AC Joint Sprain

Written By Martin Law

You tripped, landed hard on your shoulder, and now lifting your arm feels impossible. What’s going on up there?

What is it

The AC joint (short for acromioclavicular joint) is located at the top of your shoulder, where your collarbone (clavicle) meets a part of your shoulder blade called the acromion.

A sprain occurs when the ligaments that stabilise this joint are stretched or torn, typically when force exceeds what the ligament can tolerate. This often happens from a fall directly onto the shoulder or onto an outstretched hand. 

AC joint sprains lie on a classification called the Rockwood classification (a scale from 1-6), which classifies the severity of the AC joint sprain with reference to an X-ray. The higher the classification, the more damage there is to certain structures.

Type I – Mild (Sprain)

  • What it is: Minor stretching of the AC ligaments, no tearing

  • What you see: No visible deformity, normal X-ray

  • Function: Shoulder is stable, but tender to touch

  • Recovery: 1–2 weeks with conservative care

Type II – Partial Dislocation

  • What it is: Tear of the AC ligament, but coracoclavicular (CC) ligaments are still intact

  • What you see: Mild bump or prominence at the joint

  • Function: Slight loss of motion or discomfort with overhead use

  • Recovery: 2–4+ weeks with rehab

Type III – Complete Dislocation

  • What it is: Full tear of both AC and CC ligaments

  • What you see: Visible step deformity ("bump") on top of the shoulder

  • Function: More noticeable weakness or instability

  • Recovery: Often managed without surgery, but surgical consideration for active individuals or cosmetic concerns (6-8 weeks with rehab)

Type IV – Posterior Displacement

  • What it is: The distal clavicle is pushed backward into or behind the trapezius muscle

  • What you see: Obvious deformity, often confirmed with imaging

  • Management: Usually surgical

Types IV–VI are more severe and less common, often needing surgical input. We’ll refer you to the right specialists if needed.

How Does It Happen?

Common mechanisms include:

  • Falling directly onto the tip of the shoulder

  • Landing on an outstretched arm

  • Contact injuries in sports like rugby, AFL, or cycling

These forces place sudden strain on the AC ligaments, potentially causing pain, swelling, and joint instability.

How Is It Diagnosed?

  • A physiotherapist or doctor will examine your shoulder movement, pain location, and strength

  • An X-ray may be used to confirm the grade of injury

  • In some cases, an ultrasound or MRI may help rule out other injuries

What Are the Symptoms?

  • Dull throbbing pain

  • Pain with overhead movements 

  • Pain with reaching across the body 

  • Pain with externally rotating your shoulder

  • Possible bump, swelling, or bruising over the joint

What can it be mistaken for?

  • Clavicle Fracture 

  • Rotator Cuff Strain

  • Shoulder Impingement

  • Shoulder Dislocation/Subluxation

  • If you have intense pain, visible deformity, or can’t lift your arm at all, seek a physiotherapist or doctor promptly to rule out fractures or more serious injury.

What Can We Do? 

  • Manual therapy to reduce pain and restore movement

  • Individualised rehab exercises to strengthen shoulder stabilisers, increase shoulder movement, and maintain shoulder strength while the ligament is healing

  • Education and activity guidance to avoid flare-ups and re-injury

  • Taping to help with AC joint support and posture

Can It Be Prevented? 

While it can be difficult to reduce trauma to the AC joint or shoulder during contact sports, the risk of a sprain can be reduced by:

  • Ensure a proper warm-up of your shoulder before games

  • Strengthening the shoulder muscles to attenuate impact 

  • Learning how to fall safely 

Shoulder pain bothering you?

Let’s take a look — the earlier you get on top of it, the better the outcome.

We regularly work with athletes and active individuals recovering from AC joint sprains — whether it’s post-rugby match or an awkward fall.