Synovitis

The synovial membrane is a thin tissue that lines the inside of the joint capsule in movable joints and secretes a lubricating fluid called synovial fluid. Synovitis is a collective term for tissue changes in which this membrane becomes inflamed and thickened, blood vessels increase, and immune cells accumulate.

Rather than a standalone diagnosis referring to one specific disease, synovitis is closer to a term describing a condition observed inside a joint. This means that even when people have the same finding of synovitis, the underlying disease, affected location, and duration can differ. If this word appears in a test report or an explanation during a medical appointment, it makes sense to also clarify the context in which it developed.

The synovium is the first tissue to respond to irritation inside a joint. Synovitis can therefore develop for several reasons.

• Autoimmune arthritis: In rheumatoid arthritis, persistent synovitis is studied as a key pathological change. The immune system attacks the synovium, causing it to thicken, and these changes can also affect cartilage and bone.
• Osteoarthritis: Research suggests that inflammatory changes in the synovium can also accompany osteoarthritis.
• Crystal arthritis: The synovium also reacts when crystals form inside a joint, as in gout or pseudogout.
• Injury and overuse: The synovium can become irritated and swollen after a joint is sprained, struck, or repeatedly overused.
• Infection: Inflammation of the synovium also occurs in infectious arthritis.

Synovitis can cause joint swelling, pain when pressure is applied, and warmth when the skin is touched. A joint may feel stiff on waking and gradually loosen with movement, or it may be difficult to bend or straighten fully. However, the sensations of swelling and pain can overlap with changes not only in the synovium itself but also in surrounding tissues, such as the bone beneath the cartilage, ligaments, tendons, and bursae. It is therefore difficult to conclude that the problem is in the synovium based solely on where the pain is felt.

Joint effusion, sometimes described as fluid building up in a joint, means that the amount of fluid inside the joint cavity has increased abnormally. Synovitis, by contrast, refers to inflammatory changes in the synovial tissue itself. The two often occur together, but they refer to different things, so the presence of one does not necessarily mean the other is also present. This is why joint effusion and knee effusion are often discussed alongside synovitis.

Among imaging tests, MRI is used to distinguish and examine synovial tissue and fluid inside the joint. However, a thickened-looking synovium or visible effusion on imaging does not fully reveal symptom severity or prognosis, so clinical assessment considers the patient's symptoms together with other tests.

• Rest without overdoing it. A joint swollen by inflammation is more sensitive than usual. Try adjusting your activity level to how you feel.
• Elevate the affected area when it is swollen. Many people feel more comfortable when they position an arm or leg above heart level.
• Choose cold or heat according to what feels comfortable. Refer to Heat and cold therapy to choose a comfortable approach, and place a thin towel between the skin and the heat or cold source to protect the skin.
• Move the joint within a pain-free range. To keep the joint from becoming stiff, it is helpful to slowly bend and straighten it without straining.
• Make a note of when and under what circumstances swelling occurs. This is useful for reviewing how symptoms change over time.

If a joint suddenly becomes severely swollen and hot along with a fever, several joints swell at once, morning stiffness lasts a long time, or swelling does not subside for several days, the cause needs to be examined carefully. The response to synovitis varies greatly depending on its cause, so accurately distinguishing between possible causes is important.

There is a separate condition with a similar name that can easily cause confusion. Synovial sarcoma is a malignant tumor classified as a soft tissue sarcoma. Although its name includes “synovial,” it does not mean inflammation of the joint synovium. Nor is it a disease that develops because synovitis has persisted for a long time. One is an inflammatory condition of the synovium, and the other is a malignant tumor, so they are entirely different in nature.

Synovial sarcoma accounts for a small proportion of all soft tissue sarcomas and tends to be reported more often in relatively younger age groups than other sarcomas. Its characteristic molecular genetic features are known to include the chromosomal translocation t(X;18) and the resulting formation of an SS18::SSX fusion gene. Histologically, it is classified into forms including a monophasic type composed mainly of spindle cells and a biphasic type in which an epithelial component is also observed.

Clinical descriptions commonly report that it is found in the deep soft tissues of the limbs, particularly around large joints. Despite its name, however, it does not necessarily originate in the synovium inside a joint. Symptoms vary: it may initially appear as a slowly growing, painless lump, or it may be accompanied by pain. If a lump you can feel in your body is gradually growing, it is a good idea to have it assessed at a medical appointment.