Within the knee are two crescent-shaped menisci that act as shock absorbers between the femur and tibia, dispersing axial compressive loads. The menisci are fibrocartilaginous structures mainly composed of type I collagen. They have several functions including force transmission, increased joint congruency, and secondary stabilizers of the knee. The medial meniscus is typically more C-shaped and intimately in contact with the surrounding capsule. The lateral meniscus is more circular in shape and has increased mobility due to fewer capsular attachments.
A discoid meniscus is an anatomical variant from the normal crescent-shaped meniscus, in which the discoid meniscus is often thicker and takes on a disc or saucer shape. These variants are more commonly disposed to injury in comparison to a normally shaped meniscus. When these variants are present and symptomatic, they can often lead to a popping sensation within the knee which commonly is referred to as “popping knee syndrome.”
A discoid meniscus is due to a congenital, anatomical occurrence. The cause of discoid meniscus is not presently known. Injuries to the meniscus are common and may arise following a twisting or pivoting motion through the joint; although, the lack of an inciting event is also common. The presence of a discoid meniscus is associated with an increased risk of meniscal tears.
Some studies have shown that approximately 3% to 5% of the population have a discoid meniscus. Within this population, the lateral meniscus is by far the more common anatomic variant, with only a few case reports of medial discoid meniscus reported. Up to 25% of cases of the discoid meniscus is bilateral at presentation. The discoid meniscus can be divided into one of the three following subtypes based on the orientation or pattern of the meniscus. The three types are incomplete, complete, and Wrisberg variant. Incomplete, (type 1), is simply defined by a slightly thicker and wider than normal meniscus. A complete, (type 2), discoid meniscus covers the entire portion of the superior aspect of the tibia within the affected compartment. The Wrisberg variant, (type 3), is a hypermobile discoid meniscus that lacks posterior meniscotibial attachments. Type 3 is, therefore, the most unstable of the three types of the discoid meniscus.
Overall, the incidence of discoid meniscus is much higher in Asian countries compared to Western nations. In Japan and Korea rates as high as 10-13% have been reported, almost double what is reported in the US.
The abnormal shape of the discoid meniscus predisposes the structure to injury. Partial or full tears can occur with or without a traumatic event, often leading to mechanical symptoms. A study conducted by Ouyang et al. 2015 showed that discoid meniscus tears are most often horizontal, while the majority of normal meniscus tears occur in a longitudinal or radial pattern.
Studies show that there is significant disorganization of the circular collagen bundle. In addition, the circumferentially arranged collagen fibers show a heterogeneous course. It is believed that there is a structural problem, with an abnormal vascularity and increased discoid thickness which makes it prone to tearing.
A patient that presents with symptomatic discoid meniscus will commonly have complaints of anterior and/or lateral knee pain with or without a popping sensation within the knee. This popping sensation is not always present but is often more common with activity. Other mechanical symptoms may also be reported by the patient, such as catching, locking or feelings of instability through the knee. In the setting of a displaced meniscal tear, the patient may present lacking the ability to extend the knee fully. Trauma is not necessary to cause symptomatic discoid meniscus. If a discoid meniscus is symptomatic, the patient will often present during adolescence. A comprehensive knee examination should be completed, including visual inspection, a range of motion, neurovascular evaluation, ligamentous stability testing, and meniscus-specific special testing. Special tests specific to meniscal pathology should include medial or lateral joint line tenderness to palpation, McMurray’s test, Apley’s compression test, and Thessaly’s test.
Evaluation of a discoid meniscus is prompted by pain or mechanical symptoms described by the patient, although this can also be an incidental finding on MRI or x-ray. If a symptomatic discoid meniscus is suspected, the patient should be referred to an orthopedic surgeon for an evaluation. X-rays should be done first. Common findings on an x-ray of a discoid meniscus include a widened joint space and squaring of the lateral femoral condyle. Findings on an MRI of discoid meniscus include the “bow-tie sign” with three or more 5 mm sagittal images showing meniscal continuity, an abnormally thick or flattened meniscus on the sagittal cut, and thick/flat meniscal tissue that extends over the entirety of the involved compartment on coronal views.
If a discoid meniscus is diagnosed incidentally in the absence of symptoms, no formal treatment is indicated. However, if a discoid meniscus is causing mechanical or painful symptoms, surgery is warranted. Most commonly, symptomatic discoid menisci are treated with arthroscopic saucerization, which is a procedure that utilizes shavers and biters to reshape the meniscus. The surgeon converts the discoid into a more anatomic crescent, meniscal shape. Certain tear patterns may be amenable to hybrid procedures, utilizing both saucerization and subsequent remaining meniscal repair. Hypermobile Wrisberg variants may warrant saucerization combined with meniscal fixation to the adjacent capsule to preserve native meniscal tissue. Post-operatively, weight-bearing is at the discretion of the surgeon. Often, physical therapy is prescribed for muscle strengthening, increasing range of motion, edema control, balance, and coordination.
The Discoid Meniscus per Watanabe classification:
The complete type is the disc-shaped meniscus has a normal posterior attachment and totally covers the lateral tibial plateau.
The incomplete type appears semilunar in shape and has a normal posterior attachment. However, it covers less than 80% of the lateral tibial plateau.
The Wrisberg type has a normal shape compared to the incomplete or complete type of discoid meniscus. However, the normal posterior meniscal attachment (coronary ligament) is missing. This type only has the Wrisberg ligament connecting to the posterior horn of the lateral meniscus.
Most reports indicate a good outcome after partial meniscectomy with or without repair of the meniscus. Follow up studies have not shown degenerative changes in 5 year follow up studies.
The prognosis is better in young patients.
In the setting of total meniscectomy, meniscus allograft transplantation (MAT) may be a viable option. Yoon et al. 2014, followed a patient population that underwent this procedure with a mean follow-up period of 32 months to assess clinical outcomes following MAT. He found that at a minimum of 2 years, similar outcomes in knee function and pain were achieved among patients undergoing MAT for discoid meniscal tears versus non-discoid meniscal tears.