Diagnostic pitfalls mimicking meniscal tear and post-operative meniscal changes.

Dr/ ABD ALLAH NAZEER. MD.

NORMAL ANATOMIC STRUCTURES Normal anatomic structures that lie in close proximity to the meniscus and show similar MR signal intensity to the meniscus are common mimickers of meniscal pathology. The anterior transverse ligament attaches the anterior margin of lateral meniscus to anterior horn of medial meniscus.

The anterior transverse ligament is clearly demonstrated on coronal PD-W fat sat (a) and T1-W images (b) and on axial PD-W fat sat (c) and T1-W images (d) as it courses from the anterior margin of lateral meniscus to the anterior horn of medial meniscus.

On sagittal images, a linear band of increased signal is present between the anterior horn of lateral meniscus and the anterior transverse ligament, occasionally simulating an oblique meniscal tear.

A linear band of increased signal intensity (arrowhead) is noticed on a sagittal PD-W fat sat image (j), between the anterior horn of lateral meniscus and the anterior transverse ligament (arrow), simulating an oblique meniscal tear. The course of the ligament is clearly demonstrated on adjacent sagittal PD-W fat sat images (e j). Osgood-Schlatter disease is also present with patellar tendon thickening, deep infrapatellar bursitis and tibial tubercle fragmentation.

By the same mechanism, increased linear MR signal in the medial aspect of the posterior horn of lateral meniscus at the attachment site of the meniscofemoral ligament can mimic the appearance of a vertical meniscal tear. The meniscofemoral ligament originates from the lateral meniscus posteriorly and inserts on the medial aspect of medial femoral condyle. The ligament is composed of 2 separate branches, the ligament of Humphry and the ligament of Wrisberg.

A linear band of increased signal intensity (arrowheads) is noticed on sagittal PD W fat sat images (e, f), between the posterior horn of lateral meniscus and the ligament of Humphry, simulating a vertical meniscal tear. The anterior meniscofemoral ligament (arrows) is clearly demonstrated on adjacent sagittal PD-W fat sat images (a-f).

The anterior meniscofemoral ligament (arrows) is depicted on adjacent coronal PD-W fat sat images (g, h) as it courses from the medial aspect of medial femoral condyle to the medial aspect of the posterior horn of lateral meniscus. A horizontal tear of the posterior horn of medial meniscus (arrowheads) is also noticed.

The ligament of Humphry (arrows) is demonstrated on adjacent axial PD-W fat sat images (i-l) as it courses from the medial aspect of medial femoral

condyle to the medial aspect of the posterior horn of lateral meniscus.

A linear band of increased signal intensity (arrowhead) is noticed on a sagittal PD-W fat sat image (e), between the posterior horn of lateral meniscus and the ligament of Wrisberg, simulating a vertical meniscal tear. The posterior meniscofemoral ligament (arrows) is clearly demonstrated on adjacent sagittal PD-W fat sat images (a e).

The ligament of Wrisberg (arrows) is depicted on adjacent coronal PD-W fat sat images (f, g) as it courses from the medial aspect of medial femoral condyle to the medial aspect of the posterior horn of lateral meniscus. A horizontal tear of the posterior horn of medial meniscus (arrowheads) is also noticed.

The medial and lateral oblique menisco-meniscal ligaments have a reported incidence of 1% to 4% and are an uncommon source of diagnostic difficulty. These ligaments extend obliquely from the anterior horn of one meniscus to the posterior horn of the opposite meniscus.

Sagittal T1-W (a) and PD-W fat sat images (b) through the intercondylar notch at the level of PCL demonstrate the medial oblique menisco-meniscal ligament as a thin linear structure of low signal intensity (arrows) mimicking a displaced meniscal fragment. A Baker's cyst is also noticed (arrowheads). Coronal PD-W fat sat image (c) shows the ligament as it passes through the intercondylar notch.

Bucket-handle meniscal tear of medial meniscus. The thin linear structure of low signal intensity (arrow) which courses under the PCL (arrowheads) on a sagittal T2-W image (a) represents a displaced meniscal fragment ("double PCL sign"). The meniscal fragment (arrow) is also demonstrated as it passes through the intercondylar notch under the PCL (arrowheads) on a coronal PD-W fat sat image (b).

The bursa of the popliteus tendon, which lies close to the posterolateral meniscus, may also be mistaken for a meniscal tear. Fluid within the bursa appears as high-signal intensity on T2-weighted MR images and gives the appearance of a vertical or slightly diagonal tear in the posterior horn of lateral meniscus.

Fluid within the bursa (arrowhead) appears as high-signal intensity on a sagittal PD-W fat sat image (e) and gives the appearance of a vertical or slightly diagonal tear in the posterior horn of lateral meniscus. The popliteus tendon and bursa (arrows) are clearly demonstrated on adjacent sagittal PD-W fat sat images (a-e).

Fluid within the bursa (arrowhead) appears as high-signal intensity on a coronal PD-W fat sat image (f) and resembles a vertical or slightly diagonal tear in the posterior horn of lateral meniscus. The popliteus tendon and bursa (arrows) are demonstrated on adjacent coronal PD-W fat sat images (f, g).

The lateral inferior genicular artery arises from the popliteal artery at the level of the tibiofemoral joint and courses laterally to the anterior aspect of the knee where it and other arteries compose the genicular anastomosis. Unlike the superior genicular arteries and the inferior medial genicular artery, the lateral inferior genicular artery is closely applied to the meniscus as it wraps around the knee, lying in a periarticular fat pad between the meniscus and lateral collateral ligament (LCL).

Adjacent sagittal T1-W images through the lateral meniscus (a-d) demonstrate the course of the lateral inferior genicular artery (arrows). A narrow separation (arrowhead) between the anterior horn of lateral

meniscus and the lateral inferior genicular artery (c) can sometimes be mistaken for a meniscal tear.

Meniscal flounce is an uncommon variant that can simulate meniscal pathology. It occurs in the absence of a tear and is a single symmetric fold along the free edge of the meniscus. It is a normal finding that is said to be present with ligamentous laxity, although it is not necessarily indicative of a tear in the ligament. The appearance is like that of a carpet that has a wrinkled edge and, in fact, presumably has a similar cause-that is, sliding of the tibia on the femur because of ligamentous laxity with resultant folding or buckling of the inner edge of the meniscus. It has no known significance.

Meniscal flounce of medial meniscus (arrows) which results in a slight wavy S-shaped appearance along the free edge of the

meniscus on adjacent sagittal PD-W fat sat images (a, b).

A speckled appearance of the anterior horn of lateral meniscus is a frequent finding that has been explained by fibers of the anterior cruciate ligament inserting into the meniscus. It can be seen on one or two of the most medial sagittal images. The appearance can be mistaken for a torn lateral meniscus.

A speckled appearance (white arrows) of the anterior horn of lateral meniscus on adjacent sagittal PD-W fat sat images (a, b), which is caused by the insertion of the fibers of anterior cruciate ligament (black arrow) into the meniscus (c). The finding can easily be mistaken for a torn lateral meniscus.

A discoid lateral meniscus is a normal variant seen in about 3% of knees. An uncommon variant of a discoid lateral meniscus is a Wrisberg's variant, in which the posterior horn is not attached to the capsule and is, therefore, mobile enough to move freely and sublux into the joint, causing pain and, occasionally, locking. The MR imaging appearance is a discoid lateral meniscus with no posterior horn attachment or a free-floating posterior horn. Unlike the incidental discoid meniscus, which should be asymptomatic unless torn, a Wrisberg's variant can be a source of pain and require surgery. It is most commonly found in children, although it can be seen in patients at any age.

Peripheral tear-meniscocapsular detachment (arrows) of the posterior horn of medial meniscus on sagittal (a) and coronal PD-W fat sat images (b). Bone contusions of the posterior tibial epiphysis are also noticed.

The most significant characteristic in the appearance of pediatric meniscus is the presence of intrasubstance signal that represents perforating vessels within the meniscus. This signal has been described in 82% of menisci in children and is considered to be a normal finding that should not be misinterpreted as mucinous or myxoid degeneration like in adults. The MR criteria for establishing the diagnosis of a meniscal tear are the same in children and adults. The 2 most important criteria are intrameniscal signal that extends to the surface and abnormal meniscal morphology .

The presence of intrasubstance signal in the posterior horns of medial (arrows) and lateral (arrowheads) meniscus represents perforating vessels within the menisci. This signal is considered a normal finding and should not be misinterpreted as mucinous or myxoid degeneration like in adults.

MRI ARTIFACTS MRI artifacts created by the pulse sequences used can result in image degradation and errors in diagnosis. The magic angle phenomenon occurs on short TE images in fibers that are orientated 55° relative to the static magnetic field. This MRI artifact is a cause of increased signal in the medial segment of the posterior horn of lateral meniscus. Imaging the knee joint in slight abduction can alter the orientation of the fibers in the posterolateral meniscus and eliminate this artifact. Partial volume averaging is a type of artifact that occurs when 3-dimensional volumetric data is used to create a 2-dimensional image. The attachment of the capsule produces a concavity at the outer margin of the meniscus that is filled with periarticular fat and neurovascular structures. When the signal from a normal dark meniscus is averaged with the bright, fatty tissue adjacent to the outer concave margin of the meniscus, high-signal intensity linear artifacts that may simulate a tear are created through the periphery of the meniscus on sagittal images.

Partial volume averaging produces linear artifacts mimicking horizontal tears (arrows) in the anterior and posterior horns and in the body of lateral meniscus on adjacent sagittal PD-W fat sat (a-c) and T1-W images (d-f).

Motion artifact can be particularly problematic in MRI because of the long image acquisition times. If a structure moves to different positions during image acquisition, the image appears blurred or double exposed. Intrameniscal signal resembling a tear may result. If motion artifact is present on an MRI scan and a meniscal tear is suspected by imaging criteria, the sequence should be repeated without motion.

Motion artifact mimicking a tear (arrow) in medial meniscus on a coronal PD-W fat sat image (a). The meniscus appears normal on a coronal T1-W image at the same level (b), which is obtained without motion.

Wrisberg rip and pseudotear of posterior horn of lateral meniscus.

Truncation error is a type of MRI artifact that occurs at high-contrast boundaries, such as between the articular cartilage and the menisci. Truncation artifact is a result of inherent errors in the Fourier transformation method of image reconstruction. It appears as a series of alternating parallel bands of bright and dark signal. When a line of high-signal intensity projects over a low-signal meniscus, a pseudo tear is created.

Wrisberg rip for a longitudinal tear in the posterior horn of the lateral meniscus that extends laterally from the Wrisberg ligament attachment. They feel that the tear is the result of traction from the ligament of Wrisberg in cases of anterior cruciate ligament tears.

A meniscal contusion occurs when the meniscus gets trapped between the tibia and the femur during a traumatic event-usually involving ACL tears. Increased signal in the periphery of the meniscus can resemble a tear; however, the signal intensity of a contusion is indistinct and amorphous rather than sharp and discrete. An adjacent bone contusion should alert one to the possible presence of a contusion rather than a meniscal tear .

Meniscal contusion with amorphous and indistinct signal in the periphery of the posterior horn of medial meniscus (arrows) on adjacent sagittal (a-c) and coronal PD-W fat sat images (d-f) after an acute traumatic event with bone contusions, complete ACL and partial medial collateral ligament (MCL) tear. Follow-up MRI 5 months after initial imaging revealed resolution of abnormal meniscal signal.

Grade 2C signal in the posterior horn of medial meniscus (arrows) on adjacent sagittal PD-W (a, b) and coronal PD-W fat sat images (c, d), confirmed as intact meniscus on arthroscopy.

Chondrocalcinosis is defined as a radiographically visible calcification in the cartilage of a joint. It can occur in the hyaline articular cartilage lining the articular surface or in the fibrocartilage of a meniscus. Although it can occur from many types of calcium crystals, the most commonly seen is from calcium pyrophosphate dihydrate crystal deposition in pseudogout, which is also known as calcium pyrophosphate dihydrate deposition disease. When MR imaging is performed on a meniscus with chondrocalcinosis, the T1-weighted or PD-weighted sequence shows high signal, which can be mistaken for a meniscal tear. Differentiating a meniscal tear from the high signal of chondrocalcinosis can be difficult, if not impossible. Most meniscal tears have a more linear appearance than the globular high signal seen in chondrocalcinosis. Comparison with a conventional radiograph of the knee will help avoid this pitfall. However, chondrocalcinosis can also obscure a tear and result in a false-negative report.

Chondrocalcinosis in medial meniscus. Radiograph of the knee (a) reveals meniscal calcifications in the posterior horn of the meniscus (arrow). Coronal PD-W (b) and sagittal PD-W images (c, d) show marked high signal throughout the posterior horn of medial meniscus (arrows) that resemble a tear. No meniscal tear at the arthrography.

Chondrocalcinosis in lateral meniscus. Anteroposterior knee radiograph (a) shows meniscal calcifications in lateral meniscus (arrow). Sagittal PD-W image (b) shows high signal intensity both in the anterior and posterior horn of lateral meniscus with extension to inferior articular surface (arrowheads), interpreted as meniscal tears. No meniscal tear was found on arthroscopy.

Meniscal ossification is a rare finding of unknown origin with a prevalence of 0,15% in MR examinations. Meniscal ossicles are frequently asymptomatic and discovered incidentally. They mimic intra-articular loose bodies and lead to inappropriate surgical treatment. Imaging is required to confirm the diagnosis and assess meniscal integrity. The typical appearance is that of an intrameniscal lesion which shows central high signal on T1-weighted images, due to normal fat bone marrow, surrounded by low signal on both T1-weighted and T2-weighted images related to the cortical rim. The high signal of the inner portion, which is the same as viable medullary bone, allows differentiation from loose bodies, which may present internal low signal. However, loose bodies, if ossified, may present with an internal high signal.

Ossification in the discoid lateral meniscus with signal intensities equal to bone.

Anteroposterior view of the left knee showing corticated bone densities projected in the lateral aspect of the knee.

Gas within the knee joint may represent a vacuum phenomenon or iatrogenic placement during arthrography or joint aspiration. The low magnetic susceptibility of gas produces a discrete signal void. If the gas collects adjacent to the meniscus, it can simulate a meniscal tear, abnormal meniscal volume, or a displaced meniscal fragment .

Gas within the knee joint (arrows) produces a discrete signal void on adjacent coronal T2 GRE (a, b) and T1-W images (c, d), as well as on sagittal T2-W images (e g), that simulates a radial meniscal tear in medial meniscus (arrowheads).

Post-operative changes. In recent years, there has been increasing demand for postoperative evaluation of the meniscus because of the increased number of patients who undergo meniscal surgery. The most common surgical procedures include partial meniscectomy and meniscal repair. Recognition of the normal postoperative MRI appearance of the meniscus and of abnormalities is essential for the accurate assessment of symptomatic patients. After partial meniscectomy, meniscal morphology may differ from that of a nonoperative meniscus with its postoperative appearance depending on the amount and anatomical location of meniscal tissue resected and the morphology of the previous meniscal tear. On MR imaging, such postsurgical variations in meniscal shape typically include diminution in the overall size of the meniscus or meniscal horns, blunting of the meniscal apical margin, or variable degrees of meniscal truncation.

Diagram of medial meniscus (a) illustrating the two types of partial meniscectomy: circumferential (b)-where the posterior horn and anchor, and therefore the meniscal "hoop" has been preserved and segmental (c)-where the posterior attachment has been resected and the load transmitting function has been sacrificed.

Normal appearance of a partially resected meniscus. Adjacent coronal PD-W fat sat images from anterior to posterior (a-c) demonstrate a small blunted body of medial meniscus (arrow) following circumferential partial meniscectomy.

Normal appearance of a partially resected meniscus. Adjacent coronal PD-W fat sat images from anterior to posterior (a-c) demonstrate a resected posterior horn of medial meniscus (arrows) following segmental partial meniscectomy.

Diminutive morphology of the posterior horn of medial meniscus on adjacent coronal PD-W fat sat (a, b) and sagittal PD-W images (c, d) following prior meniscectomy with intrameniscal increased signal intensity extending to the inferior meniscal surface (arrows), which was interpreted as new tear. Second-look arthroscopic evaluation revealed an intact meniscus.

Intrameniscal increased signal intensity (arrows) on adjacent coronal PD-W fat sat (a-c) and sagittal PD-W images (d, e) in the resected posterior horn of medial meniscus after partial meniscectomy was interpreted as new tear. Second-look arthroscopy revealed normal signal postoperative changes.

PD and T2W images. Prior partial meniscectomy and suture repair. At arthroscopy, there was no tear.

Patient had a suture repair. RIGHT: On new exam, there is a new tear (yellow arrow). It is not possible to tell if the old tear has healed.

MR-arthrogram: In the new tear the signal is as bright as in the synovial fluid (yellow arrows). In the healed tear the signal is not as bright.

PD and MR-arthrogram after suture repair for meniscal tear: healed tear.

Sagittal T1-W fat-sat MR arthrography image following intra-articular injection of gadolinium demonstrates contrast extending into a subtle cleft (arrow) in the posterior horn of medial meniscus corresponding to a peripheral vertical tear, which was previously missed on conventional MRI.

Meniscocapsular tear and meniscocapsular recess.

MR image 1 year after anterior cruciate ligament reconstruction and partial meniscectomy of medial meniscus shows diminutive body of medial meniscus with vertical high-signal-intensity cleft through body (arrowhead), consistent with recurrent or residual tear confirmed surgically.

Sagittal T2-weighted MR image obtained without intraarticular contrast material shows retear of repaired meniscus after meniscal repair. Fluid can be seen in the tear (arrow).

Retear of previously repaired meniscus on sagittal fat-suppressed T1-weighted MR arthrograms

Normal intrameniscal signal evolution in the site of surgery, with very good final result.

Normal healing sequence. Typical worsening in the first scans - intrameniscal signal after surgery is worse then before surgery. No healing disturbance.

Normal healing sequence. Typical worsening in the first scans - intrameniscal signal after surgery is worse then before surgery. Any of these scans without sequentional imaging would be difficult to differentiate from disturbed healing.

Abnormal healing of degenerated meniscus.

Vascular variants, such as popliteal artery entrapment, cystic adventitial disease, or deep vein thrombosis, may cause symptoms around the knee and can potentially be overlooked on MRI of the knee unless specifically reviewed.

Cystic adventitial disease of popliteal artery. Sagittal fat-suppressed T2-weighted MR image of knee after acute knee injury shows incidental extensive cystic changes in relation to popliteal artery (arrowheads).

Thank You.