Coracoacromial Ligament: Anatomy, Function, Clinical & Imaging

Topic

Coracoacromial ligament

The coracoacromial ligament (CAL) is a strong triangular band forming part of the coracoacromial arch over the shoulder joint. It extends between the coracoid process and the acromion process of the scapula, completing a protective bony-ligamentous roof over the humeral head. This ligament plays a key role in preventing superior displacement of the humeral head and serves as a crucial structure in shoulder stability and impingement syndromes.

Despite not directly attaching to the humerus, the coracoacromial ligament indirectly influences rotator cuff biomechanics and subacromial space dimensions. It may ossify or thicken with age or chronic stress, contributing to subacromial impingement.

Synonyms

Coracoacromial arch ligament
Superior scapular ligament (older term)

Origin, Course, and Insertion

Origin: Lateral border of the coracoid process of the scapula.
Course: Passes obliquely laterally and superiorly, forming a strong fibrous arch across the superior aspect of the glenohumeral joint.
Insertion: Medial border and tip of the acromion process of the scapula, often blending with the deep fascia of the deltoid and trapezius muscles.

Shape and Structure

Shape: Triangular, with the apex at the coracoid process and the base at the acromion.
Fiber orientation: Oblique lateral and posterior direction.
Texture: Dense, fibrous connective tissue; may calcify or ossify in degenerative conditions.
Thickness: Variable (2–5 mm), depending on individual anatomy and repetitive mechanical stress.

Relations

Superiorly: Deltoid muscle and subcutaneous tissue.
Inferiorly: Subacromial bursa, supraspinatus tendon, and the humeral head.
Medially: Coracoid process and coracoclavicular ligament.
Laterally: Acromion process and acromioclavicular joint capsule.
Deep to: Coracoacromial arch, which protects the rotator cuff and humeral head from direct trauma.

Nerve Supply

Sensory innervation via branches of the suprascapular nerve and lateral pectoral nerve (supplying the coracoid and acromion periosteum and adjacent fascia).

Function

Protection: Forms the superior part of the coracoacromial arch, shielding the rotator cuff tendons, subacromial bursa, and humeral head.
Prevents upward dislocation: Resists superior translation of the humeral head during shoulder movement.
Force distribution: Transmits mechanical stresses between the coracoid and acromion, stabilizing the scapular ring.
Limiting structure: Restricts excessive elevation and anterior-superior translation of the humerus.
Clinical landmark: Serves as a surgical guide in subacromial decompression procedures.

Clinical Significance

Subacromial impingement: Thickened or calcified ligament compresses the supraspinatus tendon and subacromial bursa, leading to pain and restricted motion.
Degenerative changes: Chronic mechanical stress may lead to fibrosis, calcification, or ossification (coracoacromial arch ossification).
Rotator cuff pathology: Frequently coexists with supraspinatus tears and subacromial bursitis.
Post-surgical relevance: Partial release of the ligament may be performed during acromioplasty to relieve impingement.
Imaging importance: MRI and CT demonstrate ligament morphology, tears, and osseous impingement changes.

MRI Appearance

T1-weighted images:
- Ligament: Low signal (dark band) connecting the coracoid and acromion.
- Adjacent muscles: Deltoid and supraspinatus appear as intermediate signal structures.
- Marrow in adjacent bone: Bright signal intensity due to fat content.
- Calcification: Appears as very low signal (black focus).
T2-weighted images:
- Ligament: Low signal, continuous dark band across the coracoacromial arch.
- Pathology: Partial tear or degeneration appears as focal bright hyperintensity or discontinuity.
- Subacromial bursitis: Bright hyperintense fluid signal beneath ligament.
- Bone marrow edema: Bright signal in coracoid or acromion with impingement or stress reaction.
STIR:
- Normal ligament: Low to intermediate signal, maintaining sharp margins.
- Inflamed or thickened ligament: Bright hyperintense signal indicating edema or fibrosis.
- Subacromial edema or bursitis: Strong hyperintense signal under ligament arch.
Proton Density Fat-Saturated (PD FS):
- Normal: Uniform dark low signal band.
- Tendinopathy or tear: Focal bright signal in adjacent supraspinatus tendon or bursal space.
- Useful for: Visualizing subtle thickening, partial fiber disruption, and bursitis.
T1 Fat-Sat Post-Contrast:
- Normal ligament: No enhancement (remains dark).
- Inflamed ligament or surrounding tissues: Peripheral or diffuse enhancement due to synovial reaction or fibrosis.
- Post-surgical scarring: May show mild enhancement at acromial insertion site.

CT Appearance

Non-Contrast CT:

Ligament: Seen as a thin soft-tissue density band connecting the coracoid and acromion.
Bone landmarks: Coracoid and acromion clearly defined with high-density cortical outlines.
Calcification or ossification: Appears as dense, linear or nodular foci along the ligament’s course.
Utility: Excellent for identifying bony spurs or calcific deposits contributing to impingement.