MRI of the Knee

Understanding the results of a knee MRI

MRI knee
Learn what MRI reports mean. Fuse / Getty Images

MRI is often used to help investigate the source of many potential knee problems and conditions. Use this guide to help understand some of the common conditions seen by MRI. If you have an MRI report that you are having trouble understanding, hopefully, this will help explain some of the findings. For definitions of some commonly used MRI terminology, read on to the second page of this article. Some other resources that may be helpful include:

Also, keep in mind, the MRI report is not itself sufficient to determine the treatment needed in any situation. MRIs must be used with the patient's history of injury, physical examination findings, x-rays, and other studies, to determine an appropriate treatment plan.

Meniscus Tears

The MRI is tremendously helpful when investigating problems of the meniscus. The meniscus is a wedge of cartilage within the knee joint that helps to cushion, stabilize, and transmit weight across the knee joint. For more information about what a meniscus does:

There are several conditions of the meniscus that can be seen on an MRI, the most common being a meniscus tear. A meniscus tear can be seen when either the normal shape of the meniscus is disrupted by an abnormal signal, or if a portion of the meniscus is missing.

A normal meniscus appears as black triangles on the MRI images. When a meniscus is torn, the normal triangular appearance is disrupted. The torn portion of the meniscus may also be seen in an abnormal position, often in the center or the knee joint. This may be referred to as a "bucket-handle tear" of the meniscus.

MRI reports commonly use language about "intrasubstance signal" within the meniscus. The normal meniscus has a uniform signal (appearance) on the MRI image. In some situations, an abnormal signal will be seen within the meniscus, this is the so-called "intrasubstance signal." Not all intrasubstance signals are bad, and they do not necessarily mean the meniscus is torn. If the abnormal signal is entirely within the boundaries of the meniscus, then there is unlikely to be a tear. Intrasubstance signal may represent normal aging (degeneration) of the meniscus, or increase vascularity of the meniscus (commonly seen in children and young adults). Therefore, just because an MRI report describes abnormal intrasubstance signal, you do not necessarily need surgery!

Ligament Injuries

Several knee ligaments are easily seen on the MRI. The two ligaments in the center of the knee are the ACL and PCL. The ACL can be more difficult to see, but tears of the ACL can be seen about 90% of the time when the ligament is injured. Other signs of an ACL tear that can be seen include specific bone bruises and fractures. These associated findings occur when the ACL is injured and can be used to help confirm the diagnosis.

The PCL is seen more easily than the ACL since it is about twice the size of the ACL. Isolated PCL tears are relatively uncommon injuries. A PCL tear is usually easily seen by MRI as a disruption of the fibers of the ligament.

On the outside and inside of the knee, the medial and lateral collateral ligaments are seen. Injuries to these ligaments are suggested when either there is fluid (swelling) around the ligaments or if the fibers are seen to be disrupted. Injuries to the collateral ligaments are graded as follows on MRI scans:

  • Grade I: Fluid around the ligament
  • Grade II: Fluid around the ligament with partial disruption of the ligament fibers
  • Grade III: Complete disruption of the ligament fibers

Tendon Problems

The two tendons best seen in on MRI are the quadriceps tendon and the patellar tendon. MRI can detect both complete rupture of the tendons (although this is usually obvious on physical examination) or abnormalities within the tendon (tendinopathy). Most commonly, patients with chronic tendonitis are seen to have this abnormal appearance to the tendons. These are commonly seen in problems such as "jumper's knee," or chronic patellar tendonitis. These areas of abnormal signal intensity may represent areas of inflammation and scarring within the tendons.

MRI reports are filled with complex language and can be difficult to understand. If you are having trouble reading an MRI report, look through the following list of commonly used words to help your understanding. Also, review some common findings of MRIs of the knee.

  • Signal
    An MRI uses a strong magnet to produce an image of the knee joint. For information about how an MRI works, click here. The image that the MRI creates looks black, white, and all shades of gray. This different shade is called "signal." A high signal is a white appearing shade, and a low signal is a dark shade. The radiologist and orthopedist use these signal characteristics to determine if the appearance in normal or abnormal.
  • Edema
    Edema is another word for swelling and is indicative of injury to an area. Edema is commonly seen around an area that has sustained an injury. Therefore, the structure may appear normal, but the edema may show where the injury is located. Edema may be seen within a bone (a "bone bruise") or within the soft-tissues.
  • Sequence
    MRI machines use different sequences to help show different types of injuries. Common MRI sequences include T1 and T2 images, and sometimes notations such as "spin," "echo," and "STIR," will be used. These don't really matter for understanding the report; rather they are used by the radiologist to communicate which images show a characteristic finding, and how these images were obtained.
  • Bone Bruise
    A bone bruise is an area of bone that has been injured and has swelling, or edema, within the bone. This is a sign that the bone has been injured sufficiently to have inflammation within the bone itself. Specific bone bruises may not require any treatment themselves, but may be a sign of other injuries such as an ACL tear.
  • Anterior or Posterior Horn
    The anterior and posterior horns of the meniscus help the radiologist communicate where in the meniscus he or she sees an abnormality. The anterior horn of the meniscus is in the front of the knee, and the posterior horn is in back. The posterior horn of the meniscus is more commonly injured.

    These are just some of the commonly used words in an MRI report. MRI reports are written for the radiologist to communicate with the orthopedist (and other physicians). Without medical school and residency training, it can be difficult to understand these reports, so do not be discouraged! Discuss the MRI findings with your doctor. He or she can explain both what the findings mean, and whether or not the findings are significant.

    Keep in mind, the MRI report may indicate findings that are not necessarily "normal," but that does not mean those findings are causing your problem. The MRI must be used in association with the physical examination, history of injury, and other studies, in order to determine a proper treatment plan.

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