JOINT CAPSULE: CHARACTERISTICS, COMPONENTS AND FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2025

The joint capsule is a dense and fibrous connective tissue structure that is located around the joint. It is closely attached to the bones and other surrounding structures such as cartilage, tendons, fasciae, and muscles, some of which contribute to its formation.

The joints are sealed within this capsule. It provides passive and active stability thanks to its composition, which gives it both firmness and flexibility. It is found in all joints of the body regardless of size, but only the largest contain a fluid inside that lubricates the joint.

Joint capsule damage is common. It can be affected by traumatic events, rheumatological and immunological diseases, sedentary lifestyle, overweight and age. If these injuries are very severe they can incapacitate the patient, so their care and recovery are important for a good quality of life.

General characteristics

In each joint the joint capsule is distinguished, inside which are the articular surfaces of the bones and the joint cavity.

The capsule wraps the joint ends of the bones in the form of a sleeve and contains the synovial fluid or fluid, which fulfills several local functions, among which lubrication stands out.

Classical anatomists attribute to the joint capsule the formation of an uninterrupted mobile cavitary union. In its interior there is negative pressure, its separation being hindered due to atmospheric pressure.

This phenomenon explains the cold or painful sensation that some people suffer in their joints with climatic changes.

When the joint capsule is traumatized, air enters the cavity and the joint faces immediately separate. This causes pain and functional inability, but joint function can be preserved if there is no muscle or ligament injury. For this reason, certain joint injuries, although severe, do not completely limit their work.

Components

The joint capsule is made up of a fibrous outer membrane (fibrous membrane) and an inner synovial membrane (membrane synovialis).

Fibrous membrane

It is a fibrous, whitish and non-vascularized portion of the joint capsule. It goes from one bone to the other, helping to ensure the harmonious and functional union of the joint.

In this task it is accompanied by other surrounding structures such as muscles, tendons and ligaments, complementing each other.

This structure of the capsule, the outermost of all, can be seen directly by removing the surrounding muscle. It is slightly thicker than the inner layers, perhaps due to its more mechanical work, which requires a stronger structure. The thickness can also vary depending on the stress to which the joint is subjected.

Despite not being vascularized, the innervation of the fibrous membrane is very rich. In fact, extensions of these nerves insert into the interior of the capsule and into the surrounding muscles. They provide the usual exquisite sensitivity of any joint, which fulfills the functions of positioning, protection and warning of dysfunction.

Synovial membrane

The synovium is directed towards the inner face of the joint capsule and has a smooth and shiny appearance. Its extension always ends in the contour of the articular cartilage, which it covers from the inside. It is thinner than the fibrous membrane and its functions are more physiological and less related to torsion or force.

Depending on the joint studied, the synovium usually forms small cilia called synovial villi. Likewise, in isolated regions of the joint the synovial folds are formed.

Many of these folds are rich in fatty substance, and are now called adipose folds. The wing plicae of the knee are an example of this type of folds.

Occasionally sacciform extensions called synovial bursae or bursae form. These are located close to the local tendons and muscles.

As in the entire synovial membrane, thanks to its layers of endothelial cells, they produce the translucent and viscous liquid known as synovium, which lubricizes the movements of the joint itself and its annexes.

accessories

Ligaments are often generated within the joint cavity and extend to the surroundings. In every synovial joint there are at least two ligaments, which are basically localized thickenings of the capsule itself. Some joints have quite a few more, such as the hip and shoulder.

In many areas the tendons adhere to the capsule and in many others they replace it. The best example of this situation is the complex shoulder joint, where the tendons of up to 4 different muscles play an important role in joint support and in the formation of the rotator cuff.

Muscles are an active part of the joint capsule; not just through its tendons, but directly with its fibers. In fact, the work of the joints is closely linked to the work of the local musculature. The muscle complex that surrounds the joint is known as the complementary apparatus.

Features

Multiple are the tasks carried out by the joint capsule. The various components fulfill particular tasks, including the following:

Stability

Both passive and active stability depend on the joint capsule. This task is performed by the fibrous membrane and the complementary apparatus.

Limit unnecessary movements and support the joint. It is even capable of minimizing joint damage thanks to the compensation of activities between the structures involved.

Movements and position

A healthy joint capsule supports normal joint mobility. In addition, thanks to its generous innervation, it serves to inform the central nervous system about the location of the joint and the body in general. This function is supplemented by synovial fluid.

Cushioning

Synovial fluid serves to lubricate joint surfaces, like oil in an engine, and to cushion shocks.

These impacts are not only traumatic, since the function of the locomotor system is already considered an effort, such as walking. The synovium in the knee not only lubricates the cartilage, but protects it at every step.

Metabolism

The synovium participates in the metabolism of fluids and helps in the dissolution of foreign elements to the joint. Its alteration provides valuable information on the status of the joint when it is studied.

The excessive production of synovial fluid is also a pathological finding that helps when diagnosing rheumatic diseases.

References

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2. Bolton, Naomi (2017). What Is the Purpose of the Fibrous Capsule? Recovered from: sciencing.com
3. Healthline Medical team (2015). Knee joint capsule. Recovered from: healthline.com
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5. Prives, M.; Lisenkor, N. and Bushkovich, V. (1975). General syndesmology. Human Anatomy, second edition, volume I, 125-139.
6. Wikipedia (last edition 2018). Joint capsule. Recovered from: en.wikipedia.org
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