The meniscus is a wedge-shaped piece of cartilage that sits in between the bones of the knee and acts as a cushion to protect the bones during movement. Each knee has two menisci.
Healthy menisci are shaped like crescent moons (in fact, “meniscus” comes from the Greek word for crescent). A discoid meniscus is thicker than normal, and often oval or disc-shaped. It is more prone to injury than a normally shaped meniscus.
People with discoid meniscus may go through their entire lives and never experience any problems. If a discoid meniscus does not cause pain or discomfort, no treatment is needed. Some people, however, will have knee problems related to the discoid meniscus. Symptoms often begin during childhood.
The menicus acts as a “shock absorber” between your femur (thighbone) and tibia (shinbone). It protects the thin articular cartilage that covers the ends of the bones and helps the knee to easily bend and straighten.
There are two menisci in the knee: the medial meniscus on the inside of the knee, and the lateral meniscus on the outside.
The menisci are attached to the femur bone by a strong tissue called the meniscofemoral ligament. This ligament also provides a blood supply to a small portion of the meniscus.
In many cases, a discoid meniscus is shaped like a half moon or complete circle, rather than a crescent moon. It most often occurs on the lateral side (outside) of the knee, and can sometimes be found in both knees.
There are three types of discoid menisci:
A discoid meniscus is more prone to injury than a normal meniscus. The thick, abnormal shape of a discoid meniscus makes it more likely to get stuck in the knee or tear. If the meniscofemoral ligament attachment to the femur is also missing, the risk for injury is even greater.
Once injured, even a normal meniscus is difficult to heal. This is because the meniscus lacks a strong blood supply and the nutrients that are essential to healing cannot reach the injured tissues.
In many cases of discoid meniscus, patients experience symptoms without there being any injury to the meniscus.
The cause of discoid meniscus is not known. It is a congenital (present at birth) defect.
Injuries to the discoid meniscus often occur with twisting motions to the knee, such as during sports that require pivoting or fast changes in direction.
The most common symptoms of a discoid meniscus or torn discoid meniscus are:
After discussing your child’s medical history and any events that occurred before symptoms started, your doctor will examine your child’s knee.
Your child may or may not have tenderness where the bones meet.
To test for discoid meniscus, your doctor will twist your child’s knee with the knee bent and straightened. In many cases of discoid meniscus, there is a popping or clunking sensation. This can sometimes even be heard. In extreme cases, part of the meniscus will pop out of the knee joint and can be seen right under the skin.
X-rays. Although x-rays do not show tears of soft tissues like the meniscus, they can show whether there are other problems in the knee. In addition, because the discoid meniscus is so thick, the space between the femur and tibia on the lateral part of the knee may appear widened on an x-ray.
Magnetic resonance imaging (MRI) scan. This study can create detailed images of the soft tissues of the knee joint and is the best imaging test for discoid meniscus.
The MRI can show the abnormal shape of the discoid meniscus, as well as tears within the meniscus. However, the rare Wrisberg type of discoid meniscus cannot be tested with an MRI scan because the problem only occurs when the patient is moving.
An MRI study involves lying still for 30-45 minutes. Many young patients require some sedation or anesthesia to complete the test.