Go to A&E or an urgent treatment centre rather than booking a scan if you have: sudden calf pain and swelling, particularly with breathlessness or chest pain; a hot, red, swollen knee with fever; a knee you cannot weight-bear on after an injury; or a knee that locks and will not straighten. Calf swelling can indicate a DVT, which needs same-day assessment.
Knee ultrasound is very good at some things and the wrong test for others. It assesses tendons, bursae, joint fluid and Baker’s cysts in real time, and it can settle whether calf swelling is a ruptured cyst or something more serious. It cannot see the menisci or the cruciate ligaments — those need MRI. Knowing which question you are asking determines which scan you should book.
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Knee pain is the second most common musculoskeletal complaint after back pain, affecting people of all ages. The appropriate investigation depends on where the pain is, how it started, and what came with it.
| Condition | Typical presentation |
|---|---|
| Patellar tendinopathy | Pain at the front of the knee, below the kneecap; common in runners and jumping athletes |
| Quadriceps tendinopathy or tear | Pain above the kneecap; weakness on straightening the knee |
| Iliotibial band syndrome | Pain on the outer aspect of the knee; common in runners |
| Baker’s cyst | Swelling behind the knee; may rupture, causing calf pain and swelling that mimics DVT |
| Knee joint effusion | Swelling within the joint; may indicate meniscal tear, ligament injury or inflammatory arthritis |
| Bursitis | Localised swelling and tenderness over a specific bursa (prepatellar, infrapatellar or pes anserine) |
| Osteoarthritis | Diffuse aching pain, worse with activity; crepitus; loss of range of movement |
A knee ultrasound is appropriate when:
Doppler adds information about active inflammation, and the joint can be scanned while it moves, which static imaging cannot do.
This is where knee ultrasound is most often booked for the wrong reason.
Ultrasound is not the appropriate investigation for suspected meniscal tears or cruciate ligament injuries. The menisci sit deep between the bone surfaces and the cruciate ligaments run inside the joint; neither is reliably assessed with ultrasound. These require MRI.
If you have had a significant knee injury with instability, locking or the knee giving way, your GP should refer you for MRI rather than an ultrasound. Cartilage wear and the severity of osteoarthritis are also better assessed with a weight-bearing X-ray.
| Question | Right test |
|---|---|
| Is this tendon torn or inflamed? | Ultrasound |
| Is the lump behind my knee a cyst, and has it burst? | Ultrasound |
| Is there fluid in the joint, and can it be aspirated accurately? | Ultrasound |
| Have I torn my meniscus or cruciate ligament? | MRI |
| How advanced is my osteoarthritis? | Weight-bearing X-ray |
| Is this calf swelling a DVT? | Urgent DVT scan, same day |
A Baker’s cyst is a fluid-filled swelling behind the knee, usually connected to the joint and often secondary to something else going on inside it — arthritis or a meniscal problem. Most are painless and found incidentally.
The complication that matters is rupture. When a cyst bursts, fluid tracks down into the calf, producing sudden pain and swelling that is clinically indistinguishable from a deep vein thrombosis. Both present as a swollen, tender calf in someone whose knee has been troubling them.
Because a missed DVT can be fatal, the safe order is always to exclude DVT first. A DVT scan answers that question directly, and the same examination will usually demonstrate the ruptured cyst if that is what it is.
Baker’s cysts are a common incidental finding on knee ultrasound, but they become clinically important when they rupture. A ruptured Baker’s cyst causes sudden calf pain and swelling that is clinically indistinguishable from a DVT.
We regularly see patients who have been referred for a DVT scan after a ruptured Baker’s cyst. The ultrasound confirms the diagnosis immediately, avoiding unnecessary anticoagulation treatment.
Describes general patterns across the patients we scan, not any individual case. Sudden calf swelling should always be assessed urgently in the first instance.
No. Warmed gel is applied and a probe is moved over the skin. You may be asked to bend and straighten the knee during the scan, but the examination itself is painless.
No. The menisci sit deep within the joint and are not reliably assessed with ultrasound. MRI is the appropriate investigation for suspected meniscal or cruciate ligament injury, particularly where there is locking, instability or giving way.
Most swellings behind the knee are Baker’s cysts, which are benign. If the swelling has suddenly moved down into your calf with pain, seek same-day medical assessment, because a DVT needs excluding before anything else.
Yes. A Doppler examination assesses the deep veins for clot and will also demonstrate fluid tracking from a ruptured cyst. Because the two present identically, the vein assessment is done first.
No. You can book a knee ultrasound directly with us, and we can send the report to your GP or physiotherapist at your request.
A knee ultrasound is £235. You receive instant verbal results at the appointment from the consultant performing your scan, with the written report following within 24 hours.
Related: Achilles tendon pain, hip and groin pain, and our guide to MSK ultrasound.
CQC-registered clinic · HCPC-registered sonographers · BMUS member · ICO-registered. Reports can be shared securely with your NHS GP or specialist at your request.
