Structures of a Synovial Joint

Author: Matt Quinn

Last modified: June 22, 2014

This synovial joint is the most common, and most complex type of joint found in the body. In this article, we shall look at the anatomy of the structures of a synovial joint, how they work together, and how they can go wrong.


Synovial Fluid

Synovial fluid has three primary functions:

  1. Lubrication,
  2. Nutrient distribution
  3. Shock absorption.

This fluid is found in the synovial cavity of a joint, which is the space enclosed by the articular capsule.


Articular Capsule

This is a fibrous capsule which is continuous with the periosteum of articulating bones. It consists of two layers.

The outer fibrous layer is made up white fibrous tissue, called the capsular ligament. Holds together articulating bones and supports the synovium.

The synovium is the inner synovial layer, a highly vascularised layer of connective tissue. It absorbs and secretes synovial fluid, and is responsible for the mediation of nutrient exchange between blood and joint.

Fig 1.0 - The structures of a typical synovial joint. Synovial fluid is found in the joint cavity

Fig 1.0 – The structures of a typical synovial joint. Synovial fluid is found in the joint cavity


Articular Cartilage

The bones of a synovial joint are covered by a thin layer of hyaline cartilage which serves to line the epiphysis of the bone. The smooth surface it provides has two functions – it minimises friction upon joint movement and absorbs shock.


Accessory Ligaments

Accessory ligaments are separate ligaments or part of joint capsule. They are made up of bundles of dense regular connective tissue, which are highly adapted for resisting strain. This prevents any extreme movements that may damage the joint.

Fig 1.0 - The joint capsule and accessory ligaments of the temporomandibular joint.

Fig 1.0 – The joint capsule and accessory ligaments of the temporomandibular joint.


Bursae

A bursa is a small sac lined by synovial membrane and filled with synovial fluid. They are placed at key points of friction in a joint, providing the joint with free movement. They can become inflamed following infection or irritation by over-use of the joint.


Innervation

Joints have a rich nerve supply provided by articular nerves. Hilton’s Law states that the nerves supplying a joint also supply the muscles moving the joint and the skin covering their distal attachments.

The nerves of a joint transmit impulses which play a key role in proprioception.


Vasculature

Joints receive blood via articular arteries which arise from the vessels around the joint. The articular arteries are located within the joint capsule, mostly in the synovial membrane.

A common feature of the articular arterial supply is frequent anastomoses (communications) in order to ensure a blood to and across the joint regardless of position of joint

The articular veins accompany the articular arteries and are also found in the synovial membrane


Clinical Relevance: Degenerative Joint Disease (Osteoarthritis)

Heavy use of articular joints over the course of many years can result in the wearing away of articular cartilage, and often the erosion of the underlying articulating surfaces of bones. This process begins early in adult life, and progresses slowly from that point. The changes which occur are irreversible and degenerative, and result in the decreased effectiveness of articular cartilage as a shock absorber and lubricated surface.

As a result of this degeneration, repeated friction can cause symptoms of joint pain, stiffness and discomfort. This condition usually affects joints that support full body weight, such as the hips and the knees.

Arthritis can also come about as a result of infection, due to the ease with which blood (and any associated bacteria) can enter the joint cavity via the synovial membrane.