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  Knee - Meniscal pathology by David Rubin and Robin Smithuis   Radiology department of the Washington University School of Medicine, St. Louis, USA and the Rijnland hospital in Leiderdorp, the Netherlands Normal Meniscal Anatomy Medial meniscus: The posterior horn is always larger than the anterior horn. Medial meniscus Both horns are triangular in shape and have very sharp points. The posterior horn is always larger than the anterior horn (figure). If this is not the case than the shape is abnormal, which can be a sign of a meniscal tear or a partial meniscectomy. LEFT: normal medial meniscal root immediately anterior to the posterior cruciate ligament. RIGHT: missing posterior root due to meniscal root tear. The posterior root is immediately anterior to the posterior cruciate ligament. If it is missing on the sagittal images, then there is a meniscal root tear (figure). The anterior horn has an insertion on the tibia and a second portion that travels from medial to lateral to connect to the anterior horn of the lateral meniscus ( intermeniscal or transverse ligament). Lateral meniscus. Both horns are about the same size. Lateral meniscus On sagittal images the posterior horn is higher in position than the anterior horn. Both horns are about the same size. Lateral meniscus: posterior horn and posterior meniscal root. The lateral meniscus posteriorly comes up higher over the tibial spine to insert near the posterior cruciate ligament. This upward position of the posterior horn may be the reason for the higher signal intensity of the posterior horn in all planes due to magic angle effect. Meniscal tears    Criteria for tears The two most important criteria for meniscal tears are an abnormal shape of the meniscus and high signal intensityon PD-images unequivocally contacting the surface . High signal intensity not unequivocally contacting surface. Small black line on inferior margin of the meniscus. At arthroscopy the meniscus was normal. It is a misunderstanding that menisci should be homogeneously low in signal intensity on proton-density images. The meniscus does not have to be black. Only when the high signal unequivocally reaches the surface of the meniscus you can make the diagnosis of a tear. If there is doubt whether the high signal touches the surface, look at all the adjacent images and if there still is doubt than do not diagnose a tear. If you have a questionmark in your head, say meniscus is normal. (figure) Basic shapes: Longitudinal, Horizontal and Radial . Nomenclature of Meniscal Tears Shapes . There are 3 basic shapes of meniscal tears: longitudinal, horizontal and radial . Complex tears are a combination of these basic shapes. Bucket handle, Horizontal Flap tear and Parrot beak. Displaced Tears  Bucket-handle tear = displaced longitudinal tear. Flap tear = displaced horizontal tear. Parrot beak = displaced radial tear. Longitudinal, horizontal and radial tears Longitudinal tears  Longitudinal tears parallel the long axis of the meniscus dividing the meniscus in an inner and outer part. So the distance between the tear and the outer margin of the meniscus is always the same (figure). The tear never touches the inner margin.   Three sagittal images of a longitudinal tear Longitudinal tears follow the collagen bundles that parallel the contour of the meniscus. If a longitudinal tear has other components (horizontal or radial) than it is a complex tear violating the collagen bundles. This requires a higher energy trauma. LEFT: abnormal shape of posterior horn. a piece is missing. RIGHT: displaced fragment in the intercondylar fossa. Longitudinal tear (2)   Bucket handle tear  is a displaced longitudinal LEFT: meniscus is abnormal in shape and there is a displaced fragment. RIGHT: Three structures in intercondylar fossa: post cruciate lig (1), ant cruciate lig (2) and displaced fragment (3). On coronal images bucket handle tears are easier to recognize. Normally there are only two structures in the intercondylar fossa: the anterior and posterior cruciate ligament. Any other structure in the intercondylar fossa is abnormal and a displaced meniscal fragment is the most likely possibility. Flipped meniscus: posterior horn is missing because it is flipped over and located on top of the anterior horn. Longitudinal tear (3)   Flipped meniscus  is a form of bucket handle tear. There is a capsular detachment or peripheral tear of the meniscus, usually the posterior horn. The posterior horn flippes over onto the anterior horn.   Horizontal tear with a meniscal cyst Horizontal tears  Horizontal tears divide the meniscus in a top and bottom part (pitta bread). If horizontal tears go all the way from the apex to the outer margin of the meniscus they may result in the formation of a meniscal cyst. The synovial fluid runs through the horizontal tear and accumulates periferally to the meniscus. The connection with the joint space is often lost, so they will not fill with contrast on MR-arthrography. The synovial fluid is absorbed and is replaced by a gelatinous substance. There are 3 criteria for the diagnosis of a meniscal cyst: 1. Horizontal tear. 2. Fluid accumulation bright on T2. 3. Flat lining against the outside margin of the meniscus. The diagnosis of a meniscal cyst is important to the surgeon because it takes one operation on the outside of the knee to remove the cyst and another operation on the inside for the meniscus. Radial tears  Radial tears are perpendicular to the long axis of the meniscus. They violate the collagen bundles that parallel the long axis of the meniscus. These are high energy tears. They start at the inner margin and go either partial or all the way through the meniscus dividing the meniscus into a front and a back piece. Radial tears are difficult to recognize. You have to combine the findings on sagittal and coronal images to make the diagnosis. LEFT: triangle missing the tip. RIGHT: disrupted bow tie. The following combination of findings is diagnostic: In one plane: triangle missing the tip and in the other plane: a disrupted bow tie. Disrupted bow tie indicating a small radial tear. Small radial tears are difficult to diagnose. Sometimes the only sign is a disrupted bow tie.

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Jul 23, 2017
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