Bucket-handle tear (BHT) of the medial meniscus accompanying an ACL rupture

Findings

A sagittal T2-weighted MR image of the left knee showed a ruptured anterior cruciate ligament (ACL) and a large suprapatellar effusion (Figure 1). A more medial sagittal T1-weighted image of the left knee showed a curvilinear low-signal-intensity band paralleling the posterior cruciate ligament (PCL), called the "double PCL" sign (Figure 2). More medially located sagittal images showed a high-signal area between the anterior and posterior portions of the low-signal medial meniscus, called the "absent bow-tie" sign (Figure 3). A coronal T1weighted image showed the bucket-handle tear with the main portion of the medial meniscus seen as a truncated wedge (Figure 4).

Discussion

A BHT is a longitudinal tear of the medial or lateral knee meniscus that extends from the posterior horn toward the anterior horn. Bucket-handle tears are usually precipitated by trauma, and up to 40% are associated with an ACL injury, as seen in Figure 1.^1,2 Medial meniscus tears account for 60% to 90% of BHTs,^1,3 and there are reports of medial and lateral BHTs in the same knee.^2,4 The radiographic diagnosis of a BHT is important because BHTs comprise 10% of all meniscal tears and usually require arthroscopic or surgical repair to prevent degenerative change.^1-3 Since the plane of a BHT is parallel to standard sagittal MRI views, the diagnosis relies more on altered meniscal morphology rather than on changes in signal intensity.³ Two classic secondary MRI signs of a BHT are the double PCL sign and the absent bow-tie sign.

The double PCL sign represents the posterior portion of the bucket handle, which is still attached to the main meniscal body but slides over the medial tibial eminence to lie in front of the PCL in the intercondylar notch (Figure 2).⁵ When the ACL is intact, this sign is specific for a medial meniscal tear, because the course of the ACL prevents lateral meniscus fragments from lying in the intercondylar notch.¹ However, in a patient with a prior or simultaneous interruption of the ACL, a lateral bucket-handle tear can also produce the double PCL sign.⁴ In the series that first described this sign, Weiss et al⁶ reported 100% sensitivity and specificity, with 7 cases and 47 controls. Subsequent studies have shown that the double PCL sign is very specific (>98%), but that it is only approximately 33% sensitive (27% to 44%).^1-4,7 Other low-signal structures that can be present in the intercondylar notch and must be differentiated from the double PCL sign include the ligament of Humphrey, a torn ACL, fracture fragments, osteophytes, or loose bodies.⁵

In sagittal MRI of the knee, a bow-tie appearance is created by the anterior and posterior horns of the meniscus, which are seen as touching low-signal-intensity triangles. With 4- to 5-mm sagittal slices, a normal meniscus, which is 9- to 12-mm wide, should look like a bow tie on at least 2 consecutive images.⁴ When a BHT forces the central portion of the meniscus into the intercondylar notch, the knot of the bow tie disappears. Thus, the absent bow-tie sign, as defined by Helms et al,⁴ is the continuity of the anterior and posterior meniscal portions in none or only one sagittal image. In the images presented in this case, the anterior and posterior horns of the medial meniscus are discontinuous and the bow-tie sign is absent (Figure 3). The absent bow-tie sign has been shown to have a sensitivity of approximately 90% (71% to 98%) and a specificity of approximately 63%.^2-4,7 The presence of this sign can also be due to congenitally hypoplastic or ring-shaped menisci, small menisci (in children and petite adults), partial meniscectomy, or arthritic degeneration.^2,4 A discoid meniscus, which has a thickened body portion, can have the normal bow-tie appearance even when a BHT is present.³

Other MRI signs of a BHT include a truncated meniscus (Figure 4) and a fragment in the intercondylar notch in coronal views.³ A flipped posterior meniscus and double (or tall) anterior horn are other signs of a BHT on sagittal images.^2,3 When the various radiographic signs are combined, the overall sensitivity and specificity of MRI for a BHT exceeds 90%.^2,3

1. 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.
2. Aydingoz U, Firat AK, Atay OA, Doral MN. MR imaging of meniscal bucket-handle tears: A review of signs and their relation to arthroscopic classification. Eur Radiol. 2003;13:618-625.
3. Dorsay TA, Helms CA. Bucket-handle meniscal tears of the knee: Sensitivity and specificity of MRI signs. Skeletal Radiol.2003; 32:266-272.
4. Helms CA, Laorr A, Cannon WD Jr. The absent bow tie sign in bucket-handle tears of the menisci in the knee. AJR Am J Roentgenol.1998;170:57-61.
5. Singson RD, Feldman F, Staron R, Kiernan H. MR imaging of displaced bucket-handle tear of the medial meniscus. AJR AM J Roentgenol.1991;156:121-124.
6. Weiss KL, Morehouse HT, Levy IM. 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.
7. Watt AJ, Halliday T, Raby N. The value of the absent bow tie sign in MRI of bucket-handle tears. Clin Radiol.2000;55:622-626.