This a congenital (inborn) condition where the meniscus, a cartilage pad in the knee joint, is in a disc shape rather than the normal C-shape. Some people with discoid meniscus never experience any symptoms and don't need treatment. But if a child with discoid meniscus feels pain or stiffness, surgery to reshape the meniscus may be needed.
Diagram of a discoid meniscus (L) and a normal meniscus (R).
Note the backwards C-shape of the meniscus on the right. Illustration by Mac O'Conor.