Sudden Swelling and Pain on Top of the Shoulder: What Is an AC Joint Injury?

The shoulder region is exposed to high loads and trauma in both daily life and sports. One of the most commonly injured structures in this area is the acromioclavicular (AC) joint, located at the top of the shoulder where the collarbone meets the tip of the shoulder blade. A fall or direct blow followed by sudden pain and swelling on top of the shoulder is often a sign of an AC joint injury.

What Is the AC Joint and What Does It Do?

The AC joint is the point where the outer end of the clavicle (collarbone) meets the acromion, a bony part of the shoulder blade (scapula). Although it is a small joint, it has an important role in transferring load between the arm and the trunk during lifting, pushing, pulling and overhead activities.

Joint stability is mainly provided by two ligament complexes:

• AC ligaments: Strengthen the joint capsule and provide horizontal stability.
• Coracoclavicular (CC) ligaments: The main structures that provide vertical stability between the clavicle and scapula.

When these ligaments are stretched or torn as a result of trauma, the relationship between the clavicle and acromion changes and different degrees of AC joint separation can occur.

How Does an AC Joint Injury Occur?

AC joint injuries typically occur after a sudden, high-energy load to the top of the shoulder. Common mechanisms include:

• Falling directly onto the shoulder (especially from a bicycle, motorcycle or scooter)
• Direct impact to the shoulder in contact sports such as football, basketball or rugby
• Falling with the arm outstretched and the shoulder forced into the ground
• Traffic accidents and falls from height

Depending on the direction and force of the trauma, the AC ligaments may be stretched or torn first, followed by injury to the CC ligaments. The spectrum ranges from mild sprain to complete disruption with a visible bump on top of the shoulder.

Symptoms of an AC Joint Injury

Typical symptoms include:

• Sudden, sharp pain on top of the shoulder
• Swelling and tenderness directly over the AC joint
• Limited range of motion, especially pain when lifting the arm
• Discomfort when putting on or taking off clothes, or when sleeping on that side
• A visible bump on top of the shoulder in higher-grade injuries
• A so-called “piano key sign” in some cases – when the clavicle can be pressed down and springs back up

In mild injuries, pain and tenderness may be the only findings. In more severe cases, the alignment of the shoulder is clearly disturbed and cosmetic concerns are common.

Rockwood Classification: Grades of AC Joint Injury

AC joint injuries are commonly classified using the Rockwood system, which describes six types based on clinical and radiological findings:

• Type I: Sprain of the AC ligaments only. No true instability. Pain and tenderness are the main complaints.
• Type II: Complete tear of AC ligaments with sprain of CC ligaments. Mild elevation of the clavicle.
• Type III: Complete rupture of both AC and CC ligaments. Clear step-off and asymmetry between both shoulders.
• Types IV–VI: More severe displacements of the clavicle (backwards, upwards or downwards) with significant soft tissue damage.

Type I–II injuries usually respond well to non-operative treatment, whereas Types IV–VI generally require surgery. Type III is evaluated individually, taking into account the patient’s age, activity level, occupation and cosmetic expectations.

How Is an AC Joint Injury Diagnosed?

Diagnosis starts with a detailed history and physical examination. The mechanism of injury, type of sport or accident, and timing of pain onset are carefully questioned. The orthopaedic surgeon palpates the AC joint, assesses shoulder range of motion and muscle strength, and rules out other shoulder pathologies.

Imaging methods commonly used include:

• Standard shoulder X-rays: To evaluate the AC joint, clavicle and overall alignment.
• Comparison with the opposite side: Measuring AC and CC distances on both shoulders.
• MRI: When more detail on ligaments and associated soft tissue injuries is needed.

Treatment Options for AC Joint Injuries

Treatment is planned according to the grade of injury, the patient’s age, activity level, occupation and expectations. In general, there are two main categories:

1. Non-operative (Conservative) Treatment

Non-operative treatment is usually sufficient for Type I and Type II injuries and for selected Type III cases. It may include:

• Ice application in the first days after the injury
• Short-term use of a shoulder sling
• Pain medication and anti-inflammatory medicines as prescribed
• Early range-of-motion exercises to prevent stiffness
• Progressive strengthening of the shoulder girdle muscles and stabilisation exercises

Most patients experience a clear reduction in pain within a few weeks and gradually return to daily activities. However, heavy lifting and direct pressure on the shoulder should be avoided for a certain period.

2. Surgical Treatment

Surgery may be considered in the following situations:

• Type IV, V and VI injuries
• Type III injuries in young, highly active patients and overhead athletes
• Persistent pain and functional limitation despite adequate conservative treatment
• Marked cosmetic deformity and shoulder imbalance

The goal of surgery is to restore the anatomical position of the clavicle and to reconstruct the AC–CC complex. Several techniques can be used, such as:

• Coracoclavicular ligament reconstruction with grafts or synthetic devices
• Stabilisation with plates, screws or button systems
• Arthroscopy-assisted, minimally invasive procedures

After surgery, a shoulder sling is used for a period of time, followed by a structured rehabilitation programme to regain range of motion, strength and function.

Recovery and Rehabilitation

Recovery time differs depending on the grade of injury and the chosen treatment method. Patients with mild sprains may return to daily activities within a few weeks, while those with high-grade injuries or surgery may require several months to fully recover and return to sports.

A well-designed rehabilitation programme typically includes:

• Gentle stretching to maintain joint mobility
• Strengthening exercises for the rotator cuff and shoulder girdle muscles
• Posture and core stabilisation exercises
• Sport-specific, gradual return-to-play protocols for athletes

When rehabilitation is properly supervised, most patients can return to an active lifestyle with good shoulder function.

When Should You See an Orthopaedic Surgeon?

You should seek medical attention from an orthopaedic and traumatology specialist if you experience:

• Sudden pain and swelling on top of the shoulder after a fall or direct blow
• A visible bump or deformity over the AC joint
• Difficulty lifting the arm or performing overhead activities
• Night pain that interferes with sleep
• Persistent symptoms that do not improve with rest and simple pain medication

Early diagnosis and appropriate treatment are essential to protect shoulder function and to reduce the risk of chronic pain and instability in the long term.

Personalised Care with Dr. Muhammed Duman

Every AC joint injury is unique. The optimal treatment plan should be tailored to the individual, taking into account not only X-ray findings but also age, occupation, sports level and cosmetic concerns. If you have noticed sudden swelling, pain or deformity on top of your shoulder, a thorough evaluation by an orthopaedic specialist is an important step for your long-term shoulder health.

Related Article 

• Runner’s Knee: Patellofemoral Pain Syndrome in Runners