Figure 1. Coronal MRI image of the left knee.
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Figure 2. Sagittal MRI image of the left knee.
A 19-year-old man presents with a 2-month history of pain in the left knee. He denies injury to the knee or any precipitating event. The patient describes the left knee as “locking up” and then “giving way,” at which time he feels a sharp, stabbing pain. Coronal and sagittal magnetic resonance imaging (MRI) of the left knee are ordered and obtained (Figures 1 and 2).
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The patient’s imaging findings are consistent with a discoid lateral meniscus tear. The MRI images display an abnormally wide and thick lateral meniscus. In the sagittal MRI view (Figure 2), a classic “bow-tie sign” is seen. A normal meniscus is a crescent-shaped cartilaginous structure that acts as a shock absorber to protect the cartilage in the knee. A discoid meniscus is an anatomical abnormality that results in a larger-than-normal meniscus.1
Most patients with a discoid meniscus have no symptoms, and the anomaly is often found incidentally on MRI or during knee arthroscopy. An enlarged meniscus is prone to cause symptoms of pain, clicking, and mechanical locking that is more pronounced during knee extension. Symptoms of discoid meniscus often begin in adolescence and are associated with physical activity.
A discoid meniscus is more prone to tearing compared with a normal meniscus. MRI is the imaging technique of choice to diagnose a discoid meniscus.1,2 Approximately 70% of cases of symptomatic discoid meniscus include a tear found during arthroscopy.3 Most cases of discoid meniscus can be treated successfully with observation. However, patients with symptomatic discoid meniscus are often treated with knee arthroscopy that may include removal of central tissue (saucerization) to create a normal C-shaped meniscus.2,3
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).
1. Abbasi D. Discoid meniscus. OrthoBullets website. https://www.orthobullets.com/knee-and-sports/3007/discoid-meniscus. Updated October 17, 2019. Accessed November 19. 2019.
2. March image quiz: discoid lateral meniscus. J Bone Joint Surg. 2016;4(2):13-15.
3. Sabbag OD, Hevesi M, Sanders TL, et al. Incidence and treatment trends of symptomatic discoid lateral menisci: an 18-year population-based study. Orthop J Sports Med. 2018;6(9): 2325967118797886.