Flag sign: a case report of an unusual magnetic resonance imaging finding of a free lateral meniscal fragment.

BACKGROUND: Meniscal injury is a common sports medicine condition. Magnetic resonance imaging (MRI) is widely used to effectively diagnose meniscal injury. A flag sign on MRI is usually typical of a meniscal root tear. We report the case of a "flag sign" caused by a free lateral meniscal fragment that mimicked the anterior cruciate ligament (ACL) signal on MRI. CASE
PRESENTATION: This was a 21-year-old male patient who suffered a knee injury (swelling and pain) playing football. A physical examination revealed positive Lachman and lateral McMurray tests. MRI images showed an ACL injury and the flag sign, and a diagnosis of ACL tear and lateral meniscal injury was made. Arthroscopic lateral meniscal repair and ACL reconstruction were performed. Six months postoperatively, MRI showed that the injuries were well healed. In this particular case, the flag sign introduced interference to the MRI assessment of ACL injury.
CONCLUSIONS: This case illustrates that clinicians should carefully identify the morphological changes in the meniscus and the relationship of the meniscus with the femoral condyle before reaching a final diagnosis.

Introduction

Meniscal injury is a common sports medicine condition; it is classified according to whether it is vertical, horizontal, or complex.[1] Magnetic resonance imaging (MRI) is widely used to diagnose knee injuries because it has a high sensitivity and specificity for detecting injuries of the knee.[2-7] The diagnosis of meniscal injuries by MRI mainly depends on the morphological changes and signal changes of the meniscus, but a displaced meniscus sometimes causes confusion. A flag sign on MRI is usually typical of a meniscal root tear. We report the case of a “flag sign” caused by a free lateral meniscal fragment that mimicked the anterior cruciate ligament (ACL) signal on MRI.

Case report

The study protocol was approved by the XiAn HongHui Hospital ethics committee, and the patient signed a written informed consent form to have his information disclosed in this report. The patient was a 21-year-old man who was injured while playing football. He was a student with no previous history of trauma. He had twisted his right knee 3 days previously, and he was referred to the sports medicine center of the local hospital on 4 May 2017 with swelling and pain. The anterior drawer test was 1+, Lachman test was 2+, McMurray test was 1+, posterior drawer test was negative, and the pivot shift test was symmetric to the contralateral knee. Routine laboratory testing showed no abnormalities. He underwent knee MRI using a Siemens 3.0-T system (Erlangen, Germany), with scanning parameters of TR=3137 ms and TE=104 ms. The MRI revealed a partial discontinuity in at least one reading plane and horizontalization of the distal fragment of the ACL on the sagittal view (Figure 1a). Bone bruises and the kiss sign were also confirmed on MRI (Figure 1b).

Figure 1.

Sagittal magnetic resonance image. (a) A partial discontinuity (arrow) and horizontalization of the distal fragment of the anterior cruciate ligament (ACL) can be seen. (b) A bone bruise and kiss sign (arrow) are shown. (c) The “flag sign.” A meniscal fragment (arrow) has been displaced into the intercondylar notch and resembles the anterior cruciate ligament (ACL). (d) The normal ACL on a coronal magnetic resonance (MR) image connects with the condyle of the femur (arrow).

Concurrently, a tissue similar to the ACL was detected on the coronal view, which confused the radiologist and the surgeons. The signal resembled a flag connecting the femur and tibia. On careful evaluation, the “flag sign” was found to be misleading. Indeed, a flag sign is usually seen in meniscal root tears, but no such tear could be detected in the sagittal plane. After careful examination, the signal was determined to be related to a meniscal fragment displaced into the intercondylar notch (Figure 1c), which is different from the normal ACL connected to the condyle of the femur (Figure 1d) on coronal MR images. If the flag sign is not effectively differentiated, misdiagnosis of an intact ACL can occur, delaying the treatment for ACL injury.

There was no “absent bow tie sign” on the sagittal view, but the posterior horn of the meniscus was smaller than normal. Arthroscopy demonstrated that a free fragment of the lateral meniscus posterior root had moved to the intercondylar eminence, explaining the “flag sign” (Figure 2a). The ACL was completely torn. No tears or other visible pathology were observed in the remainder of the meniscus. Arthroscopic lateral meniscal repair (Figures 2b, c) and ACL reconstruction were performed in the same session. The 1-year postoperative follow-up showed that the patient’s knee joint was stable, and the meniscus was well healed. Clinically, the flag sign is not rare. It is a manifestation of a displaced meniscal root tear, and the reason for its formation is shown in Figure 3.

Figure 2.

Arthroscopy. (a) A free fragment of the lateral meniscus posterior root (M) has moved to the intercondylar eminence and resembles a false “flag sign.” (b) Resetting the lateral meniscus. (c) The repaired lateral meniscus.

Figure 3.

Schematic diagram of the formation of a flag sign. (a) Normal meniscus. (b) Tear of the posterior horn of the lateral meniscus (c) The torn lateral meniscus flipping into the intercondylar region (d). The flag sign can be seen on the sagittal and coronary views, which can be easily confused with an anterior cruciate ligament (ACL) tear.

Discussion

The menisci are important structures that not only share the distribution of load over the articular cartilage and provide a shock absorption function, but also function as secondary stabilizers of the knee.[3] To effectively diagnose knee injuries such as meniscus, cartilage, and ligament injuries, MRI is widely used because it has a high sensitivity and specificity for detecting injuries of the knee. The diagnosis of meniscal injuries by MRI mainly depends on the morphological changes and signal changes in the meniscus. Meniscal injury is a common sports medicine condition. A flag sign on MRI is usually typical of a meniscal root tear. We report the case of a “flag sign” caused by a free lateral meniscal fragment that mimicked the ACL signal on MRI. The case presented here illustrates that clinicians should carefully identify the morphological changes in the meniscus and the relationship of the meniscus with the femoral condyle before final diagnosis.

There are many well-known MRI findings of meniscal tears reported in the literature, such as the “fragment within the intercondylar notch sign”, “absent bow tie sign”, “ghost sign”, “double ACL sign”, “double posterior cruciate ligament (PCL) sign”, and “double anterior horn sign”.[4] At the same time, some uncommon signs have been reported. A “reversed” double PCL sign caused by a displaced meniscal fragment that moved to the posterior region of the PCL has been reported.[6,7]

A lateral meniscal fragment displaced into the intercondylar notch was detected on the coronal MRI view in our case, which looked like the ACL on the coronal view. The most obvious difference from the ACL is that the meniscal fragment was not connected to the condyle of the femur, but it looked like a flag, so it was named the “flag sign”. Figure 3 shows the formation of the “flag sign”. There was no obvious “absent bow tie sign” on the sagittal view, but the shape of the posterior horn of the lateral meniscus was abnormal. The absence of the bow tie sign has been found to be highly associated with displaced meniscal tears,[8] and it can lead to false positives in small menisci or osteoarthritis with free edge wear. In our case, this sign was not obvious because of the scanning interval, which does not support the usefulness of this sign for meniscal tears. In our experience, a meniscal fragment displaced into the intercondylar notch is infrequent in clinical meniscal injuries.

In conclusion, the important finding in this case was that a free lateral meniscal fragment mimicked the ACL signal on MRI and appeared as a “flag sign.” Clinicians should carefully identify the morphological changes of the meniscus and their relationship with the femoral condyle before making the final diagnosis to pursue the appropriate treatment.