The Joint Capsule

The joint capsule is a very specialized anatomical structure that serves a crucial role in the body. Here we answer some of the common questions about this interesting topic. We will discuss the joint capsule of synovial joints specifically. The capsule of a joint is often considered to cause a unique pattern of movement restriction when it is injured or inflamed. This is called a capsular pattern.

What is a joint capsule?

A joint capsule, also knows as an articular capsule, is a fluid-filled fibrous structure that surrounds the synovial joints of the body. It forms a seal around joints via fibrocartilage tissue that attaches to the bones that make up a joint. A joint itself is where two or more bones meet.

What tissues make up the joint capsule?

The joint capsule is made up of two layers of connective tissue. The outside layer is composed of fibrous connective tissue. The inside synovial layer, or ‘synovium‘ or ‘synovial membrane’, is composed of highly vascular tissue. A tissue is considered vascular when it has a large amount of blood vessels and blood supply. This layer secretes synovial fluid that provides nutrition and lubrication to the joint and joint surfaces. The fibrous exterior of the capsule is tightly integrated with other structures including tendons, ligaments and bursae.

What does the joint capsule do?

The joint capsule provides passive joint stability, limiting the joint range of motion, primarily via the tough, fibrous outer layer. The capsule also provides nutrition and lubrication to the joint via the inner synovial layer. Nerve endings within the capsule contribute to the proprioception of the joint, assisting with control and awareness.

What are the synovial joints of the human body?

There are approximately 300 joints in the body and most of them are synovial joints. The major synovial joints in the human body include the shoulder, elbow, wrist, hip, knee and ankle. There are 6 major types of synovial joints; hinge joint (e.g. elbow), ball-and-socket joint (e.g. hip and shoulder), pivot joint (e.g. between C1-C2 vertebrae), condyloid joint (e.g. between carpals of the wrist), plane joint (e.g. between tarsal bones of the foot), saddle joint (e.g. between trapezium of the wrist and the first metacarpal).

What is unique about the joint capsule of the shoulder?

The joint capsule of the shoulder is the capsule of the glenohumeral joint. The shoulder joint is the most mobile major joint in the human body and much of its stability comes from the capsule and associated ligaments. There are three openings in the capsule. One of the most common conditions related to the capsule of the shoulder is frozen shoulder, also known as adhesive capsulitis. In frozen shoulder, the capsule becomes inflamed and tight causing a progressive reduction in mobility of the shoulder joint. The capsular pattern of the glenohumeral joint is lateral rotation, abduction, medial rotation.

What is unique about the joint capsule of the knee?

The capsule of the knee has embedded within it the largest sesamoid bone in the body; the patella. The knee joint has 4 major bursae including the Suprapatellar Bursa, the Prepatellar Bursa, the Infrapatellar Bursa, and the Semimembranosus Bursa. It is classified as a hinge joint. The capsular pattern of the knee is flexion more than extension.

What is unique about the joint capsule of the hip?

The capsule of the hip joint is especially strong and ligamentous. It consists of both circular and longitudinal fibers. The capsule is weakest posteriorly and strongest anterosuperiorly, an area under the most stress, especially during standing. The capsule surrounds the ball-and-socket joint formed by two bones; the pelvis and the femur. The capsular pattern of the hip is medial rotation, flexion and abduction more than extension.

What is unique about the joint capsule of the elbow?

The joint capsule of the elbow is strengthened mostly medially and laterally. This strengthening is due to the thick radial collateral and ulnar collateral ligaments. There are two distinct joints within the elbow joint capsule. These are the trochlear notch consisting of the articulation with the ulna and the trochlea of the humerus, as well as the articulation of the radius with the capitulum of the humerus.

What is unique about the joint capsule of the ankle?

The ankle joint is a hinge joint, surrounded by a relatively weak joint capsule. Similar to the elbow joint, the main strength of the ankle joint comes from medial and lateral ligaments. In the case of the ankle, these are the deltoid ligament medially (also known as the medial collateral ligament) and the lateral collateral ligament. The ankle joint consists of articulations between three bones; the tibia and fibula of the lower leg, and the talus of the foot.

What happens if you injure the capsule of a joint?

Injury to a joint capsule will usually result in an increase in laxity of the joint itself. This will make the joint more prone to further injury. Injury can be the result of trauma or be due to conditions like rheumatoid arthritis or osteoarthritis. A traumatic injury to the capsule in an individual with otherwise healthy joints may take approximately 6 weeks to heal.

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