Flipped meniscus sign | Radiology Signs

Answer:

The flipped meniscus sign is the appearance on sagittal MRI of a meniscus whose anterior horn is abnormally enlarged because a displaced fragment from the posterior horn has folded forward and come to lie on top of it, while the posterior horn appears small or absent. It is one of the signs of a bucket-handle meniscal tear, a longitudinal vertical tear in which the inner fragment displaces into the intercondylar notch. The related signs, which should be sought together because their combined sensitivity is far higher than any one alone, are the double posterior cruciate ligament sign, in which the displaced fragment lies beneath the posterior cruciate ligament and mimics a second ligament, the absent bow tie sign, in which fewer than two consecutive sagittal slices show the normal bow-tie body of the meniscus, the double anterior horn sign, the disproportionate posterior horn sign and fragment within the notch on coronal images. The double posterior cruciate ligament sign occurs only with medial meniscal tears, because the lateral meniscus is tethered by the popliteus tendon, whereas the flipped meniscus sign occurs with either. Recognition matters because a displaced bucket-handle tear causes mechanical locking, is repairable when the tear is peripheral and vascular, and the report should state the meniscus involved, the fragment location and the tear’s distance from the meniscocapsular junction.

Why is it called so?

The torn fragment is flipped forward onto the anterior horn, so the meniscus appears folded over on itself.

Pathophysiology

A longitudinal vertical tear separates an inner fragment from the peripheral meniscal rim while both ends remain attached, producing the handle of the bucket. Because the inner fragment is no longer restrained circumferentially, hoop stresses displace it centrally into the intercondylar notch, or the fragment rotates about its remaining attachments and folds anteriorly onto the anterior horn. Displacement is what generates the characteristic MRI signs and also what produces the clinical block to extension. The medial meniscus is more often involved because it is less mobile, being firmly attached to the deep medial collateral ligament and the capsule, so it is more likely to be torn than to move out of the way of compressive and rotational load.

Alternative names: Flipped meniscus; double delta sign of the anterior horn

Other associated named signs: Double PCL sign, the absent bow tie sign and the fragment-in-notch sign

References

1. Dorsay TA, Helms CA. Bucket-handle meniscal tears of the knee: sensitivity and specificity of MRI signs. Skeletal Radiol. 2003;32(5):266-72. PMID: 12719929.