CT of the shoulder joint

Upper limb terminology and structure

From the standpoint of medical nomenclature, the shoulder (brachium) is exclusively the proximal segment of the free upper limb, represented by a single humerus and located between the shoulder and elbow joints. What is commonly referred to as «shoulders» in everyday speech is designated in anatomy as the shoulder girdle (the region above the shoulder).

The free upper limb is divided into three key segments:

1. Proximal segment — the arm (brachium).
2. Middle segment — the forearm (antebrachium), including the ulna (medially) and the radius (laterally).
3. Distal segment — the hand (manus), consisting of the carpus, metacarpus, and phalanges. In the correct anatomical position, the first digit (thumb) is always abducted laterally.

Calcium hydroxyapatite (HA) deposition disease

The disease occurs in acute or chronic forms, and its etiology remains unknown to this day. Calcium hydroxyapatite crystals can deposit both within the joint cavity and in periarticular soft tissues, such as ligaments and tendons.

In large joints (especially the shoulder and hip), the pathogenesis develops through the following chain: HA accumulation within the tendon leads to a significant increase in its volume, which naturally causes inflammation in surrounding structures. Clinically, this manifests as a combination of inflammatory pain and mechanical issues, such as the development of secondary external shoulder impingement syndrome.

In most cases, the disease is self-limiting, and patients recover without surgical or complex conservative intervention. However, when deposits are localized near entheses, intraosseous spreading may occur, accompanied by intense bone marrow inflammation.

Milwaukee shoulder syndrome

The effect of intraarticular hydroxyapatite deposition on cartilage degradation is not yet fully understood, but the most well-known manifestation of this pathology is «Milwaukee shoulder» (rotator cuff arthropathy).

Characteristic imaging features of this condition include:

• Cranial migration of the humeral head, occurring against the background of a massive rotator cuff tear.
• Remodeling (shape alteration) of the inferior surface of the acromion.
• Rapid loss of bone tissue in the humeral head itself.

Researchers debate the etiopathogenesis: some authors believe that the primary factor is traumatic or degenerative damage to the rotator cuff, while HA crystal precipitation acts as a secondary process.

Diagnostic imaging of deposits

Imaging methods clearly differentiate the stages of calcium deposit maturation. Unlike soft tissue ossification, on conventional radiography, hydroxyapatite deposits are visualized as amorphous accumulations lacking an internal trabecular structure.

On ultrasound examination (US), the picture depends on the phase of the disease:

• Acute stage: inclusions appear hyperechoic but do not produce an acoustic shadow (for example, a heterogeneous echostructure with bright foci is visible in the supraspinatus tendon).
• Maturation stage: deposits become denser, a characteristic acoustic shadow appears behind the accumulation, and fragmentation of the calcification is often observed.

Adjacent structures and nerves

Grooves for neurovascular bundles are laid down along the entire length of the upper limb bones. Each nerve innervates specific digits according to the dermatome rule. For example, compression of the ulnar nerve in its natural bony groove in the forearm can lead to entrapment neuropathy. In this case, the patient experiences numbness in two digits—the ring finger and the little finger. This may be related to the anatomical depth of the groove, age-related joint changes, or displacement of the nerve trunk itself.

Class and exam questions

What else to study on this topic

• Radiological features of pyrophosphate arthropathy in the knee joint
• Differences between pyrophosphate arthropathy and primary osteoarthritis
• Periarticular hydroxyapatite deposition in the small joints of the hand and foot
• Entrapment syndromes of the free upper limb and the role of the ulnar nerve
• Mechanism of secondary impingement syndrome development in tendinitis

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.