Normal

The menisci in the knee are C-shaped, fibrocartilaginous structures that are thick peripherally and thin centrally. A sagittal slice through the body segment should show the meniscus as an elongated rectangle or a bow tie, depending on how peripheral the sagittal slice is ( Fig. 15-3 ). The medial and the lateral menisci should have two contiguous images of the body of the meniscus if 4- or 5-mm-thick slices are obtained. Three or four sagittal images should be seen through the anterior and posterior horns of the menisci ( Fig. 15-4 ), with the posterior horn of the medial meniscus usually larger than the anterior horn. The anterior and posterior horns of the lateral meniscus are equal in size. The posterior horn of either meniscus should never be smaller than the anterior horn if it is normal.

Normal body segment of meniscus. A , Schematic shows how a sagittal slice through the body of the meniscus gives an image of the meniscus that resembles a bow tie. B , Sagittal T1W image through the body of the lateral meniscus shows the normal bow tie appearance.

Normal anterior and posterior horns of the meniscus. A , Schematic shows the appearance of a sagittal slice through the anterior and posterior horns of the meniscus. B , Sagittal T1W image through the anterior and posterior horns of the lateral meniscus.

The normal meniscus is devoid of signal on all imaging sequences, with the exception of children and young adults, who typically have some intermediate to high signal in the posterior horns near the meniscal attachment to the capsule. This signal represents normal vascularity and should not be misinterpreted as meniscus degeneration. The vascularity of the meniscus is greatest near the periphery and is almost nonexistent near the free edge. Peripheral tears can be repaired, whereas more central tears cannot.

Abnormal

Several grading schemes for abnormal meniscus signal have been developed. They are not generally in widespread use because the only abnormal signal that has any real significance is that which disrupts the articular surface of a meniscus, representing a tear. Any signal that does not disrupt an articular surface, with one exception, which is covered in detail subsequently, is intrasubstance or myxoid degeneration ( Fig. 15-5 ). Presumably, myxoid degeneration is a result of aging or wear and tear, but its cause is unknown. It is not a source of symptoms, does not always lead to meniscus tears, and is not treated clinically or surgically. Why mention it? If myxoid degeneration is prominent, it can be mentioned so that anyone else looking at the study would know we saw it, but judged the signal not to be a tear, rather than thinking it was simply overlooked. Also, if it is especially prominent, there is a possibility that it might represent a meniscal cyst. Meniscal cysts are discussed in more detail later.

Myxoid or intrasubstance degeneration. Sagittal proton density image with fat suppression through the lateral meniscus shows some high signal in the anterior and posterior horns that does not disrupt an articular margin of the meniscus. This is myxoid degeneration.

Tears

If high signal clearly disrupts an articular surface of the meniscus, it is an easy call: it is a torn meniscus. If high signal comes close to the articular surface, but does not quite reach the articular surface, it is an easy call: it is not a tear, it is intrasubstance degeneration. It is not always that clear-cut. In many cases, it is too close to call. In these situations, do the same thing radiologists do all the time—hedge. We are not being facetious. If you tell your orthopedic surgeons that when you call a tear or when you call no tear they can count on a correct diagnosis in more than 90% of the cases, they will find MRI examinations and your readings to be very helpful. You will be able to give a definitive diagnosis in about 90% of cases. You should explain that about 10% of the time you will be unable to discern definitely if the meniscus is torn or not. In those cases, the clinical examination is paramount. If the patient gets better with conservative care, it was probably not a torn meniscus. If the patient does not improve, the surgeon may decide to perform an arthroscopic procedure, in which case you would have told the surgeon where to look for a possible meniscus tear. It seems that 10% is the hedge rate for the menisci.

It has been shown that sensitivity for meniscal tears decreases considerably if there is an associated ACL tear. There are several reasons for this. First, the meniscal tears that seem to occur when the ACL is torn are located in two places: the posterior horn of the lateral meniscus and in the periphery of the menisci (medial and lateral menisci). These are not the usual locations for meniscus tears and consequently often are overlooked. Also, several pitfalls occur in the posterior horn of the lateral meniscus that can be confused with meniscal tears, all of which are mentioned later in this chapter. Suffice it to say that when the ACL is torn, a close inspection should be done for a peripheral tear or for a tear in the posterior horn of the lateral meniscus.

Oblique or Horizontal Tears.

There are many types of meniscal tears ( Box 15-1 ). The most common is an oblique or horizontal tear (these are synonymous terms; some surgeons prefer one term over the other, and others use them interchangeably) that affects the undersurface of the posterior horn of the medial meniscus ( Fig. 15-6 ). These commonly are degenerative in nature, rather than as a result of trauma.

BOX 15-1

Types of Meniscal Tears

• •
  

  Oblique or horizontal

  
  
• •
  

  Vertical

  
  
  
  
  • •
    

    Flap

    
    
  • •
    

    Bucket handle

  
  
  
  
• •
  

  Peripheral

  
  
• •
  

  Medially flipped flap tear

  
  
• •
  

  Radial (parrot beak tear)

  
  
• •
  

  Meniscocapsular separation

Meniscus tear. Sagittal image through the medial meniscus reveals an oblique tear of the posterior horn ( arrow ).

Bucket-Handle Tears.

Vertical longitudinal tears ( Fig. 15-7 ) make up the common bucket-handle tear that occurs in about 10% of meniscal tears. The normal meniscus has a body width of about 9 mm, which is seen on two consecutive sagittal images as a single slab of meniscal tissue that has a shape similar to a bow tie. When the inner edge of the meniscus displaces, a bucket-handle tear is easily diagnosed by noting only one instead of the normal two body segments present on the outermost sagittal images through the meniscus ( Fig. 15-8 ). This is called the absent bow tie sign because the body segments normally have a bow tie appearance on the sagittal images. A displaced meniscal fragment should always be found, most often in the intercondylar notch ( Fig. 15-9 ). A careful search for a fragment should be made when only one body segment is seen on the sagittal images. The displaced meniscal fragment can lie in front of the posterior cruciate ligament (PCL), which is called a double PCL sign ( Fig. 15-10 ). The displaced fragment also may flip over the anterior horn of the affected meniscus, which is called an anterior flipped meniscus sign ( Fig. 15-11 ). In any case, the absent bow tie sign is a very sensitive sign for a bucket-handle tear.

Vertical longitudinal meniscus tear. A , Schematic shows a meniscus with a vertical longitudinal tear. If the inner edge displaced, it would be called a bucket-handle tear . B , Sagittal image through a meniscus with a vertical tear ( arrow ).

Use of bow tie appearance to detect a bucket-handle tear. A , Schematic shows how two sagittal images through the body normally produce two images of the meniscus that have a bow tie appearance. B , Schematic shows how, in a bucket-handle tear with the free edge of the meniscus displaced, only one sagittal image has a bow tie appearance. C , The first sagittal image through the medial meniscus in a patient with a bucket-handle tear shows the normal bow tie appearance. D , The next sagittal image in the same patient shows anterior and posterior horns, rather than another bow tie. This appearance is characteristic of a bucket-handle tear of the meniscus.

Displaced fragment in a bucket-handle tear. Sagittal image through the intercondylar notch in a patient with a bucket-handle tear shows the displaced fragment ( arrow ).

Double posterior cruciate ligament sign. Sagittal image through the intercondylar notch in a patient with a bucket-handle tear shows the displaced fragment anterior to the posterior cruciate ligament ( arrow )—the double posterior cruciate ligament sign.

Anterior flipped meniscus. Sagittal image of the lateral meniscus in a patient with a bucket-handle tear shows the displaced fragment flipped onto the anterior horn—the anterior flipped meniscus sign.

Although our original publication on the absent bow tie sign had a 97% sensitivity, a subsequent publication of ours showed a sensitivity of around 90%. Another publication looked at all the MRI signs for a bucket-handle tear and reported only a 71% sensitivity for the absent bow tie sign. Nevertheless, it was the most reliable sign for detecting bucket-handle tears. Also, in the prospective reading of their 74 cases of bucket-handle tears, the investigators had only a 46% sensitivity (using the absent bow tie sign increased their sensitivity from 46% to 71%). We now rely on the absent bow tie sign only if we can identify the displaced meniscal fragment.

Radial or Free Edge Tears.

The absent bow tie sign also is positive in free edge tears (also called radial tears or parrot beak tears ). Free edge tears are common and are an unusual source of symptoms, unless they are large. The absent bow tie sign is useful in recognizing these tears. They are easily differentiated from bucket-handle tears because the second body segment, or bow tie, has only a small gap, rather than the large gap seen in bucket-handle tears ( Fig. 15-12 ), and no displaced fragment is present. An investigation of about 200 knees that underwent arthroscopy by a single surgeon showed a 15% incidence of radial tears. In that report we described three basic appearances of radial tears: (1) ghost, (2) cleft, and (3) truncated triangle. A ghost meniscus is seen when a radial tear has completely traversed the meniscus ( Fig. 15-13 ). The MRI slice is parallel to the tear, and partial volume averaging of the adjacent meniscal tissue creates an intermediate or gray signal. This is a severe type of meniscal tear and results in profound loss of the “hoop-stress” or springlike resistance of the meniscus. The meniscus usually extrudes off of the tibia ( Fig. 15-14 ), and osteoarthritis ensues because of the lack of cushioning or protective effect of the meniscus with axial loading.

Radial tear. A , Schematic of a free edge or radial tear shows how the sagittal image has a small gap in the expected bow tie appearance. B , The sagittal image in a meniscus with a free edge or radial tear shows a small gap in the bow tie ( arrow ).

Radial tear with a ghost meniscus. A , Schematic shows how an image parallel to a large radial tear that involves the entire width of the meniscus posterior horn gives a ghost meniscus. B , Sagittal image through a large radial tear of the posterior horn with no normal posterior horn visible.

Extruded meniscus resulting from a radial tear. Coronal image through the posterior knee shows a cleft ( large arrow ) that has caused the meniscus to extrude off of the tibia ( small arrows ).

A cleft is the most reliable sign for a radial tear and is seen when the MRI slice is perpendicular to the tear (see Fig. 15-12 ). When the MRI slice is parallel to the same tear with a cleft, it results in a truncated triangle ( Fig. 15-15 ). A radial tear usually, but not always, will have two of the signs, depending on the orientation of the tear to the imaging plane. At surgery, the free edge of a meniscus with a radial tear is treated with débridement and smoothing.

Radial tear with a truncated triangle sign. A , Schematic shows how an image parallel to a radial tear gives a truncated triangle. B , Sagittal image through a radial tear of the posterior horn shows a truncated triangle ( arrow ).

Medial Flipped Meniscus.

A meniscus tear that can be seen with MRI but can be overlooked at arthroscopy is a flap tear of the medial meniscus with the flap of meniscus flipped into the medial gutter underneath the meniscus. It can be missed at surgery if the surgeon fails to probe the medial gutter and deliver the flipped fragment. These are common tears and should be considered when the body segments look thinner than normal or have a piece of meniscus missing from the undersurface ( Fig. 15-16 ). The medial flipped fragment can be seen on the coronal images lying just below and medial to the medial meniscus.

Medial flipped meniscus. A , The first sagittal image through the body of the medial meniscus in a patient with a medially displaced flap tear shows a small fragment of meniscus inferiorly displaced ( arrow ). B , The next adjacent sagittal image reveals a defect in the undersurface of the posterior portion of the body of the meniscus ( arrow ). This defect is the donor site for the displaced flap of meniscus seen in A . C , Coronal image shows the medially displaced flap of meniscus inferior to the body of the meniscus ( arrow ). D , An artist’s depiction of a medially displaced flap tear.

Cysts

Meniscal cysts occasionally are seen that involve a meniscus without the meniscus having a tear that extends to the articular surface. With weight bearing, the fluid in the cyst can be expressed into the adjacent soft tissues, where it is called a parameniscal cyst ( Fig. 15-17 ). It is important to advise the surgeon of the presence of such a cyst because without a meniscal tear the cyst can be missed at arthroscopy. Also, many surgeons decompress a meniscal cyst that does not have a meniscal tear by an extra-articular approach, rather than via arthroscopy. Most meniscal cysts do not exhibit marked high signal with T2W images, but the parameniscal component usually is very high in signal. When the cyst is confined to the meniscus, the signal resembles intrasubstance degeneration, only it is much more pronounced ( Fig. 15-18 ). If a mass effect makes the meniscus appear swollen, it is an easy diagnosis; otherwise, it is a difficult call.

Meniscal cyst. Coronal image in a patient with a meniscal cyst of the lateral meniscus. A small parameniscal cyst ( arrow ) has resulted from the fluid in the meniscus being expressed into the adjacent soft tissues.

Meniscal cyst. Medial meniscal cyst is noted ( arrow ), which gives a slightly swollen appearance to the meniscus. The lateral meniscus has marked intrasubstance degeneration, which should not be mistaken for a meniscal cyst.

These cases are handled by suggesting a meniscal cyst versus severe intrasubstance degeneration—at least the surgeon is aware of the possibility of a meniscal cyst. A meniscal cyst often is noticed first by seeing on the most medial or lateral sagittal images through the body of the meniscus that the normal bow tie appearance has a horizontal stripe, which represents the collapsed meniscal cyst ( Fig. 15-19 ). This often is called a horizontal cleavage tear , but that term is inappropriate because a true meniscal tear is not always present.

Meniscal cyst. A , Sagittal gradient echo image through the body of the lateral meniscus shows a high signal stripe ( arrow ) bisecting the meniscus. B , Coronal image reveals a meniscal cyst with a small parameniscal cyst attached.

Discoid Meniscus

If more than two body segments are present on the sagittal images, a discoid meniscus should be considered ( Fig. 15-20 ). A discoid meniscus is most likely a congenital (although some insist it is acquired) malformation of the meniscus in which the meniscus, in the most extreme form, is disk-shaped rather than C-shaped. Most discoid menisci are not completely disk-shaped, but have a wider than normal body of the meniscus. The lateral meniscus is most commonly affected, with an incidence reported of around 3%, whereas the medial meniscus is uncommonly affected. Often a discoid meniscus is enlarged and affects the anterior or posterior horns of the meniscus asymmetrically. In such a case, the anterior or posterior horn is much larger than its counterpart. Although often encountered incidentally, discoid menisci are more prone to undergo cystic degeneration with subsequent tears than a normal meniscus. Even without cystic changes or a tear, a discoid meniscus can cause symptoms and require surgery.

Discoid lateral meniscus. A-C , Successive sagittal images through the lateral meniscus show a bow tie appearance, indicating the body segment is present on more than two images. This appearance should suggest a discoid meniscus. D , Coronal image reveals that the meniscus extends almost into the intercondylar notch ( arrow ), indicative of a discoid lateral meniscus.

A discoid meniscus that can cause symptoms without being torn is a Wrisberg variant of a discoid lateral meniscus. This is a discoid meniscus that lacks attachments to the capsule via the normal struts or fascicles and lacks attachment to the tibia via the coronary or meniscotibial ligaments at the posterior horn of the meniscus ( Fig. 15-21 ). This allows the posterior horn to sublux or fold into the joint with knee flexion, akin to a rug sliding or folding up on a slippery floor if it is not attached. In a Wrisberg variant of a discoid lateral meniscus, the only attachment to the posterior horn is the Wrisberg ligament—hence the name. It is important to inspect every discoid lateral meniscus closely for the normal struts or fascicles that surround the popliteus tendon and attach the meniscus to the capsule. If recognized, the surgeon can reattach the meniscus to the capsule and the tibia, rather than performing a meniscectomy. These are typically seen in children, and a meniscectomy at an early age leads to advanced osteoarthritis.

Wrisberg variant of a discoid lateral meniscus. Sagittal image in a young boy with a discoid lateral meniscus reveals no attachment of the meniscus to the capsule ( arrows ), which is indicative of a Wrisberg variant.

It is extremely valuable to recognize the normal bow tie appearance of the body segments in the medial and the lateral menisci as seen on sagittal images on every examination. Many of the aforementioned abnormalities can be recognized easily by noting the absence of the normal two body segments. These include the bucket-handle tear, radial tear, medially flipped flap tear, and meniscal cyst ( Box 15-2 ). A discoid meniscus exhibits more than two body segments. Our routine search pattern on sagittal images includes a close inspection of the body segments to be certain there are two, and only two, bow ties that are not deformed in any way. Exceptions to this expectation would include a large patient (Shaquille O’Neal would undoubtedly have three or four bow ties), or very thin slices (the two bow tie rule applies only to slice thicknesses of 4 or 5 mm), and, as mentioned previously, a discoid meniscus.

Exceptions to the two bow tie rule that can mimic a bucket-handle tear ( Box 15-3 ) can be seen in children or small adults in whom the menisci are small—only one bow tie is seen, but this is followed by only two or three images showing the anterior and posterior horns, rather than the usual three or four images; also, it occurs in both menisci. Simultaneous medial and lateral bucket-handle tears are rare. Another exception is in the postoperative knee where the free edge of the meniscus has been débrided. To recognize this situation, we have every patient fill out a form before the MRI examination that asks about prior surgery. The two bow tie rule can be broken if severe osteoarthritis is present or in older patients (>65 years old). These patients can wear down the free edge of the meniscus, leaving a very thin body segment that can be confused with a bucket-handle tear. None of these exceptions to the two bow tie rule would have a displaced meniscus as would a bucket-handle tear.

Pitfalls

A few pitfalls involving the menisci warrant mention.

Transverse Ligament.

An easy pitfall to recognize is the insertion of the transverse ligament on the anterior horns of the menisci. The transverse ligament runs across the anterior aspect of the knee in Hoffa’s fat pad from the anterior horn of the medial meniscus to the anterior horn of the lateral meniscus. Its function is unknown, and it is not present in every knee. At its insertion on the anterior horn of the lateral meniscus, it often has the appearance of a meniscus tear ( Fig. 15-22 ). It can reliably be differentiated from a tear by following it across the knee in Hoffa’s fat pad on sequential sagittal images. It uncommonly causes a similar pseudotear appearance on the medial meniscus.

Transverse ligament. Sagittal image through the lateral meniscus shows a transverse ligament inserting onto the anterior horn ( arrow ), creating a pseudotear.

Speckled Anterior Horn Lateral Meniscus.

The anterior horn of the lateral meniscus occasionally has a speckled appearance, which can resemble a macerated or torn anterior horn ( Fig. 15-23 ). This appearance is caused by fibers of the ACL inserting into the meniscus. It is reported to be seen in 60% of normal patients.

Speckled anterior horn lateral meniscus. Sagittal image through the lateral meniscus shows the anterior horn with a speckled appearance. This is a normal variant created by fibers of the anterior cruciate ligament inserting into the meniscus.

Meniscofemoral Ligament Insertion.

The posterior horn of the lateral meniscus has several pitfalls that mimic tears. Insertion of the meniscofemoral ligament of Humphry or Wrisberg can give the appearance of a meniscal tear ( Fig. 15-24 ). A meniscofemoral ligament is present in about 75% of knees. It originates on the medial femoral condyle and runs obliquely across the knee in the intercondylar notch ( Fig. 15-25 ), anterior (ligament of Humphry) or posterior (ligament of Wrisberg) to the PCL ( Fig. 15-26 ), and inserts into the posterior horn of the lateral meniscus. When considering a pseudotear from the insertion of one of the meniscofemoral ligaments, one needs to follow the ligament through the intercondylar notch to the PCL on sequential sagittal images. In 2% to 3% of knees, both ligaments (Humphry and Wrisberg) are present. The function of the meniscofemoral ligament has not been clearly established, and no injury to it has been described.

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