Anatomy and Function of Synovial Joint Components

1. Articular Capsule

The articular capsule is a sleeve-shaped membrane that surrounds the whole joint cavity. Its primary mission is to provide stability to the entire joint. In some areas, this capsule is very thick and strong, almost functioning as a ligament. In other joints, it can be very thin or almost nonexistent.

2. Synovial Membrane

The synovial membrane is a sac-like structure that, along with the joint capsule, surrounds the joints and transforms the area into a closed compartment. It provides the inner lining of the joint cavity, which is smooth and shiny. It focuses on the formation of a highly viscous liquid called synovium (or synovial fluid), which has two distinct functions:

• Provide proper lubrication for the articular cartilage.
• Constitute the primary means of feeding these cartilage cells, which lack blood vessels.

3. Articular Cartilage

This tissue is a type of cartilaginous tissue formed by cells and elastic, resilient fibers. All joint surfaces are coated with a thin layer of hyaline cartilage (also called an articular coating). Hyaline cartilage has the mission of providing a smooth slide and preventing the erosion of the bone ends. Its elasticity also helps prevent injuries by dampening shocks, preventing the transmission of impact to more distant parts of the body.

4. Joint Cavity

The joint cavity is the small space between the articular faces of the two bones of the joint.

5. Menisci

The menisci are fibrocartilaginous structures. They are interposed between the bone ends of some joints of the body. Their purpose is to fit the bone ends more perfectly and increase the contact surface between the bones, thereby spreading the load more evenly. The knee is the typical joint fitted with menisci. Injuries to the menisci are frequent due to the high stress that is required very often.

6. Ligaments

Ligaments are fibrous structures, like cords, which are near the joints. Thanks to them, the bones remain properly linked together and are constrained in the extent of some movements. There are different types of ligaments:

• Broad
• Elastic
• Short
• Tough
• Etc.

Some, like the round ligament of the hip or the cruciate ligaments of the knee, are located within the joint cavities.

7. Bursae (Synovial Sacs)

Some joints contain a pillow-shaped structure called a bursa (or sac), consisting of synovial membrane and filled with synovial fluid. Bursae tend to be associated with bony prominences (as in the knee or elbow), cushioning the joint and facilitating tendon movement.

Bursitis

Bursitis, or inflammation of the bursae, is a relatively common disorder, most often caused by repeated or excessive exercise or a sudden trauma to the joint.