the hip and shoulder,  the hinge joint and pivot joint of the elbow,  the plane joint in the wrist, the condyloid (or ellipsoidal) joint in the fingers, and the saddle joint in the thumb. Also the joint of the jaw (temporomandibular joint) is a synovial joint.

All these joints allow movement and thus require a lubrication for the joints to function properly: synovial fluid.

Knee Diagramme

Synovial Joint - Finger Joint

What is synovial fluid?

The synovial fluid in the joint capsule has four important functions: 

  • it keeps the bones slightly apart, protecting their cartilage coverings from wear and tear
  • it absorbs shocks, again protecting the cartilage
  • it lubricates the joint, helping it to work freely and easily
  • it acts as a filter, letting nutrients reach the cartilage, but blocking the passage of harmful cells and substances.

The most important component of synovial fluid is a substance called sodium hyaluronate. It is this substance that lets synovial fluid perform its four different functions all at the same time. Most of the joints in your body are synovial joints. Good examples, besides the knee, include the hip and the shoulder.

Next: What happens in Osteoarthritis?