Magnetic resonance meniscus tears: basic principles of practice, anatomy, descriptions and classifications (2023)

All low-, medium-, and high-field MRI systems can produce accurate diagnostic images to identify meniscal abnormalities. For units with lower field strengths, it may be necessary to increase the number of calculated signals to obtain an adequate signal-to-noise ratio. However, this setting increases imaging time, which increases the risk of patient movement. Even slight movement can degrade images, compromising the ability to diagnose meniscal tears.[18, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57]A coil is used at the end to optimize the signal-to-noise ratio. A surface coil can be used for better detail when evaluating subtle lesions or suspicious areas.[58]

Protocols and imaging plans.

The knee is usually placed in extension with slight external rotation to facilitate imaging of the anterior cruciate ligament (ACL).

High spatial resolution is required to show subtle tearing. This requires a field of view of 16 cm or less, a section thickness of 5 mm or less (3 to 4 mm is preferred), and a matrix of at least 192 X 256 steps in the phase and frequency encoding directions. A step of 0.5 to 1 mm is used between image sections. These parameters can be achieved using a solenoid surface coil. A coil is used at the end to optimize the signal-to-noise ratio.[18, 1, 59]

If subtle lesions or suspicious areas are identified using the standard edge coil, high resolution images can be obtained using a surface coil, provided the area of ​​interest is shallow enough to be covered by the surface coil with a small field of interest. vision. . Scan parameters in this situation include the following: field of view as small as 10 X 10 cm, matrix 256 X 512 (shown at 512 X 512), section thickness of 3 mm with an intersecting gap of 0.3 mm, and 3 signals acquired.[58]

Images should be obtained in the sagittal and coronal planes. Sagittal images are obtained with the knee externally rotated to allow for in-plane ACL imaging. Meniscus and ACL injuries often coexist. Axial images are also taken to study the supporting ligaments around the knee. Changing coils during an MRI scan is not part of the standard scan, but is similar to changing transducers during ultrasound to look at deeper or superficial structures. In a small or remote radiology practice, the treating radiologist may not be available to supervise the MRI scan. In this situation, the patient may be recalled for additional imaging with the surface coil.[58]

Several factors must be considered when optimizing imaging protocols. Imaging in all 3 planes is useful; however, not all sequences need to be performed in all takes. Fluid sensitive sequences are mandatory to detect subtle areas of edema. Some T2-weighted sequence is usually performed, usually in the sagittal and axial planes. Experience with a specific sequence can overcome any theoretical improvement of a pulse sequence unknown to the operator.[59]

A repetition time (TR) between 2200 and 2800 ms is needed to generate enough sections to visualize both menisci in the sagittal plane. Short echo times (TE) are important when performing PD-weighted imaging. With an TE of less than 26 ms, more than 90% of all meniscal tears can be detected. If the TE increases to more than 60ms, less than 30% of the ruptures will be detected.[59]

To review knee MRIs, the use of meniscal windows has become popular. This method consists of a region of interest, centered on the meniscus, magnified from 1.5 to 2X. A window width of 100 to 150 and a window level of approximately 1000 are used. The data indicate no significant difference in the detection of meniscal tears using narrow windows compared to conventional window widths.[59]

sequences

T1-weighted images are not as sensitive as PD-weighted images for diagnosing meniscal tears. Recovered Gradient Echo (GRE) sequences are as accurate as conventional spin-echo images for diagnosing meniscal tears.[20]However, GRE imaging is more limited in diagnosing abnormalities of ligaments, muscles, tendons, bone marrow, and articular cartilage. It is also less specific for meniscal tears resulting from spurious signs of artifact or non-tear degeneration.[59]

Fat suppression can be applied to meniscal sensitive sequences to remove high image distracting signals originating from bone marrow and soft tissue fat. With fat suppression, the dynamic range signal from the menisci is increased, making meniscal tears more noticeable.[4, 5]No evidence indicates that fat excision improves accuracy in diagnosing meniscal tears, but this practice is gaining wide acceptance.[4, 5]

Turbo pulse or fast spin echo (FSE) sequences are not as effective as conventional spin echo sequences for diagnosing meniscal tears because images with a short effective TE, as seen in FSE images, sacrifice high-frequency spatial information for velocity. . Images of the menisci may appear blurred. Rubin and colleagues postulated that the presence of a phantom artifact (secondary to phase differences between odd and even echoes in the echo train) or loss of meniscal signal intensity in meniscal tears due to increased magnetization transfer (as seen in FSE sequences ) may be responsible. due to the lower sensitivity of this sequence.[18, 40, 60]

A review of 6 studies, including the author's, showed a clear discrepancy between the sensitivities of FSE and conventional spin-echo sequences. The sensitivity of rapid spin-echo techniques to detect a meniscus tear was approximately 80%, whereas the sensitivity of conventional spin-echo sequences averaged approximately 90%. The authors postulated that the abnormal meniscal signal may appear to extend to the surface of the meniscus due to blurring and may be incorrectly interpreted as a tear.

Alternatively, the increased blurring and decreased resolution associated with FSE imaging may contribute to false-negative results. Blurring is more evident with short TE sequences, but short TE sequences are more competent in detecting meniscal abnormalities. Blurring is also more noticeable with long echo train lengths, such as those built into FSE imaging protocols. The authors recommended abandoning FSE imaging because a sensitivity loss greater than 10% is unacceptable.[36]

When using FSE sequences, an echo train length of 4 to 6 should be used to reduce blurring. The sensitivity of FSE sequences for diagnosing meniscal tears is approximately 80% in all reports, whereas the sensitivity of conventional spin-echo techniques is at least 90%. If the sensitivity drops from more than 90% to 80% and if all that is obtained is 2 to 3 minutes in imaging time, the use of FSE sequences to image the menisci is unlikely to be justified.[4, 5, 61]

The use of high-performance gradient FSE sequences is as accurate as conventional spin-echo imaging in diagnosing meniscal tears. The following parameters are used: a TR of 1500 ms and an effective TE of 20 m, with the K space centered on the second echo with a minimum echo spacing of 2X and length of 4.[61]

Classification system for meniscal degeneration.

An MRI grading system was developed and correlated with a histological model. Regions of degeneration show increased signal intensity in a spectrum of patterns or degrees based on the distribution (morphology) of signal intensity relative to a meniscal articular surface. This base is unique to the peripheral capsular margin of the meniscus, which is considered non-articular.[40]

1st grade

Grade 1 is a non-articular, focal or diffuse region of increased signal intensity within the substance of the meniscus (see image below). This finding is correlated with early meniscal degeneration and a hypocellular or chondrocyte-deficient region. The terms mucinous, myxoid, and hyaline degeneration are used interchangeably to describe the production and accumulation of increased amounts of mucopolysaccharide ground substance in stressed areas of meniscal fibrocartilage. These changes are a response to repetitive mechanical loads.

The fat-saturated proton-density-weighted sagittal image of the lateral compartment shows the relatively equal size of the anterior and posterior horns of the lateral meniscus. The body of the meniscus has the normal configuration of a bow tie.

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This occurrence is found in healthy volunteers and asymptomatic athletes and is not clinically significant.

Grau 2

Grade 2 is a horizontal linear area of ​​increased signal intensity within the substance of the meniscus that extends to, but does not wrap around, the inferior surface (see image below). These abnormal signal regions are more extensive than in grade 1 degeneration and there is no distinctive cleavage plane or tear. Grade 2 is a continuation of progressive degeneration from grade 1 changes. Patients are usually asymptomatic.

The fat-saturated proton density-weighted sagittal image shows abnormal signal intensity in the posterior horn of the medial meniscus, which appears to extend close to the inferior surface. This represents 2C degree changes in signal strength. It can be difficult to differentiate grade 2 and 3 changes. Lesions that cause grade 2C signal intensity can progress to degenerative lesions.

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Grade 2 sign can be of 3 types.[62]Type 2A is a linear mark that is not in contact with the articular surface. Grade 2B is an abnormal signal in contact with a joint surface in a single image. Grade 2C is an extensive wedge-shaped signal abnormality that is not in contact with the articular surface.

Histologically, there is microscopic fragmentation of collagen and cracks in the hypercellular region of the fibrocartilage matrix. The median perforating collagen bundle, which divides the meniscus into superior and inferior halves, is the preferred site of accumulation of mucinous ground substance. It also represents the cutting plane of the meniscus and is also the site of origin for horizontal degenerative meniscal tears.

The posterior horn of the MM is the most common location. It is also the most common site of grade 3 meniscal tears. The presence of changes in grade 2 signal intensity does not predict future progression to grade 3 meniscal tears. Grade 2 represents a point of potential structured weakness. Grade 3 tears, when they develop, are adjacent to or in continuity with areas of grade 2 changes.

Grade 2C is a subcategory where the linear signal intensity extends to the joint surface in a single image. When found in symptomatic patients, about 50% present rupture. There are no additional features that can discriminate a torn meniscus from an intact meniscus with grade 2C signal intensity.

This is not a very common occurrence, appearing in only 3% of patients in one study. Most patients with a grade 2C sign are not treated arthroscopically because they do not have symptoms attributable to the site of the abnormality, but approximately 50% of patients with a grade 2C sign and symptoms in the knee have meniscal tears. The grade 2C mole may represent more extensive degeneration than that seen with the grade 2 mole and may progress to degenerative tears.[63]

3rd grade

Grade 3 is a region of abnormal signal intensity within the meniscus that extends and communicates with at least one articular surface of the meniscus. Multiple foci of grade 3 signal intensity changes may be present in a meniscus.

About 5% of grade 3 tears are actually intrasubstance excision tears. These are closed meniscal tears and are diagnosed only with surgical probing of the meniscus. They may not be detected on routine arthroscopy if the surface extent is not identified.[34]

False-negative correlations with arthroscopy have been described. This commonly occurs in the LM, whether peripheral or posterior, when there is an associated ACL tear.[64]The improved spatial resolution and signal-to-noise ratio achieved with a surface coil may improve diagnostic accuracy in this situation and may be related to misinterpretation of worn areas as meniscal tears. These injuries may present with pain associated with swelling and synovial membrane growth upon rupture because the meniscus is an innervated structure.[30, 58]

Criteria for meniscus tears

Two MRI criteria have been established to diagnose meniscal tears. If no previous surgery has been performed on the meniscus, the accuracy in diagnosing tears is 90%.[1]

Criterion 1

Criterion 1 is an increase in internal signal strength in the meniscus. An article discusses the concept of the "two-touch rule". The authors described a positive predictive value of 94% for meniscal tears for the medial meniscus and 96% for the lateral meniscus when the tear is present on 2 consecutive images. The positive predictive value was 55% and 36% for medial and lateral meniscal tears, respectively, when observed in a single section.[1, 30, 65]

The abnormal signal strength must be in contact with a joint surface, either on the upper or inner surface or at the tip (free edge) of the meniscus. If contact with the articular surface appears on 2 or more consecutive images, the accuracy of diagnosing a meniscal tear increases.[1, 34]

The need for short TEs is important. Most other tissue disorders are characterized by increased free water and free protons. In meniscal tears, hydrogen nuclei are attached to macromolecules. Bound protons have a shorter T2 relaxation than protons in free water. Meniscal tears also cause absorption of synovial fluid at the margins of the tear. This may be related to the loss of the normal, tight collagen spiral, resulting in increased mobility of water molecules. The water molecules are trapped, which increases the local spin density. The increased meniscal signal within the tear is likely due to an increase in local spin density rather than an increase in T2 signal.[1]

Proton rotational speed is shortened by the interaction of synovial fluid and large macromolecules, resulting in shortened T1 and T2 values, increasing the sensitivity of PD-weighted images to reveal meniscal pathology. Such changes cause a local increase in the degree of freedom of trapped water molecules, resulting in increased T2 times, allowing detection of a higher signal with short TE sequences. Increased signal within the meniscus is best seen on short TE images obtained using PD or GRE weighted sequences.[34]

Although most meniscal tears show up well on PD-weighted images, they do not visualize as well on T2-weighted images unless there is a wide cleft at the site of the meniscus tear that freely communicates with the meniscus. If such a situation occurs, confidence in the diagnosis of a meniscal tear is high. However, this finding is not common.

Criterion 2

Criterion 2 is an abnormal meniscal shape. In-depth knowledge of the normal anatomy of the menisci on MRI is required. Meniscal tears are most confidently diagnosed when viewed on sagittal and coronal images. The presence of a meniscus tear in both views decreases the rate of false positive diagnoses. However, some tears at the meniscocapsular junction can only be seen on one of these views.

When writing the MRI report of a meniscal tear, the radiologist should understand the use of standard nomenclature for meniscal tears and describe the location, plane, shape, completeness, length, and number of tears.[59]

Sizes, shapes and patterns.

Multiple cross-sectional representations of a meniscal tear should be translated into a three-dimensional (3D) description for the arthroscopist's benefit.[1, 22]

Meniscal tears occur in 2 primary planes: vertical and horizontal. The three basic forms of meniscal tears are longitudinal, radial, and horizontal. Meniscal tears are either partial or full thickness.

Vertical tears line up perpendicular to the coronal plane of the meniscus and can be subdivided into longitudinal and radial tears. They occur as traumatic injuries in younger patients.[66]A full-thickness vertical tear contacts the superior and inferior articular meniscal surfaces, completely dividing the torn portion of the meniscus into an inner and outer portion. These tears can lead to the development of bucket handle tears.[18, 67]

Longitudinal tears separate the meniscus into internal and external fragments and occur parallel to the outer edge of the meniscus (perpendicular to the tibial plateau and propagate parallel to the circumferential axis of the meniscus). These tears are equidistant from the lateral (peripheral) meniscal margin for their entire length.

Longitudinal tears are more commonly of traumatic etiology and occur in younger and more physically active patients. They are also commonly found in patients who have also experienced an acute anterior cruciate ligament tear. These meniscal tears are located in the posterior horn of the LM, in the center of the popliteus tendon.[18]Furthermore, these tears do not affect the free edge (inner part) of the meniscus in any image. They are usually located in the middle or lateral third of the meniscus and usually start in the posterior horn.[1]A longitudinal partial-thickness tear contacts only the superior or inferior articular meniscal surfaces, but not both.

Short ruptures, or those confined to the posterior horn, may only be visible on sagittal images. The longer tears spread towards the body of the meniscus. They are seen on sagittal and coronal images.

Bucket handle tears

Bucket handle tears are displaced vertical longitudinal tears and usually involve the MM (see images below). The detached central (internal) fragment, viewed axially, resembles a bucket handle. The remaining larger peripheral portion of the meniscus resembles a cube. These tears account for approximately 10% of all meniscal tears.[4, 5, 59, 68]

The coronal proton-density-weighted image shows a vertical full-thickness bucket handle tear (arrow) traversing the base of the medial meniscus (MM).

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The proton density-weighted sagittal image shows a vertical bucket handle tear (arrow) through the periphery of the posterior horn of the medial meniscus (MM). Tears at this location usually do not heal spontaneously because this portion of the meniscus has no blood supply.

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The central fragment is usually well visualized on coronal images and poorly visualized on sagittal images. The central fragments of bucket handle tears of the posterior horn of the LM are often displaced anteriorly, so that the displaced torn fragment lies over the anterior horn of the LM. This occurs because the ACL prevents the meniscus fragment from completely migrating to the intercondylar notch. In this situation, the “height” of the LM anterior horn is almost twice its normal height. This is best seen on sagittal views.[34]

Absence of the bow tie sign is helpful in diagnosing bucket handle tears in the body of the meniscus (see image below). The normal body of the meniscus is between 9 and 12 mm wide and should be seen on 2 consecutive sagittal images and, as depicted in the normal MRI anatomy, is shaped like a bow tie. When the bucket handle is torn, part of the free edge of the meniscus is missing. The inner portion of the body of the meniscus will be absent. Confirmation almost always occurs in the form of a dislocated meniscus fragment visualized in another part of the knee joint.

Proton density-weighted sagittal image of the lateral aspect of the knee showing absence of the body of the lateral meniscus (LM, arrow). The body of the meniscus should be viewed peripherally. The normal appearance of the meniscus looks like a bow tie; the bow tie is absent here.

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When no displaced fragments are found, the absence of the loop sign may be related to a normal but small meniscus. In this situation, both the medial and ML are small; Bucket handle tears of both the medial and LM, which occur at the same time, are rare. Another cause of the absence of the bow tie sign is the normal postoperative meniscus in patients undergoing PM.

These injuries can be further classified as single vertical longitudinal tears, displaced bucket handle tears, ruptured bucket handle tears, and double and triple vertical bucket handle tears. They are 3 times more frequent in MM than in LM and may be associated with ACL ruptures. Bucket handle tears are commonly seen in young adults with a history of joint locking, extension locking, or slippage due to displacement of the central fragment into the intercondylar notch.

Radial features (transverse features)

Radial tears are vertical tears and propagate perpendicular to the main axis of the meniscus. These injuries are devastating because a complete tear destroys the integrity of the meniscus (ie, the ability of the meniscus to distribute circular stress; circular stress is the normal external force generated in the meniscus in all directions as a result of weight bearing).

The force is distributed in the meniscus by collagen fibers, located around the circumference of the meniscus, from the apex to the periphery. Collagen preserves the normal shape and integrity of the meniscus by supporting weight. Radial breaks traverse these fibers. The meniscus is normally attached to the tibia at its anterior and posterior ends.

During weight bearing, the meniscocapsular inserts pull the meniscus out. A radial tear that occurs between the tibial attachment points causes the free, loose edges of the meniscus at the point of the tear to be temporarily pulled outward, expanding the width of the tear, exposing a bare spot on the adjacent tibia and femur, allowing for abnormalities. . stress. on articular cartilage and unprotected bone surfaces, resulting in destruction of articular cartilage and subsequent bone breakdown, leading to accelerated degenerative disease.

A complete radial tear extends the entire length of the meniscus from the apex to the periphery. When it affects the body of the meniscus, the meniscus is divided into an anterior and a posterior fragment. The middle third of the LM is a common location. This lesion begins at the free edge (inner margin) and extends a variable distance toward the periphery.[18]

Tiny tears can be difficult to recognize on MRI scans. Overlooked radial tears constitute a large proportion of the errors made in the interpretation of images of meniscal pathology. The main recognition feature is that they involve the free edge of the meniscus. Therefore, the inner point of the meniscal triangle is missing or blurred in 1 or more images. Radial tears of the meniscal body are best visualized on sagittal images. Disrupt the normal bowtie meniscus configuration in one or more images.

Like longitudinal tears, radial tears are usually traumatic and occur in younger, physically active patients. MM tears usually occur in the posterior horn and are more common in elderly patients. Small tears of 3 mm or less may be asymptomatic.

Tears located close to the posterior horn of the LM are associated with ACL tears.[64]They may be associated with more complex meniscal tears, such as vertical longitudinal tears or peripheral horizontal fissures. They are devastating because the circumferential fibers are broken. The meniscus is prevented from developing the necessary circular tension that normally helps to dissipate forces through the knee. It is common to see a radial tear as a component of a complex tear.

Oblique (parrot beak) tears or flap tears are a form of radial tears. They begin at the free (inner) edge, like other radial tears, but then curve toward a longitudinal orientation, similar to longitudinal meniscal tears, as the tear extends toward the periphery of the meniscus. As the tear is traced on sequential images, it approaches the lateral portion of the meniscus and remains equidistant from the lateral meniscal margin on subsequent images, as seen with longitudinal tears. Oblique tears are the most common meniscal tears.[1, 22, 40]

There is a high prevalence of radial tears in postoperative patients undergoing partial meniscectomy. Magee et al indicated a prevalence of 32% when viewing 100 postoperative MRIs. This may be related to changes in the biomechanics of knee function after partial removal of the meniscus. Stresses can be redistributed, which predisposes to radial fractures.[52]

horizontal tears

Horizontal tears are also called décolleté or fish mouth tears. They divide the meniscus tear into an upper (upper) and a lower (inferior) portion. They usually start on the undersurface of the meniscus.[18]

Although horizontal tears may show up on MRI and extend deep into the substance of the meniscus, as seen on arthroscopy, they may only extend a few millimeters into the meniscus from the point where the abnormal signal comes into contact with the surface. of the meniscus. When it extends to the periphery of the meniscus, the space between the fragments can allow joint fluid to escape to the meniscosynovial edge, where it can become trapped and form a meniscal cyst. Most are degenerative and occur in elderly patients with osteoarthritis.

several tears

Other forms of failures can be considered as combinations of longitudinal, radial and horizontal failures. Multiple tears can occur simultaneously in a meniscus, involving different portions or the same region. A single meniscal tear, containing a combination of longitudinal, radial, or horizontal split planes, is called a complex tear (see image below). A common type of complex rupture is composed of horizontal and radial components. Almost all of these tears are of degenerative origin.[1, 22]

The proton-density-weighted sagittal image of the midportion of the medial compartment shows a full-thickness horizontal tear of the posterior horn of the medial meniscus, extending from the base to the superior surface. Furthermore, the image shows the amputation of the lower apex of the posterior horn. The combination of these two tears involving the same part of the meniscus makes this injury a complex tear.

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Wearing or fibrillation of the free edge of the meniscus is seen as an area of ​​increased signal intensity at the apex of a normally shaped meniscus. If there is abnormal morphology (truncation and foreshortening), a rupture is likely. Wear can occur anywhere on the surface of the meniscus (see image below).[40]

The fat-saturated proton density-weighted coronal image shows an irregularity on the superior (femoral) surface of the body of the lateral meniscus (LM, outer arrow), indicating wear. Wear usually occurs at the apex of the meniscus. There are soft tissue densities (inner arrow) below the apex of the meniscus, indicating remnants or free fragments of the meniscus at this level. The body of the ML is unusually thick and longer than normal, indicating a discoid meniscus. For comparison, the body of the normal-sized medial meniscus is present. Discoid menisci occur approximately 5 times more often here than in the LM and are more prone to injury.

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Indirect signs of a LM tear include an abnormal or absent superior popliteal-meniscal tract and the presence of posterolateral pericapsular edema associated with a tear of the posterior horn of the LM. However, caution is needed in this context. Johnson and De Smet reported that the popliteal-superior meniscal tract was not seen in 2 of 66 patients with intact menisci. De Smet and Asinger demonstrated that 2 of 13 patients with posterior pericapsular edema did not have LM ruptures.[30, 69]

meniscal avulsion

The articular cartilage of the tibial plateau normally completely covers the posterior horn of the menisci. If there is fluid under the meniscus, especially in the posterior horn, detachment should be considered. This is called a floating meniscus. These menisci are usually intact, with no evidence of tearing. Arthroscopy requires saline infusion into the joint. This can reposition a dislocated but normal meniscus into its normal alignment. It is imperative that the orthopedist be aware of the presence of this entity to avoid the danger of misdiagnosis.

dislocated meniscus fragments

Displaced meniscal fragments occur in 9-24% of meniscal tears. Any form of meniscus tear can cause a dislodged fragment (see images below).[70]Diagnosis by MRI depends on visualization of the deficient and torn meniscus or the displaced medial meniscus component.[4]Dislocated meniscus fragments are often clinically significant injuries that require surgery due to pain and locking of the knee.[71]

Fat-saturated proton density-weighted sagittal image of the posterior compartment of the knee showing the normal insertion (arrow) of the semimembranosus tendon. The insertion site is close to the posterior horn of the medial meniscus (MM) and should not be confused with a displaced meniscal fragment.

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The fat-saturated proton density-weighted sagittal image shows a well-defined soft tissue density anterior to the posterior cruciate ligament (PCL). It is mottled and resembles the normal posterior medial horn of the medial meniscus (MM), but it is in the wrong place. This finding represents a displaced tear of the meniscus involving the posterior medial horn. The position of the meniscus is known as a double PCL because it appears that 2 of these ligaments are present.

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Fat-saturated proton density-weighted coronal image demonstrates a stained displaced meniscal fragment (inner arrows). An extensive horizontal tear is also present in the body of the medial meniscus (MM) with much hyperintensity in the periphery (outer arrows). This represents a meniscal cyst. Cyst repair without repair of the underlying meniscal tear results in cyst recurrence.

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Bucket handle tears are the most common pattern, occurring in 10% of meniscal tears. They result from vertical, longitudinal or oblique ruptures. These tears usually involve the entire meniscus, but are isolated tears of the anterior horn, posterior horn, or (more commonly) the posterior horn and body.

The most reliable signal is the visualization of the displaced fragment. Typical locations of the displaced fragment include the intercondylar notch anterior and parallel to the posterior cruciate ligament, or PCL (PCL dual sign), and ventral or horizontally juxtaposed to the anterior horn. A missing bowtie sign indicates that the meniscal body is missing. However, any segment of the meniscus can be affected.[5]

A flap tear is a short-segment horizontal meniscal tear with superior or inferior displacement of the meniscal fragment. This type is less frequent; superior displacement is more common.

Horizontal tears are displaced when the top or bottom is turned over or under the rest of the meniscus, or when it slides to the inside of the knee. These tears usually affect the MM.[70]

Displaced inferomedial ruptures of the MM are rare. When the displaced fragment extends inferiorly and medially to the tibial plateau, deep to the MCL, it may go unnoticed by the arthroscopist because the surface of the meniscus may appear intact.

Inward displacement of the free edge of an oblique tear produces a displaced parrot beak tear and may precipitate mechanical symptoms such as locking, entrapment, and yielding. Displaced fragments may prevent closed reductions of knee dislocations. The fragment can be completely separated from the rest of the meniscus to become a free fragment.[66]

A meniscal body that appears unusually small should prompt a careful search for a displaced fragment.[4]

Several signs may be present in the same patient.[72]A missing bowtie sign can be identified on sagittal images and represents a torn meniscal body segment. When present, it indicates a free meniscal fragment in 97% of patients.[4, 72]

Bucket handle tears may also occur with fragments centrally displaced towards the intercondylar notch. A double PCL sign is seen in 39% of patients and the notch fragment sign in 51% of patients. In this configuration, the fragment is adjacent to but not at the same level as the PCL on sagittal images. The fragment has a slightly more medial location. It has its origin in the MM. A disproportionate posterior horn signal is seen in 21% of patients. A larger central portion of the posterior meniscus, compared to the smaller periphery of the same posterior meniscus, indicates a displaced meniscal fragment. Meniscus tear usually originates in the anterior horn.[72]

Bucket handle tears can be seen with an anteriorly displaced meniscus fragment. The inverted meniscus sign is seen in 63% of patients. The criterion is rupture or lack of visualization of the posterior meniscus with a maximum height of the anterior horn of the affected meniscus greater than 8 mm.[61, 72]

The fat-saturated proton density-weighted sagittal image shows soft tissue (arrows) lying over the anterior horn of the medial meniscus (MM), separated from it by high signal intensity (glowing joint fluid). This finding represents a displaced meniscus fragment. The posterior horn of the MM has an abnormal shape and abnormal signal strength. This is the origin of the displaced fragment. The combined height of the displaced fragment and the anterior meniscus is greater than 8 mm. A meniscal tissue height equal to or greater than this number is a good sign of a displaced meniscal fragment.

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Pseudohypertrophy of the anterior horn of a meniscus occurs when an anterior horn of the meniscus appears abnormally large. The body of the meniscus or posterior horn is exceptionally small. This indicates that a torn body part or posterior horn has been turned anteriorly and lies behind the anterior horn. The abnormal fragment of the meniscus and the adjacent normal meniscus are separated by joint fluid, which shows increased signal intensity on T2-weighted images.

The differential diagnosis includes: Humphry's ligament, free bodies, osteophytes and fracture fragments.

Stable Versus Unstable Tears

Tear stability is determined by several factors, including length, location and integrity of the tear. Probing the meniscal tear during arthroscopy is essential to determine stability.

A stable vertical longitudinal tear occurs when the central (internal) fragment of a meniscal tear cannot be displaced more than 3 mm from the periphery of the intact meniscus. Any rupture of the meniscus with a displaced fragment is unstable.

Relatively long longitudinal breaks are unstable; its length is assessed in multiple 3- to 4-mm slices in any plane and extending through the entire thickness of the meniscus or containing fluid on T2-weighted images.

Some meniscal tears do not show a free fragment at the time of the MRI. However, it is considered an unstable tear as long as the medial edge of the tear can be displaced into a position where it can become trapped between the rotating femur and tibia when probed arthroscopically.

meniscus bruise

A meniscal tear occurs when the meniscus becomes caught between the tibia and femur, usually as a result of trauma. A blunt meniscus demonstrates an increased signal in its substance that can resemble a tear. The signal is in contact with a joint surface, but it is less discrete than the abnormal signal seen in tears: indistinct and amorphous, rather than the sharp and discrete signal seen in a tear. It is less defined than the sign seen in meniscal degeneration. There is often an adjacent bone bruise.[4, 62]

The cause of the abnormal signal may be related to blood products from a compressive lesion in the vascular area of ​​the meniscus that follows the meniscal fibers, resulting in increased signal.

meniscocapsular separation

Meniscocapsular separation (MCS) is a tear of the periphery of the meniscus at the meniscosynovial junction. The insertion of the MM is most commonly affected due to its tighter contact with the joint capsule. The most frequently affected site is the capsular insertion of the posterior horn of the MM in the tibia, in the meniscotibial (coronary) ligament. MCS is often associated with knee ligament injuries. They rarely spread to the periphery of the meniscus, although they may be an extension of a full-thickness tear.

An abnormal superior meniscal bundle is highly associated with a lateral meniscal tear, but the LM can rarely be normal in this scenario. This finding should intensify the evaluation of the LM to detect possible ruptures. The presence of posterior pericapsular edema is another indirect but nonspecific sign that, when combined with an abnormal superior popliteal meniscal tract, should alert the imaging operator to the possibility of a lateral meniscal tear.[30, 73]

LM SCM can occur in the superior or inferior popliteal-meniscal tract adjacent to the popliteus tendon. Rupture of the fascicular insertions between the popliteus tendon and the LM can cause severe instability and is associated with a tear of the posterior horn of the ML.[30]

Spontaneous healing is common due to the rich blood supply at the periphery of the meniscus.

MRI findings include an increased distance between the periphery of the meniscus and the tibia and fluid between the MCL and the medical meniscus.[74]

menisci cysts

Meniscal cysts occur more often in the medial compartment than anywhere else because meniscal tears are more common in the MM. This observation contrasts with previous reports, which indicate that they are more common in the lateral compartment. There are 3 reasons for this discrepancy.[75]:

  • First, prior to the use of MRI, reports were written indicating a preference for the lateral compartment. Magnetic resonance imaging is more sensitive than EP in detecting small meniscal cysts. There is relatively little soft adipose tissue on the lateral side of the knee compared to the medial side. This may explain why more lateral cysts have been reported, because in this region cysts are more likely to appear as a palpable mass.

  • Second, other diagnostic methods, such as knee arthrography, lead to underestimations of the true incidence of meniscal cysts. Diagnosis of meniscal cysts by arthrography requires extravasation of contrast material through the meniscal tear and the cyst, but this is not always the case.

  • Third, meniscal cysts are sometimes overlooked at arthroscopy because visualization of the posterior horn of the MM can be difficult. Therefore, cysts in the posteromedial compartment were underdiagnosed.

Medial parameniscal cysts are more symptomatic than other conditions due to their location adjacent to the MCL. The incidence is between 1-2% and 7-8%, and these cysts occur more frequently in men aged between 20 and 40 years. Medial meniscal cysts are usually adjacent to the posterior horn.[18]

LM cysts are most often located adjacent to the anterior horn or body. MM cysts are twice as common as LM cysts, and MM tears are twice as common as LM tears. Tears most often occur when horizontal tears extend to the periphery of the meniscus, allowing joint fluid to escape into the parameniscal soft tissue. Subsequently, the fluid encapsulates and becomes symptomatic due to the mass effect. Sometimes the cyst can be confined to the meniscus. This is known as an intrameniscal cyst.

Parameniscal cysts located adjacent to the anterior horn of the lateral meniscus are less likely to have an underlying meniscal tear than cysts in other meniscal locations.[76]

It is important to recognize the association between meniscal cysts and meniscal tears. If the cyst is removed without repairing the rupture, the cyst may reappear.

The MRI appearance is a fluid-filled region adjacent to a horizontal meniscal tear.

meniscal ossicles

Meniscal ossicles are rare and often confused with free intra-articular bodies.

These appear as circumscribed ossification with fatty bone marrow in the center. The fatty medulla helps distinguish this entity from free intra-articular bodies. The meniscal ossicles are usually located in the posterior horn of the MM.[77]They can be symptomatic due to mass effect or an associated meniscal tear.[78]

condrocalcinose

Chondrocalcinosis is calcification of the menisci, synovial membrane and/or articular cartilage due to the deposition of crystals of calcium pyrophosphate dihydrate, dicalcium phosphate dihydrate, calcium hydroxyapatite or any combination of these crystals. It is seen in conditions such as gout, degenerative diseases, hemochromatosis, crystal deposition disease, and hypercalcemia. These intra-articular crystals are weakly positive marginal monourate crystals under polarized light.

Prevalence ranges from 5 to 14% and increases with age. A multitude of conditions and mechanisms are involved in pathogenesis.

Intrameniscal calcium can demonstrate increased signal intensity, which can mimic a meniscal tear on T1-weighted, intermediate PD, or T2-weighted images. Correlation with plain radiographs, especially in patients with associated diseases, reduces the incidence of errors.[18]

meniscus extrusion

Meniscus tears remove the circular strain from the meniscus. Sustained compressive forces within the knee can cause dislocation or extrusion.

Meniscal extrusion has been reported in elderly patients with symptomatic osteoarthritis of the knee. In this group, meniscal extrusion preceded the development of degenerative joint disease. Looseness of soft tissue attachments, meniscal tears, and degenerative changes in the meniscus have been implicated as predisposing conditions.[79]

After meniscal extrusion, intact cartilage impaction leads to osteoarthritis.

Other factors that contribute to meniscal extrusion include tibial femoral cartilage damage and knee misalignment. Misalignment could increase the load transmitted to the meniscus, which could lead to extrusion. Varus and valgus angulations are associated with medial and lateral meniscal extrusion, respectively.[79]

Indirect signs of meniscal tear.

Indirect signs of meniscal tear include meniscal and parameniscal cysts, joint effusion, medial collateral ligament edema, and bone marrow edema. Bone marrow edema can be of 3 types: (1) perivascular edema occurs around the vascular channels and does not extend to the joint surface, (2) linear subchondral marrow edema occurs adjacent to the meniscus and probably reflects hyperemia and increased of liquid in the cortical layer. -chondral-meniscal interface and (3) non-linear subchondral medullary edema is associated with adjacent cartilage surface abnormalities. The presence of this type of edema reflects cartilage loss, chronicity and extent of the meniscus tear.[80]

ramp injuries

Ramp injuries can be characterized as tears that occur at the posterior meniscocapsular junction. In several studies, the incidence of ramp injuries ranged from 9 to 40%, with chronic ACL injury being an important risk factor.[29, 81, 82, 83, 84, 85, 86]According to Seil et al, ramp injuries can be expected in approximately 25% of patients undergoing ACL reconstruction, especially in cases of contact injuries in the presence of complete ACL rupture.[85]

References

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