Lesions in the menisci are depicted as signal increase in sequences using short echo times (T1-weighted or PD-weighted fat saturated (turbo) spin-echo (TSE) sequences). Meniscal alterations can be classified in three grades with MRI using the guidelines developed by Stoller et al. (1987) and modified by Reicher et al. (1986).
- grades I alterations of (globular)
- grades II (linear) are arthroscopically occult because they do not reach the surfaces of the menisci. Underlying pathology is degenerative change with mucoid transformation of meniscal tissue.
- grade III lesions correspond to arthroscopically detectable ruptures of variable depth
Grade II alterations
In grade II alterations, damage to longitudinally aligned collagen fibers is thought to explain the course of the linear signal alterations. Follow-up observations showed that grade II alteration mostly remains stable and only rarely progresses to clinically relevant ruptures or; instead, it usually remains stable, or may even disappear (Dillon et al. 1990). Triangularly shaped signal alterations entering the meniscus from its base are to be differentiated from intrameniscal degenerations, as they correspond to the fibrovascular bundles in the peripheral third of the menisci. Their shape is uniform and they are mostly symmetric. In contrast to grade I and II meniscal alterations, signal abnormalities reaching at least one of the surfaces of the menisci are to be called meniscal tears.
Such grade III lesions
The most common location is the posterior horn of the medial meniscus. This is clinically relevant because arthroscopically the posterior horn of the menisci, especially the inferior surface of the medial meniscus, is difficult to visualize during arthroscopy. Traumatic tears, however, exhibit a predominantly vertical orientation.
Bucket handle tears
Characteristic imaging findings of bucket handle tears are the “fragment-in-notch” sign and the “truncated-meniscus” sign in coronal sections. These correspond to a longitudinally separated meniscal fragment that is displaced into the intercondylar notch. In the more common case of a bucket handle tear of the medial meniscus, this fragment is displaced underneath the posterior cruciate ligament against the barrier of the ACL or its synovial coverage preventing further dislocation. In central sagittal sections, this gives rise to the “double-PCL” sign (Weiss et al. 1991).
Portions of the posterior meniscus are flipped anteriorly over the remaining anterior horn, the “flipped-meniscus” sign. This is a very conspicuous sign of serious meniscal pathology because it reflects the disturbed relationship of the volumes of the anterior and posterior horns of the menisci. In the lateral menisci, the anterior and posterior horns are of approximately the same size, whereas in the medial meniscus the posterior horn is of about twice the size of the anterior horn. Any disturbance of this volume relationship is indicative of an instable meniscal lesion or previous meniscus surgery.
The shape and course of a meniscal tear have to be precisely described because this is relevant for therapy. Especially vertically oriented peripheral (peripheral third) tears can be sutured because healing response usually is good in the vascularized peripheral third of the menisci (red zone).
The postsurgical MRI appearance of the menisci depends on the procedure that was carried out. After suturing, grade III signal alterations may persist in clinically asymptomatic patients. Only newly developing signs of meniscal tears in follow-up examinations may provide evidence of a recurrent tear. Partial resection of meniscal tissue may bring central scar tissue to the newly created surface of the operated meniscus.
MRI easily depicts intra- or parameniscal cysts and helps to determine their localization and extension. Their typical, loculated appearance with high signal intensity content in T2-weighted images as well as their topographic relationship to the menisci allow for a confident diagnosis. Usually, they are associated with (degenerative) meniscal tears. Sometimes MRI allows visualization of the communication between the cysts and meniscal tear.