What does an ultrasound of the knee show?
The knee is the most common object of ultrasound examination among joints. The method accurately detects even a small amount of free fluid in the joint cavity and in the inversions, which makes it convenient for any swollen knee of unknown origin. The lateral collateral ligaments, the patellar ligament and the quadriceps tendon, the prepatellar and infrapatellar bursae, and the popliteal fossa are clearly visible.
The classic finding is a Baker's cyst: a collection of fluid in the bursa between the semimembranosus and gastrocnemius tendons. Ultrasound not only confirms it, but also shows the size, contents and the fact of rupture - a ruptured cyst allows fluid to flow down the lower leg and clinically simulates deep vein thrombosis, which makes the study truly important.
At the same time, you need to understand the limitation: the menisci are visible only partially, in the peripheral part, and the cruciate ligaments are hidden inside the joint and are practically inaccessible to ultrasound. If the injury occurs with rotation of the tibia and there is joint blockage or a feeling of instability, an MRI is necessary.
What can be seen on an ultrasound of the joint
Ultrasound, better than any other method, shows the soft tissues surrounding the joint: tendons and their sheaths, ligaments, muscles, synovium, joint capsules and peripheral nerves. It reliably detects free fluid in the joint cavity and bursae, tendon ruptures and tears, cysts, bursitis and tendinitis.
There are also fundamental limitations. Ultrasound does not pass through the bone, so the internal structures - menisci along the entire length, cruciate ligaments, articular cartilage deep in the joint, bone marrow - remain inaccessible. They use magnetic resonance imaging, and X-rays are used to evaluate the bone itself.
The main advantage is exploration in motion
Unlike MRI, which takes a picture of a motionless joint, ultrasound allows you to look at the joint in dynamics. The doctor asks you to bend, straighten or rotate the limb straight during the scan and sees how the tendons and ligaments behave under load.
Many conditions manifest themselves only during movement: tendon subluxation from the groove, impingement, trigger finger, ligament instability. At rest, the picture looks normal, and without functional tests the diagnosis cannot be made.
When to start with ultrasound
Ultrasound is reasonable as the first method for pain and swelling of the joint without injury, for suspected bursitis, tendinitis and cysts, for monitoring the treatment of inflammatory diseases and, if necessary, to induce a puncture or blockade.
If there was a serious injury with a suspected fracture, you need to start with an x-ray. If a torn meniscus or cruciate ligament is suspected, MRI remains the method of choice.