AC Shear Test: Purpose, Procedure, Interpretation, and Clinical Significance

What is AC shear test?

The AC shear test, also known as the AC compression test, is a commonly used orthopedic special test designed to assess the integrity and detect pathology, particularly sprains or separations, of the acromioclavicular (AC) joint in the shoulder.

Understanding the Acromioclavicular (AC) Joint

The acromioclavicular (AC) joint is a small, synovial joint located at the top of the shoulder, formed by the articulation of the lateral end of the clavicle (collarbone) and the acromion process of the scapula (shoulder blade). This joint plays a crucial role in shoulder movement, allowing for a degree of rotation and gliding that facilitates the full range of motion of the arm. It is stabilized by several ligaments, including the superior and inferior AC ligaments, and more significantly, by the coracoclavicular (CC) ligaments (conoid and trapezoid), which connect the clavicle to the coracoid process of the scapula. Due to its superficial location and the forces transmitted through it during falls or direct impacts, the AC joint is susceptible to injury, commonly referred to as an AC joint sprain or separation.

Purpose of the AC Shear Test

The primary purpose of the AC shear test is to evaluate the stability and integrity of the acromioclavicular joint and its supporting ligaments. It is performed when an individual presents with shoulder pain, especially after a fall onto the shoulder or a direct blow to the top of the shoulder, which are common mechanisms of AC joint injury. The test aims to reproduce the patient's pain or elicit specific findings that suggest a sprain or separation of the AC joint. It is a provocative test, meaning it attempts to provoke symptoms by stressing the joint in a specific manner.

How to Perform the AC Shear Test

The AC shear test is typically performed with the patient seated or standing, and the examiner positioned to effectively apply the necessary forces.

• Patient Positioning: The patient is seated comfortably, with their arm relaxed at their side.
• Examiner Positioning: The examiner stands behind or to the side of the patient, facing the shoulder being tested.
• Hand Placement:
  • Place one hand on the anterior aspect of the patient's shoulder, specifically over the clavicle near the AC joint.
  • Place the other hand on the posterior aspect of the patient's shoulder, over the scapula (specifically the acromion).
• Execution of the Test:
  • Gently but firmly squeeze or compress the anterior clavicle and posterior acromion together. This action applies a shearing force across the AC joint, attempting to compress or "shear" the joint surfaces.
  • Maintain the compression for a few seconds while observing the patient's reaction and palpating the joint.
• Observation: Note any pain, crepitus (a grinding sensation), or excessive movement at the AC joint.

Interpreting the Results

Interpreting the findings of the AC shear test requires careful observation and correlation with the patient's symptoms and history.

• Positive Test: A positive AC shear test is indicated by:
  • Localized pain at the AC joint.
  • Crepitus (a grating or crunching sound/sensation) during the compression.
  • Excessive movement or instability noted at the AC joint, although this is less common with lower-grade injuries.
  • The reproduction of the patient's chief complaint of pain at the AC joint.
• Negative Test: A negative test would involve no pain, crepitus, or abnormal movement during the compression.
• What a Positive Test Suggests: A positive test strongly suggests pathology of the AC joint, most commonly an AC joint sprain or separation. The severity of the pain and the presence of crepitus can sometimes correlate with the grade of the injury, though definitive grading requires further assessment.

Clinical Significance and Next Steps

A positive AC shear test is a significant finding that warrants further investigation.

• Diagnostic Aid: While not definitive on its own, it serves as a valuable clinical indicator that helps guide the diagnostic process for shoulder pain.
• Corroborating Evidence: It is often used in conjunction with other special tests (e.g., cross-body adduction test, palpation of the AC joint), patient history, and physical examination findings to form a comprehensive clinical picture.
• Further Imaging: If the test is positive and an AC joint injury is suspected, the next step typically involves medical imaging, such as X-rays (often including stress views to assess instability), MRI, or ultrasound, to confirm the diagnosis and determine the grade of the AC joint separation.
• Referral: A positive test often leads to referral to a medical professional (e.g., orthopedic surgeon, sports medicine physician) for definitive diagnosis and management.

Important Considerations for Fitness Professionals

For fitness professionals, understanding the AC shear test is crucial for recognizing potential injuries and knowing when to refer clients to appropriate medical care.

• Scope of Practice: Performing diagnostic special tests like the AC shear test falls within the assessment phase of a fitness professional's role, primarily to identify potential contraindications to exercise or to recognize when a client needs medical evaluation. Fitness professionals do not diagnose medical conditions.
• Red Flags: Pain during this test is a significant red flag that indicates the need for a medical referral. Do not attempt to "work through" AC joint pain.
• Client Communication: If you perform this test as part of a comprehensive movement screen and observe a positive finding, clearly communicate to your client that further medical evaluation is necessary before proceeding with certain exercises or training protocols.
• Exercise Modification: Until cleared by a medical professional, activities that place stress on the AC joint (e.g., overhead presses, push-ups, dips, bench press) should be avoided or significantly modified.

Conclusion

The AC shear test is a simple yet effective orthopedic special test used to assess the integrity of the acromioclavicular joint. By applying a compressive force, it helps identify potential sprains or separations, guiding clinicians toward a more definitive diagnosis. For fitness professionals, it serves as a critical tool in pre-participation screening and ongoing client assessment, ensuring that individuals with suspected AC joint pathology are appropriately referred for medical evaluation, thereby prioritizing their safety and long-term joint health.