Limb Length Discrepancy

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X-Ray Exam: Leg Length

What It Is

A leg length X-ray is a safe and painless test that uses a small amount of radiation to make images of the bones in a child's legs, to measure and compare their length. During the examination, an X-ray machine sends a beam of radiation through the legs, and an image is recorded on special film or a computer. The image shows the soft tissues and the bones in the legs, which includes the femur, tibia, and fibula.

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 the skin and muscles, allow the X-ray beams to pass through them and appear darker.

An X-ray technician takes the X-rays. In younger kids, one picture is taken of both legs at the same time, from the front while the child is standing. In older kids, three separate pictures might be taken, of the hips, knees, and ankles. This is done while the child is lying down.

Why It's Done

Although no one is exactly symmetric, some kids may have significant differences in the length of their legs, a condition known as leg length discrepancy. This can have several causes, as well as several symptoms. Some kids don't feel anything at all, while others might have knee, hip, or back pain. Some kids with leg length discrepancy limp or may get tired easily while walking.

In addition to physically examining the child, a doctor requests a leg length X-ray to help with the measurement. It's important to determine the exact difference in leg length before a doctor can decide on a treatment plan. If a leg length discrepancy is verified, the X-ray might be repeated at regular intervals to see if the difference is getting greater or to monitor the effects of treatment.

Preparation

A leg length 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 you suspect that your daughter is pregnant, it's 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 15 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 come in with 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 will position your child (either lying or standing), then step behind a wall or into an adjoining room to operate the machine. Older kids will be asked to stay still for a couple of seconds while the X-ray is taken; infants may require gentle restraint. Keeping the legs still is important to prevent blurring of the X-ray image.

What to Expect

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

The position required for the X-ray may feel uncomfortable, but it needs to be held for only a few seconds. If your child 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-ray is taken, you and your child will be asked to wait a few minutes while the image is processed. If it's blurred or unclear, some of the views may need to be redone.

Getting the Results

The X-rays will be looked at by a radiologist (a doctor who's 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.

Risks

In general, X-rays are very safe. Although exposure to radiation poses some risk to the body, the amount used in a leg length 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 leg length 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 keeping 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 leg length 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