Acromioclavicular Joint Dislocation Classification Explained

By Dr Christopher M. Phoon MBBS BSc(Med) FRACS(Orth) FAOrthA Orthopaedic Surgeon, Queenstown Joint Clinic

Introduction

Acromioclavicular joint (ACJ) dislocation— often called “shoulder separations” — are extremely common, especially in active people who ski, bike, or play contact sports. When the ligaments that stabilise the joint between the collarbone and shoulder blade are damaged, the result can range from a simple sprain to a complete dislocation.

normal stabilizers of the AC joint. (yellow circle)

To understand this injury it helps to know that the AC joint is held stable by two main structures:

1. The AC joint capsule and ligaments, which control forward–backward (anterior–posterior) motion.

2. The coracoclavicular (CC) ligaments, which prevent the collarbone lifting upward.

The most widely used way to describe these injuries is the Rockwood classification, which grades ACJ dislocations from Type I to Type VI. It’s a system almost every doctor and radiology report will mention when discussing your injury.

History and Limitations

The Rockwood classification was first published in 1984. It expanded an older three-grade system to six, aiming to better describe the direction and amount of clavicle displacement.

In theory, the grading is based on how much the coracoclavicular (CC) distance — the gap between the collarbone and coracoid process — increases compared to the uninjured side:

• Type II: less than 25% increase

• Type III: 25–100%

• Type V: more than 100%

  
  

In practice, this measurement is very inconsistent.The X-ray angle, whether the shoulder is relaxed, or if the arm is hanging with weights can all drastically change the apparent separation. Different observers often reach different conclusions from the same image.

For this reason, the Rockwood classification is useful as a descriptive language, but not as a precise tool for deciding who needs surgery. That judgment comes from clinical function, symptoms, and experience — not just numbers.

Types I–III: The Practical End of the Spectrum

Rockwood classification grades 1 to 3.

Type I

This is a sprain of the AC joint capsule. The main stabilising ligaments are intact, and X-rays are usually normal. These injuries settle quickly with rest and physiotherapy.

Type II

Here, the AC ligaments are torn, but the CC ligaments remain largely intact. There may be a small step at the joint or mild tenderness, but the shoulder is stable. The separation is less than 25%. Again, this heals without surgery in nearly all cases.

Type III

Both the AC and CC ligaments are completely torn. The collarbone is now free to lift up and out of joint, producing a visible lump on the shoulder. This is the true dislocation most people think of.

This is the stage where surgery can make a clear difference for some patients — particularly those who are physically active, have heavy work demands, or feel persistent weakness despite good rehabilitation.

Grades IV to VI — The Uncommon and Often Misleading End

Rockwood classification grades 4 to 6.

Beyond Type III, the classification becomes less useful in day-to-day practice:

Type IV

In this pattern, the collarbone has shifted backwards behind the acromion. This is difficult to see on standard X-rays and usually only confirmed with 3D imaging like a CT scan. In my experience, most dislocations have some posterior component due to the pull of the trapezius muscle, even if not labelled “Type IV.”

Type V

Defined as “greater than 100% displacement.” In reality, this is the same injury as Type III — the difference is only how the X-ray was taken or how the patient was standing. Both involve complete rupture of the stabilising ligaments.Calling one “Type V” and another “Type III” can falsely imply a treatment difference when there really isn’t one.

Type VI

An extremely rare injury where the collarbone dislocates underneath the coracoid process. Only about a dozen true cases have ever been reported since it was first described in 1967. I’ve never seen one in over 20 years of shoulder surgery. It’s often emergency injury because of the risk to the nearby nerves and arteries.

A Surgeon’s Perspective

At Queenstown Joint Clinic, this is one of the most common injuries we see, driven by our large downhill mountain biking and snow sports population.

After more than 20 years working in orthopaedics, my experience has led me to a practical approach that focuses on the degree of instability and on carefully balancing the risks and benefits of surgery. From a surgical perspective, the Rockwood classification is helpful for describing grades I to III, but the higher grades — IV, V, and VI — add little practical value. Surgery is generally only considered for complete dislocations — that is, anything beyond a grade I or II injury. However, not all complete dislocations require an operation. The decision is more about whether a patient is willing to accept deformity or some residual disability, and about balancing this against the risks of surgery, especially since later operations can be technically more difficult. Whether surgery is appropriate should always be discussed with an experienced shoulder surgeon, who can explain the benefits, risks, and recovery expectations in detail. I’ll cover that discussion — including how surgical repair is performed and how it can improve outcomes — in a future post.