Meniscal Tears on MRI

Overview and Anatomy

In many cases, MRI results lead to changes in the proposed management. One study determined that about one third of all diagnostic arthroscopies need not be performed if MRI is used; another study showed that the use of MRI prevented 51% of diagnostic arthroscopic procedures; and a third study showed that with the use of MRI, the morbidity associated with arthroscopy was avoided.^[15]
MRIs show many of the essential characteristics of meniscal tears critical to management, such as their location, shape, length, and depth. In this way, MRI helps to make an accurate assessment of stability and of the likelihood of tear propagation, and it enables one to determine whether the meniscal tear can be repaired. It is advantageous to know ahead of time if a given meniscal tear can be repaired, because the additional equipment, surgical assistants, and time needed for repair can be anticipated. Patients also benefit from knowing early on whether surgery is necessary. The recovery time for meniscal repair is longer than that for partial meniscectomy (PM). Patients may want to time surgery to fit with their other obligations.^{[1, 15, 17, 18]}
When combined with clinical data, such as the patient's age, athletic requirements, and physical findings (eg, possible associated ligamentous injuries), a treatment plan may be developed by assessing the need for and timing of surgery and by determining the type of surgery (meniscal debridement, rasping, repair, partial or total resection, or meniscal transplantation). MRI may be used to identify other injuries, such as ligament tears, especially anterior cruciate ligament (ACL) tears, the presence of which may also influence the decision whether to perform surgery.^{[1, 15, 17]}
With MRI, physicians may obtain images in several planes, providing multiple perspectives on meniscal and ligamentous injuries. Other advantages include the following:

• MRI does not expose the patient to ionizing radiation
• MRI does not normally involve the intravenous administration of contrast material, the use of which is associated with a small but definite number of adverse effects
• MRI does not require joint manipulation
• MRI is painless and can be performed in less than 35 minutes
• MRI does not require the intra-articular injection of iodinated radiographic contrast material, which is needed for arthrography

Arthrography has been supplanted by MRI except for patients who are too large to fit into the MRI unit or for patients who have contraindications to MRI (eg, intracranial aneurysm clips, orbital metallic foreign bodies are present). Magnetic resonance arthrography is used to evaluate residual or recurrent meniscal tears after meniscal surgery. The detection of residual or recurrent meniscal tears following meniscectomy or meniscal repair is difficult with conventional MR images.^[19]

Normal Anatomy on MRIs

Structures in the sagittal plane
Centrally, the normal meniscus is composed of 2 separate triangular structures: the anterior horn and the posterior horn. The apices (free edges or inner margins) appear as sharp points of the triangle facing each other (see the first image below). On the lateral side of the knee, the triangular anterior and posterior horns of the LM are equal in size (see the second image below). On the medial side of the knee, the posterior horn of the MM is larger than the anterior horn (see the third image below).^[13]
Coronal fat-saturated proton density–weighted image shows abnormal signal intensity in the posterior horn of the medial meniscus (MM) extending to the undersurface near the junction with the joint capsule. Such tears may be missed on arthroscopy because that part of the knee joint is difficult to access. Also present is a tear to the posterior medial horn of the lateral meniscus (LM) as it slopes obliquely inward. A false-positive diagnosis of meniscal tear can be made when one evaluates this region because of the magic angle effect. Tears persist when the echo time (TE) is varied and when T2-weighted images are obtained. True tears can also be confirmed by visualizing them on sagittal or axial projections. Sagittal fat-saturated proton density–weighted image of the lateral compartment shows the relative equal size of the anterior and posterior horns of the lateral meniscus. The meniscal body has the normal configuration of a bow tie. Sagittal fat-saturated proton density–weighted image of the medial compartment shows the larger posterior horn (arrowhead) and the smaller anterior horn. Peripherally (medially for the medial meniscus [MM] and laterally for the lateral meniscus [LM]), the menisci have a bow-tie configuration, as shown in the images below. The anterior and posterior horns are taller than the thinner and interposed body of the meniscus.
Sagittal fat-saturated proton density–weighted image demonstrates the concave superior meniscal surface (arrows), which improves contact with the femoral epicondyles, and a flat undersurface, which improves contact with the tibial plateau. The periphery (outer edges) is thicker than the central portion (arrowhead), allowing for firm attachment to the joint capsule. Note the normal bow-tie appearance of the meniscal body. Sagittal fat-saturated proton density–weighted image shows the inferior fascicle. In this location, the superior fascicle is not present. Note the normal bow-tie appearance of the meniscal body. Coronal proton density–weighted image shows extensive grade 2 signal intensity in the anterior and posterior horns of the medial meniscus. However, the signal intensity does not extend to a joint surface. Both menisci have anterior and posterior roots, which attach the anterior and posterior horns to the tibial plateau, on either side of the centrally placed tibial spine (see the image below). These attachments are referred to as roots.^{[13, 20]}
Sagittal proton density–weighted image shows the tibial insertion site of the posterior horn of the medial meniscus (MM). Popliteus tendon and sheath
The popliteus tendon and its accompanying sheath course through the posterolateral portion of the posterior horn of the LM in an oblique anterosuperior to posteroinferior direction. It is seen on the more lateral images of the LM (see the images below).
Coronal proton density–weighted image shows the ligament of Wrisberg originating from the posterior medial horn of the medial meniscus and passing obliquely upwards (arrow) to attach to the posterolateral aspect of the medial femoral epicondyle. Coronal fat-saturated proton density–weighted image shows the popliteus tendon originating from an undulation of the lateral femoral condyle. From there, it passes through the popliteus recess to insert on the proximal posterior tibial metaphysis. A radial tear (arrow) is present in the posterior horn of the medial meniscus. Two fascicles connect the posterior horn of the LM at the popliteus tendon sheath level to the joint capsule. The inferior fascicle is seen on the more lateral images through the tendon. Here, the superior fascicle is absent. More medially, both superior and inferior fascicles are present. The most medical images through the tendon show the superior fascicle and absence of the inferior fascicle. The thickness of the popliteus tendon sheath varies in size from a thin line to a thick band.
Structures in the coronal plane
This is the best plane in which to image the meniscal bodies (see the images below). Each meniscal body looks like a triangle with the pointed apex in the innermost part of the meniscus. The anterior and posterior horns appear as flat slabs. The root of the posterior horn of the LM is directed obliquely upward from a lateral to medial direction. The popliteus recess is located in the outer portion of the lateral joint compartment. It can be identified either by the presence of joint fluid within it or by the popliteus tendon originating from the distal lateral femur, above the joint, and passing through the sheath to insert on the back of the proximal tibia.^{[13, 21]}
Coronal fat-saturated proton density–weighted image of the mid knee shows the normal appearance of the body of the medial and lateral menisci. The apices (inner portions) are the thinnest part of the meniscus and are more central in the knee joint. The periphery, meniscal bases, outer portion (arrow and arrowhead) is the thickest part and contains the blood vessels supplying the meniscus. Coronal fat-saturated proton density–weighted image shows the relative size of the posterior horns of the medial and lateral menisci. The posterior horn of the medial meniscus (left arrow) is thicker than the posterior horn of the lateral meniscus (right arrow). Note the normal dark appearance (relative lack of signal intensity) in the menisci. The medial portion of the posterior horn of the lateral meniscus (ie, the meniscus on top of the fibula) is directed upward obliquely, from a lateral to medial direction. This is its normal course. Coronal fat-saturated proton density–weighted image shows the popliteus recess containing joint fluid and located between the lateral aspect of the posterior horn of the lateral meniscus and the joint capsule. An extensive tear is present in the posterior horn of the medial meniscus (arrow). Note the normal oblique upward orientation of the posterior medial horn of the lateral meniscus. Coronal fat-saturated proton density–weighted image shows the dark appearing popliteus tendon (arrows) passing through the popliteus recess. The posterior medial horn of the lateral meniscus is directed obliquely upward. Coronal fat-saturated proton density–weighted image shows abnormal signal intensity in the posterior horn of the medial meniscus (MM) extending to the undersurface near the junction with the joint capsule. Such tears may be missed on arthroscopy because that part of the knee joint is difficult to access. Also present is a tear to the posterior medial horn of the lateral meniscus (LM) as it slopes obliquely inward. A false-positive diagnosis of meniscal tear can be made when one evaluates this region because of the magic angle effect. Tears persist when the echo time (TE) is varied and when T2-weighted images are obtained. True tears can also be confirmed by visualizing them on sagittal or axial projections. Coronal fat-saturated proton density–weighted image of the posterior portion of the knee joint. A circular, fluid-filled structure (arrow) is present in the upper portion of the most medial portion of the posterior horn of the medial meniscus; it represents a meniscal cyst. The insertion of the semimembranosus tendon is located posterior along the subarticular surface of the medial aspect of the proximal tibial metaphysis (see the images below). This is not to be confused with a displaced meniscal fragment.
Sagittal fat-saturated proton density–weighted image of the posterior knee compartment shows the normal insertion (arrow) of the semimembranosus tendon. The insertion site is near the posterior horn of the medial meniscus (MM), and it is not to be mistaken for a displaced meniscal fragment. Sagittal fat-saturated proton density–weighted image of the paramedian portion of the lateral joint compartment. The transverse intermeniscal ligament (arrowhead) is about to unite with the anterior horn of the medial meniscus (MM, arrow). Fat is normally present in this region and can mimic a ligament or meniscal tear. By carefully following the course of the ligament on sequential images and by observing a uniformly well-defined, hypointense structure on every image, this pitfall can be avoided. A small, ill-defined, linear soft tissue density is present under the anterior horn. It is separated from the anterior horn by bright fluid. This is a rare tear in this region. The brightness is joint fluid in the tear. Meniscal flounce
A meniscal flounce is an uncommon meniscal variant characterized by a single symmetrical fold along the free edge of the meniscus. It appears as an S -shaped fold along the free edge on sagittal images and is associated with a truncated but normal meniscus on coronal images.
Normal meniscal signal intensity
The normal meniscus shows uniform, low signal intensity on T1- and T2-weighted images obtained with both conventional and fast-spin echo (FSE) sequences. The low signal is related to a lack of mobile protons in the meniscal fibrocartilage. Subsequent dephasing of hydrogen nuclei results in T2 shortening, contributing to the low signal intensity on all pulse sequences.
Fascicles of the posterior horn of the LM are best evaluated on T2-weighted sagittal images. This is due to the bright fluid in the popliteus tendon sheath and joint space contrasting with the low signal intensity of the fascicles.^[22]
Discoid meniscus
Differentiation between a true discoid meniscus (DM) and a slightly larger but normal meniscus may be difficult.
On sagittal images, the DM has a thickened, bow-tie appearance on 3 consecutive sagittal images. The anterior and posterior horns of the normal meniscus are seen on several images near the intercondylar notch. With a complete DM, no distinct anterior or posterior horn is present. The normal meniscus rapidly tapers from the outer periphery to the center. The presence of equal or nearly equal meniscal height on 2 adjacent peripheral 5-mm-thick images indicates a DM. The anterior and posterior horns of the LM are normally equal in height. An asymmetric discoid LM may have an abnormally large anterior or posterior horn.^[23]
On coronal views, the abnormal meniscal body extends more medially toward the intercondylar notch (see the image below).
Coronal fat-saturated proton density–weighted image shows irregularity to the upper (femoral) surface of the body of the lateral meniscus (LM, outer arrow), indicating fraying. Fraying usually occurs at the meniscal apex. Soft tissue densities (inner arrow) are present under the apex of the meniscus, indicating debris or a free meniscal fragment at this level. The body of the LM is unusually thick and longer than usual, indicating a discoid meniscus. The normal-sized medial meniscal body is present for comparison. Discoid menisci occur about 5 times more often here than in the LM, and they are more prone to injury. The posteromedial horn of the MM and the anterior horn of the MM near the roots may have a normal speckled appearance (see the images below).
Sagittal fat-saturated proton density–weighted image of the paramedian portion of the medial knee. The transverse intermeniscal ligament is about to insert on the anterior horn of the medial meniscus (MM). The anterior horn is normally speckled. The anterior horn is partially displaced off the anterior surface of the tibia by a radial tear more laterally (picture is not shown). A tear involves the posterior horn of the MM (arrow). Sagittal fat-saturated proton density-weighted image shows a well-defined, soft tissue density in front of the posterior cruciate ligament (PCL). It is speckled and looks like the normal posterior medial horn of the medial meniscus (MM), but it is in the wrong place. This finding represents a displaced meniscal tear involving the posterior medial horn. The position of the meniscus is referred to as a double PCL because it looks like 2 of these ligaments are present. Coronal images show the smallest width of the meniscal body, making this plane the most sensitive for showing meniscal enlargement. An asymmetric DM with an enlarged body may have a wide meniscal body on coronal images but normal anterior and posterior horns on sagittal images, emphasizing the need for coronal images. Incomplete DM may not extend into the intercondylar notch.
In children, grade 2 signal is frequently seen within the posterior meniscal horns. This is thought to represent normal vasculature, seen in the meniscus of a child. This disappears in adulthood.
Regarding the meniscofemoral ligaments, either the anterior or posterior ligament is present on 33% of MRIs. Both ligaments are present on 3% of examinations. One of the 2 ligaments predominates. The ligament of Humphry is best seen on sagittal images. It is occasionally seen on coronal images. The ligament of Wrisberg is best seen on posterior coronal images.^{[23, 24]}
Meniscal degeneration
Local increases in the degree of freedom of trapped water molecules within the substance of the meniscus occurs with age, resulting in increased T2 times. The appearance is that of increased signal intensity within the substance of the meniscus on short-TE images.^[22]