What It Is

A knee X-ray is a safe and painless test that uses a small amount of radiation to take a picture of a person's knee. During the examination, an X-ray machine sends a beam of radiation through the knee, and an image is recorded on a computer or special film. This image shows parts of the bones of the knee, including the femur (the bone above the knee), the tibia and fibula (the lower leg bones), the patella (kneecap), and the soft tissues.

The X-ray image is black and white. Dense body parts that block the passage of the X-ray beam through the body, such as bones, appear white on the X-ray image. Softer body tissues, such as muscles, allow the X-ray beams to pass through them and appear darker.

X-rays are performed by an X-ray technician in the radiology department of a hospital, a freestanding radiology center, or a health care provider's office. Three different pictures are usually taken of the knee: from the front (anteroposterior view or AP), from the side (lateral view), and one of the kneecap when the knee is bent (sunrise view).

Why It's Done

A knee X-ray can help find the causes of common signs and symptoms such as pain, tenderness, swelling, or deformity of the knee. It can detect broken bones or a dislocated joint. After a broken bone has been set, the image can help determine whether the bone is in proper alignment and whether it has healed properly.

If knee surgery is required, an X-ray may be taken to plan for the surgery and assess the results of the operation. Also, a knee X-ray can help to diagnose later stages of infection, as well as cysts, tumors, or other diseases in the bone.

Preparation

A knee X-ray doesn't require any special preparation. Your child may be asked to remove some clothing, jewelry, or any metal objects that might interfere with the image.

If your daughter is pregnant, it is important to tell the X-ray technician or her doctor. X-rays are usually avoided during pregnancy because there's a small chance the radiation may harm the developing baby. But if the X-ray is necessary, precautions can be taken to protect the fetus.

Procedure

Although the procedure may take about 10 minutes, actual exposure to radiation is usually less than a few seconds.

Your child will be asked to enter a special room that will most likely contain a table and a large X-ray machine hanging from the ceiling or wall. Parents are usually able to accompany their child to provide reassurance. If you stay in the room while the X-ray is being done, you'll be asked to wear a lead apron to protect certain parts of your body. Your child's reproductive organs will also be protected with a lead shield.

The technician or radiologist will position your child (on the table or standing), then step behind a wall or into an adjoining room to operate the machine. Three X-rays are usually taken (from the front, side, and with the knee bent to get a good picture of the kneecap), so the technician will reposition the leg for each X-ray. Rarely, another view or two might be needed. Occasionally doctors request an X-ray of the opposite knee as well, for comparison.

Older children will be asked to stay still for a couple of seconds while the X-ray is taken; infants may require gentle restraint. Keeping the knee still is important to prevent blurring of the X-ray image.

If your child is in the hospital and can't easily be brought to the radiology department, a portable X-ray machine can be brought to the bedside. Portable X-rays are sometimes used in emergency departments, intensive care units (ICUs), and operating rooms.

What to Expect

Your child won't feel anything as the X-rays are taken. The X-ray room may feel cool due to air conditioning used to maintain the equipment.

The positions required for the X-rays may feel uncomfortable, but they need to be held for only a few seconds. If your child has an injury and can't stay in the required position, the technician might be able to find another position that's easier on your child. Babies often cry in the X-ray room, especially if they're restrained, but this won't interfere with the procedure.

After the X-rays are taken, you and your child will be asked to wait a few minutes while the images are processed. If they are blurred or unclear, the X-rays may need to be redone.

Getting the Results

The X-rays will be looked at by a radiologist (a doctor who is specially trained in reading and interpreting X-ray images). The radiologist will send a report to your doctor, who will discuss the results with you and explain what they mean.

In an emergency, the results of an X-ray can be available quickly. Otherwise, results are usually ready in 1-2 days. In most cases, results can't be given directly to the patient or family at the time of the test.

Risks

In general, X-rays are very safe. Although there's some risk to the body with any exposure to radiation, the amount of radiation used in a knee X-ray is small and not considered dangerous. It's important to know that radiologists use the minimum amount of radiation required to get the best results.

Developing babies are more sensitive to radiation and are at greater risk for harm, so if your daughter is pregnant, make sure to inform her doctor and the X-ray technician.

Helping Your Child

You can help your child prepare for a knee X-ray by explaining the test in simple terms before the procedure. It may help to explain that getting an X-ray is like posing for a picture.

You can describe the room and the equipment that will be used, and reassure your child that you'll be right there for support. For older kids, be sure to explain the importance of staying still while the X-ray is taken so it won't have to be repeated.

If You Have Questions

If you have questions about why the knee X-ray is needed, speak with your doctor. You can also talk to the X-ray technician before the procedure.

Reviewed by: Yamini Durani, MD
Date reviewed: August 2011