Knee ultrasound: what it is used for, what happens and what to expect

An ultrasound scan uses high-frequency sound waves to create an image of the area being examined. The Royal National Orthopaedic Hospital describes it as a widely used diagnostic test to examine the organs and soft tissues of the body. A small handheld probe is moved over the skin with a lubricating gel, and the picture appears on a screen as the scan is carried out.

For the knee, ultrasound is used to examine the soft tissues around the joint, such as tendons and fluid-filled swellings like a Baker’s cyst. The NHS notes that you may need to change position during the test.

It is carried out by a radiologist or a sonographer, both trained professionals, and the report is usually reviewed before it goes to the doctor who requested it.

A GP or specialist may request an ultrasound when the cause of knee pain, swelling or a lump is unclear and a soft tissue cause is suspected. The NHS knee pain page lists many possible causes, from sprains and strains and tendonitis to osteoarthritis, bursitis and torn ligaments, tendons or meniscus, and says a doctor may refer you to hospital for a scan or specialist treatment.

The NHS pages on specific conditions mention scans in several places. For a Baker’s cyst, a soft fluid-filled lump at the back of the knee, the NHS says the GP may send you for an ultrasound or MRI scan, or refer you to a specialist. For bursitis, it says a GP may take a sample of fluid with a needle and that you may have other tests such as a blood test or a scan. Ultrasound can also be used to guide a needle for aspiration or an injection.

• A lump or swelling at the back or front of the knee, such as a suspected Baker’s cyst.

• Swelling around the knee when the cause is not obvious.

• Pain over a tendon, for example below the kneecap, when tendonitis or a tendon injury is suspected.

• Warm, red, swollen areas that might indicate bursitis, when examination alone is not enough.

• Guiding an injection or fluid sample to the right place.

The NHS says the hospital or clinic should tell you if there is anything you need to do beforehand. For a knee scan there is usually no special preparation, although the department will tell you if there is. The RNOH leaflet notes that any preparation will be given at booking or in the appointment letter, and that medication can still be taken with water before your scan.

Wear clothing that allows easy access to the knee, such as loose trousers or shorts, as you may be asked to remove clothing covering the area. You can ask for a chaperone, and you may bring a friend or relative. Remove any jewellery from the area being scanned, and tell the team about anything you are worried about.

According to the NHS, you will be asked to lie or sit on a flat bed next to a machine with a screen. The healthcare professional will put a slippery gel on the skin over the knee, move the probe over the area, and may stop to make notes or take measurements. You may need to change position. The lights may be dimmed to help them see the images.

You will not feel the ultrasound itself. The RNOH leaflet notes that the operator may need to press the probe down gently to get a good view, which can be uncomfortable if the knee is tender, and asks you to say if it hurts so adjustments can be made. When the scan is finished, the gel is wiped off with paper towel and you can get dressed. The NHS reminds you that you are in control of the scan and can ask for it to stop at any time.

The person scanning may not be able to tell you the result straight away. The Lymphoma Action leaflet on ultrasound notes that staff in the scanning department usually cannot give the results, because the images are interpreted by a specialist alongside other information, and the NHS says results go to the doctor who referred you. You may be given them at the end of the appointment, or it can take a few weeks, and you may need a follow-up appointment with your doctor.

If you have not heard anything after a few weeks, the NHS advises contacting the specialist or your GP surgery. Sometimes other tests are needed depending on why you had the scan. An ultrasound result is one piece of information: findings are interpreted alongside your symptoms and examination, and some abnormal findings can be present without causing symptoms.

The RNOH leaflet explains that, depending on signs and symptoms, alternatives to ultrasound include CT or MRI scans. The choice depends on what the doctor is trying to find. In general terms, ultrasound is suited to the soft tissues near the surface and to fluid, and is quick and does not use radiation; other tests may be chosen when the structures inside the joint, such as ligaments or cartilage, need to be seen, or when bone is the concern.

The NHS notes that for osteoarthritis, further tests such as X-rays or blood tests are not usually necessary but may be used to rule out other causes, such as rheumatoid arthritis or a fractured bone. Knee pain after an injury, especially if the knee is unstable, locks or gives way, usually needs clinical assessment first; the NHS advises getting advice from NHS 111 if the knee is very painful, you cannot put weight on it, it is badly swollen or has changed shape, or you have a high temperature with redness or heat around the knee.

In the NHS, a knee ultrasound is requested by a doctor or other health professional as part of an assessment. The NHS also notes that in many areas you can get help such as physiotherapy from community musculoskeletal (MSK) services for back or joint problems without needing a GP referral.

Private ultrasound scans are available in many towns and may be quicker to arrange. If you pay privately, make sure the report goes to your GP or specialist and that a clinician will explain the findings and next steps, because a scan result on its own does not decide treatment.