The Science of Bursitis / Tendinitis
Shoulder bursitis, specifically subacromial bursitis, involves inflammation of the small fluid-filled sac (bursa) that sits between your rotator cuff tendons and the bony roof of your shoulder (acromion). This bursa normally allows smooth gliding of the rotator cuff tendons beneath the acromion during arm movement. When the subacromial space becomes narrowed due to bone spurs, poor posture, muscle imbalances, or repetitive overhead activities, the bursa becomes compressed and irritated. This compression leads to inflammation, thickening of the bursa walls, and production of excess synovial fluid, creating a cycle of swelling and further compression. The condition often coexists with rotator cuff tendinopathy and shoulder impingement syndrome, as they share similar mechanical causes. The inflamed bursa can contribute to pain and dysfunction, but it's usually a secondary problem rather than the primary issue. Understanding this relationship is crucial because treating only the bursitis without addressing underlying mechanical problems often leads to recurrence. Acute bursitis may result from direct trauma or sudden overuse, while chronic bursitis typically develops gradually from repetitive microtrauma and sustained mechanical irritation.
Contributing Factors
The subacromial space is a narrow area between your humeral head and the undersurface of the acromion. During arm elevation, this space normally maintains about 6-14mm of clearance, but various factors can reduce this critical space.
Poor scapular mechanics significantly contribute to subacromial crowding. When your scapula doesn't rotate properly during arm elevation, it fails to maintain optimal clearance between the acromion and the underlying structures. This is often seen with weakness in the serratus anterior and lower trapezius muscles.
Forward head posture and rounded shoulders, common in our computer-based society, alter the orientation of the acromion and reduce subacromial space. This postural pattern also changes the resting length and activation patterns of the rotator cuff muscles, making them less effective at maintaining proper humeral head position.
Rotator cuff weakness, particularly in the posterior and inferior aspects, allows superior migration of the humeral head during arm elevation. This upward translation narrows the subacromial space and increases compression forces on the bursa and surrounding structures.