In the series reported by LaPrade and Konowalchuk,25 all of the patients who experienced symptoms during figure-4 testing were found, on arthroscopic examination, to have lateral meniscal hypermobility caused by tears of the popliteomeniscal fascicles. Despite the success of those authors in using the figure-4 technique for diagnosis, others have reported that the accuracy of the clinical examination (vs arthroscopy) in diagnosing Wrisberg-variant discoid lateral menisci ranges from 29% to 93%.5,26,27 This emphasizes the importance of diagnostic imaging in the work-up of patients with suspected Wrisberg-variant discoid lateral menisci.
Imaging Features
Radiography
In 1964, Picard and Constantin28 recommended that patients with suspected discoid lateral menisci undergo standard anteroposterior, lateral, tunnel, and skyline radiographs as part of the diagnostic work-up. In patients with discoid lateral menisci, plain film radiographs are often normal10 but may demonstrate lateral femoral condyle squaring, widening of the lateral joint line, lateral tibial plateau cupping, tibial eminence hypoplasia, and fibular head elevation.5,29 Plain radiography is unreliable, however, and patients often require advanced imaging, such as knee magnetic resonance imaging (MRI).10
Magnetic Resonance Imaging
Because it clearly depicts soft-tissue structures, MRI is widely used to diagnose musculoskeletal pathology in and around the knee. Criteria for the diagnosis of discoid menisci include meniscal width of 15 mm or more, ratio of minimum meniscal width to maximum tibial width on coronal slice of more than 20%, ratio of sum of width of both lateral horns to meniscal diameter (on sagittal slice showing maximum meniscal diameter) of more than 75%, and continuity of anterior and posterior horns on at least 3 consecutive sagittal slices (bow tie sign).5,30,31 Even in the presence of a tear, the described ratios have sensitivity and specificity of 95% and 97% in detecting discoid lateral menisci.30
However, the Wrisberg variant, which may consist of only a thickened portion of the posterior horn, is often more difficult to diagnose using these criteria and can even appear normal on MRI.26,32 In a series by Neuschwander and colleagues,7 none of the Wrisberg-variant menisci had a true discoid shape, suggesting that the size of the lateral meniscus may appear normal in affected patients. Appropriate positioning during MRI evaluation of patients suspected of having the Wrisberg variant was emphasized by Moser and colleagues,33 who described a case of discoid lateral meniscus not observable on initial MRI but visible on MRI performed with the affected knee extended in the locked position.
The unstable lateral meniscus may be seen subluxed anteriorly or laterally because of lack of posterior attachments. A deficiency of normal popliteomeniscal fascicles and coronary ligaments is represented by a high T2 signal interposed between the discoid lateral meniscus and the posterior joint capsule, simulating a vertical peripheral tear and suggesting presence of the Wrisberg variant (Figures 1A–1C). In addition, the posterior horn of the enlarged discoid lateral meniscus may connect to a prominent and thickened meniscofemoral ligament of Wrisberg. Despite these characteristic imaging features, some studies have found low sensitivity of MRI in the diagnosis of Wrisberg-variant discoid lateral menisci.26
Ultrasonography
There is a growing interest in using ultrasonography in the diagnosis of Wrisberg-variant discoid lateral menisci because of its availability, multiplanar capability, and lower cost compared with MRI. Ultrasonographic criteria for the diagnosis of discoid menisci include absence of normal triangular shape, presence of abnormally elongated and thickened meniscal tissue, and demonstration of a heterogeneous central pattern.5 Through use of a high-resolution probe, which better fits the anatomical concavity of the popliteal fossa, a positive predictive value of 95% and a negative predictive value of 100% have been reported for ultrasonography in the diagnosis of meniscal tears.34
Perhaps the main advantage of ultrasonography is the possibility of performing a dynamic study to evaluate the extrusion of the meniscus into the lateral gutter and to correlate this with knee snapping (Figures 2A, 2B).35 One technique for sonographic evaluation of a hypermobile lateral meniscus involves placing the patient supine with the high-resolution (9 or 12 MHz) linear transducer along the lateral knee joint line. The patient is then asked to place the foot of the affected extremity on the contralateral knee; the combination resembles the numeral 4 (figure-4 test) (Figures 3A, 3B). In a symptomatic patient, this results in clicking, snapping, and/or pain along the lateral joint line, and the lateral meniscus is noted sonographically to extrude into the lateral gutter (Figure 2B), either the result of torn popliteomeniscal fascicles or the increased meniscal mobility of Wrisberg variants.
The main drawback of ultrasonography is operator dependence. As clinicians become more familiar with ultrasonography, dynamic ultrasonography should be used for what is often a difficult diagnosis both clinically and with nondynamic imaging.