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Scapular spinal part of deltoid muscle

The scapular spinal part of the deltoid muscle, also known as the posterior deltoid, is the posterior division of the deltoid muscle. It arises from the inferior lip of the spine of the scapula and forms the posterior contour of the shoulder. This portion plays a crucial role in shoulder extension, horizontal abduction, and external rotation, acting in coordination with the latissimus dorsi and teres major.

Compared to the anterior and acromial (middle) parts, the scapular spinal part has a more oblique and posteriorly directed fiber orientation, giving it a distinct function in posterior shoulder movement and stabilization. It is a key dynamic stabilizer of the glenohumeral joint, counterbalancing anterior deltoid activity.

Synonyms

  • Posterior deltoid

  • Spinal deltoid

  • Scapular part of the deltoid

Origin, Course, and Insertion

  • Origin: Inferior edge of the spine of the scapula.

  • Course: Fibers run anteriorly and laterally from the scapular spine, converging to join the common deltoid tendon.

  • Insertion: Deltoid tuberosity on the lateral surface of the humerus (posterior aspect of the deltoid insertion zone).

Tendon Attachments

  • Fibers blend anteriorly with those of the acromial part before inserting into the deltoid tuberosity.

  • Tendon fibers interdigitate with fascial slips of the lateral head of the triceps brachii posteriorly, contributing to shoulder contour.

Relations

  • Anteriorly: Acromial part of deltoid, posterior fibers of the acromion, and subdeltoid bursa.

  • Posteriorly: Infraspinatus muscle, teres minor, and posterior shoulder capsule.

  • Superiorly: Subcutaneous tissue and skin of the posterior shoulder.

  • Inferiorly: Merges with lateral head of triceps brachii near posterior humerus.

  • Medially: Spine of scapula and fascia separating deltoid from infraspinatus.

Nerve Supply

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

Arterial Supply

  • Posterior circumflex humeral artery (from the axillary artery).

  • Additional branches from the deltoid branch of the thoracoacromial artery.

Venous Drainage

  • Accompanies arterial supply, draining into the axillary vein via deltoid and circumflex humeral veins.

Function

  • Shoulder extension: Extends the arm posteriorly, especially from a flexed position.

  • Horizontal abduction: Moves the arm backward in the transverse plane.

  • External rotation: Rotates the humerus laterally with the infraspinatus and teres minor.

  • Stabilization: Counteracts anterior translation of the humeral head; balances anterior deltoid pull.

  • Functional synergy: Works with latissimus dorsi and teres major for powerful posterior movements (e.g., rowing, pulling).

Clinical Significance

  • Tears or strain: May occur in athletes during overextension or throwing motions.

  • Denervation: Axillary nerve injury causes weakness in shoulder extension and flattening of posterior contour.

  • Muscle imbalance: Overdevelopment can alter scapulohumeral rhythm and posture.

  • Imaging relevance: Important in evaluating deltoid atrophy (post-axillary nerve injury), intramuscular edema, or tears.

  • Surgical relevance: Serves as a landmark for posterior shoulder approaches and reconstructive grafts.

MRI Appearance

  • T1-weighted images:

    • Muscle: Intermediate signal intensity, showing clear fascicular architecture.

    • Tendon: Low signal (dark) at insertion onto deltoid tuberosity.

    • Fat planes: Bright, delineating muscle boundaries.

    • Atrophy: Appears as increased fatty infiltration (bright on T1).

    • Tears or scarring: Irregular low-signal bands within muscle.

  • T2-weighted images:

    • Normal muscle: Intermediate-to-low signal, slightly darker than T1.

    • Tendon: Uniformly low signal.

    • Pathology: Strain or edema appears bright hyperintense; chronic atrophy appears intermediate with fat replacement.

  • STIR:

    • Normal muscle: Intermediate-to-dark signal.

    • Acute injury or inflammation: Bright hyperintense focal or diffuse areas indicating edema or partial tear.

  • Proton Density Fat-Saturated (PD FS):

    • Normal muscle: Intermediate-to-dark, uniform appearance.

    • Pathologic: Bright hyperintensity in acute muscle strain, tendinitis, or myofascial injury.

    • Excellent for detecting small partial tears and intermuscular edema.

  • T1 Fat-Sat Post-Contrast:

    • Normal muscle: Mild homogeneous enhancement.

    • Myositis or strain: Focal enhancement along fascial planes.

    • Chronic denervation: Minimal or no enhancement, with fatty replacement visible on pre-contrast T1.

CT Appearance

Non-Contrast CT:

  • Muscle: Homogeneous soft-tissue density posterior to humerus.

  • Tendon: Seen as thin, low-density linear structure inserting on deltoid tuberosity.

  • Fat planes: Distinct separation from infraspinatus and triceps.

  • Pathology: Muscle tears or atrophy visible as loss of bulk and increased fat density.

  • Calcification: May be seen in chronic tendinopathy.

Post-Contrast CT (standard):

  • Muscle: Enhances uniformly.

  • Inflamed or injured tissue: Shows focal enhancement with perimuscular fat stranding.

  • Useful for:

    • Identifying hematoma, abscess, or soft-tissue mass.

    • Evaluating post-traumatic changes or denervation atrophy.

MRI image

scapular spinal part of deltoid muscle AXIAL cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

scapular spinal part of deltoid muscle sagittal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image

scapular spinal part of deltoid muscle CT SAG IMAGE