Knee X-ray: why it is done, what happens and what it can show

An X-ray is a test that takes pictures of the inside of your body using a small amount of radiation, according to the NHS. A knee X-ray focuses on the joint where the thigh bone (femur), shin bone (tibia) and kneecap (patella) meet. The radiographer, the health professional who takes X-rays in hospital, positions you and lines up the machine with the knee.

Most X-rays are done in hospitals, including emergency departments, minor injuries units and X-ray departments. The test uses a standard X-ray machine, which the NHS lists as the type used mainly to check the bones. Other scans, such as an MRI or ultrasound scan of the knee, are used when soft tissues like ligaments, cartilage or tendons need to be examined.

The NHS says an X-ray may be used to check your bones for a fracture after an accident and to find out if symptoms you are having could be caused by a condition. For the knee, common reasons include the following.

• A recent injury, such as a fall or twisting injury, where a broken bone is possible. The NHS says to get an urgent GP appointment or call 111 for very bad knee pain after a fall or injury, being unable to walk or put weight on the knee, or being unable to bend or straighten the leg.

• Persistent pain or swelling that has not improved after a few weeks. The NHS advises seeing a GP if knee pain does not improve within a few weeks.

• A suspected joint problem such as osteoarthritis, which the NHS says most often causes problems in the knees, hips and small joints of the hands. X-rays are not usually necessary to diagnose it, but may be used to rule out other possible causes such as a fractured bone or rheumatoid arthritis.

• Checking a joint after surgery or following up a known condition.

You will be told before your X-ray if there is anything you need to do. The NHS says that depending on the type, you may be asked not to wear metal such as jewellery, clothes with zips or poppers, bras or hair clips. You may also be asked to change into a gown, depending on the part of the body being X-rayed.

Tell the healthcare professional if you are pregnant or might be pregnant, are breastfeeding, take any medicines or have any allergies. You can also ask for a chaperone, either someone you know or a trained member of staff. The NHS lists fasting only for certain types of X-ray, such as those involving barium.

The NHS describes the sequence: the X-ray machine is lined up with the part of the body being imaged, the person taking the X-ray (and any chaperone) moves behind a screen or leaves the room, and you are asked to keep very still for a few seconds while one or more images are taken. You may be asked to move into different positions. You will not feel anything when the images are taken.

After a standard X-ray you can usually leave as soon as it is done.

Because the standard X-ray is used mainly to check bones, it is good at showing breaks and some changes in the bones and the space between them. It does not show ligaments, cartilage or other soft tissue in detail. The NHS describes CT scans as the option when more detailed pictures are needed, and the knee pages suggest that a doctor may refer you to hospital for a scan or specialist treatment depending on the cause.

A normal X-ray does not rule out all knee problems. The NHS lists tears of a ligament, tendon or meniscus, and cartilage damage as possible causes of a knee that is unstable, gives way, cannot be straightened or makes a popping sound during injury. For a meniscus tear, the NHS says arthroscopy (keyhole surgery) may be needed if the damage is severe. Soft tissue injuries are therefore often assessed clinically and, if needed, with MRI or ultrasound.

The NHS says you should be told when you will get the results of an X-ray. This may be on the same day, at an appointment, or a few weeks after. If you have not heard anything after a few weeks, contact the doctor who referred you, or your GP. A healthcare professional should explain the results and whether any next steps are needed, which may include other tests.

If a fracture or another problem is found, the clinician who requested the X-ray will explain the next steps.

The NHS says X-rays are usually very safe and serious complications are rare. If you have lots of X-rays, there is a very small risk that radiation from them could cause cancer, and your doctor will assess the risks before offering one and discuss this with you. If you are pregnant, tell the radiographer before the test.

On the NHS, a knee X-ray is requested by a GP, an emergency or urgent care clinician or an NHS musculoskeletal service, which the NHS says you may be able to access in many areas without a GP referral. Private imaging centres also offer X-rays, but results should still be reviewed by a clinician who knows your history.