Bucket-handle tear of the lateral meniscus: Flipped meniscus sign

A 17-year-old male patient with history of acute injury of the posterolateral corner of the left knee, left untreated, returned to the emergency department with worsening pain and incapable of performing full knee extension.

Magnetic resonance imaging (MRI) performed on the first encounter showed an isolated lesion of the posterolateral corner of the left knee, with meniscocapsular separation (Fig. 1). Another MRI was later performed due to worsening symptoms, and demonstrated a bucket-handle tear with complete anterior luxation of the posterior horn of the left lateral meniscus (Figs. 2a, 2b, 2c). On sagittal proton-density and T2-weighted images, this lesion was demonstrated by sensitive but nonspecific signs, such as the flipped meniscus sign, empty meniscus sign and absence of bow-tie sign (Fig. 2a). Axial images demonstrate the anteriorly displaced posterior meniscal horn and empty posterior meniscal compartment (Fig. 2d). The meniscal bucket-handle tear was surgically treated by arthroscopy.

The menisci are fibrocartilaginous structures, located between the tibia and the femur, that allow maximum congruence of the joint, absorb shock, distribute axial load, lubricate and contribute to proprioception [1,2]. MRI provides an accurate method for detecting meniscal tears and to identify the location of the meniscal fragments, which is very important to promote an adequate treatment [1,2,3]. Bucket-handle ruptures are a type of longitudinal tear (vertical or oblique) associated with a displaced central fragment that moves in a medial direction towards the center of the joint (handle). Such tearing causes partial detachment of the central meniscus and considerable reduction of the peripheral portion which remains fixed (bucket). The displaced fragment, however, remains partially attached to peripheral meniscus at the anterior and posterior horns [4]. The clinical manifestations are knee joint pain and flexion blockage, limiting full extension of the knee. This lesion is more frequent in the medial meniscus and there is a strong association with rupture of the anterior cruciate ligament [5]. MRI findings are characteristic, which vary according to the location of the displaced fragment. The most frequent imaging findings are: flipped meniscus sign, double posterior cruciate ligament sign, absent bow-tie sign, notch fragment sign, double anterior horn sign or double delta sign and disproportionate posterior horn sign [1,3,4,6]. Flipped meniscus sign represents a large anterior horn, caused by a ruptured fragment of the posterior horn displaced anteriorly, and it is more frequent on the lateral side [3,4,7,8]. The double posterior cruciate ligament sign describes a displaced meniscal fragment lying anterior and parallel to the posterior cruciate ligament (PCL), seen on a midline sagittal MRI of the knee as a low-signal intensity linear band [3, 7, 8, 9]. Absent bow-tie sign is present when less than two sagittal consecutive slices demonstrate a normal body of the meniscus, even though several pitfalls must be considered [3, 4, 8]. Notch fragment sign corresponds to a displaced meniscal fragment lying in the intercondylar notch, but not paralleling the PCL [3, 8]. Double anterior horn sign corresponds to a displaced and intact meniscal fragment anterior to the horn, which are not vertically in juxtaposition but located next to one another in the same horizontal plane [3, 10]. Disproportionate posterior horn sign corresponds to a larger meniscal posterior horn in the central sections in comparison to the periphery, viewed on the sagittal view, and indicates a meniscal fragment displaced posteromedially [3, 11]. Written informed patient consent for publication has been obtained.