There is pain and swelling, but where is the source?

This 27-year-old male presents with pain and swelling in the anterior left knee. The symptoms began eight months prior. There was no known injury or previous surgery.

Have a look at images 1 through 3, and see if anything stands out. Do not be distracted by the obvious finding.

Sagittal PD FSE Fat Sat

Sagittal T2

Axial T2 FSE

The images show normal menisci, ligaments, and tendons. There is a large joint effusion. A normal plica is visible, which is not thickened.

If you look closely in the posterior aspect of the suprapatellar joint capsule, there is frond-like synovitis or pannus (images 4 through 6, green arrows). Therefore, the most likely diagnosis is rheumatoid arthritis or seronegative arthropathy. In certain continents, Lyme disease would be a differential. Gout is more common in older patients.

It is important to correlate the findings, and not be distracted by prominent plica, which would not cause the degree of joint effusion.