Screening for dysplasia in infants
The main indication for a bilateral study is screening for hip dysplasia in children in the first year of life. In newborns, the femoral head and acetabulum still consist of cartilage, which is not visible on x-rays, but is completely accessible to ultrasound. Therefore, it is ultrasound, and not x-rays, that is the method of choice at this age.
The study is performed using the Graf method: the alpha angle, reflecting the development of the bony part of the roof of the acetabulum, and the beta angle, characterizing the cartilaginous part, are measured in a standard plane. According to their values, the joint belongs to one of the types - from mature to decentered and dislocated. Both joints must be assessed: dysplasia is often asymmetrical, and comparing the sides helps not to miss a milder degree with one of them.
The optimal screening period is 1–1.5 months. Early detection is essential: at this age, orthopedic devices and the correct position of the legs are sufficient, while late detected dislocation requires long-term treatment, and sometimes surgery. In adults, a bilateral study is prescribed for coxarthrosis, systemic arthritis and bilateral trochanteric bursitis.
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.