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Inferior acromioclavicular ligament

The inferior acromioclavicular ligament (IACL) is the lower fibrous band of the acromioclavicular (AC) joint capsule, reinforcing its inferior aspect. It forms part of the acromioclavicular joint complex, connecting the acromion process of the scapula to the lateral end of the clavicle.

This ligament, though thinner than the superior component, contributes to joint stability, particularly in preventing inferior displacement of the acromion relative to the clavicle. It also resists rotational and shear stresses that occur during shoulder movement.

Synonyms

  • Inferior AC ligament

  • Inferior capsular ligament of the AC joint

  • Lower acromioclavicular capsular thickening

Location and Structure

  • Position: Inferior aspect of the acromioclavicular joint, forming part of the fibrous capsule.

  • Composition: Dense fibrous connective tissue, continuous with the articular capsule.

  • Attachments:

    • Superiorly: Inferior margin of the lateral clavicle.

    • Inferiorly: Inferior surface of the acromion process.

  • Continuity: Blends laterally with the articular capsule and anteriorly/posteriorly with the superior acromioclavicular ligament.

  • Associated structures: Closely related to the articular disc (when present) and the coracoclavicular ligament complex below.

Relations

  • Superiorly: Acromioclavicular joint capsule and articular disc (if present).

  • Inferiorly: Subacromial bursa and supraspinatus tendon.

  • Anteriorly: Deltoid muscle fibers.

  • Posteriorly: Trapezius muscle and posterior joint capsule.

  • Inferolaterally: Coracoacromial ligament and coracoclavicular ligament (trapezoid portion).

Nerve Supply

  • Articular branches of the suprascapular nerve and lateral pectoral nerve innervate the acromioclavicular joint and its ligaments.

Function

  • Joint stability: Reinforces the inferior capsule of the acromioclavicular joint.

  • Prevents inferior displacement: Restrains downward motion of the acromion relative to the clavicle.

  • Assists rotation control: Provides passive stability during scapular rotation and arm elevation.

  • Force distribution: Transmits shear forces during upper limb movement.

  • Supportive role: Works with superior and coracoclavicular ligaments to maintain AC joint integrity.

Clinical Significance

  • Injury: Frequently involved in AC joint sprains and separations, especially in grade II–III injuries.

  • Degeneration: Chronic microtrauma may cause capsular thickening and fibrosis visible on imaging.

  • Inflammation: May occur in acromioclavicular arthritis or impingement syndromes.

  • Surgical relevance: Preservation or reconstruction is critical in AC joint stabilization procedures.

  • Imaging relevance: MRI helps evaluate tears, capsular distension, and associated ligamentous injuries.

MRI Appearance

  • T1-weighted images:

    • Ligament: Low signal (dark linear band) spanning acromion to clavicle.

    • Bone marrow: Bright due to fatty composition.

    • Adjacent muscle and capsule: Intermediate signal.

    • Pathology: Partial tear shows focal discontinuity or thickening with intermediate signal; complete tear—loss of continuity with retraction.

  • T2-weighted images:

    • Normal ligament: Low signal (dark band).

    • Partial tear or strain: Bright hyperintense foci within ligament fibers.

    • Joint effusion or capsular edema: Bright hyperintense fluid outlining the ligament.

    • Associated injuries: Often accompany coracoclavicular ligament tears in high-grade separations.

  • STIR:

    • Normal ligament: Dark linear band.

    • Pathology: Bright hyperintense signal in edema or capsular inflammation.

    • Sensitive for detecting soft-tissue swelling, sprains, or joint effusion.

  • Proton Density Fat-Saturated (PD FS):

    • Normal: Low signal, clearly delineated against intermediate background of capsule and muscle.

    • Tear: Bright focal or diffuse hyperintensity with fiber discontinuity.

    • Capsulitis or synovitis: Surrounding bright signal indicating inflammatory thickening.

  • T1 Fat-Sat Post-Contrast:

    • Normal ligament: Minimal to no enhancement.

    • Inflammation: Diffuse or peripheral enhancement along capsule.

    • Chronic degeneration: Patchy enhancement due to fibrotic vascular proliferation.

CT Appearance

Non-Contrast CT:

  • Ligament: Not well visualized directly due to thin fibrous texture.

  • Joint margins: Sharp, with visible cortical bone of acromion and clavicle.

  • Pathology: Detects joint widening, subluxation, or distal clavicle osteolysis.

  • Indirect signs: Irregular joint margins or capsular calcification in chronic inflammation.

Post-Contrast CT (standard):

  • Ligament visualization: Enhanced contrast around joint capsule outlines ligamentous structures.

  • Inflammatory conditions: Capsular enhancement and pericapsular soft-tissue swelling visible.

  • Useful for: Confirming AC joint injury extent, chronic osteolysis, and periarticular pathology when MRI is contraindicated.

MRI image

Inferior acromioclavicular ligament axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Inferior acromioclavicular ligament coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

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Inferior acromioclavicular ligament sag cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000