A synovial joint is characterised by the presence of a fluid-filled joint cavity contained within a fibrous capsule. It is the most common type of joint found in the human body, and contains several structures which are not seen in fibrous or cartilaginous joints. In this article we shall look at the anatomy of a synovial joint - the joint capsule, neurovascular structures and clinical correlations.
The three main features of a synovial joint are (i) articular capsule, (ii) articular cartilage, (iii) synovial fluid. All of these structures work together to ensure the joint functions correctly by reducing friction, distributing nutrients, and providing shock absorption.
The articular capsule surrounds the joint and is continuous with the periosteum of articulating bones. It consists of two layers:
The articulating surfaces of a synovial joint (i.e. the surfaces that directly contact each other as the bones move) are covered by a thin layer of hyaline cartilage. The articular cartilage has two main roles- (i) minimising friction upon joint movement, and (ii) absorbing shock.
The synovial fluid is located within the joint cavity of a synovial joint. It has three primary functions-
Articular cartilage is relatively avascular, and is reliant upon the passive diffusion of nutrients from the synovial fluid.
The accessory ligaments are separate ligaments or parts of the joint capsule. They consist of bundles of dense regular connective tissue, which is highly adapted for resisting strain. This resists any extreme movements that may damage the joint.
A bursa is a small sac lined by synovial membrane, and filled with synovial fluid. Bursae are located at key points of friction in a joint. They afford joints greater freedom of movement, whilst protecting the articular surfaces from friction-induced degeneration.
Synovial joints have a rich supply from articular nerves, which provide proprioceptive feedback. The innervation of a joint can be determined using Hilton’s Law - ‘the nerves supplying a joint also supply the muscles moving the joint and the skin covering their distal attachments.’ Arterial supply to synovial joints is 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.
Osteoarthritis is the most common form of joint inflammation (arthritis). It is characterised by joint pain and decreased mobility owing to the degeneration of the articular cartilage, as well as changes to the joint capsule and synovial fluid. Treatment for osteoarthritis is generally focused on managing pain, improving joint function, and slowing the progression of the disease.
Synovial joints provide greater range of motion and protect the articular surfaces from degeneration caused by friction. However, overuse of the joint can cause inflammation, or bursitis. The understanding of synovial joints is important to clinical relevance, since they can become affected by osteoarthritis, an issue that can cause joint pain, stiffness, and discomfort.
Synovial joints have a high supply of articular nerves. This supply of nerves can be evaluated using Hilton's Law, which states that the nerves supplying a joint also supply the muscles that move the joint, as well as the skin covering their distal attachments. These articular nerves will transmit afferent impulses, which can include proprioceptive (joint position) and nociceptive (pain) sensation.
The arterial supply to synovial joints is through articular arteries, which come from the vessels surrounding the joint. These articular arteries are usually located in the synovial membrane. An interesting feature of this arterial supply is that the connections between the arteries (anastomoses) often change depending on the position of the joint, which allows for a blood supply to and across the joint no matter the position. These articular veins accompany the articular arteries and can be found in the synovial membrane as well.
Osteoarthritis is the most common type of joint inflammation. It is caused by a number of factors, the most common being the prolonged use of the articular joints. This causes the cartilage to wear away, which can leave the articular surfaces of the bone eroded as well. This process is irreversible and degenerative. The loss of the cartilage results in the joint having reduced ability to act as a shock absorber and lubricating surface, as well as the joint edges becoming more prone to causing further damage. As a result, the joint will experience pain, stiffness, and discomfort. Osteoarthritis usually targets joints that have to manage the full weight of the body, such as the hips and the knees. Other causes of arthritis include infection of the joint, autoinflammatory issues, and infection not directly involving the joint itself (reactive arthritis).
