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Joints and ligaments · Upper limb

AC ligament !

The acromioclavicular ligament holds the outer end of the collarbone to the tip of the shoulder blade; it stops front-to-back sliding.

Official name: Acromioclavicular ligament

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What it is

The band over the top of the joint between the collarbone and the acromion.

What it does

Stops the collarbone sliding forward and back on the acromion.

Key facts

Attaches
Acromion to the acromial end of the clavicle (superior, inferior)
Function
Horizontal (front-back) AC stability; first to tear in a separation
Tight in
Anterior-posterior translation of the clavicle

Relations

Above
Deltotrapezial fascia and skin; the joint lies just under the skin
Below
AC disc and joint, then the subacromial bursa and the supraspinatus tendon
Medial
Acromial end of the clavicle
Lateral
Acromion
In front
Origin of the front deltoid
Behind
Insertion of the upper trapezius

How to find it

The joint is felt as a small step at the outer end of the clavicle, a little medial to the lateral edge of the acromion; it moves under the finger when the arm is raised or the shoulder is shrugged.

Examination

  • Cross-body adduction (scarf) test: pain on top of the shoulder as the arm is brought across the chest points to the AC joint
  • Paxinos test: the thumb pushes the back of the acromion up while the index finger pushes the clavicle down; pain at the joint suggests AC pathology
  • O'Brien active compression test: pain felt on top of the shoulder (not deep inside) with the thumb down suggests the AC joint
  • Horizontal instability: holding the acromion and moving the end of the clavicle front to back shows abnormal translation when the AC ligaments are torn

On imaging

Zanca view (AP aimed at the AC joint with 10–15° of cephalic tilt and a softer exposure); bilateral views compare the coracoclavicular distance; an axillary view or a cross-body adduction (Basamania) view shows backward displacement and horizontal instability.

Common problems

  • Shoulder separation: a fall on the point of the shoulder sprains or tears the AC ligament, then the coracoclavicular ligaments.
  • In severe cases the collarbone end sticks up as a visible bump.
  • Later wear and arthritis of the AC joint.

How it is diagnosed

Tenderness and a step at the top of the shoulder. X-rays (sometimes comparing both sides) grade the separation.

Treatment

  • Sling, ice and early motion for mild and moderate separations.
  • Surgery to reconstruct the coracoclavicular ligaments for severe separations or ongoing pain.

Clinical notes

  • Grades I–II are usually treated without surgery and IV–VI with surgery; the management of grade III is debated and depends on the person's demands and symptoms
  • Type IV is easily missed on a single AP view because the clavicle is pushed backward into or through the trapezius; an axillary view shows it
  • Persistent pain or instability after a grade I–II injury may lead to a distal clavicle excision; combined AC and CC reconstruction restores stability in both planes

Test yourself

Think of the answer, then tap the question to see it.

Which ligaments give horizontal and which give vertical stability at the acromioclavicular joint?

The AC ligaments give front-back (horizontal) stability; the coracoclavicular ligaments give vertical stability.

What is the usual mechanism of AC ligament injury?

A fall on the point of the shoulder with the arm at the side, driving the acromion down under the clavicle.

In Rockwood grade III, which ligaments are torn?

Both the AC ligament and the coracoclavicular ligaments.

Why is a Rockwood type IV AC injury easily missed?

The clavicle is pushed backward into the trapezius, which a single AP view does not show; an axillary view does.

Which test brings the arm across the chest to provoke AC joint pain?

The cross-body adduction (scarf) test: pain on top of the shoulder points to the AC joint.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2)
ligamentum acromioclaviculare TA2 1745
Official Spanish (DeCS)
Ligamento acromioclavicular
English (TA2)
Acromioclavicular ligament

Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Acromioclavicular Joint.

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Representative model, for educational purposes only.

AC ligament highlighted on the Durgical 3D model