BUCKET-HANDLE TEARS OF THE KNEE MENISCI: PITFALL IN INTERPRETATION AT MR IMAGING Gianvincenzo Sparacia, MD - Antonio Lo Casto, MD - Angelo Iovane, MD Leone Filosto*, MD - Marcello De Maria, MD - Roberto Lagalla, MD Institute of Radiology "P. Cignolini" - University of Palermo, Italy *Day Hospital "La Maddalena" - Palermo, Italy Presented at the 83rd Scientific Assembly and Annual Meeting of the Radiological Society of North America (RSNA) November 30 - December 5, 1997 - McCormick Place, Chicago, Illinois INTRODUCTION A meniscal bucket-handle tear is a longitudinal tear of a meniscus with an attached fragment displaced away from the meniscus that can be difficult to diagnose. Bucket-handle tears of the menisci usually involves the medial meniscus and begin with vertical or oblique orientation that arises in the posterior horn and propagates longitudinally and anteriorly. The central portion of the torn meniscus usually undergoes to varying degrees of displacement into the intercondylar notch due to compressive forces. The term bucket-handle is derived from appearance of the tear, in which the inserted peripheral portion of the meniscus resembling the bucket and the inner displaced fragment constitutes its handle. This lesion is frequently seen in young adults with a history of locking or extension block due to displacement of the central fragment into the intercondylar notch. Singson et al. and Weiss et al. described the displaced-bucket-handle fragment within the intercondylar notch, anteriorly to the posterior cruciate ligament (PCL), as the "double posterior cruciate ligament" or "third cruciate ligament", whereas Haramati et al. described the appearance of bucket-handle tear in which the meniscal fragment, rather than migrating toward the intercondylar notch, moves anteriorly to lie directly on the anterior horn of the ipsilateral meniscus: the "flipped meniscus" sign. This exhibit presents these findings and gives some clues in interpretation of normal and abnormal imaging findings that can mimic MR appearance of bucket-handle tear.   MATERIALS AND METHODS The MR examinations of 29 patients affected by traumatic lesions of the knee with arthroscopically proved bucket-handle tears (25 of the medial and 4 of the lateral meniscus) were retrospectively reviewed. A torn anterior cruciate ligament was associated to bucket-handle tear of the medial meniscus in 10 patients. Diagnosis on MR images relied on the recognition of the traditional MR findings for bucket-handle tears: a) the "double PCL" sign on sagittal images - the inner fragment of the meniscus is located anteriorly to the PCL simulating two ligaments; b) the "flipped meniscus" sign - the inner fragment of the meniscus is flipped over the anterior horn of the ipsilateral meniscus, so the anterior horn appears large (8 mm or more); c) the presence of a fragment of meniscus in the intercondylar notch. MR examinations were performed with a 0.5 T superconducting magnet, surface coil, spin-echo (SE) T1-weighted (TR 600/TE 20) and gradient-echo (GRE) T2*-weighted (TR 500/TE 30/Flip angle 30°) sequences. Four-millimeters sagittal and coronal scans were obtained with an intersection gap of 1 mm. Four signal averages were used. The image matrix was 256 x 256. The field of view was 18 cm.   PATTERN RECOGNITION FOR BUCKET-HANDLE TEARS The double PCL sign and the presence of a fragment of meniscus in the intercondylar notch The double PCL sign for medial meniscus is seen on sagittal images (this sign was present in 9 patients in our series): the inner fragment of the medial meniscus is flipped anteriorly to the PCL (Fig. 1a). On coronal images the peripheral portion of the meniscus has a truncated triangle shape whereas the central fragment of the meniscus displaced in the intercondylar notch is seen as a low intensity band between the PCL and the tibial plateau (this sign was present in 24 patients in our series) (Fig. 1b,2). The double PCL sign is not seen for lateral meniscus tears probably because the more laterally located ACL acts as a barrier to lateral meniscal fragments as proposed by Wright et al.. The flipped meniscus sign This sign was present in 8 patients in our series. The inner fragment of the meniscus, rather than migrating toward the intercondylar notch, moves anteriorly to lie directly on the anterior horn, so on sagittal images the anterior horn appears larger than normal (8 mm or more) and the posterior horn of the ipsilateral meniscus shows a frank tear or is not visualized (Fig. 3). Often a band of high signal intensity is seen differentiating the native anterior horn from the meniscal fragment (Fig. 3c,d). The appearance of a flipped meniscus anteriorly has to be differentiated from a simple tear of the anterior horn or from the origin of the transverse geniculate ligament.   PITFALL IN INTERPRETATION The normal ligament of Humphry The ligament of Humphry is a normal accessory meniscofemoral ligament that is closely related to the anterior margin of the PCL (it extends from the posterior horn of the lateral meniscus to the lateral aspect of the medial femoral condyle). On sagittal MR images it appears as a small, rounded, low-signal structure, much smaller than the bucket-handle fragment, and its close relationship to the PCL is another clue to differentiate among them (Fig. 4). Its prevalence on MR has been reported as 34% and it must not be mistaken also for meniscal fragment or osteochondral loose bodies. Pedunculated fragment of the anterior horn of the meniscus Posterior displacement of a fragment of the anterior horn of the meniscus with a ÒpedunculatedÓ aspect can mimic a bucket-handle tear. The meniscal fragment, while still attached to the meniscus posteriorly, moves into the intercondylar notch. Its connection with the posterior horn of the meniscus allows a confident diagnosis of this lesion on sagittal images (Fig. 5). Loose bodies Loose bodies of osteochondral origin or deriving from osteophytes and fracture fragments, must not be mistaken for a fragment of meniscus in the intercondylar notch. Their true nature should be assessed by careful analysis of sequential multiplanar MR images (Fig. 6) as well as routine radiographs. The torn anterior cruciate ligament A torn ACL, when detached from its femoral insertion, can appear as a low-signal band within the intercondylar notch lying on the tibial plateau or located anteriorly and paralleling to the PCL (Fig. 7). It thus can mimic the "double posterior cruciate ligament" sign (Fig. 1a). However the ACL usually has higher signal intensity compared with low-signal meniscal fragment.   CONCLUSION Bucket-handle tear of the meniscus may be a major source of clinical complaint due to the displaced meniscal fragment that causes locking knee and therefore is important to identify. MR imaging plays an important role in the detection of displaced fragments, most notably for bucket-handle tears involving the entire meniscus. A displaced fragment may be seen as a fragment in the notch, as the double posterior cruciate ligament sign, or as the flipped meniscus sign. It is important to remember that some bucket-handle tears may displace in the interim between MR imaging and arthroscopic surgery. Furthermore, bucket-handle tears have a high association with ACL tears (10 out of 29 reported cases in this study, 34%) and therefore when an ACL tear is encountered a bucket-handle tear should be searched very carefully. Knowledge of these traditional MR findings for bucket-handle tears as well as other normal and abnormal imaging findings such as the normal ligament of Humphry, the presence of loose bodies, and the torn anterior cruciate ligament that can mimic MR appearance of bucket-handle tear, will help radiologist in pattern recognition when encountered with this condition.   CAPTIONS Fig. 1 - The double PCL sign. A) Sagittal and B) coronal T1-weighted MR images. Bucket-handle tear of the medial meniscus. A) "double posterior cruciate ligament" or "third cruciate ligament" aspect of the displaced mesial fragment (arrow), paralleling beneath the PCL (arrowhead). B) Mesial fragment of the medial meniscus (arrow), localized between the PCL (arrowhead) and the medial tibial eminence (*). Fig. 2 - The presence of a fragment of meniscus in the intercondylar notch. Coronal A) T2*-weighted and B) T1-weighted weighted MR images. Bucket-handle tear of the medial meniscus. Truncated triangular shape of the peripheral fragment of the meniscus (arrowhead). Mesial fragment displaced into the intercondylar notch (short arrow), localized between the PCL (long arrow) and the medial tibial eminence (*). Fig. 3 - The flipped meniscus sign. A-B) Sagittal T1-weighted MR images. Bucket-handle tear of the lateral meniscus. Abnormally large anterior horn (arrow) lateral meniscus due to superimposed meniscal fragment displaced or "flipped" from the torn posterior horn of the ipsilateral meniscus. Note the band of increased signal intensity within the anterior horn in B differentiating the native anterior horn from the meniscal fragment. This should not be mistaken for a simple tear of the anterior horn of the lateral meniscus. Fig. 4 - The normal ligament of Humphry. A) Coronal T2*-weighted and B) sagittal T1-weighted MR images. Accessory meniscofemoral ligament of Humphry is seen as a thin irregular low-signal band (arrow) apposed to the PCL (arrowhead). It must not be mistaken for meniscal fragment or osteochondral loose bodies. Fig. 5 - Pedunculated fragment of the anterior horn of the meniscus. A) Coronal and B) sagittal T1-weighted MR images. A) Pedunculated fragment of the anterior horn of the medial meniscus (arrowhead). Its connection with the posterior horn of the meniscus is clearly evident on the sagittal image (arrowhead). Fig. 6 - Loose bodies. A) Coronal T1-weighted and B) sagittal T2*-weighted MR images. Loose body (arrowhead) simulating on coronal plane a fragment of meniscus in the intercondylar notch. Pitfall of interpretation is avoided on the sagittal image where the loose body (arrowhead) is depicted behind the posterior capsule. Fig. 7 - The torn anterior cruciate ligament. Sagittal T2*-weighted MR image of a subacute tears of the ACL. The torn ACL detached from its femoral insertion appear as a low-signal band within the intercondylar notch located anteriorly and paralleling the PCL. This appearance, simulated by the ACL that has torn proximally, mimic the "double posterior cruciate ligament" sign.   REFERENCES Crues JV, Mink J, Levy TL, et al. Meniscal tears of the knee: accuracy of MRI. Radiology 1987; 164:445-448 Dandy DJ. The arthroscopic anatomy of symptomatic meniscal lesions. J Bone Joint Surg Br 1990;72-B:628-633 Haramati N, Staron R, Rubin S, et al. The flipped meniscus sign. Skeletal Radiol 1993;22:273-277 Mink JH, Reicher MA, Crues III JV. 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AJR 1995;165:621-625 Corresponding Author Gianvincenzo Sparacia, MD Institute of Radiology University of Palermo Via del Vespro, 127 90127 - Palermo, Italy Phone +39-91-655.2330 Fax +39-91-655.2337 Email radpa@mbox.unipa.it URL: http://www.unipa.it/~radpa/radpa.html