The Good, the Bad, and the Meniscus

As a radiologist with special training in musculoskeletal imaging, I interpret MRIs of the knee on a daily basis, and probably the most common abnormality I see is a tear of the meniscus. While more common as we age, meniscal tears do not discriminate and can happen to anyone. Certainly these tears can cause pain, and they often occur in combination with other injuries and degenerative conditions of the knee.

Before we get into the thick of it, a note on grammar (yes, grammar). The plural of meniscus is menisci. Because each knee has two—one on the inside (medial meniscus) and one on the outside (lateral meniscus)—I should technically use menisci when referring to one knee. However, sometimes doctors use “meniscus” even when talking about both menisci in one knee (I’m not sure why). You may notice I do this as well, and I apologize for any confusion.

“I can’t live with or without you, meniscus.” – Knee

Although meniscal tears and degeneration often result in knee pain, the knee needs the meniscus to function properly. The meniscus allows the bottom of your femur (thigh bone) to “articulate” smoothly with the top of your tibia (lower leg bone). Articulate is a fancy way of saying that your joint fits together and moves normally.

The three-dimensional shape of the meniscus is unusual and a bit hard to describe. Two crescent-shaped structures—the medial and lateral menisci— attach to the top of the tibia and sit right in the middle of your knee joint. If you were to magically remove your femur and look down on the top of your tibia, the meniscus would look like two letter Cs with the open ends facing each other in the middle (see figure below). These open ends attach to the top of the tibia at the two ends of each C, with the part that attaches called a root. The lateral meniscus is smaller and the C is more curved, almost meeting to form an O; the medial meniscus is larger and more open in the middle. The outer perimeter of each C is thicker than the inner edge, resulting in a triangle-shaped cross section if you were to cut the meniscus with a scissors and look from the side.

How does the meniscus actually allow the knee work properly? Without the meniscus, the femur and tibia just don’t get along. Areas of contact between the ends of these bones could damage the cartilage and eventually lead to arthritis. Because of its shape and position in the knee, the most important job of the meniscus is to distribute the load-bearing forces that might otherwise damage the cartilage. With a straightened knee, an estimated 50% of the load-bearing forces are borne by the meniscus; with a bent knee, that number is closer to 80%.

The meniscus also acts as a secondary stabilizer of the knee, with knee ligaments providing primary stabilization. The lower end of the femur and upper end of the tibia are covered with smooth, slippery cartilage—a great design for sliding around on each other but not ideal for stability. Luckily, evolution stuck a meniscus in the middle, which aids in this stabilization.

Let’s get microsopic

The meniscus is composed of a type of cartilage, similar to our ears, our noses, and shark skeletons. Microscopically, the most important component is collagen fibers (type 1) arranged circumferentially. Due to their strength and orientation, these fibers primarily account for the meniscus’ load-bearing ability.

Let’s begin our discussion of the blood supply of the meniscus with a relevant quote from Deep Thoughts by Jack Handey: “If you ever drop your keys in a river of molten lava, let ’em go, because man, they’re gone.” Likewise, if you tear your meniscus, just let it go, because it’s almost certainly gone. The meniscus heals poorly due to its relative lack of blood supply. In general, the body responds to an injury by increasing blood flow to the affected area so that oxygen and other essential cells and chemicals can begin the healing process. In adults, most of the meniscus has minimal or no blood supply, with the exception of a few millimeters on the peripheral edges. Poor blood supply = poor healing (sad emoji). There is some potential for healing in these peripheral tears, which can be treated non-operatively (more on this below).

The wide world of meniscal tears

Broadly speaking, meniscal tears can be classified as degenerative tears or acute tears. Acute tears occur in an otherwise normal meniscus in the setting of a twisting injury or other trauma. Degenerative tears occur in a degenerative meniscus, a meniscus which—through years of wear and tear—has lost strength and elasticity and developed many microscopic tears. Think of an acute tear as a tree falling and slicing your brand-new Mercedes in half, and a degenerative tear as a stiff breeze destroying your jalopy, held together with gum and duct tape.

Meniscal tears are also categorized based on location and orientation. Rather than describe these tears in painstaking detail, I have always found it much easier to visualize them, so please refer to the diagram below along with accompanying MRI examples.

A specific type of tear worth mentioning, and one difficult to represent in just a few images, is called a bucket handle tear. A large vertical tear propagates around most of the medial or lateral meniscus, and the inner rim of the torn meniscus then flips over into the center part of the knee. One could imagine holding this flipped part of the meniscus like—you guessed it—a bucket handle. These tears are usually large and tend to cause significant clicking and locking symptoms.

Another special type of meniscal tear is an avulsion of one of the meniscal roots. The meniscal roots are the parts that attach to the tibia. The medial and lateral meniscus each has two roots: anterior and posterior. A root avulsion happens when one of these roots is torn from the tibia, more commonly one of the posterior roots. Often the underlying root is abnormal, i.e., these are often degenerative tears.

If it is broke, don’t fix it?

As with most areas of medicine, the optimum treatment regimen for meniscal tears is in constant flux as new research becomes available. Currently, evidence points to non-surgical therapy as a first-line treatment, including pain relief with NSAIDS (such as ibruprofen) and physical therapy. Thus, all patients in which a meniscal tear is discovered should attempt an initial trial of non-surgical therapy before operative treatments are considered. This is especially true for tears that occur in the peripheral, more vascularized part of the meniscus, which has a better chance at healing.

About a third of patients will not achieve adequate pain relief after physical therapy, and will require surgical intervention. Two options exist: meniscal repair and partial meniscectomy (removal). As an oversimplification, tears that involve the inner part of the meniscus with a poorer blood supply are treated with partial meniscectomy, and those involving the outer more vascularized part with meniscal repair. If possible, meniscal repair is the current treatment of choice because it maintains the critical load-bearing function of the meniscus. Partial meniscectomy, on the other hand, decreases the size of the meniscus; load-bearing is transferred to the knee cartilage, resulting in earlier cartilage damage and arthritis.

The primary indication for knee MRI arthrogram—dye injected into the joint before MRI is performed—is to evaluate for meniscal tear in a patient with prior meniscal surgery. When the meniscus has been operated on, it can be difficult to differentiate its post-surgical appearance from a new meniscal tear. Dye leaking into the meniscus indicates a new tear is present.

We are lucky to have a meniscus, but unlucky that it is often damaged and a source of pain. So please take are of your menisci, you only have four of them.