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
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: one from the front (anteroposterior view or
AP), one from the side (lateral view), and one of the kneecap when the knee is bent
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,
later, to see 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.
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
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.
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 usually
can 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 also will be protected with a lead
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, the 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 also order an X-ray
of the opposite knee 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.
In general, X-rays are very safe. Although there's some risk to the body with any
exposure to radiation, the amount 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 more 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 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.