Shoulder Joint (Glenohumeral Joint): Anatomy, Function, Imaging

Topic

Shoulder joint (glenohumeral joint)

The shoulder joint, or glenohumeral joint, is a synovial ball-and-socket joint formed between the head of the humerus and the glenoid cavity of the scapula. It is the most mobile joint in the human body, allowing an extensive range of motion including flexion, extension, abduction, adduction, rotation, and circumduction.

Due to its wide range of motion, the joint sacrifices stability for mobility and relies heavily on the rotator cuff muscles, joint capsule, ligaments, and glenoid labrum for support. It is the most commonly dislocated large joint in the body.

Synonyms

Glenohumeral joint
Scapulohumeral articulation
Shoulder articulation

Location and Structure

Articular surfaces:
- Humeral head: Large, spherical, covered with hyaline cartilage.
- Glenoid cavity: Shallow, pear-shaped socket on lateral scapula, deepened by the glenoid labrum (fibrocartilaginous rim).
Type: Synovial ball-and-socket joint.
Joint capsule: Thin, loose, fibrous capsule that attaches medially to the margin of the glenoid and laterally to the anatomical neck of the humerus.
Synovial membrane: Lines the capsule and forms bursae such as the subscapular bursa.

Relations

Superiorly: Coracoacromial arch, subacromial bursa, deltoid muscle.
Inferiorly: Long head of triceps, axillary nerve, posterior circumflex humeral vessels.
Anteriorly: Subscapularis muscle, coracoid process, and axillary vessels.
Posteriorly: Infraspinatus and teres minor muscles, posterior joint capsule.

Ligamentous Attachments

Glenohumeral ligaments (superior, middle, inferior): Thickenings of the capsule reinforcing the anterior aspect.
Coracohumeral ligament: Extends from the coracoid process to the humerus, strengthening the superior capsule.
Transverse humeral ligament: Holds the long head of the biceps tendon in the intertubercular groove.
Glenoid labrum: Fibrocartilaginous ring that deepens the glenoid cavity and stabilizes the humeral head.

Nerve Supply

Suprascapular nerve (posterior capsule and superior joint).
Axillary nerve (inferior and posterior aspects).
Lateral pectoral nerve (anterior capsule).

Arterial Supply

Anterior and posterior circumflex humeral arteries (from axillary artery).
Suprascapular artery (from thyrocervical trunk).
Subscapular and circumflex scapular arteries (from axillary artery).

Venous Drainage

Anterior and posterior circumflex humeral veins, draining into the axillary vein.
Suprascapular and subscapular veins accompanying their respective arteries.

Function

Mobility: Provides the widest range of motion of any joint.
Movement planes:
- Flexion–extension
- Abduction–adduction
- Internal–external rotation
- Circumduction
Stabilization: Maintained by capsule, glenoid labrum, ligaments, rotator cuff muscles, and negative intra-articular pressure.
Load transmission: Transfers upper limb forces to scapula and thorax during movement.

Clinical Significance

Dislocation: Most frequently dislocated major joint (commonly anterior).
Labral tears: SLAP and Bankart lesions often result from trauma or repetitive overhead activity.
Rotator cuff tears: Degenerative or traumatic, causing pain and restricted movement.
Adhesive capsulitis (frozen shoulder): Fibrosis and thickening of joint capsule.
Arthritis: Osteoarthritis or inflammatory arthritis leading to cartilage degeneration.
Imaging importance: MRI and arthrography are essential for assessing soft-tissue structures and labral integrity; CT best for fractures and bony architecture.

MRI Appearance

T1-weighted images:
- Cortex: Low signal (dark).
- Marrow: Bright (fatty signal).
- Cartilage: Intermediate-to-low signal, smooth over humeral head and glenoid.
- Labrum: Triangular low-signal structure at glenoid rim.
- Joint capsule and ligaments: Low signal bands.
- Rotator cuff muscles: Intermediate signal; tendons low signal.
T2-weighted images:
- Cortex: Low signal.
- Marrow: Bright
- Cartilage: Intermediate-to-bright depending on fluid content.
- Fluid or effusion: Hyperintense, outlining joint recesses.
- Pathology:
  - Labral tear: Linear bright signal extending into labrum.
  - Rotator cuff tear: Bright fluid signal breaching tendon.
  - Synovitis or bursitis: Hyperintense capsular and peribursal fluid.
STIR:
- Normal marrow: Intermediate-to-dark.
- Abnormal marrow or soft-tissue edema: Bright hyperintensity.
- Useful for detecting rotator cuff edema, marrow contusion, or synovial inflammation.
Proton Density Fat-Saturated (PD FS):
- Cartilage and tendons: Low-to-intermediate signal.
- Joint fluid: Bright.
- Labral tears: Bright signal within dark labral structure.
- Excellent for evaluating cuff tendons, labrum, and subtle fluid collections.
T1 Fat-Sat Post-Contrast:
- Normal capsule: Mild homogeneous enhancement.
- Synovitis, infection, or tumor: Diffuse or nodular enhancement.
- Postoperative scar vs. retear: Enhancing fibrous tissue vs. non-enhancing defect.

MRI Arthrogram Appearance

Direct MR Arthrography (gadolinium contrast):
- Contrast fills the joint cavity and outlines capsule, ligaments, and labrum.
- Labral tear: Contrast enters between labrum and glenoid rim (Bankart lesion).
- SLAP tear: Contrast tracks superiorly between labrum and biceps anchor.
- Capsular laxity or tear: Overdistention and contrast extravasation.
- Rotator cuff tear: Contrast extends into subacromial or subdeltoid bursa.
- Gold standard for subtle labral and capsuloligamentous pathology.

CT Appearance

Non-Contrast CT:

Cortex: High attenuation, sharply defined.
Trabecular bone: Fine and homogeneous.
Joint space: Uniform and smooth in normal anatomy.
Pathology:
- Detects fractures of humeral head or glenoid rim.
- Shows osteophytes, sclerosis, subchondral cysts in arthritis.
- Evaluates prosthesis alignment post-surgery.

Post-Contrast CT (standard):

Capsule and synovium: Enhanced in inflammation or infection.
Bone lesions or tumors: Heterogeneous enhancement.
Useful for: Assessing neoplastic, infectious, or inflammatory processes when MRI contraindicated.

CT Arthrogram Appearance

Direct CT Arthrography (iodinated contrast):
- Contrast outlines the labrum, capsule, ligaments, and cartilage surfaces.
- Labral tears: Contrast interposed between labrum and glenoid.
- Cartilage defects: Contrast-filled irregularities on humeral or glenoid surfaces.
- Rotator cuff tears: Contrast tracking superiorly into subacromial bursa.
- Ideal for evaluating bony Bankart lesions and post-surgical labral integrity when MRI is unavailable.