AC Joint Injury: Classification, Diagnosis, and Treatment Protocols

In 2012, my father overlooked a hole on the ski slope and fell on his shoulder. He immediately felt something snap in his shoulder. After an X-ray of the shoulder, doctors told him he had torn the ligaments in the joint between the clavicle and the scapula (AC joint). Initially, they advised against surgery (citing age and healthcare cost savings), but later, due to his unwavering determination, they operated on him. Had he not had the surgery, the joint would never have been as mobile as before. The answer lies in the anatomy of the joint, so let’s take a look.

In conclusion, the rehabilitation programs after AC joint injury are presented.

The Acromioclavicular (AC) joint is a small but important joint at the top of the shoulder that connects the clavicle with the acromion (a bony projection of the scapula). Both joint surfaces are covered with fibrous cartilage, and there is often a small disc (similar to a meniscus) between them.

The stability of the joint is provided by two groups of ligaments:

1. Acromioclavicular (AC) ligaments: These ligaments surround the joint capsule and primarily provide horizontal stability (preventing the clavicle from moving forwards and backwards).
2. Coracoclavicular (CC) ligaments: These are two strong ligaments (trapezoid and conoid) that run from the coracoid process of the scapula to the clavicle. They provide the main vertical stability and prevent the clavicle from elevating too high above the scapula.

AC joint injury most often occurs during a direct fall onto the shoulder (e.g., during cycling, skiing, hockey), when the force of the impact pushes the scapula downwards while the clavicle remains in place or moves upwards.

We know several grades of injuries, most accurately described by the Rockwood classification (a more modern version of the older Tossy scale). Classification is crucial for the treatment decision.

 

✚ TYPE 1:
Sprain of AC ligaments. CC ligaments are intact. The joint is stable, tender to the touch, but there is no visible deformity or “step-off.”

✚ TYPE 2:
Complete rupture of AC ligaments and sprain of CC ligaments. The clavicle is slightly elevated, so a small “step-off” may be visible or palpable at the top of the shoulder.

✚ TYPE 3:
Complete rupture of AC and CC ligaments. The clavicle is visibly elevated (25-100% higher than normal), causing an obvious “step-off.” The joint is unstable both vertically and horizontally.

✚ TYPE 4:
Same as Type 3 (all ligaments torn), but the clavicle is also displaced backward (posteriorly) and lodges in the trapezius muscle.

✚ TYPE 5:
A very severe form of Type 3. Complete rupture of AC and CC ligaments and muscle attachments. The upward displacement of the clavicle is extreme (more than 100% higher than normal).

✚ TYPE 6:
A very rare injury. The clavicle is displaced downward (inferiorly) under the coracoid process, often alongside severe chest injuries.

 

 

Some types of injury are also visible to the naked eye, in 3 body planes.

 

Diagnosis is made with clinical examination and imaging diagnostics.

🎯 Clinical Tests

During the examination, the joint is painful to the touch. Pain is characteristic when lifting the arm above 90 degrees and during cross-body movement (e.g., touching the opposite shoulder). The tests mentioned in the original article are provocative tests that attempt to elicit pain in the AC joint:

• 1. O’Brien’s Test: Although more commonly used for SLAP labrum injuries, pain localized directly to the AC joint can be a sign of AC joint pathology.
• 2. Scarf Test (Cross-body adduction test): The patient actively or passively pushes the arm across the chest towards the opposite shoulder. Pain at the top of the shoulder is a positive sign.
• 3. Shear Test and 4. Paxinos Test: The therapist attempts to elicit pain or instability by squeezing the clavicle and acromion.

You can view each test by clicking on the image below:

1. O’Brien’s Test:	2. Scarf Test:
3. Shear Test	4. Paxin’s Test

 

🎯 Imaging Diagnostics

✚ X-ray (RTG): Is the gold standard. Often used:

➜ Weighted View (Stress X-ray): The patient holds a weight (approx. 5 kg) in the hand on the injured side. This reveals the actual degree of vertical instability and helps distinguish Type 2 from Type 3.

➜ Zanca View: A special angle of imaging that best shows the AC joint without overlapping other bones.

✚ Ultrasound (US) and MRI: Are used less frequently, mainly to assess soft tissues (ligaments, disc) or if there is suspicion of other simultaneous injuries (e.g., rotator cuff).

 

Treatment is completely dependent on the type of injury.

 

🎯 Conservative Treatment (Type I and II)

 

Type I and II injuries are always treated conservatively (without surgery).

✚ Phase 1 (Acute): The goal is pain management. Includes brief rest, cooling (ice), and the use of a simple sling for relief (usually 1-3 weeks).

✚ Phase 2 (Subacute): Gradual introduction of mobility. Starts with pendulum exercises, active assisted movements, and light isometric exercises for scapular stabilizers.

✚ Phase 3 (Strengthening): Once mobility is full and painless, targeted strengthening of the rotator cuff muscles and scapular stabilizers (muscles around the shoulder blade) begins, which must take over part of the stabilization role of the injured ligaments.

 

🎯 Treatment of Type III Injuries (“Gray Zone”)

This is the area where guidelines have changed the most. Most studies today show that conservative treatment yields equally good or even better long-term results than surgery.

✚ Conservative Treatment (Recommendation for most): The approach is the same as for Type II, but rehabilitation is longer and more demanding. The cosmetic “step-off” on the shoulder usually remains, but it causes no functional problems if the muscles around the joint are strong enough.

✚ Surgical Treatment (Exception): Surgery is recommended only in specific cases: younger, highly active athletes (especially in contact or overhead sports) or heavy manual laborers, where long-term stability is crucial, or if conservative treatment is unsuccessful after 3-6 months.

 

🎯 Surgical Treatment (Type IV, V, and VI)

These injuries are severely unstable and almost always require surgical treatment for repositioning and stabilization of the clavicle. Rehabilitation after surgery is lengthy and follows a strict surgeon’s protocol.

 

 

⚠️ Chronic AC Joint Problems (Additional)

Pain in the AC joint is not always the result of an acute fall. Two chronic conditions are also common:

✚ AC Joint Arthrosis: Wear and tear of the cartilage in the joint, causing dull pain at the top of the shoulder. Pain is typically worse when lying on the injured side or during cross-body arm movements.

✚ Distal Clavicle Osteolysis:  A condition often called “weightlifter’s shoulder.” Due to repetitive stress (e.g., bench press), there is bone resorption at the end of the clavicle, causing chronic pain. Treatment involves modifying training, physiotherapy, and sometimes surgery (removal of the distal end of the clavicle).

 

 

In the manual below, you can read everything about the AC joint rehabilitation program (access by clicking on the image below or the link on the right).