Knee Joint Effusion on MRI: What Your Report Means

Report wording you may see

• “Small joint effusion” or “trace joint fluid”
• “Moderate to large knee joint effusion distending the suprapatellar recess”
• “Joint effusion with synovial thickening” or “effusion-synovitis”
• “Hemarthrosis”, meaning blood in the joint after an injury

How effusion is graded, and what the grade cannot tell you

Research scoring systems for knee MRI, such as WORMS and MOAKS, grade effusion from 0 to 3 by how far the fluid stretches the joint capsule. Clinical reports usually use words instead: trace, small, moderate or large.

• Fluid is common on MRI. In one study of older adults without knee pain, a moderate or larger effusion was seen in 11% to 16% of knees.
• In knees with osteoarthritis, moderate or large effusions were more frequent in people with knee pain than in people without it.
• The grade does not say why the fluid is there. The rest of the report usually points to the cause, such as a ligament or meniscus injury, cartilage wear or a thickened joint lining.

When an effusion matters

After a sports injury, a knee that fills quickly with blood often has a significant injury inside: in one study of 106 athletes with a hemarthrosis, the ACL was torn in 43. Without an injury, effusion is read together with the cartilage, meniscus and bone findings, for example in knee osteoarthritis. Fluid can also flow into a Baker's cyst behind the knee. A hot, red, very painful swollen knee, especially with fever, needs prompt medical attention.

Questions for your doctor

• Is the effusion small, moderate or large, and is the joint lining thickened?
• What does the rest of the report suggest is causing the fluid?
• If I was injured, does the fluid point to a ligament or bone injury?
• Does the fluid need to be drained or tested?

Sources

1. Peterfy CG, Guermazi A, Zaim S, et al. Whole-Organ Magnetic Resonance Imaging Score (WORMS) of the knee in osteoarthritis. Osteoarthritis Cartilage. 2004;12(3):177-190.
2. Hunter DJ, Guermazi A, Lo GH, et al. Evolution of semi-quantitative whole joint assessment of knee OA: MOAKS (MRI Osteoarthritis Knee Score). Osteoarthritis Cartilage. 2011;19(8):990-1002.
3. Hill CL, Gale DG, Chaisson CE, et al. Knee effusions, popliteal cysts, and synovial thickening: association with knee pain in osteoarthritis. J Rheumatol. 2001;28(6):1330-1337.
4. Maffulli N, Binfield PM, King JB, Good CJ. Acute haemarthrosis of the knee in athletes. A prospective study of 106 cases. J Bone Joint Surg Br. 1993;75(6):945-949.