Acromioclavicular (AC) Joint Pain: Causes, Symptoms, Diagnosis, and Treatment

What is Acromioclavicular Joint Pain?

Acromioclavicular (AC) joint pain refers to discomfort and dysfunction originating from the joint where the collarbone (clavicle) meets the shoulder blade (scapula), often resulting from acute injury, overuse, or degenerative changes.

Understanding the Acromioclavicular Joint

The acromioclavicular (AC) joint is a small but critical articulation located at the top of the shoulder. It connects the acromion, a bony projection of the scapula (shoulder blade), to the distal end of the clavicle (collarbone). This joint is crucial for the full range of motion of the shoulder complex, acting as a pivot point for the scapula and allowing for subtle gliding movements that facilitate overhead arm elevation and other complex shoulder movements.

The AC joint's stability is primarily maintained by:

• AC joint capsule and ligaments: Directly surround and reinforce the joint.
• Coracoclavicular (CC) ligaments: Two strong ligaments (conoid and trapezoid) that connect the clavicle to the coracoid process of the scapula, providing significant vertical stability and preventing superior displacement of the clavicle.

Common Causes of AC Joint Pain

AC joint pain can arise from several mechanisms, each with distinct characteristics:

• Acute Injury (AC Joint Separation/Sprain): This is often the most dramatic cause, typically resulting from:
  • A direct fall onto the shoulder (e.g., cycling accident, sports tackle).
  • A direct blow to the top of the shoulder. These injuries are graded I to VI, with Grade I being a mild sprain of the AC ligaments and Grade III involving complete rupture of both AC and CC ligaments, leading to noticeable deformity. Higher grades involve more extensive soft tissue damage and displacement.
• Overuse Injuries (Distal Clavicle Osteolysis/Acromial Osteolysis): Also known as "weightlifter's shoulder," this condition is common in individuals engaged in repetitive overhead activities or heavy lifting, particularly exercises like bench press, military press, and dips. The constant compressive and shearing forces across the AC joint can lead to microtrauma, bone resorption, and inflammation at the distal clavicle and/or acromion.
• Degenerative Arthritis (Osteoarthritis): Similar to other joints in the body, the AC joint can develop osteoarthritis due to age-related wear and tear or as a long-term consequence of a previous AC joint injury. The articular cartilage breaks down, leading to bone-on-bone friction, osteophyte (bone spur) formation, and inflammation.

Recognizing the Symptoms

Symptoms of AC joint pain vary depending on the cause and severity but commonly include:

• Localized Pain: Sharp or aching pain directly on top of the shoulder, often described as radiating slightly into the neck or upper arm.
• Tenderness to Palpation: The joint is exquisitely tender when pressed.
• Pain with Movement:
  • Overhead Arm Elevation: Reaching up or lifting objects above shoulder height.
  • Cross-Body Adduction: Reaching the affected arm across the body towards the opposite shoulder.
  • Heavy Lifting: Especially pushing movements like bench press.
• Swelling and Bruising: More common with acute injuries.
• Clicking or Grinding: May be present with degenerative changes.
• Visible Deformity: In higher-grade AC joint separations, a noticeable bump may appear on top of the shoulder due to the clavicle displacing upwards.

Diagnosis of AC Joint Pain

Accurate diagnosis involves a combination of clinical assessment and imaging:

• Clinical Examination:
  • History Taking: Understanding the mechanism of injury, onset of pain, and aggravating factors.
  • Palpation: Direct pressure over the AC joint to elicit tenderness.
  • Range of Motion Assessment: Evaluating active and passive shoulder movements, specifically looking for pain with cross-body adduction or end-range overhead elevation.
  • Provocative Tests: Specific orthopedic tests designed to stress the AC joint, such as the cross-body adduction test, O'Brien's test, or AC shear test.
• Imaging:
  • X-rays: Standard views can identify fractures, significant displacement (AC separations), and degenerative changes. Stress views (taken while holding a weight) can sometimes reveal subtle AC joint instability.
  • MRI (Magnetic Resonance Imaging): Provides detailed images of soft tissues, useful for assessing ligamentous injury (AC and CC ligaments), cartilage damage, and inflammation.
  • CT Scan (Computed Tomography): Offers excellent bony detail, helpful for evaluating subtle fractures, osteolysis, or complex degenerative changes.

Treatment and Management Strategies

Treatment approaches vary based on the specific cause and severity of AC joint pain.

• Conservative Management: This is the initial approach for most AC joint issues, especially lower-grade sprains, overuse injuries, and mild arthritis.
  • Rest and Activity Modification: Avoiding activities that aggravate the pain.
  • Ice/Heat Application: To manage pain and inflammation.
  • Non-Steroidal Anti-Inflammatory Drugs (NSAIDs): Over-the-counter or prescription medications to reduce pain and inflammation.
  • Corticosteroid Injections: Injections into the AC joint can provide temporary pain relief for inflammatory conditions or arthritis.
  • Physical Therapy/Rehabilitation: A cornerstone of conservative treatment, focusing on restoring function and preventing recurrence.
• Surgical Intervention: Reserved for specific cases, such as:
  • High-grade AC joint separations (Grade III and above, especially in active individuals).
  • Persistent pain from distal clavicle osteolysis or severe arthritis that has failed conservative management. Surgical procedures aim to stabilize the joint, remove damaged bone, or reconstruct ligaments.

Rehabilitation and Exercise Considerations

From an exercise science perspective, rehabilitation for AC joint pain is critical for restoring function, strength, and preventing recurrence.

• Acute Phase (Pain Management & Protection):
  • Focus: Reduce pain and inflammation, protect the joint from further stress.
  • Activities: Gentle, pain-free range of motion exercises (e.g., pendulum swings), isometric contractions of surrounding musculature (rotator cuff, scapular stabilizers) if pain allows. Avoid direct AC joint loading.
• Sub-Acute/Strengthening Phase (Restoration of Strength & Stability):
  • Scapular Stability: Emphasize exercises for the periscapular muscles (serratus anterior, rhomboids, middle and lower trapezius). These muscles are vital for proper scapular rhythm and reducing stress on the AC joint. Examples include scapular squeezes, prone Y/T/I raises, push-up plus.
  • Rotator Cuff Strengthening: Improve dynamic stability of the glenohumeral joint, indirectly supporting the entire shoulder complex. Focus on internal and external rotation exercises with light resistance.
  • Gradual Strengthening: Progress from isometric to isotonic exercises, starting with light resistance and increasing progressively. Focus on controlled movements within a pain-free range.
  • Movement Pattern Correction: Address any compensatory movements or muscle imbalances that may contribute to AC joint stress.
  • Exercise Modifications: Temporarily avoid or modify exercises that directly compress or shear the AC joint, such as deep dips, heavy bench press, or overhead pressing to end range. Consider neutral grip presses or incline presses as alternatives.
• Return to Activity Phase (Functional Integration & Prevention):
  • Sport-Specific Drills: Gradually reintroduce movements specific to the individual's sport or activity.
  • Progressive Loading: Slowly increase resistance, intensity, and volume.
  • Education: Reinforce proper lifting mechanics, posture, and training principles to prevent recurrence.
  • Balanced Training: Ensure a balanced strength program that doesn't overemphasize pushing movements without adequate pulling and scapular stability work.

When to Seek Professional Medical Advice

It is advisable to consult a healthcare professional, such as a physician, physical therapist, or sports medicine specialist if you experience:

• Sudden, severe pain after a fall or direct trauma to the shoulder.
• A visible deformity or "bump" on top of the shoulder.
• Inability to move your arm or perform daily activities due to pain.
• Persistent pain that does not improve with rest and basic self-care within a few days.
• Numbness, tingling, or weakness radiating into the arm or hand.