Citation, DOI and article data
Discoid menisci are those that have a body that is too wide, usually affecting the lateral meniscus. They are incidentally found in 3-5% of knee MRI examinations.
Discoid menisci are congenital, frequently bilateral (up to 50%) and have been reported in twins, although no genetic locus has been identified 2. There is a higher prevalence in Asians without any gender predilection 7. Lateral discoid meniscus is far more common than medial discoid meniscus, with the latter being rare.
Although frequently asymptomatic, discoid menisci are prone to cystic degeneration with subsequent tear. Clinical presentation may, therefore, be either incidental or with pain, locking or a 'clunk'.
Discoid menisci have decreased collagen fibers and loss of normal collagen orientation, which predisposes them to intrameniscal mucoid degeneration 7.
Classification is based on the degree of peripheral attachments to the tibial plateau, and the shape of the meniscus itself:
- complete vs incomplete
- 80% coverage of the tibial plateau is often used as the cut-off between incomplete and complete 7
- stable vs unstable
- stable: normal peripheral attachments with an intact posterior meniscofemoral ligament
- unstable (also known as a Wrisberg variant): lack or tear of a posterior meniscocapsular (in particular meniscopopliteal) ligaments with an attachment only from the meniscofemoral ligament of Wrisberg 4
- anterior or posterior mega horn
Radiographs may well suggest the diagnosis with the widening of the lateral joint space and cupping of the lateral tibial plateau, which is normally flat or even slightly convex. Additionally, there may be associated hypoplasia of the lateral tibial spine.
On coronal imaging, meniscal body width of 15 mm or more is typically considered diagnostic of discoid morphology. Alternatively, a ratio of the minimal meniscal width to the maximal tibial width of more than 20% may be used 7,8.
On sagittal imaging, the body of the lateral meniscus is normally only seen on two adjacent slices. A discoid meniscus is usually present if the meniscal body is seen on three or more standard sagittal slices - the opposite of the absent bow tie sign.
Treatment and prognosis
Ideally, the meniscus is preserved with conservative management. If this is unsuccessful, then partial or total resection may be carried out.
- meniscal tears
- intrasubstance mucoid degeneration
- early bony degenerative change
- 1. Kaplan P. Musculoskeletal MRI. W B Saunders Co. (2001) ISBN:0721690270. Read it at Google Books - Find it at Amazon
- 2. Davies AM, Cassar-Pullicino VN. Imaging of the Knee, Techniques and Applications. Springer Verlag. (2003) ISBN:3540002502. Read it at Google Books - Find it at Amazon
- 3. Bancroft LW, Bridges MD. MRI Normal Variants and Pitfalls. Lippincott Williams & Wilkins. (2008) ISBN:0781783143. Read it at Google Books - Find it at Amazon
- 4. Singh K, Helms CA, Jacobs MT et-al. MRI appearance of Wrisberg variant of discoid lateral meniscus. AJR Am J Roentgenol. 2006;187 (2): 384-7. doi:10.2214/AJR.04.1785 - Pubmed citation
- 5. Nguyen JC, De Smet AA, Graf BK, Rosas HG. MR imaging-based diagnosis and classification of meniscal tears. (2014) Radiographics : a review publication of the Radiological Society of North America, Inc. 34 (4): 981-99. doi:10.1148/rg.344125202 - Pubmed
- 6. De Smet AA. How I diagnose meniscal tears on knee MRI. (2012) AJR. American journal of roentgenology. 199 (3): 481-99. doi:10.2214/AJR.12.8663 - Pubmed
- 7. Kim JG, Han SW, Lee DH. Diagnosis and Treatment of Discoid Meniscus. (2016) Knee surgery & related research. 28 (4): 255-262. doi:10.5792/ksrr.16.050 - Pubmed
- 8. Samoto N, Kozuma M, Tokuhisa T, Kobayashi K. Diagnosis of discoid lateral meniscus of the knee on MR imaging. (2002) Magnetic resonance imaging. 20 (1): 59-64. Pubmed