Rotator cuff and compartment syndrome
The shoulder is the joint where ultrasound really competes with MRI. The rotator cuff - a complex of tendons of the supraspinatus, infraspinatus, teres minor and subscapularis muscles - is located superficially and is perfectly accessible to the sensor. Partial and complete ruptures, calcific tendinitis, and degenerative thinning of the tendons are clearly visible and dynamic.
The most common cause of shoulder pain in middle-aged people is subacromial impingement syndrome, in which the supraspinatus tendon and bursa become pinched between the head of the humerus and the acromion when raising the arm. Ultrasound reveals it in motion: the doctor asks you to slowly move your arm to the side and sees the moment of pinching and a thickened bursa that does not slip under the acromion. This mechanism is not reproduced in a static MRI image.
The tendon of the long head of the biceps muscle is also assessed: fluid around it, subluxation or complete dislocation from the intertubercular groove, rupture. The limitation of the method is the labrum and internal parts of the joint, which are inaccessible to ultrasound; if damage is suspected, an MRI is prescribed, with contrast if 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.