The acromiohumeral interval (AHI) is the space within the shoulder joint between the acromion, a part of the shoulder blade, and the humeral head, the top of the upper arm bone. This space is necessary for smooth, unhindered shoulder movement. A reduction in the size of this interval can be associated with shoulder discomfort and changes in function.
Anatomical Significance of the Acromiohumeral Space
The acromiohumeral space is a biological passageway bordered by the acromion above and the humeral head below. The distance between these two bony structures defines the interval’s size. The soft tissues contained within this space are what make its dimensions meaningful for shoulder health.
Within this channel lie rotator cuff tendons, most notably the supraspinatus tendon, which helps stabilize the shoulder and enable a wide range of motion. Also located in this space is the subacromial bursa, a fluid-filled sac that acts as a cushion. It reduces friction between the acromion and the underlying supraspinatus tendon during arm movement.
For the shoulder to move freely when lifting the arm, these soft tissues must glide without being pinched. The supraspinatus tendon and subacromial bursa need sufficient room to move. If the space narrows, these structures can be compressed, leading to mechanical irritation and affecting their function.
Measuring the Acromiohumeral Interval
Clinicians measure the acromiohumeral interval to evaluate the spatial relationships within the shoulder joint. This is most commonly performed using standard shoulder X-rays. While other imaging techniques like ultrasound and MRI can also measure this space, X-rays are often the initial tool used.
These imaging tools show the distance between the inferior surface of the acromion and the superior aspect of the humeral head. A healthy adult shoulder has an acromiohumeral interval ranging from 7 to 14 millimeters. This distance is considered sufficient to accommodate the subacromial bursa and the supraspinatus tendon.
A measurement below this normal range is described as a narrowed acromiohumeral interval. A distance of less than 7 millimeters is often indicative of rotator cuff pathology. An interval measured at 6 to 7 millimeters may suggest thinning of the supraspinatus tendon, while a measurement below 6 millimeters is more strongly associated with a tear.
Causes of a Narrowed Interval
Several factors can lead to a reduction in the acromiohumeral interval, disrupting the normal mechanics of the shoulder.
- Rotator cuff dysfunction: The rotator cuff muscles work to keep the humeral head centered in its socket. If these muscles are weak or torn, they may fail to adequately stabilize the humeral head, allowing it to migrate superiorly and close the gap with the acromion.
- Imbalance with the deltoid muscle: The powerful deltoid muscle pulls the arm upward. If the rotator cuff is not functioning properly to counteract the deltoid’s strong upward pull, the deltoid can draw the humeral head too high, narrowing the interval.
- Poor posture: Chronic slouching with forward-rounded shoulders causes the scapula to tilt forward and downward. This altered scapular position changes the orientation of the acromion, effectively lowering the “roof” of the subacromial space.
- Anatomical variations: The development of bone spurs, known as osteophytes, on the underside of the acromion can intrude upon the interval. Similarly, variations in the natural shape of the acromion, such as a more “hooked” appearance, can create a smaller space from the outset.
Relationship to Subacromial Impingement Syndrome
A narrowed acromiohumeral interval is directly linked to subacromial impingement syndrome, a common condition causing shoulder pain. The reduced space is an anatomical finding, while the syndrome refers to the resulting pain and inflammation. This occurs when the soft tissues within the subacromial space become mechanically compressed.
The mechanism involves the repeated pinching of the supraspinatus tendon and the subacromial bursa between the humeral head and the acromion. This compression occurs during movements when the arm is lifted overhead or forward. With less space, these soft tissues are subjected to increased friction and pressure, leading to irritation.
This persistent mechanical irritation can provoke an inflammatory response. The subacromial bursa may become inflamed (bursitis), causing it to swell and occupy more of the limited space. The supraspinatus tendon can also become inflamed and degenerate (tendinopathy), which can progress to a tear if not addressed.
Management Strategies for a Reduced Interval
The primary approach to managing a reduced acromiohumeral interval is conservative, focusing on the functional causes of the narrowing. Physical therapy is the main component of this strategy. A targeted rehabilitation program aims to correct the biomechanical faults that led to the reduction of the subacromial space.
A key goal of physical therapy is strengthening the rotator cuff muscles. These exercises improve endurance and coordination so the muscles can stabilize the humeral head and counteract the deltoid’s upward pull. Another focus is improving scapular control through exercises that train the muscles that position the shoulder blade, helping to correct postural deficits.
Alongside therapeutic exercise, activity modification is often recommended. This involves temporarily avoiding or altering activities that provoke pain, especially repetitive overhead motions. Reducing the frequency of irritating movements allows inflammation to subside and creates a better environment for healing.
In cases where pain is significant, other interventions may provide relief. Anti-inflammatory medications can help manage symptoms, and a corticosteroid injection into the subacromial space can reduce severe inflammation. Surgical intervention, such as a subacromial decompression to increase the space, is considered only after a comprehensive course of conservative management has failed.