Disclaimer: The following case is for education purposes only and does not depict any actual person or patient.
A 16-year-old female presents with a primary complaint of a loud popping sound when she flexes her knee. She is a dance performer that participates in several types of dance including hip-hop, jazz, and contemporary. She reports that there is no pain or physical limitations secondary to the popping, but occasionally it does cause discomfort. She does not recall any specific swelling or instability of the knee.
On physical exam, there is noted swelling or tenderness to palpation about the knee. She has full range of motion of the knee. All special tests including, Lachman's Test and McMurray's Test are negative. The sole positive exam finding is when she takes her knee from a partially extended position to a fully flexed position there is an audible pop. An example of the popping is noted below (turn sound on).
X-ray obtained at the initial visit
MRI of the right shoulder illustrates the diagnosis
Case courtesy of Dr Hani Makky ALSALAM, Radiopaedia.org, rID: 35594
What is the anatomic cause of the audible popping sound with a discoid meniscus?
In the cases of a discoid meniscus, there are several variations of the shape of the meniscus. A normal meniscus is shaped like a "C" which allows the femur to glide smoothly on top of the tibia. However, in an incomplete discoid the shape is a bit thicker and wider. Lastly, in a complete discoid the shape is a solid disc that covers the majority of the tibia. With a discoid meniscus, when the knee bends the abnormal tissue obstructs the proper flexion and extension of the knee and can result in an audible pop.
What type of imaging is typically used to confirm the diagnosis?
In general, MRI can be used to confirm the diagnosis of a discoid meniscus. A discoid meniscus can be suspected using MSK ultrasound, however, frequently the final report tends to be non-specific.
Once the diagnosis is confirmed what are the next steps in management?
In some cases, a discoid meniscus can be an incidental finding on MRI when evaluating another aspect of the knee. In addition, even when the condition causes an audible popping sound, the patient may have no pain or swelling. In these instances, I typically will simply take a conservative approach and observe with no intervention and no restriction from activity. On the other hand, if the discoid meniscus is associated with a tear or pain I will recommend surgical intervention to remove the excess tissue and reshape the meniscus.
A symptomatic discoid meniscus may present in a very young patient (school age) as simply a swollen knee that is having mechanical symptoms, typically secondary to a tear within the oversized meniscus.
A Discoid meniscus has a high rate of occurrence bilaterally and has been reported to occur in both knees up to 20% of the time. Discoid menisci almost always occur on the lateral side.
The increased size and mechanical blocking mechanism of a discoid meniscus do make it more susceptible to tearing compared to a normally shaped meniscus.
Kim JG, Han SW, Lee DH. Diagnosis and Treatment of Discoid Meniscus. Knee Surg Relat Res. 2016;28(4):255-262. doi:10.5792/ksrr.16.050