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Acromioclavicular joint

The acromioclavicular (AC) joint is a small synovial joint between the lateral end of the clavicle and the acromion process of the scapula. It is part of the shoulder girdle complex and plays a key role in transmitting loads from the upper limb to the axial skeleton and allowing smooth scapuloclavicular motion. Despite its small size, it contributes significantly to shoulder stability, flexibility, and coordinated movement.

The joint allows gliding and rotational movements that accompany scapular motion during arm elevation, abduction, and rotation. It is stabilized by a combination of capsular, acromioclavicular, and coracoclavicular ligaments, as well as surrounding musculature.

Synonyms

  • AC joint

  • Claviculoacromial joint

  • Shoulder apex joint

Location and Structure

  • Articulating surfaces:

    • Lateral end of the clavicle: Oval facet facing medially and slightly downward.

    • Medial surface of the acromion: Small facet facing upward and medially.

  • Joint type: Synovial plane (gliding) joint.

  • Cartilage: Fibrocartilage (not hyaline) covering both articular surfaces.

  • Joint capsule: Fibrous, relatively weak, but reinforced superiorly by acromioclavicular and coracoclavicular ligaments.

  • Articular disc: Incomplete fibrocartilaginous wedge or pad may divide the joint cavity.

  • Synovial membrane: Lines the capsule and secretes synovial fluid for lubrication.

Relations

  • Superiorly: Trapezius muscle and aponeurosis.

  • Inferiorly: Supraspinatus tendon and subacromial bursa.

  • Anteriorly: Deltoid muscle fibers.

  • Posteriorly: Upper fibers of trapezius and posterior capsule.

  • Medially: Lateral end of clavicle and coracoclavicular ligament complex.

Ligamentous Attachments

  • Acromioclavicular ligament: Reinforces superior part of capsule, prevents horizontal displacement.

  • Coracoclavicular ligament: Main stabilizer, consisting of:

    • Conoid ligament (medial, fan-shaped, vertical orientation).

    • Trapezoid ligament (lateral, oblique orientation).

  • Capsular ligaments: Surround joint and restrict excessive translation.

Nerve Supply

  • Lateral pectoral nerve

  • Suprascapular nerve

  • Axillary nerve

Function

  • Load transmission: Transfers forces from upper limb to clavicle and axial skeleton.

  • Mobility: Allows gliding, rotation, and slight elevation/depression movements.

  • Scapular rhythm: Enables scapular rotation during arm elevation.

  • Joint stability: Maintains alignment of clavicle and acromion during shoulder motion.

Clinical Significance

  • AC joint separation: Common sports injury due to fall on the shoulder; graded I–VI based on ligament disruption and displacement.

  • Arthritis: Degenerative joint disease leading to pain and osteophyte formation.

  • Osteolysis of distal clavicle: Repetitive stress injury seen in weightlifters.

  • Joint effusion and synovitis: May cause localized swelling and tenderness.

  • Post-traumatic deformity: Chronic instability or prominence of clavicle.

  • Imaging relevance: MRI and CT essential for evaluating joint integrity, ligament injury, and degenerative changes.

MRI Appearance

  • T1-weighted images:

    • Cortex: Low signal (dark).

    • Marrow: Bright, fatty signal in normal clavicle and acromion.

    • Fibrocartilage: Intermediate-to-low signal outlining joint space.

    • Ligaments: Low signal linear structures.

    • Pathology: Disruption of low-signal ligament fibers or bright edema in bone marrow indicates injury.

  • T2-weighted images:

    • Cortex: Low signal.

    • Marrow: Bright, especially with bone marrow edema or contusion.

    • Fibrocartilage: Intermediate signal with joint fluid appearing hyperintense.

    • Effusion or inflammation: Bright hyperintensity within joint space or periarticular soft tissues.

    • Ligament tears: Bright linear fluid signal interrupting dark ligament bands.

  • STIR:

    • Normal marrow: Intermediate-to-dark signal.

    • Injury: Bright hyperintensity in marrow, capsule, or ligament attachments.

    • Excellent for detecting marrow edema, sprain, and soft-tissue inflammation.

  • Proton Density Fat-Saturated (PD FS):

    • Normal bone and ligament: Intermediate-to-dark signal.

    • Abnormal: Bright hyperintensity in capsule or ligament (sprain, partial tear).

    • Sensitive for joint effusion, bursitis, and degenerative changes.

  • T1 Fat-Sat Post-Contrast:

    • Normal joint: Minimal enhancement.

    • Inflammation or arthritis: Diffuse or patchy synovial enhancement.

    • Post-traumatic: Enhancement around disrupted ligaments and periarticular tissues.

CT Appearance

Non-Contrast CT:

  • Cortical bone: High attenuation, sharply defined.

  • Joint space: Narrow, uniform gap between acromion and clavicle.

  • Articular surfaces: Smooth, covered by fibrocartilage.

  • Pathology: Detects fractures, subluxations, osteolysis, or osteophyte formation.

  • Chronic arthritis: Sclerosis, cystic changes, and joint space narrowing.

  • Dislocation: Clavicle elevated relative to acromion; coracoclavicular interval widened.

Post-Contrast CT (standard):

  • Enhancing synovium and capsule: Indicate active inflammation or infection.

  • Periarticular enhancement: Seen in arthritis, osteomyelitis, or soft-tissue injury.

  • Useful for: Defining ligament attachment injuries, fracture extension, and postsurgical evaluation.

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