AC Joint Test: Purpose, Common Maneuvers, and Interpretation

What is an AC Test?

An AC test refers to a specific set of orthopedic physical examination maneuvers designed to assess the integrity and stability of the acromioclavicular (AC) joint, a crucial articulation in the shoulder complex.

Understanding the Acromioclavicular (AC) Joint

The acromioclavicular (AC) joint is a synovial joint located at the top of the shoulder, formed by the articulation between the acromion process of the scapula (shoulder blade) and the lateral end of the clavicle (collarbone). This joint plays a vital role in shoulder movement, particularly during overhead activities, and transmits forces from the upper extremity to the axial skeleton. It is stabilized by several ligaments, including the superior and inferior AC ligaments, and, more significantly, the coracoclavicular (CC) ligaments (conoid and trapezoid), which provide strong vertical stability.

Purpose of AC Joint Tests

AC joint tests are performed by healthcare professionals, such as physicians, physical therapists, or athletic trainers, to evaluate suspected injuries or dysfunction of the AC joint. The primary indications for conducting these tests include:

• Localized Pain: Patients presenting with pain directly over the AC joint, especially after a fall onto the shoulder or an outstretched arm.
• Swelling or Deformity: Visible swelling or a prominent "bump" at the top of the shoulder.
• Mechanism of Injury: A history of trauma consistent with an AC joint sprain or separation (e.g., direct blow to the shoulder, fall on the tip of the shoulder).
• Limited Range of Motion: Painful or restricted shoulder movement, particularly abduction and horizontal adduction.

These tests aim to provoke pain or instability at the AC joint, helping to differentiate AC joint pathology from other shoulder conditions like rotator cuff injuries, labral tears, or cervical spine issues.

Common AC Joint Provocation Tests

Several specific maneuvers are commonly employed to assess the AC joint. While no single test is 100% diagnostic, a combination of tests, along with a thorough history and other clinical findings, provides valuable information.

Cross-Body Adduction Test (Horizontal Adduction Test or Scarf Test)

• Procedure: The patient is seated or standing. The examiner passively flexes the patient's affected arm to 90 degrees and then horizontally adducts it across the body towards the opposite shoulder.
• Positive Finding: Pain localized to the AC joint, particularly at the end range of horizontal adduction. This test compresses the AC joint, which can reproduce pain if the joint capsule or ligaments are inflamed or injured.

AC Shear Test (AC Compression Test)

• Procedure: The patient is seated or standing. The examiner places one hand on the anterior aspect of the patient's clavicle and the other hand on the posterior aspect of the scapula's spine. The examiner then compresses the two bones together, shearing the AC joint.
• Positive Finding: Pain localized to the AC joint, often accompanied by a "grinding" sensation or movement at the joint. This test directly compresses and shears the joint surfaces, which can be painful in cases of AC joint pathology.

Palpation and Localized Tenderness

• Procedure: While not a "test" in the same dynamic sense, direct palpation of the AC joint is a crucial component of the assessment. The examiner gently presses on the superior aspect of the AC joint.
• Positive Finding: Exquisite tenderness directly over the AC joint, often correlating with the patient's chief complaint of pain. This is a very sensitive indicator of AC joint pathology.

Interpreting Test Results

A "positive" AC test result typically means that the maneuver reproduced the patient's pain at the AC joint or elicited a palpable instability. However, it is crucial to understand that:

• No Single Test is Definitive: A positive result on one or more of these tests strongly suggests AC joint involvement but is not solely diagnostic of a specific grade of injury.
• Clinical Correlation is Key: Test results must always be interpreted in the context of the patient's history, mechanism of injury, other physical exam findings, and the examiner's clinical judgment.
• False Positives/Negatives: It's possible to have a positive test without significant AC joint injury, or a negative test despite some pathology, due to factors like pain tolerance, chronicity of injury, or concomitant injuries.

Limitations and Considerations

While valuable, AC joint tests have limitations:

• Specificity and Sensitivity: The diagnostic accuracy (sensitivity and specificity) of these tests can vary, meaning they might not always correctly identify or rule out an AC joint injury on their own.
• Differential Diagnosis: Shoulder pain can arise from numerous sources. These tests help narrow down the possibilities but do not rule out other conditions such as subacromial impingement, rotator cuff tendinopathy, or cervical radiculopathy.
• Need for Imaging: For definitive diagnosis and grading of AC joint separations, imaging studies such as X-rays (especially weighted views), MRI, or ultrasound are often required to assess the extent of ligamentous damage and joint displacement.

What Happens After a Positive AC Test?

If a clinician identifies a positive AC test and suspects an AC joint injury, the next steps typically involve:

• Further Diagnostic Imaging: Referral for imaging studies to confirm the diagnosis and determine the grade of AC joint separation (e.g., Type I-VI).
• Consultation with a Specialist: Depending on the severity and symptoms, the patient may be referred to an orthopedic surgeon for further evaluation and management.
• Treatment Plan: Based on the diagnosis, a treatment plan will be developed. This can range from conservative management (rest, ice, pain medication, physical therapy for lower-grade injuries) to surgical intervention (for higher-grade separations or persistent instability).

Conclusion

AC tests are fundamental components of a comprehensive shoulder examination, providing critical insights into the integrity of the acromioclavicular joint. While they are powerful tools for guiding clinical suspicion, their results must always be integrated with a thorough patient history and, when necessary, advanced imaging, to formulate an accurate diagnosis and an effective treatment strategy. For anyone experiencing shoulder pain, a professional evaluation is essential to ensure appropriate care.