DOI: 10.1148/radiol.2332020945
(Radiology 2004;233:503-504.)
© RSNA, 2004
The Double Posterior Cruciate Ligament Sign1
Marc A. Camacho, MD, MS
1 From the Department of Radiology, Medical College of Virginia, Virginia Commonwealth University Health System, 401 N 12th St, PO Box 980615, Richmond, VA 23298-0615. Received July 30, 2002; revision requested September 30; revision received April 24, 2003; accepted June 19. Address correspondence to the author (e-mail: macamach@vcu.edu).
Index terms: Knee, injuries, 452.4852 • Knee, ligaments, menisci, and cartilage, 452.4852 • Magnetic resonance (MR), three-dimensional, 452.121419 • Signs in Imaging
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APPEARANCE
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The double posterior cruciate ligament (PCL) sign is seen on midline sagittal magnetic resonance (MR) images of the knee as a low-signal-intensity band that is parallel and anteroinferior to the PCL (1).
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EXPLANATION
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The double PCL sign is associated with bucket-handle tears of the medial meniscus that occur in the presence of an intact anterior cruciate ligament (ACL). A bucket-handle tear is a longitudinal tear of a meniscus that results in a displaced but attached meniscal fragment (2). The fragment may become displaced into the notch between the PCL and the medial tibial eminence in the midline, with the fragment orientated parallel to the PCL (1). The intact ACL serves as a barrier that prevents further lateral displacement of the meniscal fragment. The ACL is pivotal in the resulting alignment of the structures that are responsible for the depiction of the double PCL sign. Since ligaments and menisci demonstrate a hypointense signal with all pulse sequences, the displaced fragment will mimic a second PCL that is anterior and inferior to the true ligament, hence the name "double PCL sign" (Fig 1).
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Figure 1. Sagittal spin-echo MR image (2300/45 [repetition time msec/echo time msec]) with fat saturation demonstrates a large, superoposterior low-signal-intensity arcuate structure (arrow), representing the PCL. The smaller, low-signal-intensity band (arrowhead) located anteroinferior and parallel to the PCL is the displaced medial meniscal fragment responsible for the double PCL sign. A large joint effusion (*) is also noted.
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DISCUSSION
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The spectrum of meniscal tears ranges from small, subtle tears of questionable clinical importance to bucket-handle tears, as described above. Early identification of bucket-handle tears is critical because, depending on how peripheral and complex the tear is, any delay can compromise the chances for repair. If the tear is not reduced, the meniscal fragment risks further maceration. These lesions cause substantial symptoms, including knee locking or a lack of full extension. Arthroscopy is required to excise or reattach the free fragment (2–4). The prevalence of bucket-handle tears in symptomatic patients is 9%–19% (2,4,5). Various studies have consistently demonstrated a 2:1 ratio of medial-to-lateral involvement in bucket-handle tears (2,4–6).
MR imaging has proved valuable in the diagnosis of meniscal tears. The sensitivity of MR imaging for the diagnosis of bucket-handle tears, however, is lower than that for other meniscal tears and is reported to be approximately 60%–88% (2,5–7). Earlier studies demonstrated the sensitivity of the double PCL sign for bucket-handle tears to be 100% (1,3); however, more recent studies have reliably demonstrated sensitivities of 27%–53% (2,4,5,8). The double PCL sign remains a highly specific indicator of a bucket-handle tear, with a specificity range of 98%–100% (1,5) and a positive predictive value of 93% (5).
A potential pitfall of the double PCL sign is the presence of a normal accessory meniscofemoral ligament that is known as the ligament of Humphry, which extends from the posterior horn of the lateral meniscus to the lateral aspect of the medial femoral condyle (Fig 2) (1,3). The ligament of Humphry can be differentiated from a bucket-handle fragment because the ligament is smaller and thinner and because the ligament is in extremely close proximity to the PCL (3). Another potential pitfall is the presence of the oblique meniscomeniscal ligament (9). This relatively uncommon normal anatomic variant has one of two configurations, which are both named for the anterior attachment site and which have a reported combined frequency of 1%–4%. The medial oblique meniscomeniscal ligament originates from the anterior horn of the medial meniscus and inserts into the posterior horn of the lateral meniscus. The lateral oblique meniscomeniscal ligament originates from the anterior horn of the lateral meniscus and inserts into the posterior horn of the medial meniscus. These ligaments traverse the intercondylar fossa, passing between the ACL and PCL, and, with this midline course, may mimic the double PCL sign (9). To avoid this pitfall, one must trace the course of these ligaments entirely from origin to insertion, as well as confirm the normal morphologic features of the adjacent menisci (9).
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Figure 2. Paramedian sagittal spin-echo MR image (2300/45) with fat saturation demonstrates the ligament of Humphry (arrowhead), a normal accessory meniscofemoral ligament that is immediately inferior to the PCL (arrow). The appearance of this ligament differs from the thicker band that is seen with the double PCL sign.
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As MR technology and pulse sequences advanced, other signs of bucket-handle tears have been introduced, including the central meniscal fragment (5), the fragment-in-notch sign (2,8,10), the flipped meniscus sign (2,8,10), and the absent bow tie sign (4,5). As with the double PCL sign, the reported sensitivities of these signs for bucket-handle meniscal tears vary. Overall, the absent bow tie sign is consistently the most sensitive sign, followed by (in decreasing order) the central meniscal fragment, the fragment-in-notch sign, flipped meniscus sign, and double PCL sign (2,4,5,8). The sensitivity of the combined fragment-in-notch, double PCL, and flipped meniscus signs as an indicator of bucket-handle tears increased from 60% to 93% with the addition of coronal short inversion time inversion-recovery images (8). The sensitivity of a central meniscal fragment as an indicator of bucket-handle meniscal tears has increased from 62% to 74% with the use of a three-dimensional volume sequence (5). Watt et al (5) used a three-dimensional dual-echo free induction in steady state sequence that consisted of 10-cm volume acquisitions with one signal acquired, which resulted in heavily T2-weighted images with a section thickness of 1.2 mm in all planes. When comparing features of the bucket-handle tear, investigators found that sensitivities were higher with larger tears (especially with those involving the entire meniscus), medial meniscal tears, and meniscal tears that were not associated with an ACL tear (2,6).
The sensitivity of the double PCL sign and other imaging findings that are associated with bucket-handle tears is determined based on the standard of reference, arthroscopy, which invariably is performed some time after the MR examination. Various authors concede that, in an unstable knee, this time interval may introduce the opportunity for a small meniscal tear to extend and become displaced, thereby becoming a bucket-handle tear. Such a displacement would artificially degrade the calculated sensitivity of the MR study (2,6).
The double PCL sign is a specific indicator for medial meniscal tears that occur in the presence of an intact ACL. It is presumed that, given its normal lateral-to-midline position, the ACL serves as a barrier preventing the medial displacement of the fragment that results from a lateral meniscal bucket-handle tear (2,4,5). A double PCL sign has also been reported in which the displaced fragment originated from a lateral meniscal tear (4). This was associated with a torn ACL.
In summary, the double PCL sign is a low-signal-intensity band that is parallel and anteroinferior to the PCL on sagittal MR images and is a highly specific indicator of a bucket-handle meniscal tear. The double PCL sign is most frequently seen with medial meniscal tears that occur in the presence of an intact ACL.
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FOOTNOTES
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Author stated no financial relationship to disclose.
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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REFERENCES
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