Dorsal Scapular Nerve: Anatomy, Course, Function, Imaging

Topic

Deltoid muscle

The deltoid muscle is a large, triangular-shaped muscle covering the shoulder joint and giving the shoulder its rounded contour. It is the primary abductor of the arm and plays a crucial role in shoulder stability, lifting, and rotational control. The deltoid forms the outer layer of the shoulder and acts synergistically with the rotator cuff muscles to maintain smooth glenohumeral movement.

It consists of three distinct parts (heads):

Anterior (clavicular) fibers — flex and medially rotate the arm.
Middle (acromial) fibers — abduct the arm beyond 15°.
Posterior (scapular spinal) fibers — extend and laterally rotate the arm.

The deltoid is essential in nearly all upper-limb movements and acts as a dynamic stabilizer of the shoulder joint.

Synonyms

Shoulder cap muscle
Deltoideus

Origin, Course, and Insertion

Origin:
- Anterior fibers: Lateral third of the clavicle
- Middle fibers: Acromion process of the scapula
- Posterior fibers: Inferior lip of the spine of the scapula
Course: Fibers converge downward and laterally to form a thick tendon
Insertion: Deltoid tuberosity on the lateral surface of the humerus

Relations

Superficial: Skin and subcutaneous tissue of the shoulder region
Deep: Shoulder joint capsule, subacromial/subdeltoid bursa, rotator cuff tendons (mainly supraspinatus)
Medially: Pectoralis major (anteriorly) and infraspinatus/teres minor (posteriorly)
Inferiorly: Continuous with deep fascia of the arm

Nerve Supply

Axillary nerve (C5–C6) — branch of the posterior cord of the brachial plexus

Arterial Supply

Posterior circumflex humeral artery (branch of the axillary artery)
Deltoid branch of thoracoacromial artery

Venous Drainage

Accompanying veins of the arteries, draining into the axillary vein

Function

Abduction: Principal abductor of the shoulder (middle fibers).
Flexion and medial rotation: Anterior fibers.
Extension and lateral rotation: Posterior fibers.
Stabilization: Maintains humeral head position during shoulder motion.
Postural support: Provides upper-limb elevation and balance during movement.

Clinical Significance

Deltoid paralysis: Caused by axillary nerve injury, leading to loss of shoulder contour and impaired abduction.
Tears or strains: Common in athletes or due to trauma; may involve partial detachment from the clavicle or acromion.
Injection site: Common site for intramuscular injections due to muscle mass and accessibility.
Atrophy: Seen in chronic shoulder disuse or nerve damage.
Imaging relevance: MRI is used to assess tendon tears, denervation, atrophy, or deltoid bursitis.

MRI Appearance

T1-weighted images:
- Muscle fibers: Intermediate signal intensity, uniform striated texture.
- Tendon: Low signal (dark) as it inserts into humerus.
- Fat between muscle fascicles: Bright signal outlining muscle segments.
- Pathology: Muscle tears appear as focal discontinuities with bright hemorrhagic signal.
T2-weighted images:
- Normal muscle: Intermediate-to-low signal, slightly darker than on T1.
- Tendon: Dark, continuous band; tears show bright focal hyperintensity.
- Pathology:
  - Tears or strains: Bright intramuscular signal at myotendinous junction.
  - Edema or inflammation: High signal intensity.
  - Chronic atrophy: Muscle volume loss with increased fatty infiltration.
STIR:
- Normal deltoid: Intermediate-to-dark signal.
- Edema, strain, or inflammation: Bright hyperintensity.
- Highly sensitive for early muscle injury and soft-tissue swelling.
Proton Density Fat-Saturated (PD FS):
- Normal muscle: Intermediate-to-dark signal, homogeneous.
- Acute injury: Focal bright signal in myofascial or myotendinous regions.
- Chronic injury: Patchy low signal (fibrosis) or bright fatty infiltration.
T1 Fat-Sat Post-Contrast:
- Normal muscle: Mild homogeneous enhancement.
- Tendinitis or myositis: Diffuse or focal enhancement at insertion or within fibers.
- Tears: Peripheral enhancement around hematoma or fluid collection.
- Chronic denervation: Minimal enhancement with high fat signal intensity.

CT Appearance

Non-Contrast CT:

Muscle belly: Soft-tissue density with clear margins; separates from subcutaneous fat.
Tendon: Seen as a dense linear structure at humeral insertion.
Chronic injury: Atrophic muscle with fatty replacement (low-density areas).
Acute injury: May show intramuscular hematoma or swelling.

Post-Contrast CT (standard):

Normal muscle: Homogeneous enhancement.
Inflammatory changes or myositis: Focal enhancement at myotendinous junctions.
Tears or hematoma: Peripheral rim enhancement around low-density collections.
Useful for: Detecting soft-tissue masses, calcific tendinitis, or deltoid avulsion fractures.