Spectrum of meniscal lesions: An MR teaching atlas

Spectrum of meniscal lesions: An MR teaching atlas Poster No.: C-2284 Congress: ECR 2010 Type: Educational Exhibit Topic: Musculoskeletal - Joints Authors: C. Leal, P. Alves, H. A. M. R. Tinto, J. Raposo, T. Bilhim, A. Vasconcelos, R. Marques; Lisbon/PT Keywords: meniscal lesions, MR of the knee, meniscal tears Keywords: Musculoskeletal joint, Musculoskeletal system DOI: 10.1594/ecr2010/C-2284 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. www.myesr.org Page 1 of 78

Learning objectives -To illustrate the MR spectrum of meniscal lesions. -To describe the varied morphology of meniscal tears, it's grading system and associated findings, emphasizing the features that influence management. Page 2 of 78

Background The meniscus is known to play an important role in the complex biomechanisms of the knee. It is involved in joint stability, load sharing and transmission, shock absorption, nutrition and lubrication of the articular cartilage. Meniscal injuries are a common sports-related problem and the most frequent injury to the knee joint. Anatomic considerations The menisci are C -shaped fibrocartilaginous structures composed of thick collagen fibers. The limbs of the C face centrally and the superior meniscal surface is concave to improve contact with the curvilinear-shaped femoral condyle. Conversely, the undersurface of the meniscus is flat, improving contact with the flattened tibial plateau. The thick convex periphery allows for a firm attachment to the joint capsule. The normal meniscus is 3-5 mm in height and is arbitrarily divided into anterior horn, body, and posterior horn. The adult meniscus is relatively avascular, except for the peripheral 10-25% (the "red (3) zone"). The meniscus of the child is better vascularized than that of the adult, but this regresses with advancing age. Page 3 of 78

Fig.: Peripheral meniscal vascularization in a child (coronal fat-saturated proton density-weighted image). Page 4 of 78

Fig.: Peripheral meniscal vascularization in an adolescent patient (sagittal fatsaturated proton density-weighted image). Typically, the medial meniscus (MM) is larger and covers 50% of the articular contact area of the medial joint compartment. It has a larger posterior horn and is more "open" to the intercondylar notch (has a more crescentic shape). The attachment of the MM to the joint capsule is more rigid, making the MM more susceptible to injury than the LM. Page 5 of 78

The posterior horn attaches to the tibia posterior to the intercondylar fossa between the attachment of the posterior cruciate ligament (PCL) and the more anteriorly located posterior horn of the LM, by the meniscotibial ligament. Usually, the anterior horn is attached to the tibial plateau anterior to the anterior cruciate ligament (ACL), but in 11% (3) of patients it is not attached to the tibia. In this situation, it attaches directly to the transverse ligament or the anterior cruciate ligament. The tibial attachment is also called "root" of the meniscus. The periphery of the MM is attached to the joint capsule throughout its length. At its mid portion, the body of the MM is more firmly attached to the tibia and femur through the deep medial collateral ligament (MCL), a thickening of the joint capsule. The tibial portion of the capsular attachment of the MM is called the coronary ligament and the femoral attachment is by way of the meniscofemoral ligament. The body is tapered when compared with the wider anterior and posterior horns. Page 6 of 78

Fig.: Rupture of the coronary ligament (coronal fat-saturated T2-weighted image). The lateral meniscus (LM) is smaller and more "closed" towards the notch (has a more circular shape) but covers more of the tibial plateau (about 70%). It is more uniform in width from anterior horn to posterior horn. The anterior horn of the LM attaches to the tibia between intercondylar eminence and the insertion of the ACL. The fibers of the anterior horn and ACL insertion may be contiguous. Page 7 of 78

The posterior horn is secured to the tibia between the intercondylar eminence and the insertion of the MM. The LM has a loose attachment to the joint capsule and to the femur and tibia by means of the meniscal ligaments and is not anchored to the lateral collateral ligament (LCL). Fig.: The lateral meniscus is not anchored to the lateral collateral ligament and the medial meniscus is firmly attached to the deep medial collateral ligament. Note the lateral meniscal attachments to the joint capsule, visible in the presence of joint fluid (coronal fat-saturated T2-weighted image). At the posterolateral corner of the knee joint, the LM is attached to the joint capsule by the superior and inferior fascicles. Page 8 of 78

Fig.: Depiction of posterior capsular atachments of the posterior horn of the lateral meniscus (sagittal fat-saturated proton density-weighted image). The LM contains the popliteus recess in its posterolateral margin, allowing passage of the popliteus tendon and sheath between the lateral recess and the joint capsule. The popliteomeniscal fascicles (superior and inferior) are synovial attachments of the posterior horn of the LM that extend around the popliteus bursa. Disruption of these fascicles allows increased meniscal movement, meniscal subluxation and even locking of the knee. Page 9 of 78

Fig.: Passage of the popliteus tendon between the lateral recess and the joint capsule (sagittal fat-saturated proton density-weighted image). Page 10 of 78

Fig.: Popliteus tendon mimicking a vertical tear in the posterior horn of the lateral meniscus (sagittal fat-saturated T2-weighted image). The LM attaches to the lateral aspect of the medial femoral condyle through the meniscofemoral ligaments that originate from the posterior horn: ligament of Wrisberg (posterior to the PCL) and the ligament of Humphrey (anterior to the PCL). Page 11 of 78

The attachments of the LM to the femur and popliteus tendon enable it to couple its motion with that of the femur during rotation. Its greater mobility makes it less likely to become injured. Transverse or intermeniscal ligament - Connects the anterior horns of the MM and LM. It originates anterolateral to the central rhomboid attachment of the LM and inserts on the anterosuperior aspect of the anterior horn of the MM. This ligament is absent in up to 40% of individuals (3). Fig.: Intermeniscal ligament seen at its connection to the anterior horn of the meniscus (sagittal fat-saturated proton density-weighted image). Page 12 of 78

Mensicomeniscal ligament - It is an uncommon normal variant that runs from the anterior horn of one meniscus to the posterior horn of the other meniscus. It is named according to its anterior attachment. Meniscal variants The variants more commonly described include the discoid meniscus, meniscal ossicles and meniscal flounce. Discoid meniscus (DM) It's a dysplastic meniscus, enlarged and shaped like a half moon, associated to higher incidence of degeneration and tears. The DM can be bilateral but is more common in the LM. The DM may be classified as complete or incomplete, with both types firmly attached to the tibia and complete DM extending to the intercondylar notch, or as Wrisberg variant which lacks any attachment to the posterior tibia and joint capsule. In this case, the meniscus retracts anteriorly unopposed from the pull of the meniscofemoral ligaments, becoming at risk for hypertrophy and trauma. On MR, the diagnosis is suggested by either: -Meniscal tissue on three contiguous sagittal 4mm-thick slices; -Meniscal body on coronal images greater than 15mm wide or extending into the intercondylar notch (1). Page 13 of 78

Fig.: Discoid lateral meniscus (coronal fat-saturated T2-weighted image). Page 14 of 78

Fig.: Discoid lateral meniscus (sagittal fat-saturated T2-weighted image). Meniscal ossicles Meniscal ossicles are small ossified foci found typically in the posterior horn of the medial meniscus. They are though to be either developmental or posttraumatic and are associated with meniscal tears. They can be asymptomatic or clinically simulate a torn meniscus with a flap component. Page 15 of 78

They are often mistaken for intra-articular loose bodies, but differentiation is made by the presence of fatty bone marrow signal in their center. Meniscal flounce Meniscal flounce is a wavy appearance along the free edge of the meniscus. Previously, it was thought to be identified only at arthroscopy, with the knee flexed or in the setting of a ligament injury. As recently observed, the meniscal flounce can indeed resolve in extension, may simulate a tear or degeneration and can be seen with meniscal tears (1) Fig.: Meniscal flounce - Wavy appearance of the free edge of the LM (sagittal proton density-weighted image ). Meniscal lesions Page 16 of 78.

-Meniscal tears -Meniscocapsular separation -Meniscal cysts -Meniscal contusion -Meniscal degeneration Meniscal tears - Etiology The prevalence of meniscal tears increases with age. We can have: -Increased force on a normal meniscus # usually results in longitudinal or radial tears. -Normal forces on a degenerative meniscus # usually produce horizontal tears in the posterior half of the meniscus. Tears are more common in the MM, probably because it is less mobile and bears more force during weight-bearing, with 56% of meniscal tears involving the posterior horn of the MM (1). LM tears are more common in younger patients who have a higher incidence of sportsrelated tears and this is likely explained by the higher incidence of concomitant ACL tears. Meniscal tears in children are often associated with ACL tears. The degenerative tears are more common in older patients (> 40 years) with no history of trauma. Meniscal tears - Diagnostic MR Features The normal meniscus has low signal intensity on all MR imaging sequences. The diagnostic criteria for a meniscal tear are an area of abnormal signal within the meniscus on at least one image extending to the meniscal articular surface or abnormal meniscal morphology. Page 17 of 78

An accurate description of a meniscal tear has great importance in the treatment planning, because of long-term complications of complete meniscectomy, which include degenerative changes. Location in the body, anterior horn or posterior horn. Location in the peripheral third (vascularized red zone), in the inner two thirds or both. Meniscal tears can be described by the zone of injury: Zone 0 - Injury to the synovial-perimeniscal vascular plexus Zone 1 - Injury to the red or vascularized zone of the meniscus Zone 2 - Injury to the red-white junction of the meniscus Zone 3 - Injury to the white avascular zone Complete, extending from one articular surface to another, or incomplete. Morphology: horizontal, vertical (longitudinal, radial or parrot-beak) or complex. Length of the tear: important to determine if the tear is repairable. Stable or unstable (at arthroscopy, an unstable tear is displaceable into the joint with probing and more often resected or repaired). MR technical protocol considerations The knee is usually positioned in extension with slight external rotation to facilitate imaging the ACL. High spatial resolution is required to show subtle tears, with a field of view of 16 cm or less, a section thickness of 5 mm or less (3-4 mm is preferred) and a matrix of at least 192 X 256 steps in the phase- and frequency-encoding directions. An extremity coil is used to optimize the signal-to-noise ratio. The sagittal plane is used more commonly, but coronal plane improves the detection and characterization of radial, bucket-handle, horizontal and displaced tears of the body. The axial plane assists in diagnosing radial, vertical, complex, displaced and LM tears. Several factors should be considered in optimizing the imaging protocols. Page 18 of 78

Short echo times (TE) are important when PD-weighted imaging is performed. With a TE of less than 26 ms more than 90% of all meniscal tears can be detected and with a TE greater than 60 ms less than 30% of tears are detected (3). The most commonly used sequences include spin-eco or fast spin-eco (FSE) proton density with or without fat saturation, T1 and gradient echo (GRE) (1). T1-weighted images are not as sensitive as PD-weighted images for diagnosing meniscal tears. GRE imaging is more limited in diagnosing ligament, muscle, tendon, bone marrow, and articular cartilage abnormalities. It is also less specific for meniscal tears as a consequence of spurious signal from artifacts or degeneration without a tear. No evidence indicates that fat suppression increases the accuracy in diagnosing meniscal (3) tears, but this practice is gaining widespread acceptance. With fat suppression, the dynamic range signal of the menisci is increased, what can make meniscal tears more conspicuous. Turbo or fast spin-echo (FSE) pulse sequences may be less effective because images with short effective TE sacrifice high-spatial-frequency information for speed and images of the menisci may appear blurred. Page 19 of 78

Imaging findings OR Procedure details Based on a review of several cases of our institution, we illustrate the various types of meniscal tears, its morphology, grades of injury and associated findings. Classification of meniscal tears Horizontal tears Horizontal or cleavage tears are parallel to the tibial plateau and divide the meniscus into upper and lower segments. Fig.: Horizontal meniscal tear. Page 20 of 78

Fig.: Horizontal tear at the posterior horn of the medial meniscus (sagittal T2 starweighted image). Page 21 of 78

Fig.: Horizontal meniscal tear at the posterior horn, with extension to the body shown by the anterior truncated appearance (sagittal fat-saturated proton density-weighted image). Page 22 of 78

Fig.: Oblique horizontal tear of the posterior horn of the MM (sagittal fat-saturated proton density-weighted image). Page 23 of 78

Fig.: Oblique horizontal tear of the posterior horn of the MM, in the same patient, with involvement of the body (sagittal fat-saturated proton density-weighted image). Page 24 of 78

Fig.: The truncated meniscal appearance indicates tear of the body in this lateral meniscus (sagittal fat-saturated proton density-weighted image). Vertical tears Vertical meniscal tears can be longitudinal, radial or with parrot-beak morphology. A vertical longitudinal tear occurs perpendicular to the tibial plateau and parallel to the long axis of the meniscus, between the circumferential collagen fibers and equidistant from the peripheral edge of the meniscus. Page 25 of 78

Fig.: Longitudinal radial meniscal tear. Page 26 of 78

Fig.: Longitudinal vertical tear at the posterior horn of the medial meniscus (sagittal fat-saturated T2-weighted image ). A vertical radial tear occurs on a plane perpendicular to the tibial plateau and perpendicular to the long axis of the meniscus, transversing the circumferential collagen fibers what disrupts the ability to distribute the hoop stresses associated with weightbearing. Page 27 of 78

Fig.: Vertical radial meniscal tear. Page 28 of 78

Fig.: Vertical radial tear of the lateral meniscus (sagital proton density-weighted image). Partial thickness radial tears can ultimately result in two separated pieces of meniscus and allow contact between the articular contact surfaces with consequent accelerated degenerative changes. There are four useful radiological signs: - Ghost appearance # absent section of the meniscus or area of high signal in the shape of the meniscus on a single image that is parallel to the tear. - Marching cleft # presents more commonly with a radial tear at the junction of the posterior horn and body and appears to "move" across the meniscus on successive images. - Truncated triangle sign # abrupt truncation of the inner point of the normal meniscus. Page 29 of 78

- Cleft sign # when there is abnormal signal in the meniscus perpendicular to the image plane. A vertical parrot-beak tear is radial at the inner meniscal edge and longitudinal more peripherally within the meniscus. They are difficult to detect with MR imaging. Fig.: Vertical parrot-beak tear. Complex tears Complex tears have two or more tear configurations or are not categorized easily into a certain type of tear. Page 30 of 78

Fig.: Complex tear of the posterior horn of the medial meniscus (sagittal fat-saturated proton density-weighted image). Page 31 of 78

Fig.: Complex tear of the anterior horn of the medial meniscus with degenerative associated aspects (sagittal fat-saturated proton density-weighted image). Bucket-handle tears A bucket-handle tear occurs when the inner meniscal segment of a longitudinal or oblique tear "flips", more commonly into the intercondylar notch, becoming disrupted or remaining intact. Page 32 of 78

Fig.: The inner meniscal segment of a longitudinal or oblique tear may "flip" resulting in a bucket-handle tear. Page 33 of 78

Fig.: The inner meniscal segment of a longitudinal or oblique tear may "flip" resulting in a bucket-handle tear. The entire meniscus is often involved, but also the posterior horn and body or only one horn. It is the more common type of displaced "flap" tear. The radiological signs are: - Double posterior cruciate ligament (PCL) sign # results from meniscal material in the notch, inferior and parallel to the PCL in the same sagittal plane. It is noted only in medial meniscal bucket-handle tears unless there is an ACL tear. - Fragment in notch sign # occurs when the displaced fragment is in the notch but not in the same sagittal plane as the PCL. It is seen more often in lateral bucket-handle tears. Page 34 of 78

- Absent bow tie sign # when the meniscal body is not identified on at least two adjacent sagittal 4 to 5 mm thick images. False positives can occur in children or adults of short stature, in degenerative menisci, in radial tears or in postsurgical changes. False negatives can occur in bucket-handle tears of discoid meniscus. - Tuncated meniscus on coronal images. - Disproportional posterior horn # when there is a larger posterior horn on sagittal images closer to the root attachment than peripherally because of a centrally displaced fragment of the more peripheral posterior horn. - Quadruple cruciate sign # when there is medial and lateral bucket-handle tears with both fragments displaced into the notch. - Flipped meniscus sign # when the fragment is flipped anteriorly adjacent to the ipsilateral anterior horn, which normally does not measure more than 6mm in height. - Double anterior horn sign # it is the same as the flipped meniscus sign, but with two separate "anterior horns" identified. This two last signs are usually associated with intercondylar meniscal displacement. Page 35 of 78

Fig.: Bucket-handle tear of the posterior horn of the LM with internal displacement of the meniscal fragment near the notch (coronal fat-saturated T2-weighted image). Page 36 of 78

Fig.: Bucket-handle tear of the posterior horn of the LM with internal displacement of the meniscal fragment near the notch. In this case, there is an associated acute tear of the ACL (not shown here) (coronal fat-saturated proton density-weighted image). Page 37 of 78

Fig.: Bucket-handle tear of the anterior horn (sagittal plane). Page 38 of 78

Fig.: Bucket-handle tear - the meniscal fragment is displaced posteriorly (sagittal proton density-weighted image). Page 39 of 78

Fig.: Bucket-handle tear - the meniscal fragment is displaced posteriorly and located behind the PCL (sagittal proton density-weighted image). Page 40 of 78

Fig.: Small posterior horn as a sign of displacement of meniscal tissue - bucket-handle tear (sagittal proton density-weighted image). Displaced meniscal injuries can occur in both the medial and lateral meniscus and include flap tears, bucket-handle tears and free fragment displacement. By far, buckethandle tears are the most common, occurring in about 10% of patients. Flap tears with displacement A flap tear or a displaced flap tear is a short-segment horizontal tear with displaced fragments into the notch or into superior or inferior gutters. Usually, this type of meniscal injury involves the medial meniscus with superior displacement. Inferomedial displaced tears from the MM are uncommon and when the displaced fragment extends inferior and medial to the tibial plateau, deep to the MCL, it may go unnoticed by the arthroscopist because the meniscus surface may appear intact. Page 41 of 78

It is important to recognize and describe this type of tears because they are unstable and especially the inferior gutter is a difficult area for the surgeon to visualize, with poor outcome after meniscal resection. The normal rectangular meniscus is not identified on the most peripheral sagittal image and meniscal tissue is noted inferior to the body segment. The coronal plane is useful to confirm the displacement. Fig.: Tear of the posterior horn of the medial meniscus with a predominant horizontal component and an anterior flap (sagittal plane). Page 42 of 78

Fig.: Displaced inferior flap tear on the LM (coronal fat-saturated proton densityweighted image). Page 43 of 78

Fig.: Inferiorly displaced flap tear on the posterior horn of the LM (same patient) (sagittal fat-saturated T2-weighted image). Free fragments A free fragment displacement is rare. Page 44 of 78

Root tears Occurs at the tibial attachment or "root" of the meniscus, like a full thickness radial tear at or adjacent to the root. It has been described only posteriorly. It's a difficult tear to diagnose because meniscal tissue is noted only on one side of the tear. A medial meniscal root tear is easier to recognize, with more common and pronounced meniscal extrusion. Also, there is a close anatomic relationship between the posterior horn of the medial MM and the tibial attachment of the PCL: the normal meniscus should be seen on the image medial to the PCL attachment, on 3mm sagittal images. Coronal images are useful to confirm the suspicion. A lateral meniscal root tear is diagnosed when the posterior horn of the LM does not cover the most medial aspect of the ipsilateral tibial plateau on at least one coronal image. Fig.: Degeneration of the LM's root without a tear (coronal fat-saturated proton density-weighted image). Page 45 of 78

Fig.: Tearing of the tibial insertion of the LM, with presence of joint fluid (coronal T2 star-weighted image). Meniscocapsular separation Is more common medially and associated with other lesions. Page 46 of 78

Suggestive signs are the following, being the best predictors the first two: Irregular outer margin of the meniscus body at coronal images; Presence of perimeniscal fluid; Increased distance between the meniscus and the median collateral ligament (MCL) or fluid between the meniscus and the MCL; Displacement of the meniscus relative to tibial margin; Extension of the tear into the superior or inferior corner of the peripheral meniscus. These lesions occur in an area with extensive blood supply, so many heal conservatively. Grades of injury The meniscal injuries can be graded in severity according to the increased signal intensity morphology: Grade 1 globular increased signal intensity that does not communicate with the articular surface. Grade 2 linear increased signal intensity that does not communicate with the articular surface. Grade 3 linear increased signal intensity that communicates with the articular surface, a true tear. Grade 3a and 3b more extensive articular involvement. Grade 4 complex tears with distortion of the meniscus. Page 47 of 78

Fig.: Grade 2 lesion - linear increased signal intensity that does not communicate with the articular surface, at the posterior horn of the medial meniscus (sagittal fat-saturated proton density-weighted image). Page 48 of 78

Fig.: Grade 2 lesion - linear increased signal intensity that does not communicate with the articular surface, at the posterior horn of the lateral meniscus (coronal fat-saturated T2-weighted image). Page 49 of 78

Fig.: Grade 4 - Complex tear at the anterior horn of the lateral meniscus (sagittal fatsaturated proton density-weighted image). Meniscal pitfalls Causes of false positives are the followings: Page 50 of 78

- False negatives on arthroscopy # Regarding the limitations of this technique, the undersurface of the posterior horn is an arthroscopic blind spot. - Healed meniscal tears # Healed meniscal tears often continue to show increased signal intensity that contacts a meniscal surface. - Postoperative menisci # See below. - Magic-angle phenomenon # Meniscal tears become less visible when TE values are longer than 16 to 20 ms. However, using TEs of shorter than 16 ms may result in increased signal within the normal meniscus because of the magic-angle phenomenon. The normal posterior horn of the LM slopes obliquely upward from lateral to medial, as it ascends from the lateral tibial plateau to its insertion on the posterior tibial eminence. On coronal images, it often achieves an angle of 55 relative to the long axis of the leg. This is the angle where the magic-angle phenomenon occurs when short TEs are used (3). So, this orientation leads to shortening of the apparent T1 relaxation time, resulting in an increase in signal intensity within the posterior horn, which may simulate a tear or grade 2 signal-intensity changes. Increasing the TE, magic-angle findings disappear and true meniscal tears remain visualized. Also, imaging the knee in slight abduction alters the orientation of the posterior horn of the LM in relation to the main magnetic field (B0). - Vacuum-joint phenomenon # Gas, normally present in tissues, can become intraarticular when traction is applied to the joint, as a result of reduced pressure. This leads to magnetic susceptibility artifacts, which appear as low signal intensity on T1-weighted images or as a blooming artifact on GRE images, what may be mistaken for a meniscal tear. Also, gas collecting in the medial joint space between the articular cartilage of the femur and tibia can appear as a triangular signal void that simulates meniscal tears or an abnormal volume of meniscal tissue resembling a DM or a displaced torn meniscal fragment. - Truncation artifact # Truncation artifacts are lines of high signal that run parallel to the meniscal surface traversing the menisci which can be confused with tears and are problematic when a 128 X 128 matrix is used with a 128 phase-encoded axis orientated in the superior to inferior direction. One manifestation is the ringing artifact (Gibbs phenomenon), which occurs near highly contrasting interfaces, and these lines extend beyond the boundary of the meniscus. The truncation artifact can be attenuated if the phase-encoded axis in a 128 X 256 matrix is orientated anterior to posterior but the most effective way is to use an acquisition matrix of 192 X 256 or 256 X 256. Page 51 of 78

- Increased conspicuity of intrameniscal signal intensity (grade 2 signal intensity) on GRE images - Misinterpretation of normal anatomic structures and anatomic variants (some of these errors can be avoided by following the anatomic structures on subsequent images): -Attachment points of the transverse intermeniscal ligament to the anterior meniscal horns # fat surrounding the transverse intermeniscal ligament may mimic a meniscal tear. -Meniscal attachments of the meniscofemoral ligaments from the posterior horn of the LM # may mimic a vertical tear through the posterior horn of the LM, due to interposition of fat between the ligament and its meniscal attachment. The fat signal intensity disappears on fat-suppressed images. -Popliteus tendon passing next to the posterior-lateral corner of the LM # True tears of the periphery of the LM in this region usually have a different orientation than that of the tendon sheath. However, a vertical tear of the posterior horn of the LM may parallel the popliteus tendon sheath. -Medial and lateral oblique meniscomeniscal ligaments # the obliquely orientated meniscomeniscal ligament may be confused with a meniscal fragment in the intercondylar notch region. Careful attention to axial and coronal images will avoid this pitfall. -High signal striations from the ACL # structures of high signal intensity in the anterior root of the LM are caused by normal contributions of the root from the ACL. Such striations do not actually contact the meniscal surface and should not be confused with meniscal tears. -Lateral inferior genicular artery # arising from the popliteal artery at the level of the knee joint, courses anteriorly around the lateral aspect of the knee and participates in the rich collateral circulation in this area. It is closely applied to the LM through its course, lying within a region of periarticular fat between the meniscus and the LCL. When the artery lies adjacent to the anterior horn of the LM, it can appear as a tear on sagittal images. - Intra-articular loose bodies # Can obscure the meniscal margins and mimic a tear. - Meniscal contusion # See below. Page 52 of 78

- Chondrocalcinosis # Intrameniscal calcium crystals deposition is seen in conditions such as gout, degenerative disease, hemochromatosis, crystal deposition disease and hypercalcemia. It can demonstrate increased signal intensity on T1-weighted, PDweighted or T2-weighted images, which may mimic a meniscal tear. Correlation with plain radiographs is necessary. - Degenerated meniscus # Initial MRI changes of intrameniscal signal intensity in patients with osteoarthritis who later go on to develop grade 3 signal-intensity abnormalities in the same region, may represent an early and symptomatic precursor of a horizontal cleavage tear. Regarding grade 2 versus grade 3 signal-intensity changes in the meniscus, determining whether the signal intensity in the meniscus extends to the articular surface can be difficult. Evaluating for any change in the morphology of the meniscus at this site may be helpful. If distorted or abnormally shaped, a tear is likely. As causes of false negatives we have: - Small meniscal tears # These tears may be seen in only 1 plane and small radial tears can be difficult to diagnose. This type of tear is stable and treated conservatively. Most are not diagnosed directly at arthroscopy but rather observed on the basis of the meniscus behavior to probing or compression. Radial tears of the posterior meniscal roots can be overlooked. - Abnormalities involving the meniscal free edge. Associated lesions Traumatic meniscal tears are frequently associated with ACL tears and less commonly with PCL. In the setting of an acute ACL tear, the LM is torn twice as often as the MM, with approximately one half representing peripheral longitudinal tears most commonly located in the posterior horn of the LM. The increased shear forces on the less mobile posterior horn may be responsible. The peripheral location makes these tears amenable to repair or conservative treatment. Also in this setting, displaced meniscal tears like bucket- Page 53 of 78

handle tears are more common and the sensitivity for diagnosing meniscal tears is decreased. With an ACL tear, the LM root is torn more often than the medial root and can present with lateral meniscal extrusion greater than 1mm. With an intact root, the meniscal extrusion reveals another type of meniscal tear, often a radial or complex tear. With a chronic ACL tear, the incidence of subsequent MM tears increases and these lesions are often degenerative and complex and less amenable to surgery. The MM is a significant restraint to anterior tibial translation after ACL disruption, suggesting the possibility of increased sheer forces. The relatively immobile MM may also be injured during abnormal knee motion during periods of instability. With both ACL and MCL tears, LM tears are more common. Many of the uncommon MM injuries in this setting are actually ruptures of the medial joint capsule and the deep fibers of the MCL in conjunction with peripheral meniscocapsular separation. Page 54 of 78

Fig.: ACL tear. This patient had also a bucket-handle tear of the posterior horn of the lateral meniscus (sagittal fat-saturated T2-weighted image). Page 55 of 78

Fig.: Rupture of the vastus lateralis muscle. This patient had also a tear of the posterior horn of the MM (coronal fat-saturated proton density-weighted image). Page 56 of 78

Fig.: Associated lesion of the MCL (coronal fat-saturated proton density-weighted image). Page 57 of 78

Fig.: Posterior joint capsule lesion. The patient had also a tear of the anterior horn of the lateral meniscus (sagittal plane). Page 58 of 78

Fig.: This is the patient refered to in the last image. Here is shown the tear of the anterior horn of the lateral mensicus (coronal plane). Assessment of stability The stability of tears is determined by a number of factors, including the length, location, and completeness of the tear. Longitudinal tears that are relatively long are unstable and their length is assessed on multiple 3- to 4-mm sections in either plane, extending through the full thickness of the meniscus or containing fluid on T2-weighted images. A stable vertical longitudinal tear occurs when the inner fragment of a meniscal tear cannot be displaced more than 3 mm from the intact meniscal periphery. Any meniscal tear with a displaced fragment is unstable. Probing the meniscal tear during arthroscopy is determinant for stability assessment. If the inner margin of the tear can be displaced to a position where it can be entrapped Page 59 of 78

between the rotating femur and tibia when probed at arthroscopy, it is considered an unstable tear. Unstable meniscal tears do poorly without surgery because they tend to propagate and lead to accelerated osteoarthritis. Factors with therapeutic and prognostic relevance MRI is the most accurate noninvasive method for diagnosing meniscal tears and has influenced clinical practice and patient care by avoiding unnecessary diagnostic arthroscopies and by identifying alternative diagnosis that can mimic meniscal tears. MR imaging show many of the essential characteristics of meniscal tears critical to management, such as their location, shape, length, and depth. In this way, MR helps to provide an accurate assessment of stability, the likelihood of tear propagation and a determination of whether it can be repaired. When the possibility of meniscal repair is considered, the following considerations must be evaluated: - Chronicity of the tear (meniscal repairs are reported to heal better when performed closer to the time of injury); - Associated injuries, especially ACL and PCL tears (longitudinal tears of the MM in the setting of acute ACL tears need to be repaired, because of a higher propensity to propagate over time due to the higher stresses the MM is subject to); - Extent and type of tear; - Location of the tear. Partial-thickness horizontal tears, shallow radial tears of 5 mm or less and short vertical or oblique longitudinal tears shorter than 8 mm and involving 50% or less of the thickness of the meniscus can be treated conservatively, especially when the knee ligaments are intact or have been successfully repaired. Postoperative imaging Page 60 of 78

Surgical management of meniscal tears includes total or partial meniscectomy and primary meniscal repair, always with the goal to preserve as much meniscal material as possible. Sometimes, a portion of the cleaved meniscus is preserved and a meniscus with preoperative grade 2 signal intensity can falsely appear after as grade 3, a phenomenon called intrameniscal signal conversion. Thus, basic criteria helpful in diagnosing meniscal tears in preoperative patients are unreliable in the postoperative knee. Postoperative imaging of the meniscus is complicated, as also in those patients receiving conservative treatment. In the stable knee, meniscal tears in the periphery with less than 1 cm in length and displacement of 3 mm or less are usually treated conservatively. When fluid signal is noted in the meniscus on T2-weighted images or a displaced meniscal fragment is identified, the specificity for diagnosis is increased. However, in the early postoperative period (<12 weeks), the repair site may show increased T2-weighted signal due to granulation tissue. After injury, the signal intensity of the meniscus may even never return to normal and meniscal healing may appear as lines of hyperintensity reaching the articular surface of the meniscus, mimicking new tears unless preoperative images are available for comparison. So, the use of gadolinium with either indirect or direct arthrography has been proposed to detect residual or recurrent tears. Indirect arthrography involves the acquisition of MR images 10 to 20 minutes after intravenous injection of gadolinium (0,1mmol/kg), occurring synovial excretion of the contrast. Direct arthrography involves the intra-articular injection of approximately 20 ml of a gadolinium-saline mixture (1:150 solution) into the knee. There is a high incidence of radial tears in patients who have undergone partial meniscectomy. Meniscal cysts Page 61 of 78

Meniscal cysts occur more frequently in the medial compartment, twice as common as LM cysts. The most widely accepted cause of meniscal cysts is extension of fluid through a meniscal tear. Most commonly, they are formed when horizontal tears extend to the meniscal periphery, allowing joint fluid to escape into the parameniscal soft tissues. The fluid subsequently encapsulates and becomes symptomatic due to mass effect. Occasionally, the cyst can be confined to the meniscus, being referred to as an intrameniscal cyst. MR features include well-defined high-signal intensity lesion adjacent to or partially including the meniscus on T2-weighted sequences and meniscal cysts may be septated and up to 5 cm in size. Ganglion cysts and cruciate ligament cysts may be confused with meniscal cysts. Fig.: Meniscal cyst associated with a horizontal tear of the posterior horn of the medial meniscus (sagittal fat-saturated proton density-weighted image). Page 62 of 78

Fig.: Meniscal cyst of the medial meniscus (coronal proton density-weighted image). Page 63 of 78

Fig.: Meniscal cyst of the medial meniscus (coronal fat-saturated proton densityweighted image). Meniscal contusion Meniscal contusion occurs when the meniscus gets trapped between the tibia and femur, usually as a result of trauma. It demonstrates increased signal within its substance that Page 64 of 78

might resemble a tear. The signal is indistinct and amorphous, rather than sharp and discreet like in a tear and there is often an adjacent bone contusion. Meniscal degeneration Local increases in the degree mobility of trapped water molecules within the substance of the meniscus occurs with age, resulting in increased signal intensity within the meniscus on short TE images. Degenerative meniscal changes include the grade 1 and the grade 2 of the MRI grading system concerning meniscal injuries: Grade 1 is found in healthy volunteers and asymptomatic athletes and not clinically significant. Grade 2 is a continuation of progressive degeneration from grade 1 changes. Patients are usually asymptomatic. The posterior horn of the MM is the most common location. It also is the most common site for grade 3 meniscal tears. The presence of grade 2 signal-intensity changes is not predictive of future progression to grade 3 meniscal tears, but grade 2 represents a point of potential structured weakening. Grade 3, a real tear, when develops is adjacent to or in continuity with areas of grade 2 changes. Page 65 of 78

Fig.: Extrusion and abnormal morfology of the MM - degenerative aspects (coronal fatsaturated proton density-weighted image). Page 66 of 78

Fig.: Discoid lateral meniscus with tear and degeneration of the anterior horn (sagittal fat-saturated T2-weighted image). Page 67 of 78

Fig.: Extrusion and fragmentation of the medial meniscus - severe degenerative aspects. Radial fissure and internal high-signal of degenerative nature on the lateral meniscus (coronal T2 star-weighted image). Meniscal extrusion The meniscus is considered extruded when it extends beyond the tibial margin. Major extrusion (>3mm) is usually associated to severe degeneration, large radial tears, complex tears or tears involving the meniscal root (10). These lesions result in disruption of the meniscal stability. Detection of meniscal extrusion is important not only because it may be associated with an underlying meniscal tear but also because meniscal extrusion itself is thought to be related to the development of osteoarthritis. Page 68 of 78

Fig.: Extrusion of the medial meniscus with an associated meniscal cyst superiorly located (coronal T2 star-weighted image). Page 69 of 78

Images for this section: Fig. 0: Horizontal meniscal tear. Radiology, Hospital de São José, Centro Hospitalar de Lisboa central, Hospital de São José, Centro Hospitalar de Lisboa Central - Lisbon/PT Page 70 of 78

Fig. 0: Longitudinal radial meniscal tear. Radiology, Hospital de São José, Centro Hospitalar de Lisboa central, Hospital de São José, Centro Hospitalar de Lisboa Central - Lisbon/PT Page 71 of 78

Fig. 0: Vertical radial meniscal tear. Radiology, Hospital de São José, Centro Hospitalar de Lisboa central, Hospital de São José, Centro Hospitalar de Lisboa Central - Lisbon/PT Page 72 of 78

Fig. 0: Vertical parrot-beak tear. Radiology, Hospital de São José, Centro Hospitalar de Lisboa central, Hospital de São José, Centro Hospitalar de Lisboa Central - Lisbon/PT Page 73 of 78

Fig. 0: The inner meniscal segment of a longitudinal or oblique tear may "flip" resulting in a bucket-handle tear. Radiology, Hospital de São José, Centro Hospitalar de Lisboa central, Hospital de São José, Centro Hospitalar de Lisboa Central - Lisbon/PT Page 74 of 78

Conclusion MR imaging is a highly accurate diagnostic tool for detecting and grading meniscal tears, avoiding unnecessary diagnostic arthroscopies and identifying alternative diagnosis that can mimic meniscal compromise. Page 75 of 78

Personal Information Cecília Leal, Hugo Rio Tinto, Joana Raposo and Alexandra Vasconcelos are Residents of Radiology at "Serviço de Radiologia do Hospital de São José - Centro Hospitalar de Lisboa Central (CHLC)". Tiago Bilhim is Resident of Radiology at "Serviço de Radiologia do Hospital de Santo António dos Capuchos - Centro Hospitalar de Lisboa Central (CHLC)". Pedro Alves is Radiology Consultant and Rui Mateus Marques is a Senior Consultant of Radiology (Chief of Departmen) at "Serviço de Radiologia do Hospital de São José - CHLC". Page 76 of 78

References 1- Fox, M.; MR imaging of the meniscus: Review, current trends and clinical implications; Radiol Clin N Am, 2007; 45: 1033-1053. 2 - Kelley E.A., Berquist T.H. - in Berquist T.H.; MRI of the Musculoskeletal System, th Lippincott Williams & Wilkins, 2006, 5 edition, pp.303-373. 3 - Aiello M.R.; Knee, Meniscal Tears (MRI); www.emedicine.com; updated Jan, 26, 2009. 4 - Vande Berg B.C., Malghem J., Poilvache P., Maldague B., Lecouvet F. E.; Meniscal Tears with Fragments Displaced in Notch and Recesses of Knee MR Imaging with Arthroscopic Comparison; Radiology 2005; 234(3): 842-850. 5 - Vande Berg B.C. et al; Lesions of the Menisci of the Knee - Value of MR Imaging Criteria for Recognition of Unstable Lesions; American Journal of Roentgenology(AJR) 2001; 176 (3): 771-776. 6 - Boxheimer L., Lutz A. M., Zanetti Met al.; Characteristics of Displaceable and Nondisplaceable Meniscal Tears at Kinematic MR Imaging of the Knee; Radiology, 2006; 238(1): 221-231. 7 - Englund M., Guermazi A., Gale D. et al.; Incidental Meniscal Findings on Knee MRI in Middle-Aged and Elderly Persons; N. Engl. J. Med., 2008; 359(11): 1108-1115. 8- Helms, C.A.; The Meniscus: Recent Advances in MR Imaging of the Knee; AJR, 2002; 179: 1115-1122. 9- Lecas L. K., Helms C. A., Kosarek J., Garret w. E.; Inferiorly Displaced Flap Tears of the Medial Meniscus: MR Appearance and Clinical Significance; AJR 2000; 174:161-164. 10- Costa C.R., Morrison W.B., Carrino J.A.; Medial Meniscus Extrusion on Knee MRI: Is Extent Associated with Severity of Degeneration or Type of Tear?; Am. J. Roentgenol. 2004; 183 (1): 17. Page 77 of 78

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