Glossary
Synovium and synovial fluid
Quick answer
The synovium is the thin lining inside the knee joint capsule. It produces synovial fluid, a clear, slippery liquid that lubricates the joint and feeds the cartilage. When the knee is injured or inflamed, the synovium makes extra fluid and the knee swells, explains Dr. Pablo Gelber, knee surgeon at Aman Hospital in Doha.
What it is
The synovium covers the inner surface of the capsule and the fat pads, but not the cartilage or menisci. A healthy knee contains only a small amount of synovial fluid, a few millilitres, thick like egg white because of a substance called hyaluronic acid. The fluid reduces friction, carries nutrients to the cartilage and removes waste as the knee moves.
Why it matters for patients
Swelling, which patients often call water on the knee, is extra fluid in the joint. How quickly it appears is a useful clue. Swelling within an hour or two of an injury often means bleeding inside the joint, as with an ACL tear or fracture. Swelling that builds overnight or the next day is more typical of a meniscus tear or cartilage irritation. Swelling without injury can come from arthritis, gout or inflammatory conditions.
When the synovium itself is inflamed, it is called synovitis. A hot, red, very painful swollen knee with fever needs urgent same-day assessment to rule out infection.
How it is examined
A doctor can feel and see fluid during examination, and ultrasound or MRI confirms it. Drawing off fluid with a needle (aspiration) relieves pressure and allows testing for infection, crystals and blood.
Frequently asked questions
Is it bad to drain fluid from the knee?
Not when done for a reason. Aspiration can ease pain and help with diagnosis. Fluid can return if the underlying cause is not treated.
Are hyaluronic acid injections the same as synovial fluid?
They contain a substance found naturally in synovial fluid. They may help symptoms in some people with arthritis, but they do not replace a healthy joint lining.
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