Why look at both knees at once?
Comparison with the opposite side is the basic principle of ultrasound diagnostics of the musculoskeletal system. The thickness of the synovial membrane, the volume of small physiological effusion, and the width of the joint space vary markedly from person to person, and there are no unambiguous standards here. The healthy knee of the same patient serves as an ideal standard, so when examining one joint, the doctor often still briefly looks at the second.
But there are situations when a full examination of both knees is necessary initially. These are rheumatoid arthritis, reactive arthritis, gout and other systemic diseases in which the lesion is often symmetrical, and the severity of the right and left differs. This is gonarthrosis, where it is necessary to assess the stage on each side and decide on the order of treatment. This also includes therapy control: the dynamics of synovial thickness and blood flow activity can be correctly monitored in two joints at once.
A separate category is athletes and people with high loads on the knees, in whom overload changes in the ligaments and tendons develop on both sides, but appear earlier on the dominant leg. Bilateral examination allows you to see the problem on the asymptomatic side before pain occurs.
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.