The menisci of the knee are crescent-shaped pieces of cartilage which lie between the femur and tibia, and serve to cushion the weight of the body and reduce friction during movement. Here is a view of the menisci from above, looking down on the tibia, depicting the medial meniscus (blue arrows) and the lateral meniscus (green arrows):
You can see why these pieces of cartilage are called menisci, from the Greek "meniskos", or "lunar crescent". Consider the similarity to the crescent of the moon:
Images from a knee MRI of a 59 year old male, who heard a "pop" walking down some steps, and had subsequent knee pain:
This coronal T2 fatsat image identifies a torn medial meniscus, with an inferiorly displaced meniscal fragment (red arrow). There is a trace of marrow edema (yellow arrow) subjacent to the torn meniscus.
A series of sagittal images (medial to lateral) identifies the displaced meniscal fragment (red arrows) as well as the adjacent truncated undersurface of the meniscus (blue arrows):
An axial intermediate image also visualizes the displaced meniscal fragment:
A magnification of this axial view better shows the meniscal fragment (red arrows) underneath the posterior oblique ligament (green arrow):
Inferiorly displaced flap tears were first described in the radiology literature by Lecas et al. (AJR 2000; 174:161-164). More recently, McKnight et al. (Skel Rad 2010: 39:279-283) have described common patterns of displaced meniscal tears on MRI. They noted that meniscal fragments can be displaced inferiorly or superiorly, and displaced tears can affect both the medial and lateral meniscus. As noted by both authors, flap tears that are displaced peripherally can be overlooked at arthroscopy, as they are not immediately visible to the arthroscopist.
The patient in this case was taken to arthroscopy. In this arthroscopic image, we see the torn meniscus located between the femur and tibia. Note that the displaced meniscal fragment is not apparent:
The arthroscopist was alerted to the presence of the fragment by the preoperative MRI, and used a hook to probe the region in question:
This enabled the arthroscopist to deliver the displaced fragment back into the joint:
The displaced fragment was then resected, and the meniscus trimmed back to a stable rim.
This case illustrates the type of important information a preoperative MRI can provide to the orthopedic surgeon. This information enhances appropriate pre-operative planning, and maximizes the ability of the surgeon to provide optimal care to the patient.
Vic David MD