AC Joint Injuries: Understanding, Diagnosis, and Testing Methods

How do you test for AC joint?

Testing for an acromioclavicular (AC) joint injury involves a comprehensive clinical assessment, including a detailed history, physical examination with specific provocative tests, and often imaging studies, to accurately diagnose the extent and nature of the injury.

Understanding the Acromioclavicular (AC) Joint

The acromioclavicular (AC) joint is a small but crucial articulation located at the top of the shoulder, connecting the acromion (part of the shoulder blade) to the clavicle (collarbone). This joint plays a vital role in shoulder movement, especially during overhead activities, and helps stabilize the scapula. It is primarily stabilized by the AC joint capsule and the strong coracoclavicular (CC) ligaments (conoid and trapezoid ligaments). Injuries to this joint, commonly known as AC separations or sprains, range from mild ligamentous strains to complete dislocations, often resulting from a direct blow to the shoulder or a fall onto an outstretched arm.

Grades of AC Joint Injury

AC joint injuries are typically graded based on the severity of ligamentous damage and displacement:

• Grade I: A sprain of the AC ligaments without significant displacement. Mild tenderness and pain.
• Grade II: Tear of the AC ligaments with partial disruption of the CC ligaments. Moderate pain, slight instability, and a subtle "step-off" deformity may be present.
• Grade III: Complete tears of both AC and CC ligaments, leading to significant superior displacement of the clavicle relative to the acromion. Obvious deformity, significant pain, and instability.
• Grades IV-VI: More severe injuries involving posterior displacement (Grade IV), superior displacement through muscle tissue (Grade V), or inferior displacement (Grade VI), often requiring surgical intervention.

Clinical Assessment: The Foundation of Diagnosis

The initial steps in testing for an AC joint injury involve a thorough clinical evaluation by a healthcare professional.

• Patient History:
  • Mechanism of Injury: Understanding how the injury occurred (e.g., direct fall onto the shoulder, fall onto an outstretched arm) provides crucial clues.
  • Symptoms: Location, character, and severity of pain; presence of clicking, popping, or instability; limitations in movement.
  • Prior Injuries: Any history of shoulder trauma or AC joint issues.
• Observation and Palpation:
  • Visual Inspection: Observing the shoulder for swelling, bruising, and most importantly, any visible deformity or "step-off" at the AC joint, indicative of clavicular displacement.
  • Palpation: Gently feeling the AC joint for localized tenderness. Tenderness directly over the joint line is a hallmark sign of AC joint pathology. Palpation can also help assess the degree of clavicular mobility or instability.
• Range of Motion Assessment:
  • Assessing active and passive range of motion of the shoulder, noting any pain or limitations, particularly with cross-body adduction or overhead movements.

Specific Provocative Tests for AC Joint Integrity

Several specific physical examination tests are used to provoke pain at the AC joint, helping to confirm a diagnosis and differentiate it from other shoulder pathologies.

• Cross-Body Adduction Test (Horizontal Adduction Test):

  • Procedure: The examiner passively adducts the patient's arm across the body, bringing the elbow towards the opposite shoulder.
  • Positive Sign: Pain localized to the AC joint, especially at the end range of motion.
  • Clinical Significance: This test compresses the AC joint, making it highly sensitive for AC joint pathology, including sprains, arthritis, or distal clavicle osteolysis.

• AC Joint Shear Test (Compression Test):

  • Procedure: The examiner cups the patient's shoulder, placing one hand over the clavicle and the other over the scapular spine. The hands are then squeezed together, compressing the AC joint.
  • Positive Sign: Pain or abnormal movement/crepitus at the AC joint.
  • Clinical Significance: Directly stresses the AC joint and its ligaments.

• O'Brien's Test (Active Compression Test):

  • Procedure: The patient flexes the shoulder to 90 degrees with the elbow extended and the arm adducted 10-15 degrees. The arm is then internally rotated so the thumb points down. The examiner applies a downward force while the patient resists. The test is repeated with the arm externally rotated (thumb up).
  • Positive Sign: Pain in the AC joint during the first part of the test (thumb down) that is relieved or reduced during the second part (thumb up).
  • Clinical Significance: While often associated with superior labral anterior-posterior (SLAP) tears, a positive test with pain specifically localized to the AC joint suggests AC joint pathology.

• Paxinos Sign:

  • Procedure: The examiner places their thumb on the posterior aspect of the acromion and their fingers on the anterior aspect of the distal clavicle. Pressure is applied, compressing the AC joint.
  • Positive Sign: Pain at the AC joint.
  • Clinical Significance: A direct compression test of the AC joint.

• Resisted Shoulder Abduction Test:

  • Procedure: The patient attempts to abduct their arm against resistance applied by the examiner.
  • Positive Sign: Pain at the AC joint.
  • Clinical Significance: While primarily testing the deltoid and supraspinatus, pain at the AC joint can indicate inflammation or injury exacerbated by muscle contraction pulling on the joint.

Imaging Studies: Confirming the Diagnosis

While clinical tests are highly indicative, imaging studies are often necessary to confirm the diagnosis, determine the grade of injury, and rule out other conditions.

• X-rays:
  • Standard Views: Anteroposterior (AP) and lateral views of the shoulder can show significant displacement.
  • Zanca View: A specific AP view angled 10-15 degrees cephalic, which isolates the AC joint and reduces superimposition.
  • Stress Views: AP views taken with weights held by the patient's wrists (typically 10-15 lbs) can exaggerate subtle superior displacement of the clavicle, revealing mild separations not visible on standard views.
• Magnetic Resonance Imaging (MRI):
  • Provides detailed images of soft tissues, including the AC joint capsule, articular disc, and especially the coracoclavicular ligaments. It is excellent for assessing the extent of ligamentous injury, bone bruising, and ruling out other soft tissue pathologies like rotator cuff tears.
• Ultrasound:
  • Can be used dynamically to assess AC joint stability and visualize swelling or effusion. It's a less expensive and non-ionizing alternative for initial assessment, particularly for visualizing soft tissue swelling.
• CT Scan:
  • Rarely used for AC joint separations unless there is suspicion of associated fractures or complex dislocations, particularly for pre-surgical planning in severe cases.

Differential Diagnosis

It's important to differentiate AC joint pain from other conditions that can cause similar symptoms, such as:

• Rotator cuff tendinopathy or tear
• Biceps tendinopathy
• Subacromial impingement syndrome
• Cervical radiculopathy (pain referred from the neck)
• Glenohumeral joint arthritis

The Importance of Professional Diagnosis

While understanding these tests is valuable for fitness enthusiasts and trainers, it is crucial to emphasize that self-diagnosis of an AC joint injury is not recommended. Accurate diagnosis requires the expertise of a qualified healthcare professional, such as a physician, physical therapist, or sports medicine specialist. They can integrate the findings from the patient's history, physical examination, and imaging studies to provide an accurate diagnosis and develop an appropriate management plan, ranging from conservative treatment (rest, ice, physical therapy) to surgical intervention for more severe injuries.

Conclusion

Testing for an AC joint injury is a multi-faceted process that combines a detailed clinical examination with specific provocative tests and, when necessary, advanced imaging. Understanding the anatomy and biomechanics of the AC joint, along with the purpose and execution of these diagnostic tools, empowers both patients and professionals to approach shoulder pain with precision, leading to effective treatment and optimal recovery.