Adipose Tissue (Shoulder): Anatomy, Relations, Function, Imaging

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Adipose tissue (Shoulder)

Adipose tissue in the shoulder region consists of organized fat pads and connective tissue layers distributed around the glenohumeral joint, rotator cuff tendons, deltoid, and axillary spaces. It plays vital roles in cushioning, friction reduction, and facilitating smooth tendon gliding during shoulder movement.

Fatty tissue serves as both a mechanical buffer and a metabolic structure, containing vascular and lymphatic channels. Key adipose compartments include the subacromial-subdeltoid bursa fat, infraspinatus and supraspinatus fossae fat, subcoracoid and axillary fat pads, and the pericapsular fat of the glenohumeral joint.

Synonyms

Shoulder fat pad
Subacromial adipose tissue
Axillary fat compartment

Location and Structure

Superficial fat: Lies beneath the skin and subcutaneous fascia, surrounding the deltoid and trapezius regions.
Deep fat pads: Occupy spaces between muscles, tendons, and bursae, including:
- Subacromial-subdeltoid fat pad: Between deltoid and supraspinatus tendon.
- Subcoracoid fat pad: Anterior to subscapularis tendon and coracoid process.
- Axillary fat pad: Fills axilla around neurovascular bundle, acting as protective cushion.
- Infraspinatus and supraspinatus fossae fat: Posteriorly between muscle planes and scapula.
Composition: Lobules of white adipocytes surrounded by thin fibrous septa and small vessels.

Relations

Superiorly: Subcutaneous fascia and deltoid muscle.
Inferiorly: Rotator cuff tendons and glenohumeral capsule.
Anteriorly: Pectoralis major and subscapularis.
Posteriorly: Supraspinatus, infraspinatus, and teres minor.
Medially: Glenohumeral joint capsule and scapula.
Laterally: Deltoid and acromion process.

Nerve Supply

Cutaneous branches: Supraclavicular nerves and axillary nerve branches.
Deep fat innervation: Minimal sensory supply; some sympathetic fibers accompany small vessels.

Function

Cushioning: Protects tendons, vessels, and nerves from mechanical friction.
Lubrication: Facilitates gliding between deltoid, rotator cuff, and subacromial structures.
Thermal regulation: Maintains local temperature for muscle and joint function.
Vascular and metabolic support: Houses small vessels and lymphatics.
Shock absorption: Dissipates forces transmitted across the shoulder during motion or impact.

Clinical Significance

Inflammation: Fat pads may become inflamed in bursitis or synovitis (e.g., subacromial impingement).
Lipoma or lipomatosis: Benign fatty tumors or diffuse fatty overgrowths may occur in the subdeltoid or axillary regions.
Atrophy: Seen in chronic rotator cuff tears or disuse; associated with loss of shoulder contour.
Fatty infiltration: Common in chronic muscle denervation or tendon tears (especially supraspinatus and infraspinatus).
Post-surgical or post-traumatic changes: Fat necrosis or fibrosis can alter local signal patterns.
Imaging importance: MRI distinguishes normal fat (bright) from inflammation, fibrosis, or edema (signal loss).

MRI Appearance

T1-weighted images:
- Adipose tissue: Bright (high signal intensity), homogeneous appearance.
- Fibrous septa: Thin low-signal lines traversing fat lobules.
- Inflammation or fibrosis: Focal or patchy intermediate-to-low signal replacing normal bright fat.
- Lipoma: Uniform bright signal, well-circumscribed.
T2-weighted images:
- Normal fat: Bright, slightly less intense than on T1.
- Edematous fat: Heterogeneous bright signal with internal streaks.
- Inflammatory changes: Areas of mixed hyperintensity.
- Fat necrosis: Central low signal with peripheral bright ring.
STIR:
- Normal fat: Dark (signal suppressed).
- Pathology: Bright hyperintense where inflammation or edema replaces fat.
- Useful for detecting bursitis, fat infiltration, or necrosis.
Proton Density Fat-Saturated (PD FS):
- Normal fat: Dark (signal suppressed).
- Inflamed or fibrotic fat: Bright hyperintense due to retained water content.
- Differentiates normal adipose tissue from edema, tumor, or postoperative scarring.
T1 Fat-Sat Post-Contrast:
- Normal fat: No significant enhancement.
- Inflamed fat: Mild diffuse or nodular enhancement.
- Lipoma: Minimal or no enhancement; liposarcoma shows irregular or septal enhancement.

CT Appearance

Non-Contrast CT:

Adipose tissue: Low attenuation (-80 to -120 HU), homogeneous, easily distinguishable from muscle or bone.
Fibrous septa: Appear as thin soft-tissue-density strands.
Inflammation: Shows fat stranding or increased attenuation.
Lipoma: Well-circumscribed hypodense lesion with smooth borders.
Fat necrosis: Focal density variation with possible calcifications.

Post-Contrast CT (standard):

Normal fat: No enhancement.
Inflammation or infection: Linear or hazy enhancement along septa or fascial planes.
Neoplasm: Liposarcoma shows irregular septal or nodular enhancement.
Postoperative regions: Fat grafts or necrosis may demonstrate peripheral enhancement.

MRI images

CT image

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