Double Posterior Cruciate Ligament Sign

The double posterior cruciate ligament (PCL) sign refers to the presence of a linear hypointense band parallel and anteroinferior to the posterior cruciate ligament on mid-sagittal images of the knee and has a high specificity (98%-100%) for a bucket handle tear of the medial meniscus.

The double posterior cruciate ligament sign is created when the meniscal fragment is displaced into the intercondylar notch anterior and parallel to the posterior cruciate ligament and above the medial tibial eminence. An intact anterior cruciate ligament is said to be needed to prevent further lateral displacement of the meniscal fragment; however, the patient in the first panel had a tear of her anterior cruciate ligament graft and the patient in the second panel had also torn his anterior cruciate ligament.

Other things that may simulate a double posterior cruciate ligament sign on a single image:

• Normal ligament of Humphrey: May appear as a hypointense band anterior to the PCL, but is usually much smaller and thinner than a bucket-handle fragment and closer to the PCL.
• Normal ligament of Wrisberg: Normally a dot on sagittal images, it may be parallel the PCL if the knee is in external rotation or when the ligament of Wrisberg is lax. Trace it to its origin to confirm.
• Normal meniscomeniscal ligaments: Can be seen in up to ~5% of cases. Travel between the anterior horn of one meniscus to the posterior horn of the contralateral meniscus between the anterior and posterior cruciate ligaments. As such, they can mimic a double PCL sign, but are thinner and are found lower in the intercondylar notch. Trace their course to confirm.

The case in the first panel is courtesy of Dr. Elizabeth Barak.

References

Venkatanarasimha N, Kamath A, Mukherjee K, Kamath S. Potential pitfalls of a double PCL sign. Skeletal Radiol. 2009 Aug;38(8):735-9.