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The Absent Bow Tie Sign1
APPEARANCE
The normal meniscus of the knee resembles a bow tie
on sagittal magnetic resonance (MR) images (Fig 1). The absent
bow tie sign is present when fewer than two distinct bow tie
segments of the meniscus are present on sequential sagittal
MR images(Fig 2).
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Figure 1a. Normal bow tie segment of meniscus.
Two sequential, sagittal, intermediate-weighted MR
images (1,200/28 [repetition time msec/echo time msec]),
starting (a) medially and moving (b) laterally, show
characteristic bow tie segments of the medial meniscus
(arrow).
Figure 1b. Normal bow tie segment of meniscus.
Two sequential, sagittal, intermediate-weighted MR
images (1,200/28 [repetition time msec/echo time msec]),
starting (a) medially and moving (b) laterally, show
characteristic bow tie segments of the medial meniscus
(arrow).
Figure 2a. Absent bow tie sign of bucket-handle
tear. Sagittal, intermediate-weighted MR images (1,200/28).
(a) First of three sequential images shows the characteristic
bow tie segment of the medial meniscus (arrow). (b)
Second and (c) third images reveal absence of the
distinct bow tie segment of the medial meniscus (arrow).
(d) More lateral image of the medial meniscus in the
same patient as in a-c reveals the displaced handle
fragment (arrow).Note the posterior cruciate ligament
(arrowhead)
Figure 2b. Absent bow tie sign of bucket-handle
tear. Sagittal, intermediate-weighted MR images (1,200/28).
(a) First of three sequential images shows the characteristic
bow tie segment of the medial meniscus (arrow). (b)
Second and (c) third images reveal absence of the
distinct bow tie segment of the medial meniscus (arrow).
(d) More lateral image of the medial meniscus in the
same patient as in a-c reveals the displaced handle
fragment (arrow).Note the posterior cruciate ligament
(arrowhead).
Figure 2c. Absent bow tie sign of bucket-handle
tear. Sagittal, intermediate-weighted MR images (1,200/28).
(a) First of three sequential images shows the characteristic
bow tie segment of the medial meniscus (arrow). (b)
Second and (c) third images reveal absence of the
distinct bow tie segment of the medial meniscus (arrow).
(d) More lateral image of the medial meniscus in the
same patient as in a-c reveals the displaced handle
fragment (arrow).Note the posterior cruciate ligament
(arrowhead).
Figure 2d. Absent bow tie sign of bucket-handle
tear. Sagittal, intermediate-weighted MR images (1,200/28).
(a) First of three sequential images shows the characteristic
bow tie segment of the medial meniscus (arrow). (b)
Second and (c) third images reveal absence of the
distinct bow tie segment of the medial meniscus (arrow).
(d) More lateral image of the medial meniscus in the
same patient as in a-c reveals the displaced handle
fragment (arrow).Note the posterior cruciate ligament
(arrowhead).
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EXPLANATION
The body of the normal meniscus averages 9–12 mm in
width. Sequential, sagittal, 4–5-mm-thick images should
reveal the body of the meniscus as distinct bow tie segments
on two consecutive images .
The absent bow tie sign is an indication of a possible bucket-handle
tear . The term "bucket-handle
tear" relates to its appearance, in which the inner,
displaced meniscal fragment resembles a handle and the peripheral,
nondisplaced part resembles a bucket .
Because of the displaced inner fragment, fewer than two
bow tie segments will be seen on sequential, sagittal MR
images.
DISCUSSION
MR imaging is very sensitive for identifying meniscal abnormalities.
There currently is excellent correlation between MR imaging
and surgical observations in the diagnosis of meniscal tears
. The MR sensitivity
for distinguishing a fragment from a bucket-handle tear
is 84% when the tear involves the entire lateral or medial
meniscus . Despite
extensive meniscal involvement, bucket-handle tears are
often missed at MR imaging because they are oriented parallel
to the sagittal plane .
The bucket-handle tear is one of three basic displaced
meniscal injuries. As reported by various investigators
, the bucket-handle
tear is the most frequent pattern of displaced meniscal
injury; it is present in up to 80% of the displaced meniscal
injuries. The other two are the flap tear with displacement
and free fragment displacement.
The bucket-handle tear consists of a longitudinal tear
of the meniscus, most commonly the medial meniscus. A longitudinal
tear runs parallel to the main axis of the meniscus .
The bucket-handle tear usually involves the entire meniscus,
but isolated involvement of the anterior horn, posterior
horn, or body segment of the meniscus may occur.
Longitudinal tears with displaced fragments are best identified
at sagittal MR imaging, with the displaced fragment often
lying within the intercondylar notch or parallel to, in
front of, or below the posterior cruciate ligament .
The bucket-handle tear is commonly seen in young people
with a history of locking, extension block, or joint instability
due to displacement of the medial fragment toward
the intercondylar notch. Identification of displaced meniscal
fragments is important since arthroscopy is required to
resect or reattach the fragment.
Normal anterior and posterior horns of the menisci are
visualized at MR imaging as triangular areas of homogeneously
low signal intensity. The two triangles with a connecting
body segment produce a normal bow tie appearance.
In addition to the absent bow tie sign, several other signs
may be present at sagittal MR imaging for the identification
of bucket-handle tears. The flipped meniscus sign consists
of a meniscal tear in which the displaced meniscal fragment
migrates anteriorly to sit directly on the anterior horn,
which produces an abnormally tall (>6-mm) anterior horn
.
The double anterior horn sign has a meniscal fragment displaced
into the anterior meniscal compartment. Thus, two anterior
horns are visualized in the sagittal plane, with the anterior
triangle representing the normal anterior horn and the posterior
triangle representing the displaced bucket-handle fragment
. The double posterior
cruciate ligament sign consists of the displaced bucket-handle
fragment that lies inferior to the posterior cruciate ligament
. Coronal MR images
can also help identify a bucket-handle tear by showing a
truncated, deformed meniscus and by demonstrating the displaced
handle fragment.
Misinterpretations of bucket-handle tears can easily occur
at MR imaging. Examples of conditions that can mask a bucket-handle
tear include a nondisplaced longitudinal tear and a discoid
meniscus. A discoid meniscus has meniscal tissue that partially
or completely covers the tibial plateau, which allows for
greater than two bow tie segments to be present, even with
a bucket-handle tear. The lateral meniscus is more commonly
involved with this type of bucket-handle tear .
Mimics of the displaced meniscal fragments include loose
bodies and normal anatomy of the knee. These normal structures
include the transverse meniscal ligament, the accessory
meniscal ligaments, and the popliteus tendon. There are also a few instances in which
the number of bow tie meniscal segments can be fewer than
the normal two, which simulates a bucket-handle tear. Examples
include prior meniscectomy, in which the free edge has been
resected; osteoarthritis with an irregular, free edge; age-related
wearing of the free edge in older patients; and a small
meniscus in which a narrow body segment is present .
Footnotes
A trainee (resident or fellow) wishing to submit a manuscript
for Signs in Imaging should first write to the Editor for
approval of the sign to be prepared, to avoid duplicate
preparation of the same sign.
References
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http://radiology.rsnajnls.org/cgi/content/full/215/1/263#REF3
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