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What is a knee ultrasound scan?

What is an ultrasound?

Ultrasound is high-frequency sound waves produced by a special machine. They can provide very useful images of the tissues within the body, and it is extremely useful in assessing various musculoskeletal conditions involving tendons, muscles, joints and adjacent soft tissues. Unlike X-rays, ultrasound is harmless and involves no radiation or other harmful side effects.

Do I need a knee ultrasound scan?

Knee ultrasound is very useful in assessing certain knee conditions. These include:

  • Knee joint effusion. This refers to excess fluid build-up within the knee joint, which can happen for various reasons (knee injury, knee arthritis, meniscal injury, etc). An ultrasound scan can assess the size of the effusion and if there is any active inflammation (which can be seen in certain types of inflammatory arthritis “like rheumatoid arthritis”). Furthermore, ultrasound can be used to guide a knee aspiration. This is a special procedure where the excess fluid within the knee is removed by inserting a small needle under ultrasound guidance.

  • Ultrasound scan is useful to assess the ligaments that run inside and outside of the knee (called the collateral ligaments).

  • Assessment of Baker’s cyst is excellent with ultrasound. The scan will confirm the diagnosis and assess the size of the cyst and suitability for ultrasound-guided aspiration.

  • Assessing soft tissue lumps around the knee joint. Ultrasound is usually the first choice of imaging to assess for swelling around the knee.

  • Assess a special group of tendons that run at the inner aspect of the upper leg, just below the knee (pes anserine tendinosis and bursitis).

  • Assess for any fluid build-up in front of the knee cap (called pre patellar bursitis).

  • Probably the most important use of ultrasound in the knee is to guide injections and treatment. Ultrasound allows for direct visualisation of the needle to ensure that the medicines are injected into the exact targeted site. There is significant evidence that ultrasound-guided injections provide better levels of pain relief and longer effects compared with injections done without imaging guidance. Injecting under ultrasound guidance allows for visualising the adjacent structures and reduces the risk of tissue injury at the injection site.

The different conditions where ultrasound-guided treatment can be useful include:

Different treatment options/injections can be done under ultrasound guidance to help manage knee pain. These include:

What are the limitations of a knee ultrasound scan?

The deep structures of the knee cannot be adequately assessed with ultrasound. These include:

· The menisci. The meniscus is a C-shaped structure made of a special type of cartilage (called hyaline cartilage). They are very important to support normal knee function and can undergo tears. A meniscal tear can be difficult to pick up on ultrasound, and an MRI examination would be preferred.

· The cruciate ligaments: These are two ligaments located very deep inside the knee joint and, therefore, difficult to visualise with ultrasound. A knee MRI examination is preferred.

· While ultrasound is excellent in assessing soft tissue lumps, an assessment with a knee x-ray or MRI would be more useful if there is a possible bone lesion.

Knee ultrasound vs x-ray and MRI

A knee X-ray (or knee radiograph) is a black-and-white image produced by sending an X-ray beam through the knee area and getting the images on a film. It is usually the first type of imaging done when there is a knee problem, particularly if there is suspicion of knee arthritis or fracture around the knee. The bones absorb the X-ray beam more than the other surrounding soft tissues, and therefore, bones are usually very well seen on X-rays, unlike ultrasound. However, they cannot assess the soft tissues like ultrasound and involve a small dose of radiation. To find out more, please see our article about knee X-rays.

An MRI examination uses strong magnetic fields and radio waves to produce detailed knee images. MRI is preferred when there is significant knee trauma with suspicion of multiple injuries to the ligaments and internal structures. MRI, however, is more expensive and requires you to stay still for 20-30 minutes inside a special machine. To find out more, please see our article about knee MRI scans.

To learn more, please see our article about the different types of scans used for knee problems.


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