Infraspinatus Tendon: Anatomy, Function, Clinical Significance, Imaging

Topic

Infraspinatus tendon

The infraspinatus tendon is the strong fibrous extension of the infraspinatus muscle, one of the four muscles comprising the rotator cuff of the shoulder. It plays a crucial role in external rotation of the humerus, stabilization of the glenohumeral joint, and dynamic control during arm movement.

The tendon arises from the infraspinous fossa of the scapula and converges laterally to form a flat, thick tendon inserting onto the middle facet of the greater tubercle of the humerus. It blends with the joint capsule and fibers of the supraspinatus and teres minor tendons, forming part of the rotator cuff tendinous complex.

Injury or degeneration of the infraspinatus tendon is common in rotator cuff tears, overhead athletes, and chronic shoulder impingement syndromes.

Synonyms

Tendon of infraspinatus
Infraspinatus insertion
Posterior rotator cuff tendon

Origin, Course, and Insertion

Origin: From the infraspinatus muscle, which arises from the infraspinous fossa of the scapula and the infraspinatus fascia.
Course: The muscle fibers converge laterally, forming a thick tendon that passes posterior to the glenohumeral joint capsule.
Insertion: Onto the middle facet of the greater tubercle of the humerus; the tendon blends anteriorly with the supraspinatus tendon and inferiorly with the teres minor tendon.

Tendon Attachments

Capsular attachment: Deep surface of the tendon fuses with the fibrous capsule of the shoulder joint, contributing to its posterior reinforcement.
Adjacent tendinous attachments:
- Superiorly: Supraspinatus tendon
- Inferiorly: Teres minor tendon
Bursal relation: Covered by the subacromial–subdeltoid bursa, which reduces friction during movement.

Relations

Superiorly: Supraspinatus tendon and subacromial bursa
Inferiorly: Teres minor tendon
Anteriorly: Glenohumeral joint capsule
Posteriorly: Deltoid muscle and infraspinatus fascia
Laterally: Greater tubercle of the humerus and posterior aspect of rotator cuff interval

Nerve Supply

Suprascapular nerve (C5–C6) — same as its parent muscle

Function

External rotation: Rotates the humerus laterally at the shoulder joint.
Joint stability: Compresses the humeral head into the glenoid fossa, preventing superior translation during arm elevation.
Co-contraction: Works with the subscapularis to balance anterior–posterior shoulder forces.
Dynamic support: Prevents posterior humeral subluxation during throwing and reaching activities.

Clinical Significance

Tendinopathy: Common in overhead athletes and older adults due to overuse and degeneration.
Partial-thickness tears: Often articular-sided near insertion; may coexist with supraspinatus tears.
Full-thickness tears: Result in weakness in external rotation and posterior shoulder pain.
Calcific tendinitis: Deposition of calcium hydroxyapatite crystals within tendon fibers causing pain and stiffness.
Posterior impingement syndrome: Seen in throwing athletes where the infraspinatus tendon becomes entrapped between humeral head and glenoid rim.
Posterosuperior cuff tear: Involves both supraspinatus and infraspinatus tendons.
Imaging role: MRI is the gold standard for assessing tears, atrophy, and tendon retraction; CT can evaluate bony changes and post-surgical repair integrity.

MRI Appearance

T1-weighted images:
- Tendon: Uniform low signal (dark) band with smooth margins.
- Muscle belly: Intermediate signal intensity with visible fascicular pattern.
- Fatty infiltration: Bright high-signal foci within muscle fibers.
- Tear or retraction: Discontinuity, fluid-filled gap, or tendon thinning at humeral insertion.
- Normal marrow of humeral head: Bright on T1 due to fat content.
T2-weighted images:
- Normal tendon: Low signal (dark).
- Muscle: Intermediate-to-low signal, slightly darker than T1.
- Tendinopathy or tear: Bright hyperintense signal at the insertion or within tendon substance.
- Joint fluid and bursa: Hyperintense signal outlining the tendon margins.
- Chronic tears: Retraction, muscle atrophy, and fatty replacement visible as high T1 signal within muscle belly.
STIR:
- Normal tendon: Dark (low signal).
- Pathology: Bright hyperintense signal in partial tears, peritendinous edema, or bursitis.
- Excellent for detecting early inflammatory or edematous changes in acute tendinopathy.
Proton Density Fat-Saturated (PD FS):
- Normal tendon: Homogeneously dark.
- Partial tears: Bright hyperintense streaks or foci within tendon substance.
- Complete tears: Fluid-filled bright defect with tendon discontinuity.
- Muscle edema: Patchy hyperintensity near myotendinous junction.
T1 Fat-Sat Post-Contrast:
- Normal tendon: Minimal enhancement.
- Inflamed tendon or bursa: Focal or diffuse enhancement around tendon sheath.
- Post-surgical repair or granulation tissue: Moderate enhancement at insertion site.
- Chronic scar tissue: Low-signal, non-enhancing fibrotic bands.

CT Appearance

Non-Contrast CT:

Tendon: Soft-tissue density along the posterior humeral head; margins may appear ill-defined in chronic tears.
Bone: High attenuation cortical outline of the greater tubercle clearly visualized.
Calcific tendinitis: Appears as localized dense foci within the tendon near insertion.
Chronic tears: Bony irregularity, cortical erosion, or sclerosis at tendon footprint.
Useful for: Surgical planning, bone tunnel evaluation, and postoperative anchor assessment.

Post-Contrast CT (standard):

Normal tendon: No enhancement.
Inflammation or postoperative scar: Enhancing soft tissue around tendon insertion.
Bursal enhancement: Suggestive of active bursitis or reactive inflammation.
Helpful in differentiating infectious, inflammatory, and postoperative changes in the rotator cuff region.

MRI image

CT image

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