AC Joint Injury: Understanding, Clinical Tests, and Diagnostic Imaging

How to test for AC joint injury?

Testing for an AC joint injury involves a combination of subjective assessment, visual inspection, specific orthopedic tests performed by a healthcare professional, and often diagnostic imaging to confirm the diagnosis and assess severity.

Understanding the AC Joint and Injury

The acromioclavicular (AC) joint is a crucial articulation in the shoulder complex, connecting the acromion process of the scapula (shoulder blade) to the distal end of the clavicle (collarbone). This joint, though small, plays a vital role in shoulder movement and stability, particularly during arm elevation. Its stability is primarily maintained by the AC ligaments (superior and inferior) and, more importantly, the stronger coracoclavicular (CC) ligaments (conoid and trapezoid), which connect the clavicle to the coracoid process of the scapula.

An AC joint injury, commonly known as a shoulder separation, occurs when these ligaments are stretched or torn. The severity is typically graded based on which ligaments are involved and the degree of displacement of the clavicle from the acromion. Common causes include direct falls onto the shoulder (e.g., during sports or accidents), direct blows to the top of the shoulder, or falls on an outstretched arm. Symptoms often include localized pain at the top of the shoulder, swelling, tenderness, bruising, and potentially a visible deformity or "step-off" at the joint line.

Initial Self-Assessment and Red Flags

While a definitive diagnosis requires professional evaluation, initial self-assessment can provide clues and help determine the urgency of seeking medical attention.

• Subjective Assessment:
  • Pain Location: Is the pain directly on top of the shoulder, localized to the AC joint? Can you pinpoint it with one finger?
  • Mechanism of Injury: Did the injury result from a direct fall onto the shoulder or a forceful impact?
  • Movement Limitations: Is it painful to raise your arm overhead, reach across your body, or sleep on the affected side?
• Visual Inspection:
  • Deformity: Look for any visible bump or "step-off" deformity at the top of the shoulder, indicating the clavicle has displaced upwards. Compare it to the uninjured side.
  • Swelling and Bruising: Observe for any localized swelling or discoloration around the AC joint.
• Palpation:
  • Gently press directly on the AC joint. Is there acute tenderness?
  • Follow the clavicle out to its end where it meets the acromion.
• Red Flags for Immediate Medical Attention:
  • Severe, unmanageable pain.
  • Significant, obvious deformity of the shoulder.
  • Complete inability to move the arm or shoulder.
  • Numbness, tingling, or weakness in the arm or hand (suggesting nerve involvement).

Clinical Examination by a Healthcare Professional

A thorough clinical examination by a physician, physical therapist, or other qualified healthcare provider is crucial for accurate diagnosis. This typically involves:

• Comprehensive History Taking: Detailed questions about the injury mechanism, onset of symptoms, pain characteristics, and aggravating/alleviating factors.
• Observation: Assessing posture, symmetry, and any visible deformities or swelling.
• Palpation: Systematically palpating the AC joint, clavicle, acromion, and surrounding structures to identify areas of tenderness, crepitus, or step-off deformities.
• Range of Motion (ROM) Assessment: Evaluating active and passive ROM in all planes (flexion, abduction, internal/external rotation, horizontal adduction). Painful arcs, especially in abduction beyond 90 degrees and horizontal adduction, are common.
• Strength Testing: Assessing strength of shoulder girdle muscles, often against resistance, to identify weakness or pain that might indicate associated injuries or confirm AC joint involvement.

Specific Orthopedic Tests for AC Joint Injury

Several specific orthopedic provocative tests are commonly used to assess the integrity of the AC joint. These tests aim to compress or shear the joint, eliciting pain if an injury is present.

• Cross-Body Adduction Test (Horizontal Adduction Test):
  • Purpose: To compress and shear the AC joint.
  • Procedure: The examiner passively or actively brings the patient's arm across the body towards the opposite shoulder, causing horizontal adduction.
  • Positive Sign: Sharp pain directly at the AC joint, which suggests AC joint pathology.
• AC Joint Shear Test (AC Compression Test):
  • Purpose: To directly compress the AC joint.
  • Procedure: The examiner places one hand on the clavicle and the other on the spine of the scapula (or acromion), then compresses the two structures together.
  • Positive Sign: Pain or excessive movement/crepitus at the AC joint.
• O'Brien's Test (Active Compression Test) - Modified for AC Joint:
  • Purpose: While primarily for labral tears, it can also provoke AC joint pain.
  • Procedure: The patient stands with the shoulder flexed to 90 degrees, elbow extended, and horizontally adducted 10-15 degrees. First, the arm is internally rotated with the thumb pointing down, and the examiner applies downward resistance. Then, the arm is externally rotated with the thumb pointing up, and resistance is applied again.
  • Positive Sign: Pain localized to the AC joint during the thumb-down (internal rotation) position that is reduced or eliminated in the thumb-up (external rotation) position.
• Paxinos Test:
  • Purpose: A specific test to compress the AC joint.
  • Procedure: The examiner places their thumb on the posterior aspect of the acromion and the fingers of the same hand over the anterior aspect of the clavicle, then applies a compressive force.
  • Positive Sign: Pain elicited directly at the AC joint.
• Resisted Extension Test (AC Resisted Extension Test):
  • Purpose: To stress the AC joint through resisted movement.
  • Procedure: The patient's arm is positioned in slight extension (behind the body), and the examiner applies resistance as the patient attempts to extend the arm further or flex it forward.
  • Positive Sign: Pain localized to the AC joint, particularly with resisted extension.

Diagnostic Imaging

While clinical tests are highly valuable, diagnostic imaging is often necessary to confirm an AC joint injury, assess its severity (grading), rule out fractures, and guide treatment.

• X-rays:
  • Purpose: The initial imaging choice to visualize bone structures, rule out fractures of the clavicle or acromion, and assess the degree of AC joint separation.
  • Key Views: Standard anterior-posterior (AP) views, Zanca view (a specific AP view angled to better visualize the joint), and an axillary view.
  • Stress Views: Often performed with the patient holding weights in each hand. This can highlight subtle AC joint instability by accentuating the separation under load. Bilateral views (both shoulders) are typically taken for comparison.
• MRI (Magnetic Resonance Imaging):
  • Purpose: Provides detailed images of soft tissues, including the AC and coracoclavicular ligaments. It is excellent for assessing the extent of ligamentous damage, detecting associated soft tissue injuries (e.g., rotator cuff tears, labral tears), and ruling out other pathologies.
• Ultrasound:
  • Purpose: A dynamic and cost-effective imaging modality that can visualize the AC joint and surrounding ligaments in real-time during movement. It can detect fluid accumulation, ligament disruption, and joint instability. However, its accuracy is highly dependent on the operator's skill.

Important Considerations and Professional Guidance

It is crucial to understand that self-diagnosis based solely on symptoms or these tests is not recommended. These tests are best interpreted within the context of a comprehensive clinical examination by a trained healthcare professional.

• Self-Diagnosis Limitations: Many shoulder conditions can mimic AC joint pain (e.g., rotator cuff tendinopathy, biceps tendinopathy, cervical spine issues, referred pain). A professional can perform a differential diagnosis to rule out other conditions.
• Professional Assessment: A qualified physician or physical therapist will integrate the findings from the history, physical examination, and imaging to arrive at an accurate diagnosis and develop an appropriate treatment plan.
• Rehabilitation: Once diagnosed, AC joint injuries, especially higher-grade separations, require a structured rehabilitation program focusing on pain management, restoring range of motion, improving stability, and progressive strengthening to ensure a full return to function and prevent chronic issues.