The Acromiohumeral Interval (AHI) is a specific measurement used by healthcare professionals to evaluate the condition of the shoulder joint. This measurement represents the space between two bones within the shoulder complex. Analyzing this distance provides objective data that helps determine the presence and severity of shoulder pathologies. The AHI serves as an important diagnostic indicator of the overall mechanical health and stability of the shoulder.
Defining the Acromiohumeral Interval
The AHI is the vertical distance measured between the undersurface of the acromion and the head of the humerus. The acromion is a bony projection that forms the roof of the shoulder and is part of the scapula, or shoulder blade. The humerus is the upper arm bone, and its rounded head forms the ball of the shoulder’s ball-and-socket joint.
The interval represents the space where the rotator cuff tendons, particularly the supraspinatus tendon, and the subacromial bursa pass. The measurement is taken at the shortest vertical distance between the cortical bone of the acromion and the subchondral surface of the humeral head. This anatomical corridor is subjected to high mechanical stress during shoulder movement.
Standard Measurement and Numerical Interpretation
The AHI is most commonly measured on a plain X-ray, specifically a standardized anteroposterior (AP) view of the shoulder, often taken with the patient standing or seated. The measurement involves drawing a perpendicular line from the inferior border of the acromion to the most superior aspect of the humeral head. This provides a static, two-dimensional snapshot of the space.
For a healthy adult shoulder, the normal AHI typically falls within a range of approximately 7 to 14 millimeters. A measurement below this threshold suggests the presence of shoulder pathology. An AHI of less than 6 to 7 millimeters is often considered an indicator of abnormality.
The patient’s position during imaging can affect the measurement, as gravity and arm position influence the shoulder joint’s alignment. Measurements taken on Magnetic Resonance Imaging (MRI) are consistently smaller than those from standard radiographs. An AHI below 6 millimeters on a radiograph is highly specific for advanced rotator cuff disease, indicating a high probability of a significant tear.
Clinical Significance of a Narrowed Interval
A reduced AHI is a significant finding because it is strongly associated with mechanical interference and damage to the soft tissues of the shoulder. The primary consequence of a narrowed interval is subacromial impingement, where the tendons and bursa are compressed between the two bones during arm elevation. This chronic compression can lead to inflammation and degeneration of the rotator cuff tendons.
A severely narrowed AHI is a hallmark sign of a full-thickness rotator cuff tear, particularly involving the supraspinatus tendon. When the rotator cuff is significantly damaged, it loses the downward force necessary to counteract the upward pull of the deltoid muscle. This loss of stabilization causes the humeral head to migrate superiorly toward the acromion, dramatically decreasing the AHI.
The degree of narrowing correlates with the severity of the tendon damage and the extent of fatty infiltration. A smaller AHI suggests a larger tear size and a poorer prognosis for surgical repair. Conversely, an abnormally wide interval, typically exceeding 12 millimeters, can also be clinically relevant, often suggesting acute trauma, such as shoulder dislocation, or conditions causing inferior subluxation.
Diagnostic Imaging Used to Evaluate AHI
Multiple imaging tools are used to measure the AHI, each providing different information about the static and dynamic state of the joint.
Standard X-rays provide an initial, cost-effective, and reproducible static measurement of the bony interval. This is often the first step in screening for significant changes, like superior humeral migration.
Magnetic Resonance Imaging (MRI) allows for detailed visualization of the soft tissues, including the rotator cuff tendons and bursa. MRI also provides a static AHI measurement. MRI measurements are typically performed on coronal or sagittal oblique views and can confirm the presence and size of any underlying tendon tear.
Ultrasound is another modality used to measure the AHI, offering the unique benefit of dynamic assessment. An ultrasound can measure the interval as the arm moves, allowing clinicians to observe how the space changes during shoulder elevation. This dynamic perspective can reveal impingement that might not be apparent in a static X-ray or MRI image.