Menisci are two crescent-shaped structures in the knee that act as shock absorbers between the femur and tibia, dissipating stress and dispersing compressive loads . The menisci are fibrocartilaginous structures mainly composed of type I collagen. Functions include force transmission, increase joint congruency, and secondary stabilization of the knee . The medial meniscus is typically 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. It is often thicker and is disc or saucer-shaped . 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 pain and a popping sensation within the knee which commonly is referred to as “popping knee syndrome." Three subtypes have been well described in the literature based on the pattern of the meniscus and they are classified as complete, incomplete, and Wrisberg variant . Management consists of observation versus arthroscopic saucerization with or without meniscal repair based on morphology and patient presentation .
It is suggested that a discoid meniscus is congenital. Much debate has been presented as to whether it is a true morphological versus structural variant. Histological examination of a discoid meniscus has shown decreased, disorganized, and degenerative collagen fibers in comparison to a normal meniscus .
The thickness of the meniscus is increased and has abnormal vascularity . It is suggested that these factors are associated with an increased risk of meniscal tears in the presence of a discoid meniscus ].
Many studies have shown that approximately 3% to 5% of the United States population has a discoid lateral meniscus. In Asian countries, the reported incidence is much higher, with studies reporting up to 13% 10795054 to have a discoid lateral meniscus .
A discoid medial meniscus is extremely rare, with literature suggesting an incidence of 0.1% to 0.3% . It has been reported that up to 25% of cases of the discoid meniscus are bilateral at presentation. Given that a majority of discoid menisci are asymptomatic, the true incidence is unknown .
The abnormal shape and composition of the discoid meniscus are thought to predispose the structure to injury . Partial or full tears can occur with or without a traumatic event. 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 abnormal vascularity and increased discoid thickness which makes it prone to tearing. The discoid meniscus with its abnormal collagen formation undergoes mucoid degeneration weakening its integrity in the presence of repetitive microtrauma  .
Symptomatic discoid meniscus most commonly presents with complaints of anterior and/or lateral knee pain with or without mechanical symptoms.
Discoid meniscus has classically been referred to as “snapping knee syndrome,” where at terminal flexion a clunk may be heard. This is typically related to the unstable Wrisberg variant where there are no posterior meniscotibial attachments . This popping sensation is not always present but is often more common with activity and may be associated with pain, limited range of motion and effusion . The onset of symptoms may be traumatic or atraumatic in nature. Mechanical symptoms include catching, locking, and clicking and are not necessarily related to instability .
In the setting of a displaced meniscal tear, the patient may present lacking the ability to extend the knee fully. If a discoid meniscus is symptomatic, the patient will often present during adolescence .
A comprehensive knee examination should be completed, including visual inspection, 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 palpation to assess tenderness, McMurray’s test, Apley’s compression test, and Thessaly’s test.
Evaluation of a discoid meniscus is prompted by pain or mechanical symptoms, although this can also be an incidental finding on X-ray or MRI in the setting of acute trauma .
Plain radiographs may be normal, however, there are well documented radiologic features that may be present in patients with a discoid meniscus . Common findings include widened joint space in the affected compartment, squaring of the lateral femoral condyle, and cupping of the lateral tibial plateau .
Findings on MRI of discoid meniscus include the “bow-tie sign,” which is 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 . MRI is also a useful diagnostic tool to evaluate for meniscal tearing and may aid in pre-operative surgical planning . Imaging findings should be correlated to the patient’s history and physical exam, as an incomplete discoid meniscus can appear normal on MRI .
Surgical treatment is indicated for symptomatic discoid meniscus that is causing pain and/or mechanical symptoms. Most commonly, symptomatic discoid menisci are treated with arthroscopic saucerization/partial meniscectomy, which is a procedure that reshapes the meniscus to a more anatomic crescent morphology to enhance its function . It is also important during saucerization to preserve the peripheral rim to maintain meniscus stability .
If a discoid meniscus is incidentally diagnosed in the absence of symptoms, no formal surgical treatment is required and the patient may be observed.
It is well documented that the lateral meniscus has roughly twice the amount of excursion in comparison to the medial meniscus . In cases of Type C Wrisberg variant discoid meniscus, the lateral meniscus is even more hypermobile than normal, thus likely requiring arthroscopic meniscal fixation if symptomatic at the time of saucerization to improve stability .
Post-operatively, weight-bearing is at the discretion of the surgeon. Physical therapy is frequently prescribed for muscle strengthening, increasing range of motion, edema control, balance, and coordination.
Symptomatic discoid meniscus commonly presents with knee pain, catching, locking, clicking, effusion, and limited range of motion . It is important to perform a detailed physical exam of the affected and unaffected knee and obtain appropriate imaging studies. The following conditions should be considered in the differential diagnosis:
A discoid meniscus is classified by the Watanabe classification:
The complete type has a disc-shaped meniscus that completely covers the lateral tibial plateau. It has a normal posterior meniscotibial attachments .
The incomplete type covers less than 80% of the lateral tibial plateau . It appears semilunar in shape and has a normal posterior meniscotibial attachments.
C: Wrisberg variant.
The Wrisberg type has a more normal shape compared to the incomplete or complete type of discoid meniscus. The normal posterior meniscal attachment (coronary ligament) is missing and only the Wrisberg ligament remains as a connection to the posterior horn of the lateral meniscus, making it an unstable variant .
Partial meniscectomy with or without meniscus repair for symptomatic discoid meniscus has reported good to excellent clinical outcomes in the current literature. Multiple studies with five years of follow up report favorable results in about 85% of cases .
Partial meniscectomy has been shown to be favorable in comparison to total or subtotal meniscectomy . After total meniscectomy, the increased contact pressures without adequate meniscus to function as a shock absorber is likely to increase the risk of degenerative joint disease .
Asymptomatic discoid menisci may become symptomatic in the presence or absence of trauma. They are at increased risk of tearing due to their abnormal collagen composition . Postoperatively, a potential unique complication to saucerization of the meniscus is an osteochondritis dissecans (OCD) lesion of the lateral femoral condyle. Repeated impaction of the chondral surface after partial meniscectomy is thought to predispose a patient to this issue  and the clinicians should be aware of this potential outcome.
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. The outcomes after surgery in young patients are good to excellent. A thorough discussion of the risks and benefits between operative and nonoperative management should be held between the patient and the patient's legal guardian if a pediatric patient, in order for them to make an informed decision.
The diagnosis and management of discoid meniscus are with an interprofessional team that includes a primary care provider, nurse practitioner, sports physician, orthopedic surgeon, radiologist, and a physical therapist. All personnel is essential in optimizing patient outcome .
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