Transient synovitis and trochanteric bursitis
In children, hip ultrasound is the first-line method for sudden lameness and pain. The main task is to detect effusion in the joint cavity, which is typical of transient synovitis, the most common cause of acute lameness in children of preschool and primary school age. Ultrasound detects even a small amount of fluid in the expansion of the anterior pocket and allows you to track how the effusion resolves during treatment.
In adults, a common cause of pain in the joint area is not the joint itself, but the surrounding structures. Trochanteric bursitis and tendinopathy of the tendons of the gluteus medius and minimus muscles cause pain along the outer surface of the thigh, which increases when lying on the side. These structures are located superficially and are clearly visible on ultrasound, whereas on x-ray they are not visible at all.
The limitation is significant: the head of the femur and articular cartilage in adults are covered by bone structures and cannot be fully assessed by ultrasound. If coxarthrosis, aseptic necrosis of the femoral head or neck fracture is suspected, X-rays and MRI are needed.
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.