Biomechanics and anatomical features
The shoulder joint has the greatest freedom of movement in the body. This is achieved due to the shape of the glenohumeral joint (glenohumeral joint): the shallow glenoid cavity of the scapula (glenoid) articulates with the large humeral head. Biomechanically, this resembles a golf ball on a small tee.
Because of this shape, the joint is inherently unstable. Its stabilization relies on soft tissues:
- Static stabilizers: the labrum (labrum), which increases the surface area of the socket, and the glenohumeral ligaments (capsule). The most important of these is the inferior glenohumeral ligament, which acts as a supporting "hammock" during arm abduction.
- Dynamic stabilizers: the tendon of the long head of the biceps and the rotator cuff. The cuff includes the subclavius, supraspinatus, infraspinatus, and teres minor muscles. Their tendons closely intertwine at the sites of attachment to the tubercles of the humerus.
To ensure smooth gliding of the tendons, the joint is surrounded by a network of communicating synovial bursae: the subscapularis, subacromial, and subdeltoid bursae.
Rotator cuff pathology
The leading cause of cuff tears is external impingement syndrome. During arm abduction, structures are pinched between the humeral head and the coracoacromial arch. With age (usually over 40) or due to bone spurs, the space narrows, triggering a cascade of changes: edema $\rightarrow$ tendinopathy (tendon degeneration) $\rightarrow$ tear.
Tears are divided into two categories:
- Partial tears: affect only the articular or bursal surface of the tendon without passing through its entire thickness.
- Full-thickness tears: the defect extends all the way through from the articular to the bursal surface. A pathological communication arises between the joint cavity and the subacromial bursa. Most commonly, the supraspinatus tendon (supraspinatus) is affected first.
Note: the term "tendinitis" is considered obsolete, as chronic tendon injury is of a degenerative rather than inflammatory nature.
Instability and shoulder dislocations
Traumatic instability most commonly manifests as an anterior dislocation. When the humeral head is displaced anteriorly, the structures of the anterior margin of the joint are injured.
Key traumatic injuries:
- Bankart lesion (Bankart lesion): detachment of the anteroinferior labrum. If the margin of the glenoid fractures along with the labrum, it is referred to as a bony Bankart lesion.
- Hill-Sachs defect (Hill-Sachs defect): an impaction fracture on the posterosuperior surface of the humeral head. It occurs when the humeral head impacts against the hard edge of the glenoid during dislocation.
In rare posterior dislocations (e.g., following epileptic seizures), analogous defects occur on the opposite side—referred to as a reverse Bankart and a reverse Hill-Sachs defect.
Selection of diagnostic imaging modality
A rational approach to imaging depends on the clinical task:
- Radiography: the primary method. Detects dislocations, fractures, secondary osteoarthritis, bone spurs, and calcifications. A sharp narrowing of the subacromial space on radiographs is a specific sign of a massive cuff tear.
- Ultrasound: a reliable method for evaluating superficial structures and cuff tendons. Allows visualization of fluid-filled defects. The key is to avoid the anisotropy artifact, which mimics a tear.
- MRI: the "gold standard" for soft tissue evaluation. The main sign of a full-thickness tear on MRI is a localized tendon defect combined with simultaneous fluid accumulation in the joint cavity and the subacromial bursa.
Briefly: answers to topical questions
Which radiographic views are used for reliable diagnosis of a shoulder dislocation?
Standard and additional radiographic views are used for reliable diagnosis of a shoulder dislocation.
- Anteroposterior view (AP view) — a standard projection where pathology is easily identified.
- Axial view (axial view) — used for definitive confirmation of the diagnosis, feasible in chronic instability.
- Modified 40-degree axial oblique view (modified 40-degree axial oblique) — preferred in acute trauma when arm abduction is not possible.
- 'Y' lateral view ('Y' lateral view) — used when a scapular fracture is suspected or if the modified axial view is not feasible.
Which synovial bursae normally surround the shoulder joint?
The shoulder joint is normally surrounded by several communicating synovial bursae that provide lubrication for tendon movement.
- Subscapularis bursa (bursa subtendinea m. subscapularis / Subscapularis bursa) — located between the subscapularis tendon and the anterior portion of the deltoid muscle.
- Subacromial bursa (Subacromial bursa) — located between the supraspinatus and infraspinatus muscles and the inferior surface of the acromion.
- Subdeltoid bursa (Subdeltoid bursa) — a continuation of the subacromial and subscapularis bursae, located superficial to the greater tubercle of the humerus.
- Subcoracoid bursa (bursa m. coracobrachialis / bursa subcoracoidea) — lies superficial to the subscapularis bursa and frequently communicates with it.
Which specific anatomical zones are injured in a posterior shoulder dislocation (reverse Bankart and reverse Hill-Sachs)?
In posterior shoulder dislocation, sources describe the following injuries:
- Reverse Hill-Sachs lesion (reverse Hill-Sachs defect) — a defect of the anterosuperior surface of the humeral head resulting from a posterior dislocation.
- Posterior glenoid rim fractures (posterior glenoid fractures) — injuries to the posterior margin of the glenoid identified on CT or MRI.
The term "reverse Bankart" is not described in the provided sources.
What is the anisotropy artifact in tendon ultrasound and why does it occur?
The anisotropy artifact (Anisotropy) is a specific ultrasound artifact that occurs when examining tissues with a parallel linear fibrillar structure (tendons, ligaments, muscles). It arises when the ultrasound beam does not strike strictly perpendicular to the direction of the fiber bundles. As a result, the tissue appears hypoechoic on the monitor screen. This phenomenon represents a frequent diagnostic pitfall, as the artificially created hypoechoic area can be mistakenly interpreted as tissue degeneration or a tear. To prevent this artifact, the angle of the transducer must be adjusted.
To describe the localization of labral tears, radiologists use the "clock face rule": 12 o'clock represents the biceps tendon insertion on the glenoid, and 3 o'clock represents the anterior equator of the glenoid.
Class and exam questions
Why is the term "tendinitis" not recommended for use?
This term implies an inflammatory process. Chronic tendon pathologies are degenerative in nature without classic inflammation, so it is more accurate to use "tendinopathy" or "tendinosis".
How to prove on MRI that a tendon tear is full-thickness?
A full-thickness tear is indicated not only by the tendon defect itself, but also by the simultaneous presence of fluid in the joint cavity and the subacromial bursa, confirming a through-and-through communication between them.
What is the "glenoid track" concept?
This is a biomechanical model. It compares the amount of bone loss on the glenoid with the size of the Hill-Sachs defect on the humerus to predict the risk of recurrent dislocation and select the type of surgery.
What else to study on this topic
- Anatomy of the rotator interval and the stabilizing "biceps pulley"
- Differential diagnosis of the magic angle artifact on MRI
- Imaging appearance of calcific tendinopathy at different stages
- Secondary osteoarthritis and rotator cuff tear arthropathy
- Evaluation of glenoid version and its clinical significance
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