Ultrasound of the articular joints

A joint capsule is a sac containing synovial fluid. They are most often located at the attachment point of tendons, under the tendon above the bone. Joint capsules are divided into those communicating with the joint cavity and those not communicating. Non-communicating ones are most common. Joint capsules can also be divided into superficial (subcutaneous) and deep.

Methodology for examining joint capsules.

The choice of sensor depends on the type of joint capsule being examined. For superficial ones, it is better to use a sensor with a frequency of 10-15 MHz and a large amount of gel. For deep ones - from 3 to 7.5 MHz, depending on the constitution of the patient. Comparison with the contralateral side helps in assessing the amount of synovial fluid inside the capsule. The largest bursa is the subacromial-subdeltoid, located deep under the deltoid muscle. Some of the joint capsules are directly connected to the joint cavity, such as the suprapatellar bursa or the superficially located patellar bursa or olecranon bursa.

Localization and types of joint capsules

Bag type	Localization	Name of the bag
Subcutaneous	Elbow	Olecranon bursa
	Hip	Subcutaneous bursa of the greater trochanter
	Knee	Prepatellar, subcutaneous infrapatellar, subcutaneous bursa of the tibial tuberosity
	Ankle	Subcutaneous bursa of the Achilles tendon
	Foot	Metatarsal bursa of the first toe
Deep	Shoulder	Subacromial, subdeltoid, subscapular (in 50% of cases may be associated with the joint)
	Hip	Internal obturator, iliopsoas (may be associated with the joint), deep trochanteric
	Knee	Iliotibial tract bursa, fibular collateral ligament bursa, tibial collateral ligament bursa, subfascial prepatellar, deep infrapatellar, popliteal bursa (may be connected to the joint), gastrocnemius semimembranosus (may be connected to the joint), suprapatellar (may be connected to the joint)
	Ankle	Heel bag

The echo picture of the joint bags is normal.

Normally, the cavity of the joint capsule looks like a thin hypoechoic strip about 1-2 mm thick, surrounded by hyperechoic lines - the walls of the capsule. Normally, the suprapatellar bursa contains about 3-5 ml of fluid. By squeezing the lateral sections of the capsule toward the center, the visualization of the capsule can be improved.

Ultrasound signs of pathology of joint capsules.

Bursitis is an inflammatory reaction in the joint capsule. The degree of inflammatory reaction can be from minimal, in the form of synovitis, to abscess formation. The echo picture depends on the presence or absence of a connection between the joint capsule and the joint cavity. Treatment consists of limiting movements in the joint. Sometimes anesthetics and corticosteroids are injected into the joint capsule to reduce the inflammatory reaction of the synovial membrane.

Pathology of joint capsules that do not communicate with the joint cavity.

Acute posttraumatic bursitis. The mechanism of bursitis development is repeated trauma. It occurs as a protective reaction to frequent joint trauma. In acute bursitis, the volume of the joint capsule increases, anechoic contents appear. The walls of the capsule are thin, which is a defining feature in differentiating from chronic bursitis. The most typical localizations of acute bursitis are: prepatellar bursa, deep infrapatellar bursa, retrocalcaneal bursa of the Achilles tendon, olecranon bursa, subcutaneous bursa of the greater trochanter of the femur.

Chronic post-traumatic bursitis. Develops with constant increased loads on a certain anatomical area. In chronic bursitis, unlike acute bursitis, the walls of the bursa are thick. The contents may be anechoic, hypo- or hyperechoic with the presence of fibrous septa. Hyperechoic calcifications may be detected against the background of the contents of the bursa. Women very often develop chronic bursitis of the metatarsal bursa of the first toe, which is caused by wearing shoes that are too tight.

Hemorrhagic bursitis. The causes of occurrence are different: from simple trauma, rupture of adjacent tendon, bone fracture to increased bleeding and disorder of hemostasis system. In this case, the joint capsule is enlarged in size more than in simple acute bursitis, which arose as a result of excessive load. In the acute period, the echostructure of the contents of the capsule is uniform due to finely dispersed suspension, later it becomes heterogeneous, due to the presence of echogenic clots, fibrin threads and anechoic fluid.

Pathology of the joint capsules communicating with the joint cavity.

Effusion in the joint capsule and intra-articular pathology. The connection of the joint capsule with the joint cavity develops gradually and is often observed after 50 years. For example, bursitis of the iliopsoas bursa in aseptic necrosis of the femoral head or the appearance of Baker's cysts (gastrocnemius semimembranosus bursitis) in athletes. The connection of the subacromial bursa with the shoulder joint appears only in case of rotator cuff ruptures.

The presence of fluid in the subacromial bursa may be an early sign of impingement syndrome.

Ultrasound examination can reveal a connection between the bursa and the joint through a narrow channel, for example, in Baker's cysts at the medial edge of the popliteal fossa.

An increase in the size of the joint bags may be a manifestation of a number of pathological conditions and diseases of the joints: osteochondritis dissecans, osteonecrosis, osteoarthritis, defects of the cartilaginous plate, damage to the meniscus, intra-articular body ("joint mouse"). In all these diseases, the fluid in the bag is anechoic. Ruptures occur in rheumatoid bursitis.