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# 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 points

- Highly variable: a complete disc is uncommon; most AC joints have a partial, meniscus-like wedge hanging down from the upper capsule
- Its early wear explains why narrowing and osteophytes of the AC joint on X-ray are very common in people without symptoms
- Osteophytes on the underside of the AC joint sit right above the supraspinatus tendon and can add to subacromial impingement

## 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](https://durgical.com/en/anatomy/clavicle/)

**Lateral**
Articular facet on the medial edge of the acromion

**Above**
[AC ligament](https://durgical.com/en/anatomy/acromioclavicular-ligament/) and [AC capsule](https://durgical.com/en/anatomy/acromioclavicular-joint-capsule/), blending with the fascia of the deltoid and [trapezius](https://durgical.com/en/anatomy/trapezius-muscle/)

**Below**
Lower AC capsule, then the [subacromial bursa](https://durgical.com/en/anatomy/subacromial-subdeltoid-bursa/) and the [supraspinatus tendon](https://durgical.com/en/anatomy/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](https://durgical.com/en/anatomy/region/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

- [Acromioclavicular joint separation](https://durgical.com/en/conditions/acromioclavicular-joint-separation/)

## 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.

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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

- [AC ligament](https://durgical.com/en/anatomy/acromioclavicular-ligament/)
- [Clavicle](https://durgical.com/en/anatomy/clavicle/)
- [Scapula](https://durgical.com/en/anatomy/scapula/)

Sources: Terminologia Anatomica (TA2); [Anatomy, Shoulder and Upper Limb, Acromioclavicular Joint](https://www.ncbi.nlm.nih.gov/books/NBK499858/); 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.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/acromioclavicular-joint-disc/