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Radiology 2151

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					Signs in Imaging
Kristin A. Lieberman, MD

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). The bucket-handle tear is one of three basic displaced meniscal injuries. As reported by various investigators (4,6), 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 (7). 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 (4). 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 (8). 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 (6). The double posterior cruciate ligament sign consists of the displaced bucket-handle fragment that lies inferior to the posterior cruciate ligament (9). Coronal MR images can also help identify a bucket-handle tear by showing a trun-

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 (1). The absent bow tie sign is an indication of a possible bucket-handle tear (2). 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 (2). 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 (3). The MR sensitivity for distinguishing a fragment from a bucket-handle tear is 84% when the tear involves the entire lateral or medial meniscus (4). Despite extensive meniscal involvement, bucket-handle tears are often missed at MR imaging because they are oriented parallel to the sagittal plane (5).

Index terms: Knee, ligaments, menisci, and cartilage, 4524.4852, 4525.4852 Knee, MR, 45.1214 Signs in Imaging Radiology 2000; 215:263–265
1

From the Department of Radiology, SUNY Health Science CenterSyracuse, 750 E Adams St, Syracuse, NY 13210. Received June 3, 1998; revision requested July 1; final revision received September 23; accepted January 19, 1999. Address reprint requests to the author (e-mail: liebermk@vax.cs.hscsyr.edu). RSNA, 2000

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.

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Figure 1. Normal bow tie segment of meniscus. Two sequential, sagittal, intermediateweighted 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).

a.

b.

Figure 2. Absent bow tie sign of buckethandle 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).

a.

b.

c.
264 • Radiology • April 2000

d.
Lieberman

cated, 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 buckethandle 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 (7). 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 (7). 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 buckethandle 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 (2).

References 1. Silverman J, Mink J, Deutsch A. Discoid menisci of the knee: MR imaging appearance. Radiology 1989; 173:351–354. 2. Helms CA, Laorr A, Cannon WD. The absent bow tie sign in bucket-handle tears of the menisci in the knee. AJR Am J Roentgenol 1998; 170:57–61. 3. Crues JI, Mink J, Levy T, Lotysch M, Stoller D. Meniscal tears of the knee: accuracy of MR imaging. Radiology 1987; 164:445–448. 4. Wright DH, De Smet AA, Norris M. Bucket-handle tears of the medial and lateral menisci of the knee: value of MR imaging in detecting displaced fragments. AJR Am J Roentgenol 1995; 165:621– 625. 5. Herman L, Beltran J. Pitfalls in MR imaging of the knee. Radiology 1988; 167:775–781. 6. Ruff C, Weingardt JP, Russ PD, Kilcoyne RF. MR imaging patterns of displaced meniscus injuries of the knee. AJR Am J Roentgenol 1998; 170:63–67. 7. Rubin DA. MR imaging of the knee menisci. Radiol Clin North Am 1997; 35:21–43. 8. Haramati N, Staron RB, Rubin S, Shreck EH, Feldman F, Kierman H. The flipped meniscus sign. Skeletal Radiol 1993; 22:273–277. 9. Weiss K, Morehouse H, Levy I. Sagittal MR images of the knee: a low-signal band parallel to the posterior cruciate ligament caused by a displaced bucket-handle tear. AJR Am J Roentgenol 1991; 156:117–120.

Volume 215 • Number 1

The Absent Bow Tie Sign • 265


				
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