Discoid meniscus is an anatomic variant, usually affecting the lateral meniscus, in which the meniscus is abnormal in shape and/or in its attachments within the knee joint. The normal medial meniscus is a C-shaped piece of cartilage covering about 50% of the medial tibial plateau. It is fixed to the joint capsule by the meniscotibial (i.e., coronary) ligament and the meniscocapsular ligament; the latter connects to the deep medial collateral ligament.¹ The size, shape, thickness, and mobility of the lateral meniscus tend to be more variable than for its medial counterpart. In general, the lateral meniscus covers about 70% of the lateral tibial plateau and is more ring-shaped.² On average, it is approximately 12 mm in width and 4 to 5 mm in height. Connections to the joint capsule are less strong than for the medial meniscus. Similar to the medial meniscus, the lateral meniscus has a meniscocapsular ligament, but in this case, it attaches to the joint capsule only, with no attachment to the lateral collateral ligament. The lateral meniscus also lacks any fixation to the joint capsule in the region of the popliteal hiatus.¹ Consequently, the lateral meniscus is more mobile than the medial meniscus, translating 9 to 11 mm on the tibia during knee flexion³ versus 2 to 5 mm. Frequent anatomic variant attachments include the ligaments of Humphry and Wrisberg (or the anterior and posterior meniscofemoral ligaments, respectively), which attach the posterior horn of the lateral meniscus to the medial femoral condyle.²

Abnormal development of the meniscus, particularly the lateral meniscus, may cause it to be larger, thicker, and more hypertrophic than usual as well as more discoid in shape. A discoid meniscus also tends not to taper centrally. Its anomalous size and shape, and its subsequent decreased vascularity, cause it to be more prone to tears than a normally formed meniscus. In fact, the actual discoid shape of the meniscus is asymptomatic, and it tends to present to medical attention secondary to symptoms of a tear.

It is thought that discoid lateral meniscus affects 3% to 5% of the population, and in those affected, about 20% have bilateral involvement.¹,² Notably, Asian and, particularly, Japanese populations are thought to have an increased prevalence of this condition, with estimates approaching 15%.³ Discoid medial meniscus has been reported but is significantly less common. Prevalence estimates of discoid medial meniscus range from 0.06% to 0.3%, and bilateral involvement is thought to be rare.⁴ Historically, the true prevalence of discoid meniscus has been difficult to estimate given that many patients with the condition are asymptomatic, and the pathology is often noted only incidentally or in the setting of a tear.³,⁴ Of note, in children younger than 10 years of age, traumatic meniscal injury is rare in the absence of discoid meniscus, and this condition should always be suspected in a child presenting with symptoms of meniscus tear.¹,³ Discoid meniscus has also been referred to as popping knee syndrome or snapping knee syndrome. Presentation with a snapping knee is not typical, and this classic presentation occurs most commonly in the Wrisberg variant of the condition, which affects a minority (0% to 33%) of patients with discoid meniscus.¹,²,⁵,⁶,⁷,⁸ The classification of discoid meniscus variants and the importance of distinguishing between them will be discussed in the following text.

The pathologic nature of a discoid meniscus is evidenced by the fact that up to 70% of discoid menisci noted at arthroscopy have tears.³,⁹ In patients with discoid menisci, the most common tear pattern is the horizontal cleavage tear, which lies in contrast to patients who traumatically tear normal menisci, in which case, peripheral longitudinal tears are most ubiquitous.³ Of symptomatic tears of discoid menisci, between 58% and 98% have been reported to be of the degenerative horizontal cleavage type.¹,⁴,¹⁰,¹¹,¹² This information is significant, as tear type directly affects the treatment decision-making process. Assessment of the pathology of tears of discoid menisci has often revealed mucoid fibrinous degeneration, likely secondary to repetitive microtrauma and consequent delamination,
predisposing these menisci to the cleavage tears that are seen clinically.³ Although tears are less common in type III discoid menisci than in types I and II (classification discussed in the following text), these may be predisposed to complex degenerative tears of the posterior horn of the lateral meniscus, which is abnormally hypermobile and often enlarged compared to normal.³

Discoid meniscus has traditionally been classified into three categories: complete, incomplete, and Wrisberg ligament type (Fig. 28.1). These were described by Watanabe et al.¹³ in 1967 and were based on the arthroscopic appearance of the involved portion of the meniscus. The complete and incomplete types are discoid in shape and have normal anatomic attachments to the tibia, and they vary only in the amount of the tibial plateau that they cover.¹⁴ Those that cover the entire or most of the tibial plateau are considered complete or type I. Those that cover less than 80% of the tibial plateau are classified as incomplete or type II. Types I and II are abnormal both in shape and in width. The width of a type I or II discoid meniscus generally ranges from 8 to 10 mm, which is much larger than a normal meniscus.³ Type III discoid lateral meniscus is also called the Wrisberg ligament type. Kaplan⁶ initially described this type in 1957. This meniscus type may be discoid but can also be near normal in shape, with the defining characteristic being the lack of normal posterior capsular/tibial attachments. In a type III discoid meniscus, the posterior meniscotibial ligament is absent and the only posterior fixation is by the posterior meniscofemoral, or Wrisberg, ligament. The lack of attachments makes the meniscus unstable and pathologically more mobile. This hypermobility of the lateral meniscus may manifest with the classic complaint of snapping knee as the meniscus moves within the joint during knee motion.³ Namely, in the presence of a posterior meniscofemoral ligament and absence of a posterior meniscotibial ligament, during knee extension, the lateral meniscus can be drawn into the joint toward the intercondylar notch instead of its normal anteroposterior gliding motion, and this causes symptoms of locking, popping, and snapping.³