How to diagnose a meniscal tear on MRI?
First, let’s define what we mean by the term “meniscal tear.” Meniscus is a fairly dark structure on proton density images. Young patients may have some intrasubstance signal in the periphery that represents vessels, but in general, the majority of the meniscus is dark. Hyperintense signal in the meniscus will most of the time indicate some sort of an “abnormality,” especially if present in the mid zone or central zone.
Should we diagnose all hyperintense signal as a tear? Can a dark meniscus be torn?
Let’s define “meniscal tear” in the following fashion: meniscal tear is either an absent portion of meniscus or a defect that reaches tibial or femoral surface of the meniscus. You can see that a dark meniscus with a missing part is a “torn meniscus” using this definition. Hyperintense signal that involves tibial or femoral surface of the meniscus is a meniscal tear based on this definition, as well. We will still have to deal with meniscocapsular injuries and meniscal contusions separately. More on that later.
What about meniscal scars?
When you start your first radiology job, talk to your orthopedic surgeons. You may want to discuss different terms come to an agreement on what is actually meant by such terms as “scars” and “tears”. One way to look at this issue is that “scar” was a “tear” at some point, so meniscus suffered a tear. However, don’t be surprised if a surgeon asks you whether you can differentiate the two. Small scars and tears may be impossible to differentiate from each other. The larger tears usually follow the signal of joint fluid, while larger scars have signal slightly darker than the joint fluid. On some scans, such differentiation may be very difficult. If important for surgical decision making, MR arthrogram may be needed to differentiate contrast filled tear from scar. Scar will not fill with contrast.
What are the types of meniscal tears?
Let’s keep this simple:
- Subtype: Flap
- Bucket handle
Other types are mentioned in various publications, but the above will get your started.
What is the deal with meniscal root tears?
Traditionally, meniscus is divided into anterior and posterior horns and body. The anterior and posterior roots attach anterior and posterior horns onto the tibia. When scrolling through sagittal slices, it is easy to bypass complete root tear, which can be parallel to the MRI plane. “Ghost sign” is that one slice that catches the root tear, but can be hard to notice when you are on your first MRI rotation. It is important to look at the roots on the coronal images, which usually show these tears in greater detail.
How are ligamentous injuries of the knee classified?
Sprains and tears.
- Mild sprain: edema around otherwise normal ligament.
- Moderate sprain: surrounding edema + intrasubstance hyperintese signal.
- Severe sprain: hardly ever used. Usually, there is a tear associated with it, in which case it makes more sense to describe the tear. Severely thickened ligament with surrounding edema and intrasubstance abnormal signal would qualify as severe sprain.
- Tear: a measurable defect in the ligament.
If you see a measurable defect, call it a tear, either partial or complete. Complete tear can be further qualified of how far ligament fibers are retracted from the bony attachment. At times, a large ACL fragment can be seen as displaced into intercondylar notch, for example.
In older patients, it becomes difficult to differentiate an injury from degeneration. Degenerated ligamentous structures often show surrounding edema, intrasubstance signal, partial tears.