Causes / typical injury mechanism: The discoid meniscus is a congenital anatomical abnormality. Discoid menisci may or may not cause symptoms and are sometimes found incidentally on MRI or during arthroscopy. It is almost exclusively seen in the lateral meniscus and can be bilateral in up to 20% of cases. Unilateral medial discoid meniscus is very rare, and the bilateral medial variant is even rarer. Discoid menisci can be stable or unstable, with unstable discoid menisci causing symptoms of locking or catching.

Classic history and presentation: Unstable discoid menisci present as atraumatic and nonpainful knee snapping / popping at a young age (patients aged 3-5 years) that progresses to painful snapping / popping over a couple of years (patients aged 8-10 years). Stable discoid menisci present in older children with mechanical symptoms suggesting a meniscal tear. Acute traumatic tears can occur at any age and are most often related to athletic activity.

Prevalence: Of the general US population, 3%-5% are affected. Asian populations have a prevalence as high as 15%.

• Age – Presents in children in the first or second decade of life. Symptoms of snapping (if present) can begin as early as age 3.
• Sex / gender – Equal prevalence.

Risk factors: Asian descent.

Pathophysiology: Discoid meniscus is a congenital abnormality that results from failed absorption of the central part of the meniscus during embryonic development. The result is a meniscus that is larger and "discoid" shaped (as opposed to the "C" shape of a normal meniscus), which occupies a greater surface area of the tibial plateau. Discoid menisci are more prone to tearing than normal menisci due to their shape and abnormal vascularity.

Grade / classification system:

Watanabe Classification

• Type I – Stable, complete; discoid menisci with intact peripheral attachments covering the entire lateral plateau.
• Type II – Stable, incomplete; discoid menisci with intact peripheral attachments covering only a portion of the lateral plateau.
• Type III – Unstable; discoid menisci (complete or incomplete) that lack posterior capsular attachments, with the exception of the posterior meniscofemoral ligament (ligament of Wrisberg).