Discoid meniscus

A discoid meniscus is one in which the body is too thick, typically being seen on 3 or more standard sagittal images. It is most frequently seen affecting the lateral meniscus, and is incidentally found in up to 3% of knee MRIs. Although frequently asymptomatic, it is prone to cystic degeneration with subsequent tears.
X-rays may well suggest the diagnosis with widening of the lateral joint space and cupping of lateral tibial plateau, which is normally flat or even slightly convex. Additionally there may be associated hypoplasia of the lateral tibial spine.
For another image of the same case, and information about classification of discoid menisci please visit Radiopaedia.org here.
References:
Musculoskeletal MRI - Kaplan
Wheeless Online
Credit: Dr Frank Gaillard
http://www.radpod.org