All parts of both menisci are well visualized by MRI. Sagittal MR images provide good views of the anterior and posterior horns and a fair view of the body of both menisci. In more central sections, both horns of the menisci appear as wedge- shaped signal voids contrasted on their superior and inferior surfaces by the moderate signal intensity of the hyaline cartilage on the articular surfaces of the femur and tibia. In more peripheral sections, where the images are tangential to the circumference of the menisci, they appear bow tie shaped (Fig. 6-22). Coronal MR images provide the best visualization of the bodies of both menisci.
FIGURE 6-22. Sagittal MR images through a normal peripheral menisci. The anterior and the posterior (arrow ) horns appear wedge (triangular) in shape.
There are three types of meniscal findings visualized by MRI (16,39,40). One is the presence of small globular or irregular high–signal-intensity foci confined to the interior of the meniscus. This is considered to be an early type of mucoid degeneration. A second type of meniscal MRI finding is the presence of a linear region of increased signal intensity within the meniscus that does not extend to either the femoral or tibial articular surface of the meniscus but may extend to the meniscocapsular junction. Histologically, this represents fragmentation and separation of the fibrocartilage and is considered by many to be an intrameniscal tear. The significance of the globular or linear signals that do not extend to either articular surface of the meniscus is not fully agreed on (41).
Frank meniscal tears are demonstrated by MRI as linear or irregular areas of signal intensity that extend to one or both articular surfaces of the meniscus (Fig. 6-23). The high signal intensity is produced by synovial fluid in the crevices within the meniscus. These meniscal tears can be horizontal, vertical, or complex. Bucket-handle tears are vertical tears where the inner meniscal fragment is displaced toward the intercondylar notch (Fig. 6-23C,D). At times, repeated trauma or chronic degeneration may cause a gross distortion of meniscal shape, and the meniscus may then appear to have a truncated apex or to be grossly small with a free fragment.
FIGURE 6-23. Meniscal tears. A: T1-weighted MRI of a horizontal tear (arrow ) at the posterior horn of the medial meniscus that extends to its tibial articular surface. B: Coronal PD-weighted image of a bucket-handle tear. There is a displaced fragment from the lateral meniscus into the intercondylar fossa (arrow ). C: Sagittal FSE-PD fat suppressed sequence of a double “PCL sign.” The displaced fragment from a bucket handle tear (arrow ) projects anterior to the PCL. D: Sagittal PD fat suppressed sequence through the lateral tibiofemoral joint demonstrates the double delta sign. The displaced anterior horn from a large flap injury is projecting anterior to the posterior horn.
Other meniscal abnormalities well visualized by MRI include discoid meniscus, meniscal cysts, and abnormalities involving the postoperative meniscus. In discoid meniscus, typically involving the lateral meniscus, there is a continuous bridge of meniscal tissue between the anterior and the posterior horns in the central part of the joint. Meniscal cysts are usually associated with underlying horizontal meniscal tears through which synovial fluid collects at the meniscocapsular junction (42). They show high signal intensity on T2-weighted images. MRI also can be used to evaluate the postmeniscectomy patient with continuing or recurrent symptomatology (39). It can detect an incompletely excised meniscal tear, retained meniscal fragments, or a tear developing within the residual part of the meniscus. MR arthrography with gadolinium can be helpful to distinguish between retears and old healed tears that might still show increased signal intensity on T2-weighted images (43). .