Meniscal tears in MRI

 

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Introduction
Tears to the meniscus have become most common among the knee injuries. These tears occur either in combination with injuries suffered on the knee ligaments or in isolation. Symptoms associated with injuries to the meniscal include: inflammation, pain, stiffness of the knee and knee locking. Diagnosis of injuries to the menisci can be carried out through examining history of the patient, detailed physical checkup and employing magnetic resonance imaging (MRI). However, utilization of MR imaging is considered most suitable for detection of tears to the menisci because it is non-invasive and most accurate (Koenig et al., 2009).
MRI is of greatest importance to the doctors including radiologists, orthopaedic surgeons and physiotherapists in that it aids them make the right diagnostic decisions for the patients. The basic method of treatment is arthroscopic treatment whereby the tear in the meniscus is repaired or removed. Magnetic resonance imaging provides a good contrast resolution that helps in making the appropriate choice of treatment for people suffering from injuries to the ligaments, knee joints or tendons which could otherwise not be clearly visible using other methods including CT scans (Choi et al., 2010). Moreover, the use of MRI reduces the risk of exposure to x-ray radiation and offers better quality of image produced. Currently, magnetic resonance imaging is widely used as the key diagnostic method for making clinical decisions and planning for surgeries among patients showing symptoms such as derangement of the knee. MRI complements the use of physical diagnosis to enhance planning of surgeries and pre-surgical counseling of patients (Kim et al., 2006).
Magnetic resonance imaging serves a key function in accurate description of various types of tears to the meniscus and outlines the likelihood of reparability of such tears. The success of repair of menisci highly depends on the location and pattern of the tear. It has been studied that tears that are longitudinal and in minor cases oblique are reparable. On the other hand, tears that are radial, horizontal and complex are not reparable. The tears found in the periphery of vascularized meniscus are most repaired successfully (Costa et al., 2004).
Anatomy of meniscus
The menisci are fibrocartilaginous parts of the body that form a C-shape structure and are located between femoral condyles superiorly and inferior to the tibial plateau. Each knee consists of two menisci with one located inside the knee (medial) while the other located outside the knee (lateral). Blood supply to the meniscus is provided through the arteries of the medial and lateral geniculate (Kijowski et al., 2012).

Fig 1: Anatomy of the meniscus with sagittal slicing through the menisci (Mcdermott, 2006).
The menisci serve key roles in the knee joint such as allowing the knee to stabilize through allowing enough contact between the femoral chondyles and tibial plateau. It also provides for equal distribution of hoop forces across the surface of articular cartilage. In addition, the menisci help in proprioception due to the presence of nerve fibres both at the anterior and posterior thirds. The menisci are vital in ensuring proper lubrication of joints. It also absorbs shock during the process of axial loading (Kramer and Micheli, 2009).
The tissue of the human menisci constitutes 72% water, 22% collagen, 0.8% glycosaminoglycans and 0.12% DNA. The menisci are composed of collagen that make right angle crossing inside the menisci and as a result form strong bonds. Radial fibres also form part of the meniscus but are not numerous. The radial fibres hold together the circumferential fibres and thus help the menisci resist longitudinal splitting. Collagens cross link extensively using hydroxylpyridinium aldehydes. Meniscal collagen composes of ninety per cent type 1 collagen whereas type two, three and four form the remainder. This structure aids in elasticity of the meniscus and gives it the ability to tolerate high compression forces. However, the lateral meniscus is not strongly anchored in the knee which provides it with ease of mobility and therefore increases the risk of tearing (Buma et al., 2007).
The menisci occupy approximately fifty percent of the medial and seventy percent of lateral surface of tibial plateau. The medial meniscus attaches to the joint unlike the lateral meniscus. Consequently, the front third is smaller in proportion to the posterior third and is linked to the spine of the medial tibia frontal to the anterior cruciate ligament (ACL). The anterior is linked to the posterior cruciate ligament (PCL) by the posterior third. Coronary ligaments link the medial meniscus to the tibial plateau and femoral condyle forming the medial collateral ligament (MCL). The lateral meniscus is linked to the joint loosely especially at the posterolateral end which allows for attachment of capsule to the posterior third through popliteomeniscal fascicles (Park et al., 2007).
The lateral meniscus’ posterior third is linked to the posterior medial femoral condyle by the meniscofemoral ligaments (MFLs). MFL’s front limb is known as the ligament of Humphrey whereas the hind region is referred to as ligament of Wrisberg. The front ligament is found situated from the anterior thirds of menisci across to the posterior part of Hoffa fat pad and serve to bring about stabilization of the anterior third of medial meniscus.
The oblique meniscomeniscal ligament is situated at a location between the posterior third of meniscus to the anterior third of another meniscus and is described depending on its linkage to the anterior (Mcknight et al., 2010).
Function of meniscus
The menisci are located between femur (thigh) and tibia (shin) bones, acting as a filler to the joint and as a result play the role of shock absorbers for forces that are transmitted across the knee. The menisci decrease and distribute stress through a large region of contact and as a result increase the stability of the joint. 50%-90% of forces are transmitted across the knee by the menisci. Therefore, meniscal tissue loss results in high amount of stress transferred to the end of bones. This causes a decline in the load supported by the axis on the articular cartilage. For meniscal tears, it accelerates the degeneration of articular cartilage resulting in chondrosis and osteoarthritis (Sakai et al., 2006).
The meniscus also function to enhance smooth flexion and angle of rotation between the tibial articular surface and femoral. The meniscus acts as a shock absorber and allows synovial fluid distribution in the joint. In cases where there is complete damage caused to the anterior cruciate ligament, the menisci provide the knee with secondary stabilization and hinder the dislocation of the anterior tibia. The space in the joint can be decreased upto seventy percent through the process of meniscectomy resulting in increased stress on tibial plateau. The stress generated causes bone density of the trabecular in tibia’s proximal region resulting in joint degeneration (Lee et al., 2008).
Pathology of menisci
Tears to the menisci occur among a large number of human populations with majority being asymptomatic. In order to comprehend the pathology of meniscal tears, it is important to comprehend the physiological activities of menisci. Anterior horn of medial meniscus is found to cover a mean of 7.1 mm while posterior horn covers 3.9 mm with mediolateral displacement of radial cartilage by 3.6 mm (De Smet and Tuite, 2006).

Fig 2: Mean movement (mm) in each meniscus during flexion (Karachalios, 2005).

There is an increase of the anterior horn by 2.6 mm with the poster horn increasing by 2.0 mm. the lateral meniscus exhibits movement of its anterior horn by 9.5 mm with the posterior horn moving by 5.6 mm and the radial displacement is 3.7 mm. There is an increase of the anterior horn by 4.0 mm with the poster horn increasing by 2.4 mm (De Smet and Tuite, 2006).

Fig 3: Mean movement (mm) in each meniscus during flexion (Karachalios, 2005).

It is observed that there is greater movement of the lateral meniscus compared to the medial meniscus with the anterior horns also moving greater than the posterior horns. The low movement exhibited by the posterior horns of the medial meniscus could explain the high occurrence of damages to the meniscus (Choi et al., 2010).
Tears to the menisci mainly occur due to single, acute force that is directed on the knee carrying weight therefore overfilling the meniscus. Tears that are degenerative result from repeated application of sub maxial power to the menisci that already have experienced some attritional damage resulting from femoral articular plane that is irregular. Menisci are directed towards displacements carried out by femoral condyles during the process of axial rotation. They move in the opposite direction on the tibial condyles starting from neutral point. The lateral meniscus is then drawn to the tibial condyle;s anterior with the medial meniscus moving to the posterior in the process of lateral revolution. As medial rotation takes place, the medial meniscus advances with the lateral meniscus receding. As a result, the menisci are distorted along the fixed locations including their horn attachments. The menisci displacements occurring during axial revolution are inactive and are moved by the femoral condyles. Tightening of the meniscopatellar fibres causes movement of patella. The tension experienced in the fibres pulls the menisci to the anterior. The menisci can be injured during knee movement if they do not move in sync with the femoral condyles on the tibial condyles resulting in the menisci getting trapped between the two.
The major cause of injury to the menisci is acute knee trauma resulting from sporting activities. During this process, there is failure of one of the menisci to move forward and as a result in held sandwiched between the femoral condyle and the tibial condyle whereas the tibia revolves anterior-superior in relation to the femoral condyles. This results in tears that are transverse or displacement of anterior horn that eventually becomes infolded.
Fig 4: (a) Transverse meniscal injury. (b) Detachment of the anterior horn (Karachalios, 2005).

Another mechanism that produces tears on the menisci is the twisting movement experienced by the joint of the knee combining both lateral rotation and lateral displacement. The medial meniscus moves towards the middle of joint under femoral condyle convexivity. The extended joint is held in between the condyles resulting intosplitting of meniscus longitudinally, meniscus displacement from capsule and even complex tear to the meniscus (Costa et al., 2004).
Tears to the menisci are often followed by anterior cruciate ligament tears and injuries to the knee ligament. Tears to the lateral meniscus frequently occur in anterior cruciate ligament damages. On the other hand, insufficiency of anterior cruciate ligament results in tearing of the medial meniscus.

Fig 5: ACL injury with associated medial meniscal injury (Karachalios, 2005).
The occurrence frequency of tears to the menisci is less with whole medial collateral ligament tears (grade III) compared to partial tears (grade II) with the two types of tears affecting the lateral meniscus. With meniscal tearing, the damaged part becomes sandwiched between femoral and tibial condyles and does not follow normal movements (Brody, 2006).

Grading of meniscal tears
The grading system that uses the magnetic resonance imaging appearance initiated by Lotysch et al and Crues et al is currently used widely. However, it is of importance to note that not all the damages to the menisci can fit into the grading system.

Grade 1
Grade 1 signal is a high intensity signal that has irregular margin, appearing in magnetic resonance images in the form of a spot. Grade 1 signals do not extend to the articular margin. The occurrence of intrameniscal focal change is through degeneration of myxoid followed by fibrocartilage disintegration which results in tearing.

Fig 6: Grade 1 meniscal tear signal expressed by magnetic resonance image (Preda et al., 2014)
Grade 2
Grade 2 signal is linear and has high intensity with no linkage to the surface of menisci. The linkage of the capsule to the meniscus is not classified as connection to the surface. The increase in signal in the meniscus is as a result of penetration of the synovial fluid into the injury caused to the meniscus.
Fig 7: Grade 2 meniscal tear signal expressed by magnetic resonance image (Preda et al., 2014)
Grade 3
Grade 3 signals possess high intensity and are considered as the signal that relays information to the surface of the meniscus. This is due to extensive damage of the fibrocartilage. Moreover, some structures that appear normal in the joint of the knee can produce meniscal lesions which require considerations during diagnosis. This involves a large percentage of knee transverse ligament (38%), medial meniscus concavity (29%) and popliteal muscle synovial sheath (6%). Grade 3 signals exhibit linear or diffuse intensity.

Fig 8: Grade 3 meniscal tear signal expressed by magnetic resonance image (Preda et al 2014)
The appearance of menisci is indicated by the intensity of signal. Tears of the menisci or degenerative tears have high intensity of signals in magnetic resonance images appearing as bright regions in the meniscus. Grading of the menisci can be summarized according to their appearance as indicated below.
Grade Meniscus
Grade 1 Presence of high signal shadow without extending to the meniscal articular surface
Grade 2 Presence of linear high signal on image, continuation of grade 1 and moving to margin of articular surface
Grade 3 Linear or diffuse high signal moving towards surface of the joint
Table 1: Summary of grading of menisci according to appearance (Preda et al., 2014).
Classification of tears to the meniscus
Precise description of tears to the menisci is necessary because it will aid in the management of the damages. The description comprises of the location, size and orientation. The commonly used classification types of meniscal tears include: horizontal, longitudinal, displaced flap, radial, bucket handle and complex meniscal tears. The people responsible for interpreting the magnetic resonance images need to be keen with the magnetic resonance appearance of each tear type to enhance proper diagnosis and consequently provide the correct description of the tear to the treating surgeon (Jung et al., 2012).
Horizontal tears
Horizontal tears run in parallel axis to the tibial plateau dividing menisci into two segments; upper segment and lower segment.

Fig 9: Sagittal image of horizontal tear of posterior horn of medial meniscus (Fox, 2007).

Longitudinal tears
Longitudinal tears occur at right angle to the tibial plateau and they run in parallel position to the axis of the menisci dividing menisci into peripheral and central halves. Longitudinal tears are common in younger patients experiencing knee trauma.
Radial tear
Radial tears are found perpendicular to both the tibial plateau and meniscal axis. Radial tears cut through the longitudinal collagen bundles and in the process run from the free edge to the periphery.

Fig 10: Image of radial tear with the inner margin of the meniscus truncated in the middle third of lateral meniscus (Fox, 2007).
Displaced flap tear
Displaced flap tear is a description of short, horizontal tear to the meniscus with fragments displaced to either the gutters or the notch. It is important to locate the fragments before arthroscopy because they are difficult to find and if not removed can result in persistent pain and locking of the knee (Tarhan et al., 2004).
Complex tears
Complex tears extend through several planes forming separate flaps of menisci. Complex tears therefore possess two or more configurations of tears and cannot easily be classified into particular type of tear.

Fig 11: Magnetic resonance image of complex tear showing both horizontal and vertical components (Fox, 2007).

Bucket-handle tears
Bucket-handle meniscal tears occur when the inner segment of meniscus of oblique or longitudinal tears flip into intercondylar notch. This is a longitudinal tear exhibiting localized movement of fragments in the inner region (Ververidis et al., 2006).

Fig 12: Image of a bucket handle tear of the medial meniscus with part of the body located in intercondylar notch (Fox, 2007).
Diagnosis by magnetic resonance imaging
Diagnosis of tears to the menisci employs high spatial resolution in combination with signal to noise ratio that is optimized. A variety of magnetic resonance sequences have been utilized in the evaluation of meniscal tears due to their short echo time (TE), despite their variation in other factors. Short echo time has various advantages including reduced time of scan, decreased flow artifacts and decreased susceptibility, generation of high number of scan slices per sequence and increased signal to noise ratio (Welsch et al., 2012)
There are various frequently used sequences including; fast spin echo (FSE), gradient echo (GRE), and proton density (T1). Studies carried out have outline high specificity and sensitivity with magnetic resonance imaging of approximately 88% and 93% for medial and 95% and 79% for tears of lateral meniscus. Differences in specificity and sensitivity could be associated with the type of sequences used, variation among observers or size of the sample.
Low signal is experienced in normal meniscus in all the sequences of magnetic resonance images. Both front and hind horns belonging to lateral meniscus are equal whereas the anterior medial meniscus horn is smaller than the posterior horn.
The criteria for diagnosing knee meniscal tears without previous surgery are a region of abnormal signal on an image that runs through to the articular point of the meniscus. Accurate description of tears is very important for preservation and repair of meniscus. Description of the tear includes factors such as whether the tear is anterior, posterior, body or if its location is in the peripheral third of meniscus. The tear can also be described as complete, incomplete or extending from one end of an articular surface to the other. Another description for the tear could be horizontal, longitudinal, radial or complex. The length is also an important factor since it can help determine whether the tear is repairable or not. Tears can categorized according to arthroscopy as stable, unstable with lesions capable of displacement into the joint (Bergin et al., 2008).
Magnetic resonance imaging parameters
High spatial resolution MR imaging is used to increase the potential to identify meniscal tears. Improvement of spatial resolution can be done through maximizing size of matrix while retaining small viewing field. Spatial resolution and confidence of the reader are improved by higher field strengths while at the same time reducing the time of acquisition of the image.
In the recent past, three dimensional sequences (3-D) that possess isotropic resolution have been produced. These sequences are capable of producing thinner sections and also reduce the averaging of partial volumes. In addition, isotropic three dimensional imaging is capable of generating multiplanar reformations after single acquisition in any form. Two dimensional and three dimensional FSE techniques show some similarity in accuracy of detection of tears with minor difference in the low sensitivity in detecting lateral meniscus tears especially those concerned with the root during three dimensional FSE imaging (Kim et al., 2006).
Proton density weighted sequences are preferred to T2 weighted sequences for identification of tears to the meniscus. The assumption is that hydrogen nuclei bind to the macromolecules in the tear instead of being free. This in turn gives them a shorter time of relaxation T2. Therefore, interpretation of magnetic resonance images requires accurate evaluation of images obtained from all the sequences.
Magnetic resonance imaging criteria for diagnosis of meniscal tears
From late 1980s, the two key magnetic resonance criteria used in detecting tears to the menisci have not been altered. The first criterion is contact of the signal from the intrameniscus with the superior/inferior part of the menisci. The second criteria involve the disarrangement of the common appearance of the meniscus.
For proper diagnosis of a tear to the menisci using these criteria, understanding of how variations in the menisci shape and subsequent associations compare with the magnetic resonance appearance of tears to the menisci.
Methods
MRI sequences
Magnetic resonance imaging sequence consists of structured permutation of gradients pulses and RF formulated to produce data required to form an image. Data required for the formation of an MR image are obtained through a number steps. To begin with, magnetization of the tissue is excited with the help of RF impulse in presence of suitable gradient. Other required elements include phase encoding and frequency encoding which are essential for spatial localization of protons in further angles. Data is then gathered and the procedure repeated for multiple phase encoding. Parameters of MR imaging sequence are selected based on the suitability of the clinical application. The sequences to be selected include gradient echo sequence, inversion recovery sequence and spin echo (Welsch et al., 2012).
Gradient echo sequence
Gradient echo sequence is considered one among the easiest MR imaging sequence types. It is composed of excitation pulses sequences with the beats divided by repetition time (TR). Data generation is achieved at specific time following introduction of excitation pulses. It is therefore termed as echo time (TE). Echo time is lapse time linking excitation pulse midpoint and data acquisition midpoint. The figure below is an illustration of gradient echo sequence (Gandy, 2004).

Fig 13: Illustration of gradient echo sequence (Gandy, 2004).
The advantages of gradient echo sequence are that it provides for fast imaging, has low flip angle and less RF power. In gradient echo sequences, determination of flip angle is essential in obtaining T1-weighted images. Small flip angles preferably less than 900 are and short repetition sequences are generally used in gradient echo sequences.

Fig 14: choice of flip angles determining optimum contrast (Gandy, 2004).
From the above figure it can be observed that optimal angle of flip is dependent on values of T1 of the part to be imaged. Shorter T1 values lead to a larger angle of flip (Robson et al., 2003).
Spin echo sequence
Spin echo sequence exhibits some similarity with gradient echo sequence with the difference being the presence of further 1800 refocusing pulse. The refocusing pulse is located midway between the echo and excitation pulse. The magnetization vector is positioned in the transverse surface after a 900 pulse from RF.

Fig 15: Illustration of spin echo sequence (Gandy, 2004).
As a result of the dephasing of T2, there is slowing down of some spins while others increase their speed. The vectors that were initially slow are made to proceed ahead of the fast ones by application of 1800 pulse in order to cause flipping of the spin vectors. A spin echo is then formed following further delay in time (TE/2).

Fig 16: Formation of a spin echo (Gandy, 2004).
Inversion recovery sequence
Inversion recovery is known as spin echo sequence variant because it starts with an inverting pulse of 1800. The longitudinal magnetization vector is then inverted through 1800. The removal of the inverting pulse causes the magnetization vector to relax. An excitation pulse of 900 is applied after some duration from inverting pulse of 1800 called time to inversion (TI). The image produced has some contrast which is dependent on the length of time to inversion, repetition time and echo time.

Fig 17: Inversion recovery (Gandy, 2004)
The contrast of the image produced is dependent on the degree of longitudinal magnetization following the amount of delay time utilized.
Proton density weighting
The difference in proton number per unit volume in patients is crucial in formation of contrast of image t in proton density weighting. In order to achieve it, the contrast properties of T1 and T2 have to be reduced so as to allow domination of proton density weighting. Longer repetition time (TR) provides for parts to recover their longitudinal magnetization and as a result weaken T1 weighting (Harper et al., 2005).
Contrast to Noise Ratio (CNR)
Comparison between noise levels and contrast reduces the perceived contrast. Two images can possess similar contrast but differ in the levels of noise. Images that have lower contrast too noise ratio are observed to have lower contrast (Gandy, 2004).
Echo Planar Imaging (EPI)
Echo planar imaging utilizes high performance gradients for fast on and off gradient switching. EPI allows the oscillation of frequency that encodes gradient pulses and subsequent filling of gap following a solitary RF pulse (Karachalios, 2005).
3D Gradient Echo for Volumes
Three dimensional formation of images using adjacent slim slices can be accomplished using gradient echo methods. Such imaging is achieved through incorporation of phase encoding step (NZ) along the direction of slice axis (z axis) (Jung et al., 2012).
Turbo spin echo
Turbo spin echo which is commonly referred to as fast spin echo allows manipulation of normal spin echo to save on time. Fast spin echo reduces time for scanning while maintaining the SNR. This technique can better cope with magnetic fields that are poorly shimmed (McRobbie et al., 2003).

Future imaging
Further advancements are still expected in the use of imaging techniques. The optimization process of flip angle, TR and sampling of data from various parts has just begun. Low flip angle methods that are fast are likely to form a major component of ultra short echo imaging (Robson et al., 2003).
Three dimensional techniques will be of great importance towards advancement of signal to noise ratio (CNR) in addition to imaging of intricate structures including articular cartilage and joints that have slim slices in reducing the effects of partial volumes. The use of reversed radial sampling will help in advancement of signal to noise ratio (CNR) for echoes.
The combination of ultra short echo time sequences with magnetization transfer imaging will make it possible for observation of magnetization transfer effects as a result of short species of T2 by their impact on T2 species that are detectable (Robson et al., 2003).
The middle aged and older people suffering from injuries to the meniscus still form part of a challenge to the medical practitioners. This is because it is difficult to differentiate between pain caused due to tears to the menisci and the symptoms of early development of osteoarthritis of the knee. Therefore, in order to set up proper guidelines randomized control studies employing the use of sham treatment are required. In addition, arthroscopic surgery placebo effects need not be overlooked (McCauley, 2005).
There is also need to look into ways of combining the ultrashort echo time imaging and spin echoes in the future in order to decrease or completely remove susceptibility artifacts occurring due to subtraction from other gradient echoes.
In addition, 3 Tesla and other imaging systems will continue to receive expansion to allow for improvement in the future. There is need to develop new coil systems that are multichannel in order to increase the potential of parallel imaging. This will in turn increase the use of 3 Tesla and other systems of higher field (Welsch et al., 2012).
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