Anatomy

The medial and lateral menisci of the knee are fibrocartilaginous semicircular structures that act as shock absorbers and transmit forces between the femur and the tibia. The menisci are composed of longitudinal collagen bundles, circumferentially oriented in a C-shaped configuration, as well as transversely oriented collagen fibers that radiate from the free edge of the meniscus to the peripheral margin. Together, these longitudinal and radial collagen fibers act to provide hoop tensile strength, resist axial loading extrusive forces, and prevent separation of the menisci in a radial direction.

Both menisci are thicker in craniocaudad dimension along the periphery and taper to a thinner margin along the free edge. Although the medial and lateral menisci serve the same purpose in the medial and lateral compartments of the knee, they are not symmetrical in size or shape. The medial meniscus is a larger C-shaped structure, and the lateral meniscus is a tighter, near complete circle (Fig. 8-1). Because of these morphologic differences, the medial meniscus covers approximately one half of the tibial plateau contact surface, and the lateral meniscus covers approximately three quarters of the tibial plateau contact surface.

Figure 8-1 A and B, Normal meniscal anatomy. Axial T2-weighted fat-suppressed images at the level of the menisci demonstrate the larger C-shaped medial meniscus (arrows in A) and the smaller near complete circle lateral meniscus (arrows in B).

The medial meniscus can be differentiated from the lateral meniscus by position and size, and also by its distinct morphologic characteristics and regional attachments. The posterior horn of the medial meniscus is wider in an anteroposterior dimension than the anterior horn. This can be demonstrated on sagittal imaging of the knee when the posterior horn appears two to three times larger than the anterior horn (Fig. 8-2). The posterior horn of the medial meniscus attaches to the tibia at the posterior intercondylar fossa, anterior to the posterior cruciate ligament insertion, but behind the posterior horn of the lateral meniscus. The anterior horn of the medical meniscus attaches to the tibia at the anterior intercondylar fossa, in front of both the anterior horn of the lateral meniscus and the insertion of the anterior cruciate ligament. The periphery of the medial meniscus is attached to the joint capsule along its entire length via meniscotibial and coronary ligaments.¹²

Figure 8-2 Normal meniscal anatomy. Sagittal proton density image through the medial compartment shows that the posterior horn of the medial meniscus (arrow) typically appears two to three times larger than the anterior horn (arrowhead).

In comparison, the lateral meniscus is symmetrical from front to back (Fig. 8-3). Therefore, on sagittal imaging of the knee, the posterior horn and the anterior horn are similar in size. The posterior horn of the lateral meniscus attaches to the tibia behind the intercondylar eminence, anterior to both the posterior cruciate ligament insertion and the posterior horn of the medial meniscus. The anterior horn of the lateral meniscus attaches to the tibia in front of the intercondylar eminence, behind both the anterior horn of the medial meniscus and the anterior cruciate ligament insertion. The fibers of the anterior cruciate ligament partially blend with the lateral meniscus at its tibial attachment. The periphery of the lateral meniscus cannot attach directly to the joint capsule because of the intra-articular course of the popliteus tendon between the lateral meniscus and the joint capsule. The lateral meniscus actually attaches to the joint capsule through small fascicles or struts.

Figure 8-3 Normal meniscal anatomy. Sagittal proton density image through the lateral compartment shows that the lateral meniscus is symmetrical from back (arrow) to front (arrowhead).

Meniscal nutrition is supplied by two routes. The vascular supply is confined to the outer one third of the meniscus, also known as the red zone. The vessels arise from the medial and lateral genicular arteries, forming a perimeniscal synovial capillary plexus that bathes the periphery of the menisci. The central portion of the meniscus receives nutrients from the synovial fluid, which diffuses into or is forced through the joint with activity. This avascular portion of the meniscus is known as the white zone. The presence or absence of vascular supply at the location of a meniscal tear can determine whether the tear has a possibility of healing without intervention. A peripheral meniscal tear with adequate vascular supply is capable of healing and may not require surgical intervention.

Function

The menisci have many biomechanical functions. They act to increase contact area and joint congruity, transmit load and absorb shock, prevent radial extrusive forces during axial loading, and aid in joint lubrication. Because fibrocartilage is less stiff than hyaline cartilage, the menisci intrinsically have a higher shock-absorbing capacity. Functional meniscal studies have found that 50% to 85% of the load placed across the joint is transmitted by the meniscus. Following total meniscectomy, the contact area between the femur and the tibia decreases by approximately 75%; thus contact stresses between the femur and the tibia increase by more than 200%. Studies have demonstrated that contact stresses at the knee joint proportionately increase in relation to the amount of meniscus removed.¹²

Discoveries such as these have altered the surgical management of meniscal tears. Preservation and conservation of meniscal tissue are now the ultimate goals to maximize the function of the residual meniscus and prevent progression to osteoarthritis.

Magnetic Resonance Imaging of the Meniscus

The normal meniscus demonstrates homogeneous low signal intensity on all imaging sequences because of its short T2 relaxation. Increased signal intensity within the meniscus is abnormal and represents a meniscal tear or degeneration. A short time to echo (TE) imaging sequence is necessary to evaluate the meniscus on magnetic resonance imaging (MRI). This can be accomplished with proton density, gradient echo, or traditional spin echo T1-weighted imaging sequences. The utility of fast spin echo has been debated in the literature, with some describing blur artifact limitations, and others reporting similar sensitivities and specificities as conventional spin echo.

T1-weighted images (i.e., low TE images) are the most sensitive for detecting signal alteration within the meniscus; however, they are the least specific for meniscal tear. Meniscal vascularity and degeneration, as well as tear, are bright on low TE images. As TE increases, fluid in true meniscal tears becomes relatively more prominent. However, not all tears contain fluid. Therefore, T2-weighted images (i.e., high-TE images) are specific but not sensitive for tear and are more useful for confirmation, as fluid signal may be present at the site of the tear. The best imaging sequence to evaluate for meniscal tear is a proton density–weighted imaging sequence that achieves a balance between sensitivity and specificity. Sagittal proton density images are typically more valuable in diagnosing a tear of the anterior or posterior horns. However, meniscal root tears and flipped fragments may be better seen on coronal imaging, and correlation with two imaging planes has been encouraged in the interpretation of meniscal pathology.²⁵ Slice thickness can affect sensitivity as well. It has been recommended that slice thickness be no greater than 4 mm, and that minimal gap exist between each slice.

At our institution, we routinely acquire fast spin echo sequences, including sagittal proton density images, coronal T2-weighted images with fat suppression, and sagittal T2-weighted images with fat suppression. We also acquire coronal T1-weighted spin echo images, without fat suppression. We use a slice thickness of 3 mm with 0.5-mm gaps between slices. Protocols will vary depending on vendor, field strength, and user preference.

MRI Criteria for Meniscal Injuries

A meniscal tear can be diagnosed by identifying abnormal intrameniscal signal, abnormal morphology, or a displaced meniscal fragment. MRI criteria for diagnosing meniscal tear were first investigated just over 20 years ago. Abnormal MRI signal (hyperintensity) within the meniscus in symptomatic patients was evaluated and subjectively classified prior to surgery. Intrameniscal signal abnormality was graded according to its confluence and extension to the articular surface on sagittal imaging. Histologic grading of the same menisci was performed following surgery, thereby differentiating degeneration from meniscal tear. This histologic grading was correlated with MRI signal grade, as classified below:

In this study, 100% correspondence was noted between MRI grade signal alteration and histologic grade. MRI grade 1 and 2 signal alterations corresponded with meniscal degeneration. MRI grade 3 signal alteration corresponded with meniscal tear.²⁰

Later it was described that as the number of sequential images with abnormal surfacing meniscal signal increased, the accuracy of diagnosing a meniscal tear also increased. In two separate studies conducted in 1993 and 2005, the positive predictive value for diagnosing meniscal tears increased when two or more images with surfacing signal abnormality were required compared with only a single abnormal image.⁸ This concept was presented as the two-slice-touch rule and is used by many radiologists today in diagnosing meniscal tear.

These basic MRI criteria were created in the early days of MRI. Today, with higher-field-strength MRI and dedicated extremity coils and imaging systems, the original MRI diagnostic criteria for meniscal tear may not be entirely applicable. No recent studies have been performed on MRI at different field strengths to evaluate the difference in diagnostic accuracy between two sequential images with surfacing signal abnormality and only a single image with surfacing signal abnormality. With higher signal-to-noise ratio and improved imaging techniques, the two-slice-touch rule may not be necessary for accurate diagnosis of meniscal tears. Although the original MRI criteria are still used as guidelines at our institution, they are not always strictly adhered to. Furthermore, secondary signs of meniscal tear have become more important in our interpretations.

Secondary signs of meniscal tear can enhance confidence in diagnosis, particularly in cases where the signal abnormality within the meniscus is equivocal, or when the study is degraded by artifact. Indirect evidence of meniscal pathology includes adjacent cartilage loss, parameniscal cyst (also referred to as meniscal cyst), meniscal extrusion, parameniscal soft tissue edema, bowing of the ipsilateral collateral ligament, joint effusion, perivascular bone marrow edema, and subchondral bone marrow edema (Table 8-1).¹

Table 8-1 Positive Predictive Value (PPV), Sensitivity, and Specificity of Indirect Signs for Meniscal Tears at Arthroscopy

The presence of a parameniscal cyst has a 100% positive predictive value for an associated meniscal tear in some studies. Parameniscal cysts are believed to result from extruded joint fluid through an adjacent meniscal tear.² Parameniscal cysts are seen in 7% of meniscal tears (Fig. 8-4). They have the same incidence for medial and lateral meniscal tears but are seen more commonly medially owing to higher prevalence of medial tears. Medial meniscal cysts are most frequently located posteriorly, and lateral meniscal cysts are most frequently located anteriorly.²

Figure 8-4 A through C, Secondary signs of meniscal tear: parameniscal cyst. Coronal (A), sagittal (B), and axial (C) fluid-sensitive sequences depict a large parameniscal cyst (arrows) emanating from an underlying lateral meniscal tear.

Adjacent collateral ligament edema and linear subchondral bone marrow edema have been shown to have high specificity and positive predictive values in the diagnosis of meniscal tear.¹ Collateral ligament edema can be seen in the setting of primary ligamentous injury and osteoarthritis. However in the setting of meniscal tear, collateral ligament edema likely reflects inflammatory hyperemia, reactive synovitis, and increased fluid formation related to the tear (Fig. 8-5). The sensitivity of this sign is greater for medial meniscal tears, indicating the closer apposition of the medial collateral ligament to the periphery of the medial meniscus as compared with the lateral collateral ligament and the lateral meniscus. Periarticular bone marrow edema can be seen with trauma and osteoarthritis. However, in the setting of meniscal tear, linear subchondral bone marrow edema is located directly adjacent to the meniscus and probably represents hyperemia at the junction of the bony cortex, cartilage, and meniscus (Fig. 8-6). These secondary signs can help guide attention to the meniscus on MRI and can increase confidence when primary diagnostic criteria are equivocal.¹

Figure 8-5 Secondary signs of meniscal tear: collateral ligament edema. Coronal T2-weighted fat-suppressed image shows medial collateral ligament bowing and edema (arrow) related to underlying medial meniscal tear.

Figure 8-6 A through B, Secondary signs of meniscal tear: subchondral edema. Sagittal T2-weighted fat-suppressed image (A) demonstrates linear subchondral bone marrow edema (arrow) adjacent to the posterior horn of the medial meniscus, which contains surfacing signal consistent with tear. Coronal T2-weighted fat-suppressed image in a different patient (B) demonstrates cartilage loss in the medial compartment with underlying bone marrow edema (arrow), findings commonly seen in association with a meniscal tear.

Meniscal extrusion can also be used as a secondary sign of meniscal tear. It is defined as extension of the peripheral meniscus past the tibial margin, and it results from a tear that destabilizes the circumferential collagen fibers of the meniscus and allows it to expand in a radial direction (Fig. 8-7). Major meniscal extrusion (>3 mm) is more highly associated with extensive tears, advanced meniscal degeneration, complex tears, and large radial tears. Tears that extend into the meniscal root are also more likely to result in substantial meniscal extrusion. Identifying meniscal extrusion is important, not only in the detection of meniscal tear, but also because it is strongly associated with the development of osteoarthritis.^3,13

Figure 8-7 A and B, Secondary signs of meniscal tear: meniscal extrusion. Coronal T2-weighted fat-suppressed images depict extrusion of the periphery of the medial meniscus (arrow in A) beyond the periphery of the tibial margin. Major meniscal extrusion, demarcated by lines in (B), is classified as >3 mm; this finding has a high association with complex, radial, or root tear of the associated meniscus.

Errors in Interpretation

Some normal variants may cause confusion in the diagnosis of meniscal tears. For instance, the anterior horn of the lateral meniscus can have a speckled appearance with foci of increased signal. This may be related to blending of the fibers of the anterior cruciate ligament with the anterior horn, or splaying of the fibers of the meniscus at its attachment.¹¹ This abnormal signal should not be mistaken for a tear or degeneration (Fig. 8-8).

Figure 8-8 Pitfall for meniscal tear: normal intrameniscal signal. Sagittal proton density image shows fibers of the anterior horn of the lateral meniscus spreading apart at the root attachment (arrow). This creates a normal speckled pattern and should not be confused for a meniscal tear.

Meniscal flounce is a rare normal variant of the medial meniscus in which there is an undulating appearance of the inner margin, possibly related to ligamentous laxity (Fig. 8-9). This buckling along the free edge may be confused for a meniscal tear, but is not said to increase the risk of tearing. Its prevalence is approximately 0.2%.¹¹

Figure 8-9 Pitfall for meniscal tear: meniscal flounce. Sagittal fluid-sensitive sequence demonstrates buckling of the meniscal body (lateral meniscus pictured), referred to as a meniscal flounce, a normal finding.

The meniscofemoral ligaments of Wrisberg and Humphrey connect the posterior horn of the lateral meniscus to the lateral aspect of the medial femoral condyle. The ligament can divide and course anterior to the posterior cruciate ligament named the ligament of Humphrey, or posterior to the posterior cruciate ligament named the ligament of Wrisberg (Fig. 8-10). The ligaments of Humphrey and Wrisberg are noted in approximately one third of cases. If soft tissue or fluid is interposed between the origin of the meniscofemoral ligament and the posterior horn of the lateral meniscus, this interface can be misinterpreted as a meniscal tear. Care must be taken to follow the ligament over several successive images while avoiding this pitfall.¹⁵

Figure 8-10 A through D, Pitfall for meniscal tear: meniscofemoral ligament. Consecutive sagittal proton density images show the ligament of Wrisberg (arrows) coursing from the posterior horn of the lateral meniscus, posterior to the posterior cruciate ligament (PCL), inserting onto the lateral aspect of the medial femoral condyle; the ligament is seen on magnetic resonance imaging (MRI) in approximately one third of individuals. A similar structure, the ligament of Humphrey, is also seen in about one third of individuals and courses anterior to the PCL. The point of attachment on the meniscus can simulate a tear on MRI.

The transverse intermeniscal ligament courses horizontally between the anterior horns of the medial and lateral menisci, in front of the anterior cruciate ligament. The interface between the ligament and the anterior meniscal horns can also be confused for a tear.¹⁵

The popliteus tendon travels superiorly from its muscle belly in an oblique, intra-articular course, separating the lateral meniscus from the joint capsule, to insert on the popliteal groove along the lateral aspect of the lateral femoral condyle. The popliteal bursa is the opening created by the fascicles of the lateral meniscus, which allow the popliteal tendon to course from its muscle belly into its intra-articular location, and finally to insert on the femur. The medial margin of the popliteal hiatus is the body of the lateral meniscus (Fig. 8-11). Fluid within the popliteus tendon sheath or the popliteal hiatus may be mistaken for a meniscal tear.^7,23

Figure 8-11 A through D, Pitfall for meniscal tear: popliteus tendon. Consecutive coronal T2-weighted fat-suppressed images show the popliteus tendon (arrows) as it originates from the lateral femoral condyle and courses posterolaterally through the popliteal hiatus and inferiorly past the tibial plateau. As the tendon passes by the lateral meniscus, the intervening fluid can be misinterpreted for meniscal tear.

A meniscal contusion occurs during an acute traumatic event, typically described with an acute anterior cruciate ligament disruption. The meniscus is compressed between the femur and the tibia, becomes contused, and demonstrates altered signal on MRI. The increased signal within the contused meniscus is more likely to be amorphous in shape, will not extend to the articular surface, and may be accompanied by a bone bruise. This may simulate a meniscal tear and result in a false-positive MRI interpretation.¹¹

Magic angle phenomenon describes the artifact that occurs when collagen fibers are oriented at 55 degrees relative to the main magnetic field on short TE images. This artifact causes falsely increased signal intensity and can imitate a meniscal tear. This is particularly a dilemma in the posterior horn of the lateral meniscus as it angles upward from its root to the insertion on the tibia behind the intercondylar eminence.⁶

Chondrocalcinosis within the fibrocartilage of the meniscus can cause a false-positive interpretation for tear. Chondrocalcinosis results in increased signal on proton density and T1-weighted images, which can be confused with a meniscal tear.¹¹ Correlation with radiographs may help to detect and confirm the presence of chondrocalcinosis within the meniscus (Fig. 8-12).

Figure 8-12 A and B, Pitfall for meniscal tear: chondrocalcinosis. Frontal radiograph (A) shows lateral meniscal calcification (arrow) representing calcium pyrophosphate crystal deposition. Coronal T1-weighted image (B) shows increased lateral meniscal signal corresponding to chondrocalcinosis seen on radiographs. This can simulate a meniscal tear.

Some authors propose that a delay between MRI diagnosis of meniscal tear and arthroscopy may allow for spontaneous healing.¹⁷ When the tear is not identified at surgery, it is documented as a false positive. Others report that healed or surgically repaired meniscal tears may have persistent signal that extends to the articular surface and can be mistaken for a new meniscal tear or retear. Some meniscal tears are more difficult to visualize at arthroscopy, particularly along the inferior surface of the medial meniscus.⁷ If these tears are not documented by arthroscopy, which is the gold standard, then they are also reported as false positive.