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 symmetrical, 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.
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
Developing babies are more sensitive to radiation and are at more risk for harm,
so if your daughter is pregnant, inform her doctor and the X-ray technician.
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 also will be protected with a lead
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
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.
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, be sure to tell 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.