Joints and ligaments · Upper limb
AC disc !
The acromioclavicular disc is a small cushion in the joint between the collarbone and the tip of the shoulder blade; it is often incomplete.
Official name: Acromioclavicular joint disc
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What it is
A small wedge of cartilage inside the AC joint.
What it does
Cushions the joint between the collarbone and the acromion.
Key facts
- Where
- Inside the acromioclavicular joint; often incomplete
- Function
- Fibrocartilage; degenerates early (from the 2nd–3rd decade)
Relations
- Medial
- Acromial end of the clavicle
- Lateral
- Articular facet on the medial edge of the acromion
- Above
- AC ligament and AC capsule, blending with the fascia of the deltoid and trapezius
- Below
- Lower AC capsule, then the subacromial bursa and the supraspinatus tendon
How to find it
Follow the clavicle outward to its slightly raised end above the level of the acromion: the joint line, and the disc within it, lie just there. Tenderness exactly at this point, rather than over the deltoid, points to the joint.
Examination
- Cross-body adduction test: the arm is brought across the chest at 90° of flexion; pain on top of the shoulder points to the AC joint
- Point tenderness over the joint; pain relief after a diagnostic injection of local anaesthetic into the joint supports the AC joint as the source
On imaging
The Zanca view (AP aimed at the AC joint with 10–15° of cephalic tilt and a softer exposure) shows the joint without overlap. MRI shows disc tears, joint fluid and bone oedema; a 'geyser' cyst above the joint suggests a large rotator cuff tear letting joint fluid escape upward.
Clinical notes
- Distal clavicle osteolysis in weightlifters and manual workers: repeated loading resorbs the end of the clavicle next to the disc
- Distal clavicle excision (open or arthroscopic) removes the disc and joint surfaces; resecting too much, or cutting the upper and back AC ligament, leaves the clavicle unstable
Related conditions
Test yourself
Think of the answer, then tap the question to see it.
What is the usual form of the acromioclavicular disc?
A partial, meniscus-like wedge hanging from the upper capsule; a complete disc is uncommon.
Why are AC joint narrowing and osteophytes common on X-ray in people without symptoms?
The acromioclavicular disc wears early.
What is distal clavicle osteolysis?
Resorption of the end of the clavicle next to the acromioclavicular disc from repeated loading, as in weightlifters and manual workers.
Educational information, not medical advice. For any symptom, see a health professional.
Its names
- Official Latin (TA2)
- discus articulationis acromioclavicularis TA2 1746
- Official Spanish (DeCS)
- Disco articular de la articulación acromioclavicular
- English (TA2)
- Acromioclavicular joint disc
Related
Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Acromioclavicular Joint; Anatomy of the Human Body (20th ed.): Surface Anatomy of the Upper Extremity (Lea & Febiger (Gray's Anatomy, 1918), 1918) · adapted, Public domain.
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Representative model, for educational purposes only.
