The medial meniscus has a more open C-shaped structure than the lateral meniscus, covers approximately 60 % of the corresponding tibial plateau and is wider posteriorly than anteriorly (Fig. 5.1) [1]. In a loaded, in vitro situation, 50% of the axial load in the medial compartment is absorbed by the medial meniscus [4].

The lateral meniscus is more circular O-shaped than the medial meniscus and covers 80 % of the corresponding tibial plateau [1]. Its width is constant from anterior to posterior. In a loaded, in vitro situation, 70% of the axial load in the lateral compartment is absorbed by the lateral meniscus [4]. The lateral meniscus is more mobile than the medial meniscus and, compared to the medial meniscus, has a loose attachment to the knee capsule and no attachment to the lateral collateral ligament.

The medial meniscal flounce is a normal positional variant characterized by a single symmetric fold along the free edge of the meniscus [21, 22]. The flounce is less commonly seen on MRI compared to arthroscopy and is considered it to be the result of traction of the medial meniscus during tibial rotation [22]. In these cases, the flounce is a transient finding and it may disappear during maximal knee extension or flexion [22]. On MR images the body of the medial meniscus shows a wavy, irregular S-shaped along the free margin of the meniscus on sagittal planes (Fig. 5.13). A similar appearance may be seen in patients with knee instability or joint effusion when the flounce can be also seen in the lateral meniscus. Although, the meniscal flounce is a rare anatomic variant, its presence should be recognised since a meniscal tear or a discoid meniscus may resemble same appearance (Fig. 5.14). However, meniscal flounce and meniscal tears may coexist.

A discoid meniscus is a congenitally tall and elongated meniscus that can be symmetrically or asymmetrically increased in size [23]. The lateral discoid meniscus is more common (incidence of 0.4–16.6 %) than the medial discoid meniscus (incidence of 0.3 %) [24]. There are three types of discoid meniscus: complete, incomplete, and the Wrisberg type [25]. The complete and incomplete variants describe the shape of the meniscus with the incomplete type having a more semilunar shape. Both types, complete and incomplete discoid meniscus, are stable due to a normal posterior capsular and tibial attachment. The Wrisberg discoid meniscus is unstable and the only posterior attachment is the posterior meniscofemoral ligament of Wrisberg with a deficient posterior root meniscotibial ligament [26]. Clinically, patients can be asymptomatic or may complain of pain, swelling, clicking, and snapping especially in Wrisberg type due to the hypermobility of the meniscus. The MR imaging diagnosis of discoid meniscus is mainly based on the meniscus width and not on the meniscus height since it has been demonstrated that the accuracy of the meniscus height of the discoid meniscus is low [24]. Taking into consideration that the normal meniscus width is 14 mm measured on central slices, the diagnosis of discoid meniscus is suggestive when on sagittal images a continuity between the anterior and posterior horn is seen on four or more continuous slices (3 mm slice thickness) (Fig. 5.15) [27, 28]. The coronal images through the middle joint are more accurate and show an abnormally wide meniscus (>15 mm) (Fig. 5.16) [28]. The discoid meniscus may extend into the intercondylar notch in the complete type (Fig. 5.17) or may be seen in a normal position in the incomplete discoid type [29]. In the Wrisberg type, in the absence of the meniscotibial and posterior capsular attachment, an increased T2 signal intensity may be present between the capsule and the meniscus, simulating a peripheral tear [26]. Regardless the type, a discoid meniscus is prone to degeneration and tears (Figs. 5.18 and 5.19).

Meniscal avulsion or the “floating” meniscus is defined as the detachment of the meniscus from the tibial plateau without separating completely from the capsule [30]. Typically, the lesion is limited to the meniscotibial ligaments (coronary ligaments) while the meniscofemoral ligaments remain intact [31]. In the setting of acute trauma the meniscotibial ligaments may become disrupted and the meniscus is avulsed from the tibial plateau without evidence of tear within the substance of the meniscus [31]. The medial meniscus is more frequently involved than the lateral meniscus [30]. On MRI, the meniscal avulsion is best evaluated on sagittal and coronal T2-weighted images and on MR arthrography. The presence of high-intensity fluid of greater than 3 mm thickness in the long axis between the meniscus and the tibial plateau is suggestive for meniscal avulsion (Fig. 5.20) [31].

Meniscal extrusion is considered when the meniscus is displaced beyond the tibial margin [32]. Meniscal radial and oblique tears, meniscal roots tears, meniscal degeneration, cartilage defects and the absence of the meniscofemoral ligaments are the most common causes of the radial expansion of the meniscus from the joint space. The meniscal tears as well as the meniscal roots tears alter the circumferential margin of the meniscus that resists radial displacement and permit the meniscal extrusion from the joint space [33]. The meniscal roots tears are strongly associated with meniscal extrusion especially in the medial compartment. There is also a significant association between all grades of meniscal degeneration and cartilage damage of the medial compartment and the medial meniscal extrusion (Fig. 5.21) [34]. A rare cause of meniscal extrusion may be the knee effusion when the extrusion is seen in the medial compartment as a result of the distention of the capsule that is firmly attached to the medial meniscus [35]. Recognising the meniscal extrusion is of clinical importance since it may lead to development of osteoarthritis. On coronal MR images the extrusion is assessed between the tibial margins (excluding osteophytes) and the outer margin of the lateral or medial meniscus (Fig. 5.21) [34]. A distance of more than 3 mm for the medial meniscus and more than 1 mm for the lateral meniscus is considered abnormal [9].

Meniscocapsular separation is the result of the disruption of the capsular attachment of meniscus. The lesion can occur at the meniscocapsular junction or within the peripheral zone of the meniscus. Because of the strong fixation of the medial meniscus, tears of the meniscotibial or the coronary ligaments, especially of the posterior horn, are more frequently compared to the lateral meniscus. Coronal and sagittal T2-weighted MR images are particularly useful in demonstrating the lesion (Figs. 5.22 and 5.23). The presence of fluid with an increased distance between medial collateral ligament and medial meniscus, the displacement of meniscus from tibia, tear within the peripheral zone of meniscus, and irregular meniscal margins are the best predictors of medial meniscocapsular separation (Fig. 5.22) [36]. Laterally, a displaced meniscus and the disruption of the popliteomeniscal fascicles with high-signal intensity edema in the posterolateral corner of the knee are suggestive for lateral meniscocapsular separation [37]. Perimeniscal fluid may be also present in medial or lateral meniscocapsular separation (Fig. 5.23).

The degenerative meniscal changes are the result of the internal derangement of the normal meniscal architecture followed by intrasubstance mucoid degeneration. It is frequently seen in elderly but it can appear at younger ages in response to high mechanical loading. The degenerative changes involve more frequently the posterior horn of the medial meniscus and are usually asymptomatic. On MR images the lesion appears as diffuse or focal increased signal intensity changes surrounded by normal low-signal intensity of the normal meniscal substance (Fig. 5.24). The degenerative lesions do not extend to the articular surfaces of the meniscus [29]. However, in some cases the free margin may display irregularities and the mucoid changes may progress to horizontal tears. When the linear changes do not convincingly extend to the articular surface, it is better to be descriptive rather than overdiagnose an equivocal finding [29].

Meniscal contusion appears in the setting of acute trauma and it usually involves the meniscal surface [38]. Bone injury adjacent to the meniscal contusions have been described in these patients suggesting that the mechanism of these lesions may be a compressive injury [38]. Meniscal contusion is seen on MR images as a diffuse, amorphous area of abnormal signal intensity that extends to the articular surface (Fig. 5.25). The signal does not meet the definite linear pattern of a meniscal tear neither the well-defined intrameniscal signal of the mucoid degeneration. At arthroscopy these lesions do not have the appearance of typical meniscal tears [38]. On MR imaging follow-up the meniscal contusion may resolve or improve suggesting the transient character of the contusion [38].

The meniscal tear is described as a linear increased signal intensity within the meniscus. The MR report should not only identify the presence of a linear meniscal tear but also should accurately describe the orientation, location, length and stability, because these criteria are important and may influence the choice of operative or nonoperative treatment. The classifications of meniscal tears is based on different criteria (Table 5.1). According to the orientation, a meniscal tear can be vertical, horizontal, or complex. Vertical tears are further subdivided into radial (perpendicular to the long axis of the meniscus), which includes the tears of the meniscal root and longitudinal (parallel to the long axis of the meniscus) [29]. The complex tears refer either to a combination of more orientations (vertical and horizontal) including the parrot beak tear and tears with displaced fragments (bucket-handle tear, flap meniscus tear, or free meniscus fragment). Meniscal grading (grade I–III) that has been widely used in the past is now considered obsolete in the clinical routine imaging [41].