The most common causes of knee pain and disability are tears in the medial and/or lateral menisci. Pain due to meniscal tears can be mediated via the neurovascular bundle that is in the outer third of the meniscus or can occur when innervated synovium invaginates into a tear. Patients may also present with locking, which is usually related to a bucket-handle tear, or giving way, which is more often related to pain.64,117,119
Figure 7.35 Accessory gastrocnemius. Axial fat-suppressed proton density (A) and sagittal T1-weighted (B) images demonstrating an accessory lateral head of the gastrocnemius (arrow).
Figure 7.36 Axial T2-weighted image of the knee in a patient with knee pain. There is an effusion with a low-signal-intensity structure (arrow) medially. This could be mistaken for a loose body, medial patellar fragment, or thickened soft tissue structure. The signal abnormality was created by an air collection from arthroscopy 2 days earlier.
Figure 7.37 Patient with advanced osteoarthritis and a low-signal-intensity defect medially (arrows). The artifact is subtle on the coronal T1-weighted image (A) and more obvious on the gradient-echo image (B). This artifact was created by a small metal remnant from previous arthroscopy.
Figure 7.38 Coronal T1-weighted image, demonstrating artifacts from a previous screw tract (arrows) created by residual microscopic metal fragments.
Before discussing the MR features of meniscal tears, it is essential to review certain anatomic and pathophysiology aspects of meniscal injury. The lateral meniscus is C-shaped and thicker than the medial meniscus. The transverse diameter is similar in the body, posterior, and anterior horns (Fig. 7.39
). The lateral meniscus is also less firmly attached to the capsule and is in fact separated posteriorly by the popliteus tendon and tendon sheath (Figs. 7.24
, and 7.39
). The horns of the meniscus attach to the tibia in the intercondylar region (Figs. 7.14E
). The medial meniscus is more firmly attached to the capsule. The anterior horn attaches to the intercondylar eminence anterior to the ACL (Figs. 7.14E
). The transverse diameter of the anterior horn is smaller (Figs. 7.39
) than the posterior horn. The posterior horn attaches to the intercondylar eminence anterior to the PCL.19,64,65
The menisci perform an important function in load bearing and knee function. Up to 50% of load bearing is transmitted through the menisci when the knee is in extension and 85% in flexion.120
The contact area can be reduced significantly after partial menisectomy that can increase contact pressures by 350%.120
Tears in the menisci may result from acute trauma or repetitive trauma and progressive degeneration.37,61,64,121,122,123,124
Acute tears are usually due to athletic injuries, with crushing of the meniscus between the tibia and femoral condyles. Most tears extend from the
posterior to the anterior.61
Chronic repetitive trauma is common both in athletes and nonathletes with aging.75,125
Chrondrocyte necrosis and increase in mucoid ground substance can lead to meniscal tears.126
Figure 7.39 A: Tangential sections of the medial and lateral meniscus. (From Rand JA, Berquist TH. The knee. In: Berquist TH, ed. Imaging of Orthopedic Trauma. 2nd ed. New York, NY: Raven Press; 1992:333-432.) B: Sagittal turbo spin-echo fat-suppressed proton density-weighted image of the knee through the lateral meniscus demonstrating the similar size of the anterior and posterior horns (arrows). C: Sagittal turbo spin-echo fat-suppressed proton density-weighted image through the medial meniscus demonstrating the posterior horn is significantly larger than the anterior horn (arrows).
Examination techniques for meniscal pathology (see Techniques section) may vary depending on the software and preferences of the examiner.2,89,127,128
Patient throughput and ease of lesion detection are both important (Table 7.1
). Over the years, many pulse sequences have been studied to determine the optimal imaging sequence for detection of meniscal tears.2,29,41,48,54,60,89,129,130
In the past, we preferred proton density and three-dimensional gradient-echo sequences for meniscal imaging. The contrast of lesions (high signal intensity) compared with the normal low signal intensity (black) of the menisci allows defects tobe more easily appreciated. Lesions are often less conspicuous
on the second echo of spin-echo sequences. Today, turbo or FSE proton density sequences are commonly used with or without fat suppression.89,129,130
The increased signal intensity seen on proton density MR images is felt to be related to hydrogen protons attached to macromolecules in the region of the tear.89
There is still some controversy regarding the accuracy of conventional spin-echo and FSE proton density sequences. Rubin et al.41
found similar sensitivities and specificities with both conventional and FSE techniques. If using FSE techniques, Rosas and De Smet89
recommend lower echo train lengths (<
4) and larger bandwidths (>
30 mHz) to reduce blurring. We currently obtain FSE proton density sagittal images with and without fat suppression. False negatives can be reduced using fat-suppression techniques (Fig. 7.42
Statistics for fat-suppressed proton density FSE sequences result in 92% to 95% sensitivity, 92% to 93% specificity, and 92% to 93% accuracy.130
Figure 7.40 Lateral (A) and posterior (B) illustrations of the knee demonstrating the joint space and associated ligament and meniscal anatomy.
Figure 7.41 Menisci and their attachments and associated ligament and tendon anatomy.
Additional parameters include slice thicknesses of 3 to 4 mm and 14 cm field of view with matrix of greater than or equal to 256 × 256.2
Field strength does not seem to result in significant differences in accuracy whether comparing 0.1 to 1.5 T or 1.5 to 3.0 T.89,131,132
Some authors use 0.7-mm thick sections (28 contiguous), a 50/15 TR/TE, 20° flip angle, and 128 × 256 matrix with 3DFT to evaluate meniscal lesions. This technique yielded a sensitivity of 97% and specificity of 96%.2,46
Disler et al.29
found three-dimensional techniques especially useful for posterior horn lesions.
The appearance of normal menisci and meniscal tears has been well documented in MRI literature (Fig. 7.42
). Increased signal intensity has been noted on T1-weighted, T2-weighted, proton density, and GRE sequences.4,22,89,129,130,131,132,133,134,135,136
These changes can be seen with mucoid degeneration as well as meniscal tears. Differentiating degeneration from true meniscal tears can be difficult. Communication with the articular surface or meniscal distortion must be demonstrated to feel confident that a tear is present.89,137
When these criteria are documented on two or more contiguous sections, the positive predictive value for a medial meniscal tear is 94% and for the lateral meniscus 96%. When comparing data to finding the above abnormalities on only one section the positive predictive value drops to 43% for the medial and 18% for the lateral meniscus.138
Figure 7.42 Sagittal proton density fast spin-echo without (2,000/25, ET 5, 4-mm thick sections) (A) and with fat suppression (3,000/37, ET 5, 4-mm thick sections) (B) demonstrating the increased conspicuity of the peripheral medial meniscal tear communicating with the inferior articular surface (arrow) on the fat-suppressed image (B).
Grading systems for meniscal tears have been described by Stoller,81
and Mesgarzadeh et al.60
on the basis of pathologic findings in cadaver specimens and operative features (Figs. 7.43
). A grade 1 meniscal lesion is globular in nature and does not communicate articular surface. Histologically, this stage correlates with early mucoid degeneration. It is felt that these changes are not symptomatic but represent a response to mechanical stress and loading that result in increased production of mucoid polysaccharide ground substance.81
Figure 7.43 Normal and abnormal menisci. A: Normal 3.0-T sagittal turbo spin-echo fat-suppressed proton density-weighted image of the lateral meniscus. There is no signal in the meniscus. Note the popliteus tendon and sheath posteriorly. B: Grade 2 globular increased signal intensity in the medial meniscus on a sagittal 3.0-T proton density-weighted image. C: Grade 3 increased signal intensity in the posterior horn of the medial meniscus communicating with the inferior articular surface on 1.5-T proton density image (small arrowhead). D: Sagittal 1.5-T gradient-echo image demonstrating a grade 3A tear (small arrowhead) with an associated meniscal cyst (large arrowhead). E: Sagittal 1.5-T gradient-echo image demonstrating a more complex linear tear in the posterior horn of the medial meniscus. F: Grade 3B meniscal tear with a broad area of articular involvement (arrowheads) on gradient-echo 1.5-T image.
Figure 7.43 (continued)
Grade 2 signal intensity is linear (Fig. 7.44
) in nature and remains within the substance of the meniscus. Once again, there is no evidence of communication with the articular surface of the meniscus. Histologically, grade 2 menisci are characterized by more extensive bands of mucoid degeneration. Most feel that grade 2 changes represent progression of grade 1. Some authors feel that grade 2 lesions are precursors to complete tears.40,81,139
However, Dillon et al.140
found that most were stable when followed for 3 years. Reinig et al.126
found progression when evaluating football players over a period of one season.
With grade 3 tears (Fig. 7.44
) there is increased signal intensity within the meniscus that extends to the articular surface (Fig. 7.43C
). Demonstrating communication with the articular surface is important, as the tear is not likely to be confirmed arthroscopically. Some authors have used narrow “meniscal windows” to assist in confirming communication with the articular surface. In our experience and that of Buckwalter et al.,141
there is little value in this technique. In fact, if done routinely without conventional windows, bone lesions and other abnormalities can be overlooked. We prefer the “two-slice touch” rule described by De Smet and Tuite138
to confirm articular surface involvement. Grade 3 tears can be further divided into subcategories. Grade 3A signal intensity is a linear intrameniscal signal that abuts the articular margin (Figs. 7.43D
). Grade 3B is a more irregular area of signal intensity that abuts the articular margin (Figs. 7.43F
). The grade 3B lesions are most often associated with more extensive degenerative change in the adjacent areas of the meniscus associated with the tear. It is not unusual to have difficulty in differentiating grade 2 from grade 3 tears. Careful windowing and evaluating adjacent sections is useful in these cases.
Grade 4 menisci (Fig. 7.44
) are distorted (Fig. 7.45
) in addition to changes described with grade 3. With more severe meniscal tears, meniscal extrusion and associated articular cartilage loss are common (Fig. 7.46
These categories do not include all possible meniscal injuries such as truncated menisci, bucket-handle tears, and so on.60,61
The grading system is most useful in describing
significant signal-intensity changes that communicate with the articular surface.
Figure 7.44 Meniscal tear grading system.
The MR appearance of different types of meniscal tears is similar to those that have been described with arthrography (Fig. 7.47
). Vertical tears are usually traumatic compared with horizontal cleavage tears that are more often degenerative. Degenerative fraying of the surface of the meniscus may also be evident on MR images and is demonstrated as areas of irregular increased signal intensity on the meniscal surface compared with the normal dark or low intensity of the body of the meniscus.
Figure 7.45 Sagittal 1.5-T proton density-weighted (A) and coronal three-dimensional DESS (B) images demonstrate complex degenerative tearing of the anterior horn and body of the lateral meniscus. Additionally, the coronal image B shows extrusion of the fragments (white arrowhead).
Radial tears may be partial or complete and involve the free edge of the meniscus.89
Radial tears separate the longitudinal fibers resulting in loss of function and distortion of the meniscus with axial loading.89
Radial tears may be somewhat difficult to diagnose but are typically seen as areas of increased signal in the inner margin of the menisci (Figs. 7.47
). Multiple MRI signs have been described to increase the accuracy for detection of radial tears. The meniscus may appear truncated, or a missing segment may be seen when reviewing contiguous images (Fig. 7.48
Full-thickness radial tears are demonstrated on coronal and sagittal (Fig. 7.44
) images as areas of increased signal involving the entire meniscus with normal meniscal signal on adjacent coronal sections.145
Displaced menical tears require surgical intervention.89
The most common type of tear is the bucket handle tear that accounts for up to 10% of all meniscal tears. The tear may involve the entire meniscus or only the anterior or posterior portion.89
A bucket-handle tear is a tear with displacement of an attached inner fragment for variable distances (Figs. 7.47
Up to 82% of bucket-handle tears involve the medial meniscus.40
When truncated menisci are identified, one must search carefully for the displaced (bucket-handle) fragment (Fig. 7.49
). Several signs (Table 7.5
) have been described to assist in detection of bucket-handle tears. Signs of a bucket-handle tear include the double-PCL sign, flipped meniscus sign or double anterior horn sign, the absent bow-tie sign, and a fragment in the intercondylar notch.89
The double-PCL sign (Fig. 7.50
) is seen on coronal and sagittal images when the displaced fragment lies below the PCL, giving the appearance of two ligaments. This feature is more common with medial tears (53%) than lateral (14%) bucket-handle tears (Table 7.5
flipped-fragment sign (Fig. 7.51
) is seen with 44% of medial and 29% of lateral meniscal bucket-handle tears (Table 7.5
Another sign is a fragment in the intercondylar notch, which is not the same as the double-PCL sign.89
Defining a fragment in the notch may be difficult, especially when they are small and the configuration of the meniscus is not significantly truncated (Fig. 7.48
; Table 7.5
). Larger fragments are identified with 66% of medial and 43% of lateral meniscal tears.54
Detection may be improved by using coronal STIR images. Magee and Hinson48
reported detection of 93% of fragments using STIR sequences. Defining the fragments is important, as they need to be removed arthroscopically.48
The final sign is the absent “bow-tie sign,” which is seen as absence of the normal meniscal configuration on sagittal images through the meniscus (Fig. 7.52
Figure 7.46 1.5-T proton density (A) and proton density with fat suppression (B) images of a complex medial meniscal tear with associated cartilage loss (arrow) and a large popliteal cyst.
Figure 7.47 A: Types of meniscal tears. B: (1) Radial tear with cross-sectional appearance. (2) Horizontal tear that is only seen on tangential view. (3) Flap tear, oriented oblique to the long axis of the meniscus. Note the distance from the apex increases (a to b) as the tear extends into the meniscus. (4) Vertical tear. Meniscal tears seen tangentially.
Parrot-beak tears are radial tears that progress to a peripheral component giving the appearance of a parrot beak (Figs. 7.47A
These lesions are most common at the junction of the body and posterior horn of the lateral meniscus. The meniscal flap tear (Fig. 7.47A
) has a similar appearance, and though described as a different entity it is essentially the same as a parrot beak on MR images.146
These two tears are often lumped with bucket-handle tears. Flap tears are less common than bucket-handle tears but account for 19% of symptomatic meniscal injuries. Proper description of the tear and fragment location are more important than the eponym used.89
Horizontal meniscal tears extend from the free edge parallel to the articular surface (Fig. 7.54
). When the tear extends to the periphery of the meniscus it is not uncommon
to find an associated meniscal cyst. The etiology of meniscal cysts is felt to be related to extension of joint fluid through the tear.89
Horizontal meniscal tears are more common in elderly patients with degenerative disease.89,147
Figure 7.48 Bucket-handle tear with a displaced fragment in the intercondylar notch (arrow).
Meniscal root tears have been lumped into the radial tear category (Fig. 7.55
). However, these tears are often more complex resulting in management issues for the orthopedic surgeon.89,148,149
Recent data indicate that lateral meniscal root tears may be difficult to detect on MR images and arthroscopy.148,149
Detection on MRI may be affected by the magic angle phenomenon, and pulsation artifact from the popliteal artery.89
In addition, the incidence of ACL tears is significant as 8% to 9.8% of patients with ACL tears have associated lateral meniscal root tears.148,149
Medial mensical root tears are seen with ACL tears in only 3% of cases. Meniscal extrusion is seen with both medial and lateral posterior root tears.149
The incidence of meniscal extrusion with medial root tears is 88% compared with 23% with lateral root tears.149
As reported by others, we find that coronal water-sensitive sequences are most useful for diagnosis (Figs. 7.55
Table 7.5 Bucket Handle Tears: MRI Features
Double-posterior cruciate ligament sign (Fig. 7.50)
Flipped-fragment sign (Fig. 7.51)
Fragment in notch (Fig. 7.49)
From references 48, 54, 119.
Peripheral tears or separations in the meniscus can also be identified. These injuries are usually easily diagnosed with MR arthrography but can be subtle on conventional MR images due to vascular tissue and the normally increased signal intensity at synoviomeniscal junction along the margins of the meniscus (Figs. 7.23
). It is important to clearly define the location (Fig. 7.57
) (medial, lateral, inner margin, peripheral), type of tear, and associated bone and/or ligament injury.3,135,150
Grade 1 and 2 lesions will not be identified arthroscopically.150,151
Certain lesions are stable and can heal with conservative therapy, so arthroscopy may be avoided.13,16
Peripheral lesions are particularly suited for meniscal repair (Fig. 7.57
Patients with suspected meniscal tear can be accurately diagnosed with history alone in 75% of cases. Clinical examination with stress testing, specifically the Thessaly test is94% accurate for medial meniscal tears and 96% accurate for lateral meniscus tears. The Thessaly test is performed with the patient weight bearing and the knee flexed at 5° and 20° with internal and external rotation maneuvers.152
Clinical evaluation along with MRI prepare the treating physician for the most appropriate approach in a given clinical setting. Several reports in the MR literature have described the accuracy of MRI for identification of meniscal tears (Table 7.6
Sensitivity for MRI in detecting meniscal tears seen at arthroscopy ranges from 75% to 100%. Review of our data demonstrated a sensitivity of 99%, specificity of 90%, accuracy of 90%, positive predicted value of 96%, and negative predicted value of 98% for the medial meniscus. MRI of the lateral meniscus showed 97% sensitivity, 97% specificity, and 91% accuracy.
Management of meniscal tears varies significantly depending upon the patient status and location and type of tear.151,152,153,154
The type and location of meniscal tear are important in determining if the lesion is responsible for the symptoms so the appropriate therapy can be selected.3,16,150,151,152,153,154
The orthopedic surgeons and arthroscopists classify meniscal tears by location and tissue integrity.154
The meniscus is divided into anterior, middle, and posterior as well as inner, middle, and outer thirds (Fig. 7.58
). The vascular supply peripherally has lead to calling outer third tears red tears.154
Inner third tears have no blood supply and are referred to as white tears.
Recent articles have described the indications, contraindications and available treatment options for meniscal injuries.151,152,153,154,155
Noyes and Barber-Westin154
described indications and contraindications for meniscal repair.
Figure 7.49 Radial tear in the posterior horn of the lateral meniscus. Axial fat-suppressed proton density 1.5 T (A), coronal DESS (B), and sagittal proton density-weighted (C) images demonstrate a focus of free edge increased signal intensity (arrows in A and B) and a truncated appearance at the level of the tear on the saggital image (arrow in C).
Figure 7.50 Coronal fat-suppressed T2-weighted image (A) demonstrating a medial tear (curved arrow) with a large displaced fragment (black arrow) that gives the appearance of two posterior cruciate ligaments (PCL). There is also a complex tear of the lateral meniscus (white arrow) and loss of articular cartilage. Sagittal proton density-weighted image (B) demonstrating a medial meniscal tear with a large displaced fragment (small arrow), resulting in a double-PCL sign.
Figure 7.51 Flipped-fragment (large or double anterior horn) sign. A: Illustration of the flipped fragment sign seen in the axial and sagittal planes. B: Sagittal proton density-weighted MR image with a large anterior horn (arrow). A portion of the posterior horn is still evident.
Figure 7.52 The absent bow-tie sign. A: Normal fat-suppressed turbo spin-echo proton density sagittal image demonstrating the normal bow-tie configuration of the meniscus. B: Sagittal proton density-weighted image demonstrating absence of the posterior portion of the bow tie (arrow). Note the double appearance of the anterior horn of the meniscus.
Arthroscopic intervention is most optimal in active patients in the second through fourth decades. Unstable fragments greater than 10 to 12 mm in length involving the middle third (Fig. 7.58A
) of the meniscus are good candidates for repair. Meniscal tissue should appear near-normal arthroscopically and there should not be secondary tears. Patients must also agree to the postoperative rehabilitation program. Older patients (>60 years of age) or those unwilling to comply with rehabilitation programs can be treated with partial meniscectomy. Tears of the inner third (Fig. 7.58B
) are not recommended for repair. Degenerative tears, partial tears, and longitudinal tears less than 10 mm in length are also not usually repaired.154
Arthroscopic repair may be partial meniscectomy or reattachment of the torn fragment using suture material or bioabsorbable arrows.153,154
Partial resection of the meniscus often results in increasing damage to the articular cartilage over time. Therefore, repair is preferred when possible.153
Mensical transplantation is also performed on younger patients with prior meniscectomy to preserve meniscal function and preserve joint stability.154
Detection and characterization of meniscal tears can establish the need for surgery and the type of procedure. Other injuries, such as occult bone injury and injury to the capsule or supporting structures, must be evaluated along with meniscal tears as explanations of patients’ symptoms. Conservative therapy or watchful waiting has little downside risk in patients without locking of the knee.159
The meniscus serves multiple important functions. It is a shock absorber and assists in joint lubrication and
chondrocyte nutrition. Stress to the articular cartilage is also reduced. The meniscus also restricts anterior displacement of the tibia on the femur, reducing stressonthe ACL.64,160,161
Figure 7.53 Parrot-beak tear. Coronal DESS (A), sagittal proton density (B), and axial fat-suppressed proton density (C) images demonstrate a radial tear extending peripherally (arrow).
Decisions regarding surgical repair depend upon the type and location (peripheral or central) of the tear as well as what other injuries may be associated with the meniscal tear. Partial meniscectomy, complete removal, or replacement with cadaver allograft or prosthesis may be considered.160,161
Replacement procedures are relatively new and suggested for patients with previous total meniscectomy or partial meniscectomy with continued symptoms but good joint alignment.161
As noted earlier, partial menisectomy is usually performed for flap tears or tears in the inner or avascular zones of the meniscus (Fig. 7.57
Peripheral tears are frequently repaired (sutured), so the normal meniscal configuration is maintained but with potentially confusing signal intensity changes on MR images.2,62,158
MRI is also of value, though potentially more difficult to interpret, for studying patients who have had either partial or complete menisectomy or primary arthroscopic repairs of the meniscus. Postoperatively, patients are generally referred to exclude residual fragments, remnants of a tear that was not completely resected, or new tears (Fig. 7.59
MR features noted following meniscal repair are similar to preoperative findings. Increased signal intensity that communicates with the articular surface (primary repair) or a truncated margin (partial menisectomy) (Figs. 7.60
) are common findings.160,162
Although meniscal tears may fill in with fibrous tissue (dark on MR images; Fig. 7.59
), the tear can also fill in with chondrocytes or granulation tissue that will have increased intensity similar to a tear. Increased signal intensity in the meniscus can persist even though the tear is healed.164
Differentiation of postoperative changes versus residual or new tear can be difficult with short TE sequences. T2-weighted or water-sensitive
sequences are more useful in this setting as fluid signal intensity extending to the articular surface are more specific. Specificities have been reported as high as 90%.165
More recent studies suggest that the specificity ranges from 73% to 88% using the earlier criteria.89,162
Although the MR features at the operative site may be confusing, additional causes of pain such as bone or ligament lesions and residual or new meniscal fragments (Fig. 7.60
) can be identified.2,3,162
Figure 7.54 Horizontal tear of the lateral meniscus with an associated meniscal cyst. Sagittal proton density-weighted (A) and coronal DESS (B) images demonstrate a horizontal tear in the lateral meniscus (arrow) with an associated menical cyst (arrowhead).
Figure 7.55 Meniscal root tear. Axial fat-suppressed proton density-weighted (A) and coronal DESS (B) images demonstrate a radial appearing posterior medial meniscal root tear (arrow). There is also a popliteal cyst with a loose body (arrowhead).
Improved accuracy using MR arthrography for the postoperative knee has been reported.166,167
Sciulli et al.163
compared conventional arthrography with MR arthrography using iodinated contrast and gadolinium. MR arthrogram studies were most accurate (92%) and conventional
arthrograms least accurate (58%) for evaluating menisci after surgical procedures.163
The use of arthrography remains somewhat controversial.89
In a recent study, De Smet et al.168
found that similar to the nonoperative knee, any signal intensity contacting the meniscal surface on two or more contiguous sections is most likely a new tear. This group reserves MR arthrography for patients with greater than 25% of the meniscus removed.168
We have not used MR arthrography except in selected cases or when specifically requested by the orthopedic surgeons. In our experience, MRI equivocal cases are most often reexamined arthroscopically.
Figure 7.56 Medial meniscal root tear. Axial fat-suppressed proton density (A), coronal DESS (B), and sagittal proton density-weighted (C) images demonstrate a complete posterior medial mensical root tear (arrows). There is extrusion of the anterior meniscus (arrowhead in C).
Table 7.6 MRI of Meniscal Tears