A grading system exists for meniscal signal abnormalities. Higher grades are associated with more meniscal body involvement [14].

The two primary criteria for the diagnosis of meniscal tears are intrameniscal signal that contacts the superior and/or inferior articular surface and/or abnormal meniscal morphology [8].

A commonly used classification of meniscal tears includes the following types: longitudinal, radial, displaced flap, bucket-handle, and complex.

Longitudinal tears, the most frequent type, are classified as horizontal (dividing the meniscus into an upper and lower segment), or vertical (dividing the meniscus into an inner and outer segment).

Longitudinal horizontal tears, also known as cleavage tears, are oriented parallel to or at a slight angle relative to the joint surface and perpendicular to the meniscocapsular junction (Fig. 3.2). Longitudinal horizontal tears are often associated with mucoid degeneration, and the associated signals may be difficult to distinguish on MRI (Fig. 3.3).

Longitudinal vertical tears are oriented perpendicular to the joint surface (Fig. 3.4). The etiology of these tears is commonly acute injury.

When the portion of the meniscus central to a longitudinal tear becomes displaced or “flips” into the intercondylar notch, this is referred to as a “bucket-handle” tear [10] (Fig. 3.5).

Radial tears extend from the central portion of the meniscus radially outward toward the periphery and may be partial or full thickness (Fig. 3.6). These tears are oriented perpendicular to the longitudinal collagen bundles of the meniscus [20]. Such tears sometimes extend anterolaterally or posterolaterally and are then referred to as parrot-beak or flap tears. Radial and flap tears account for approximately 5–10 % of meniscal tears.

A meniscal fragment may become displaced away from the site of tear, referred to as a displaced flap tear. Patients may present with locking in addition to knee pain. Like a radial tear, a displaced flap tear is manifested by a truncated or shortened appearance, but with a meniscal fragment that is typically flipped peripherally (Fig. 3.7).

Complex tears contain tears in multiple planes, typically with extensive morphological changes of the meniscus including loss of substance (Fig. 3.8).

Another important type of meniscal tear is the root ligament tear or avulsion. Disruption of the root ligament of a meniscus disrupts the hoop tension and load distribution function of the meniscus [21]. The most common root tears occur in the posterior horn of the medial meniscus, typically representing a degenerative type of tear (Fig. 3.9a). Loss of the posterior anchor allows the meniscus to extrude medially and become less effective at shock dissipation.

A tear that occurs in the peripheral third of the meniscus may heal spontaneously or be successfully repaired, due to the vascular nature of this portion of the meniscus [18].

Meniscal cysts are frequently associated with meniscal tears [22]. A meniscal (or intrameniscal) cyst is defined as a focus of high signal within a swollen meniscus. In the setting of increased pressure, fluid from an intrameniscal cyst can be squeezed or extravasated into the surrounding peripheral soft tissues, forming a parameniscal (also known as a perimeniscal) cyst [23]. Horizontal tears are also commonly associated with intra- or parameniscal cysts, as their orientation allows for synovial fluid extravasation through the tear but then acts as a one-way valve (Fig. 3.10) [19].

Although laterally located cysts are more easily detectable clinically and potentially more symptomatic, the incidence of medial- and lateral-sided cysts is similar [24].