What is a discoid meniscus? My child has this, what does it mean?

Before we talk about a discoid meniscus, its important to first understand what a regular meniscus is.  

The meniscus is located inside of the knee, and it acts as a shock absorber to help protect the cartilage in your knee.  The meniscus lines the perimeter of your knee, and it normally has a "C"-shaped appearance.  A discoid meniscus in contrast looks like the center of the "c" is filled in with extra tissue.  There is just too much meniscus.  A discoid meniscus is just something that some people are born with.  It actually exists in about 5% of the population.

Whats the problem with a discoid meniscus...doesnt extra tissue just mean extra protection for the cartilage? The discoid meniscus is a problem because the extra tissue is not normal.  The discoid is prone to tearing, becoming unstable and causing problems inside of the knee.  It can be painful, or the tear can cause "catching" and "locking" of the knee.

The discoid meniscus is classified based on the Wantanabe Classification.  There are three types.  There is 1. Stable and Complete (covers all of the cartilage), 2. Stable and Incomplete (covers part of the cartilage), and 3. Unstable (poor attachment to the bone, it slides all around).

A discoid meniscus is diagnosed by MRI.

A discoid meniscus is treated based on the symptoms that it is causing.  If you treated every discoid meniscus that you saw, that would mean that 1 in 20 people would be getting knee surgery.  So that is not good.  A discoid meniscus rarely causes symptoms and therefore it can be left alone in most cases.  Only when symptoms occur, and these symptoms must affect daily living, is there a need for surgical intervention.

The surgery involves removing the excessive meniscus and creating a normal "C"-shape to the meniscus.  The entire discoid meniscus should no be removed because this leads to a high risk of arthritis...the meniscus is important to protecting the cartilage.  

REFERENCES

-Watanabe M, Takeda SJ, Ikeuchi HJ: Atlas of Arthroscopy, ed 2. Tokyo, Japan, Igaku-Shoin Ltd, 1969. article.

-Kocher MS et al. Discoid Lateral Meniscus in Children: Diagnosis, Management, and Outcomes. JAAOS 2017. full article.

-Ogüt T, Kesmezacar H, Akgün I, Cansü E: Arthroscopic meniscectomy for discoid lateral meniscus in children and adolescents: 4.5 year follow-up. J Pediatr Orthop B 2003;12(6):390–397. full article.

-Good CR, Green DW, Griffith MH, Valen AW, Widmann RF, Rodeo SA: Arthroscopic treatment of symptomatic discoid meniscus in children: Classification, technique, and results. Arthroscopy 2007;23(2):157–163. full article.