Radiology Services

Newspaper Column

Question: My knee has been hurting and swelling since I fell a couple of weeks ago. My doctor says the x-rays were normal, but now he wants me to get a Magnetic Resonance scan. What can that show that the x-ray didn’t?

Answer: Often, after a minor injury, a couple of days of rest, analgesics and healing, we get back to normal activity without much difficulty. If the x-rays showed no obvious fracture, and the expected recovery did not follow, your doctor is looking for what we call “internal derangement” in your knee.

This means the cartilage; ligaments, joint surfaces or the bone itself is injured, preventing the expected return to health.

When we perform MR scans of your knee, you rest on a couch with a special “coil” placed over the injured knee. The technologist places the couch into a doughnut shaped magnet while you listen to music and lie as still as you can.

The magnetic field causes your atoms to line up with the north and south pole of the magnet. Radio wave pulses are then sent to your knee, causing the atoms to tilt. When the radio wave is turned off, the atoms bounce back to the original position, and give up a tiny signal that the coil over the knee sends to the computer. The computer generates the picture of the bone, and soft tissues of your knee......... You never felt a thing!!

While x-rays have high detail of bony structures, there can be microscopic cracks in the bone that cannot be seen from any angle. Since we have been doing MR scans, we have found numerous “occult fractures” that can be seen on MR, that cannot be seen by the x-ray, even if we go back and review the x-rays knowing exactly where to look.

There is another MR discovery in relationship to bony injury that is called a “bone bruise”. This is when there is no actual fracture line, but occurs when marrow in the bone swells after an injury. Because each MR scan of the knee has hundreds of pictures made with different angles and lightness and darkness in the images, we find this kind of injury in a surprisingly frequent number of patients. In juveniles, the growth plates of the knee are carefully examined for injury.

The ligaments of the knee are wonderfully visible on knee MR. Everyone has heard of football and soccer players injuring their “ACL” or “PCL” ligament. These are the anterior cruciate and posterior cruciate ligaments. If they are injured, MR shows they turn a different shade of gray than normal, and if ruptured, can be seen as separate fragments attached to the bones at the knee. Four other major ligaments and two minor ligaments at the knee may also be injured, and must be located and examined in each scan performed.

If fluid is present in the knee joint, it is obvious in the MR scan, and swelling outside the joint is also clear.

The bones at the knee are covered with cartilage where they meet. This allows them to slide against each other when you move. The cartilage can be injured or degenerate. Those changes must also be reported.

And last, the knee has a crescent shaped cushion or cartilage at the inside and outside edges of the joint. These are called the medial and lateral meniscus. These cartilages are the ones often referred to as “torn cartilage” for which surgery may be required.

MR scans of the joints provide an internal look without pain or risk to help decide whether surgery may be required or might be avoided.