AJR 2002; 179:1115-1122
© American Roentgen Ray Society
The Meniscus: Recent Advances in MR Imaging of the Knee
Clyde A. Helms1
1 Department of Radiology, Duke University Medical Center, Box 3808, Durham NC
27710.
Received February 18, 2002;
accepted after revision April 3, 2002.
Address correspondence to C. A. Helms.
Introduction
MR imaging of the menisci of the knee has proven useful for more than 10
years, with current sensitivity and specificity for meniscal tears ranging
from 90% to 95% in most reports
[1,2,3,4,5].
In the past few years, advances in imaging the menisci, including appropriate
imaging techniques, and in imaging of bucket-handle tears, meniscal cysts,
displaced meniscal flap tears, radial tears, peripheral tears, meniscal
contusion, the effect of chondrocalcinosis, and some normal variants have been
reported. This review will discuss these recently described developments.
Imaging Techniques
A short TE is necessary for imaging linear tears in the menisci
[3,
6]. This can include
conventional spin-echo T1-weighted, proton density-weighted, or gradient-echo
sequences. Whether fast spin-echo proton density—weighted sequences are
adequate for imaging the menisci is controversial, with several reports in
favor of their use [7,
8] and others against it
[9,10,11,12].
The sensitivity described in all the reports for fast spin-echo imaging is
approximately 80% in every study (Table
1), whereas conventional spin-echo, as mentioned previously, has a
sensitivity of at least 90%. Therefore, the results for all the published
sensitivities for fast spin-echo imaging of the menisci are the same; only the
conclusions are at odds. If the sensitivity for meniscal tears drops from more
than 90% to 80% and all that is gained is 2-3 min in imaging time, the use of
fast spin-echo sequences for imaging the menisci is hardly justified (Fig.
1A,1B).
It is not known precisely why fast spin-echo proton density—weighted
images perform less well than conventional sequences, but it is presumed to be
because of the increased blurring that is inherent in fast spin-echo
sequences.
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Fig. 1A. —Conventional spin-echo versus fast spin-echo imaging for
meniscal tear in 33-year-old man. Sagittal proton density—weighted MR
image (TR/TE, 2000/20) obtained through medial meniscus shows oblique tear
(arrow) of posterior horn, which was also seen on two adjacent
images.
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Fig. 1B. —Conventional spin-echo versus fast spin-echo imaging for
meniscal tear in 33-year-old man. Sagittal fast spin-echo MR image (3000/18)
obtained through medial meniscus does not show meniscal tear.
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Although the use of fat suppression in evaluating the menisci has not been
formally discussed in the literature, many radiologists apply fat suppression
to the meniscus-sensitive sequence to rid the image of the distracting high
signal emanating from the fatty marrow in the bones and the fat in the soft
tissues. Fat suppression also increases the dynamic range of signal in the
menisci, making meniscal tears more conspicuous. To our knowledge, no evidence
indicates that the use of fat suppression increases the accuracy for meniscal
tears, but this use is gaining widespread acceptance. Fat suppression is also
useful on fast spin-echo sequences in helping to distinguish fat from fluid,
both of which have marked high signal in fast spin-echo sequences.
Much interest in MR imaging of the knee has centered on cartilage in the
past few years because surgeons have developed new techniques for dealing with
abnormal cartilage. Investigators have reported favorable results with
cartilage-sensitive sequences ranging from three-dimensional volume spoiled
gradient-echo with fat suppression sequences
[13,
14] to fast spin-echo with
[15] and without
[16] fat suppression
sequences. Gradient-echo and fast inversion recovery imaging have been touted
as accurate for cartilage [17,
18]. Thorough inspection of
knee cartilage, which covers the entire articular portion of the knee, can be
time-consuming, regardless of the sequence used.
[19].
Bucket-Handle Tears
A bucket-handle tear is a vertical longitudinal tear that comprises
approximately 10% of all meniscal tears
[20,
21]. Bucket-handle tears
typically require arthroscopic surgery to either repair or remove the meniscus
because these tears often cause knee locking. It has been reported that the
sensitivity for a bucket-handle tear is lower than that for most other
meniscal tears, with some reports showing a sensitivity as low as 44-64%
[20,
22].
Using the absent bow-tie sign, I found that the sensitivity for a
bucket-handle tear increases dramatically, although the specificity can be
less unless a few caveats are considered
[21]. The absent bow-tie sign
relies on the fact that the normal meniscus is 9-12 mm in width, and two
consecutive sagittal images of the body of the meniscus should be seen (Fig.
2A,2B).
The meniscus in these images often has the appearance of a bow tie, hence the
name of the sign. If a bucket-handle tear is present, part of the free edge of
the meniscus will be removed, and the second or inner body segment (bow tie)
will be absent Although this feature will typically be the first clue that a
bucket-handle tear is present, confirmation will almost always be found in the
form of a displaced meniscal fragment that is visualized elsewhere on the
study (Fig.
3A,3B,3C,3D).
Without a displaced fragment, one should consider one of the following causes
for the absent bow tie sign: a small meniscus in a child or a small adult with
the width of the meniscus less than normal. A small meniscus is usually easily
recognized because only two or three additional images of the meniscus are
present (a normal-sized meniscus would have four additional images), and this
finding would affect both the medial and the lateral menisci (a bucket-handle
tear of both menisci is rare). Other possible causes are a postoperative state
in which part of the free edge of the meniscus has been surgically removed or
the free edge is being worn away and leaving a thin body of the meniscus in an
older patient or one with severe osteoarthritis.
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Fig. 2A. —Schematic of absent bow-tie sign. A = first sagittal slice, B
= next adjacent sagittal slice. Drawing illustrates how two consecutive MR
images should traverse body of meniscus, giving bow-tie appearance on sagittal
images.
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Fig. 2B. —Schematic of absent bow-tie sign. A = first sagittal slice, B
= next adjacent sagittal slice. When bucket-handle tear is present and part of
free edge of meniscus is displaced, second sagittal image fails to have
bow-tie appearance.
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Fig. 3C. —Bucket-handle tear in 25-year-old man. Coronal fast spin-echo
T2-weighted MR image (3000/60) with fat suppression reveals truncated medial
meniscus (straight arrow) with displaced fragment seen in
intercondylar notch (curved arrow).
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Fig. 3D. —Bucket-handle tear in 25-year-old man. Sagittal fast
spin-echo T2-weighted MR image (3000/60) obtained through intercondylar notch
shows displaced fragment (arrows) beneath posterior cruciate
ligament.
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Although the initial report of the absent bow-tie sign had a 97%
sensitivity in 32 patients with bucket-handle tears
[21], other reports have not
shown it to be as useful. In one report, the absent bow-tie sign had a
sensitivity of only 77% [22].
However, in that study, without the absent bow-tie sign, the sensitivity for
bucket-handle tears was only 44%, and the absent bow-tie sign was the most
useful sign of several tested.
Meniscal Cysts
Several recent reports have shown that meniscal cysts occur in 4-6% of
knees studied on MR imaging
[23,
24]. These cysts occur about
twice as often in the medial meniscus and may or may not be confined to the
meniscus. One report found an associated meniscal tear in more than 90% of the
cysts [24], whereas another
found a tear in less than half of the cysts
[23]. The presence of a tear
is an important determination for many surgeons because it may alter their
surgical approach [25]. Some
surgeons percutaneously address a meniscal cyst without an associated meniscal
tear. They avoid violating the articular surface of the meniscus by
percutaneously decompressing it via its attachment to the capsule
peripherally. The cyst is then débrided and packed with a fibrin clot.
Other surgeons approach a meniscal cyst arthroscopically regardless of whether
a meniscal tear is present. If no tear is present, the surgeon will unroof the
cyst, débride it, pack it, and then suture the meniscus closed. This
procedure violates the articular surface of the meniscus. To our knowledge, no
controlled studies comparing one technique with the other have been
performed.
The etiology of a meniscal cyst is unknown. Nevertheless, the cysts should
be recognized on MR imaging because they are sources of symptoms, regardless
of whether they are associated with meniscal tears.
The diagnosis of a meniscal cyst on MR imaging is made when high signal is
seen in a swollen meniscus (Fig.
4A,4B).
For unknown reasons, the high signal is not typically as bright as fluid on
T2-weighted sequences; this finding leads many radiologists to incorrectly
assume that no cyst is present. When the fluid is expressed into the adjacent
soft tissues as a parameniscal cyst, the fluid outside the meniscus typically
becomes high in signal on T2-weighted sequences
(Fig. 5), and the swollen
meniscus decompresses to a more normal-appearing shape. If the meniscus tears
and the fluid leaks out into the joint, the swollen meniscus also returns to a
normal shape.
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Fig. 4A. —Meniscal cyst in 40-year-old woman. Sagittal proton
density—weighted MR image (TR/TE, 2000/20) with fat suppression obtained
through medial meniscus shows swollen anterior horn (arrow) with high
signal within, indicative of meniscal cyst.
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Fig. 4B. —Meniscal cyst in 40-year-old woman. Fast spin-echo
T2-weighted MR image (3000/20) with fat suppression shows parameniscal
component (arrow), which is similar to joint fluid in signal, whereas
intrameniscal signal remains intermediate in intensity.
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Fig. 5. —Meniscal cyst with parameniscal component in 27-year-old man.
Coronal fast spinecho T2-weighted MR image (TR/TE, 3000/20) with fat
suppression shows meniscal cyst (solid arrow) with intermediate
signal throughout medial meniscus with adjacent parameniscal component
(open arrows), which is similar to joint fluid in signal
intensity.
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Displaced Flap Tears
A type of meniscal tear that is easily overlooked by both the radiologist
and the surgeon during arthroscopy is an inferiorly displaced medial meniscus
flap tear [26]. This type of
tear is best seen on MR imaging by noting on the first sagittal image through
the body of the meniscus that the usual rectangular slab of the meniscus is
absent. Instead, a piece of meniscus can be seen inferior to the body segment
(Fig.
6A,6B,6C).
The adjacent body segment seen on the next sagittal image is also distorted
with the inferior surface missing a piece of meniscal tissue. The coronal
images confirm that a piece of meniscus is inferiorly displaced just medial to
the medial meniscus. This displaced meniscal flap tear can be easily
overlooked by the surgeon if the medial gutter is not vigorously probed to
deliver the fragment of meniscus.
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Fig. 6A. —Inferiorly displaced medial meniscus flap tear in 20-year-old
man. Most medial sagittal fast spin-echo MR T2-weighted image (TR/TE, 3000/60)
obtained through medial meniscus shows low-signal mass (arrow)
inferior to normal rectangular body segment.
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Fig. 6B. —Inferiorly displaced medial meniscus flap tear in 20-year-old
man. Adjacent sagittal MR image shows apparent defect in central portion of
bow tie (arrow), caused by inferior flap of meniscus displacing into
medial gutter and allowing central portion of body of meniscus to
collapse.
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Fig. 6C. —Inferiorly displaced medial meniscus flap tear in 20-year-old
man. Coronal fast spin-echo T2-weighted MR image (3000/60) shows displaced
flap (arrow) of meniscus inferior to medial meniscus in medial
gutter.
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Radial Tears
Radial tears, also called free-edge tears, involve the inner edge of the
meniscus. A radial tear with a curved inner portion is called a parrot-beak
tear. When a radial tear is large, it can diminish the ability of the meniscus
to protect the cartilage during weight-bearing by interrupting the so-called
hoop stress function of the meniscus. In these cases, it has been reported
that accelerated degenerative disease occurs
[27]. To our knowledge, little
has been published concerning the MR imaging appearance of radial tears of the
meniscus; however, one report showed an incidence of 15% in a series of nearly
200 consecutive arthroscopies performed by a single surgeon
[28]. The MR imaging of radial
tears in this report had four characteristic appearances depending on the
location of the tear and the imaging plane: ghost, cleft, truncated triangle,
and marching cleft (Figs.
7,8,9,10).
Using these four signs, the authors were able to correctly identify radial
tears in 89% of the cases. Most of their radial tears occurred in the
posterior horns of the menisci (79%), with only 5% seen in the anterior
horns.
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Fig. 7. —Radial tear in 25-year-old man, with diagram showing ghost
meniscus. Sagittal gradient-echo MR image obtained through complete radial
tear of posterior horn shows entire posterior horn as intermediate in signal
(arrow) compared with that of anterior horn.
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Fig. 8. —Radial tear in 32-year-old woman, with diagram showing
meniscal cleft. Sagittal T2-weighted MR image with fat suppression obtained
through body of meniscus with radial tear shows small gap in expected
"bow tie" (arrow).
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Fig. 9. —Radial tear in 45-year-old woman, with diagram showing
truncated triangle. Coronal T2-weighted MR image with fat suppression obtained
through radial tear in mid body shows tip of meniscus as intermediate in
signal (arrow) compared with that of remainder of meniscus.
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Fig. 10. —Radial tear in 26-year-old man, with diagram showing marching
cleft. Sagittal T2-weighted MR images with fat suppression obtained through
radial tear that is aligned obliquely to plane of imaging reveal cleft
(arrows) in each segment of meniscus that is slightly more posterior
than that in each prior image. A, B, and C = sagittal slices through
meniscus.
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Peripheral Tears
Peripheral tears occur in the portion of the meniscus near its attachment
to the capsule (Fig. 11), an
area that typically has more vascularity than other parts of the meniscus
[29]. This location makes
these meniscal tears better candidates for meniscal repair than nonperipheral
tears [30,
31]. It has been reported that
peripheral tears are often overlooked in the presence of an anterior cruciate
ligament tear [2]. Peripheral
tears and meniscocapsular injuries are reported to occur frequently when a
contrecoup bone contusion is present
[32]. This contusion of the
posterior lip of the medial tibial plateau occurs after an anterior cruciate
ligament tear, in which the tibia internally rotates on the femur, striking
the anterior or central lateral femoral condyle against the posterior lateral
tibial plateau and then rotates in the opposite direction with the medial
femoral condyle striking the medial tibial plateau.
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Fig. 11. —Peripheral meniscal tear in 24-year-old man. Sagittal proton
density—weighted MR image (TR/TE, 2000/20) with fat suppression shows
meniscal tear (arrow) in periphery of posterior horn.
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Meniscal Contusion
A meniscal contusion occurs when the meniscus gets trapped between the
tibia and the femur during a traumatic event—usually involving an
anterior cruciate ligament tear
[33]. 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
(Fig.
12A,12B,12C).
An adjacent bone contusion should alert one to the possible presence of a
contusion rather than a meniscal tear.
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Fig. 12A. —Meniscal contusion in 39-year-old woman. Sagittal proton
density—weighted MR image (TR/TE, 2000/20) with fat suppression obtained
through medial meniscus in patient with torn anterior cruciate ligament shows
increased signal in posterior horn (solid arrow), which abuts
articular surface of meniscus but is somewhat illdefined. Adjacent bone
contusion is seen in tibia (curved arrow). At surgery no meniscal
tear was found.
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Chondrocalcinosis
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 proton density—weighted sequences show high signal, which
can be mistaken for a meniscal tear (Fig.
13A,13B)
[34]. One report showed a
specificity of 71% in the lateral meniscus and 72% in the medial meniscus in
37 subjects with chondrocalcinosis compared with a specificity of 100% in the
medial and lateral menisci of 34 controls. Twelve of the 37 patients with
chondrocalcinosis had MR imaging findings of meniscal tears that were not
found at arthroscopy [35]. In
this report, the sensitivity was also decreased, presumably by the high signal
of the chondrocalcinosis obscuring a meniscal tear.
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Fig. 13B. —Chondrocalcinosis mimicking meniscal tear in 63-year-old man.
Sagittal proton density—weighted MR image (TR/TE, 2000/20) obtained
through lateral meniscus shows marked increased signal throughout meniscus
(arrows).
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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. Reviewing
radiographs of the knee can forewarn the radiologist of aberrant meniscal
signal if chondrocalcinosis is present; however, chondrocalcinosis can also
obscure a tear and result in a false-negative report.
Normal Variants
Radiologists should be familiar with several normal variants of the menisci
that have been described in recent years. These include the meniscal flounce,
speckled anterior horn of the lateral meniscus, and Wrisberg's variant of a
discoid lateral meniscus, although none of these variants are indicative of a
ligamentous tear.
A meniscal flounce is a wavy or folded appearance of the inner edge of the
medial meniscus (Fig. 14). 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. Yu et al.
[36] reported finding a
meniscal flounce in 0.2% (6/3159) of their patients. 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.
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Fig. 14. —Meniscal flounce in 21-year-old man. Sagittal proton
density—weighted MR image (TR/TE, 2000/20) obtained through medial
meniscus shows wavy appearance. Medial meniscus was otherwise normal.
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A speckled appearance of the anterior horn of the lateral meniscus is a
frequent finding that has been explained by fibers of the anterior cruciate
ligament inserting into the meniscus
[37]. It can be seen on one or
two of the most medial sagittal images
(Fig. 15). The appearance can
be mistaken for a torn lateral meniscus.
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Fig. 15. —Speckled anterior horn of lateral meniscus in 25-year-old
woman. Sagittal proton density—weighted MR image (TR/TE, 2000/20) with
fat suppression obtained through lateral meniscus shows speckled appearance of
anterior horn (arrow) due to some anterior cruciate ligament fibers
interdigitating with meniscus.
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A discoid lateral meniscus is a normal variant seen in about 3% of knees
[38,
39]. 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
[40]. The MR imaging
appearance is a discoid lateral meniscus with no posterior horn attachment or
a free-floating posterior horn (Fig.
16A,16B).
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.
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Fig. 16A. —Wrisberg's variant of discoid lateral meniscus in 16-year-old
boy. Fast spin-echo T2-weighted MR image (TR/TE, 4000/75) with fat suppression
obtained through discoid lateral meniscus shows posterior horn unattached to
capsule (arrow).
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Fig. 16B. —Wrisberg's variant of discoid lateral meniscus in 16-year-old
boy. Adjacent sagittal MR image shows no attachment of posterior horn to
capsule (arrow). At surgery, posterior horn was found to be
unattached to capsule and freely mobile.
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In conclusion, orthopedic surgeons routinely rely on MR imaging of the knee
before surgery because of its high accuracy in showing meniscal, ligamentous,
and cartilaginous abnormalities. Although MR imaging of the knee is well
established as a useful clinical tool, techniques continue to evolve and
knowledge continues to improve. This review, hopefully, will serve
radiologists who are trying to keep abreast of the improvements and to
understand abnormalities when imaging the menisci of the knee.
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Introduction
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