Anatomy of Synovium

This thin, fluid filled lining lubricates and protects your joints

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The synovium is a thin, fluid-filled lining composed of connective tissue, found inside movable joints throughout the body. Your synovial lining encloses synovial fluid, a type of fluid that lubricates and nourishes synovial joints.

The term synovium is used to describe the synovial lining, and it is often also used to describe the synovial fluid that lies within the synovial lining. Your synovial lining produces and renews synovial fluid on a regular basis.

Anatomy

The synovial lining and synovial fluid are located in the synovial (movable) joints throughout your body. Synovial joints are made up of bones, cartilage, muscles, tendons, ligaments, and synovial fluid.

Synovial fluid doesn’t flow freely within a joint because it is enclosed within the synovial lining.

Structure

The synovial lining (membrane) and its synovial fluid are described together as a unit—the synovial capsule. 

The synovial lining is composed of two layers of tissue: the internal layer, which comes into direct contact with the synovial fluid, and the external layer, which faces the other structures in the joint.

The inner layer is made of cells that produce synovial fluid and the outer layer is a thin layer of connective tissue that slides against the other parts of the joint.

Because the synovial lining is so thin, it has two characteristics: flexibility and permeability. 

  • Flexibility: As a joint moves the synovial lining, which forms a soft capsule around the synovial fluid, is squeezed and can change shape without being damaged or moving out of its place.
  • Permeability: The synovial membrane is thin enough to allow tiny components (like oxygen and nutrients) to flow into and out of the synovial fluid.

Location

Synovial membranes are located between the bones, muscles, tendons, and ligaments of synovial joints. A synovial joint may have several synovial cavities.

The synovial cavities lie underneath tendons and ligaments. Tendons connect muscles to bones and ligaments connect bones to each other. 

Synovial joints are located throughout your body. Examples of synovial joints include the joints that move your shoulders, wrists, ankles, and knees. 

Function

The synovial membrane and fluid have several functions, including lubricating the joints, providing nourishment to the joints, and removing debris from the joints. 

  • Lubrication: The presence of synovium allows the moving parts of joints, like the bones and tendons, to move without becoming damaged or irritated. The soft cushion of the synovial membrane and the thick synovial fluid provide a surface against which the joint structures can move. 
  • Nourishment: Synovial fluid contains glucose, sodium, potassium, oxygen, and other small molecules that are necessary for the health and survival of every cell in the body. These tiny molecules can flow between the synovial membrane and the structures in the joint. 
  • Defense and removing debris: The synovium contains immune cells that protect joints from infections and remove harmful material, including waste and dead infectious organisms. 

Associated Conditions 

When there are problems with the structure or function of your synovial lining, such as inflammation, you may feel discomfort or your joints may become swollen. There are effective treatments for this synovial inflammation, and serious damage or disease of the synovial lining or synovial fluid is rare.

Arthritis

Osteoarthritis, a common condition caused by wear and tear of the joints, is characterized by inflammation and breakdown of the joints. Rheumatoid arthritis is an autoimmune condition in which inflammation in and around the joints occurs because the body attacks its own tissue.

The synovial fluid and its surrounding membrane are inflamed in both of these types of arthritis.

Synovitis

Arthritis is a type of synovitis, but there are other types as well. Lupus and gout are examples of common conditions that involve inflammation of the synovium. These illnesses can cause pain and swelling of the joint, as well as limited movements. 

Infection

Joint infections are not common, but they can occur and may involve the synovial fluid or synovial membrane. Immunosuppression, which is a state of decreased immunity in which the body does not fight infection properly, can predispose you to synovial infections. 

Traumatic Injury

A joint injury can affect any region of a joint, including the synovium. It may cause the synovium to tear or leak. Major injuries can predispose you to infections as well.

Cancer

A rare tumor, pigmented villonodular synovitis (PVNS), involves thickening of the synovial tissue. This condition is often described as a tumor, but it usually does not spread to other areas of the body the way cancer does. PVNS can be surgically removed if it enlarges, becomes painful, or interferes with movement.

Cancer within the synovium is not common, and metastatic cancer from other regions of the body does not commonly spread to the synovium.

Examination of Synovium 

Examination of your synovial lining or fluid may be necessary to diagnose the cause of your joint discomfort and swelling. There are several ways that your doctor can examine your synovial fluid.

Physical examination: Your joints can appear swollen or enlarged if you have a synovial condition. Sometimes, the fluid may appear to flow within the joint, suggesting a tear in the synovial lining. Redness and warmth may suggest that you have a joint infection. 

Imaging: Imaging examinations, such as an X-ray, magnetic resonance imaging (MRI), bone scan, and ultrasound can all visualize the structures in a joint, which can help your doctors identify disease of the synovium. 

Synovial fluid analysis: A sample of synovial fluid can be obtained using a needle aspiration. This procedure can be done in the doctor’s office. It may be slightly painful, but it is safe and takes less than half an hour.

Your synovial fluid is examined based on its appearance as it is removed from your joint, and it can be examined in more detail under a microscope as well.

A microscopic examination of synovial fluid can provide information about the presence of bacteria, blood, or a change in the cell types. Sometimes, the fluid is sent for chemical analysis to determine if there is a change in the chemical composition.

Synovial biopsy: Your synovial tissue lining can be sampled with a biopsy. This is more invasive than a needle aspiration. A biopsy can identify the growth of the synovial lining, such as PVNS.

Rehabilitation

Disease of the synovium can be managed with lifestyle modifications, therapy, medication, and sometimes with interventional procedures.

Lifestyle Management

Conservative therapies include using ice for relief of pain and swelling, resting the joint (if the inflammation is due to injury or overuse), and physical therapy. The balance between rest and activity can be tricky—too much rest can cause atrophy (thin weakened muscles) or stiffness. Yet at the same time, exercising can cause further injury to healing joints.

It is important that you discuss your joint pain with your doctor or physical therapist so you can do the exercises that are safest and most productive for you. 

Medication 

Synovitis due to injuries or inflammation can be treated with over-the-counter non-steroidal anti-inflammatories (NSAIDs) such as Advil, Motrin (ibuprofen) or Aleve (naproxen).

If you have an infection then antimicrobial treatment, such as antibiotics for a bacterial infection, may be needed to help you recover. 

Interventional Procedures

Other treatments for synovial conditions include removal of excess fluid with procedures such as aspiration (physically draining the fluid with a needle). Depending on the cause of your synovial condition, you may recover after only one treatment or you may need the fluid removed repeatedly if it builds up again.

If the disease involves the synovial tissue and/or surrounding joint structures, you may need to have surgical repair of your joint.

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