How does rotator cuff tendonitis and tear cause impingement syndrome in patients, particularly those over 40 with a history of repetitive overhead activities or trauma to the shoulder?

How Rotator Cuff Tendonitis and Tears Cause Impingement Syndrome

Rotator cuff tendonitis and tears cause impingement syndrome through a biomechanical cascade: weakened or damaged rotator cuff muscles fail to maintain proper humeral head positioning, allowing superior migration of the humeral head which then repeatedly compresses the supraspinatus tendon against the coracoacromial arch. 1

The Biomechanical Mechanism

The pathophysiology follows a clear sequence:

Primary pathway: When rotator cuff muscles become weakened through tendonitis or torn, they lose their ability to keep the humeral head centered and depressed in the glenoid fossa during arm elevation. 1 This improper functioning permits the humeral head to migrate superiorly, directly causing the supraspinatus tendon to impinge against the coracoacromial arch. 1

The anatomic vulnerability: The supraspinatus complex occupies an inherently narrow space, with only light contact between the supraspinatus and the coracoacromial arch during normal abduction. 1 When rotator cuff dysfunction allows abnormal superior humeral head translation, this narrow space becomes critically compromised, leading to repeated mechanical impingement. 1

Primary vs. Secondary Impingement Distinction

In patients over 40 with repetitive overhead activities: The mechanism typically represents primary impingement, where the rotator cuff pathology itself (tendonitis or tear) is the initiating problem that subsequently causes the impingement syndrome. 2 This differs fundamentally from secondary impingement seen in younger athletes, where instability and weakness precede the tendon damage. 2, 3

The degenerative component: Hypovascularity in the region proximal to the supraspinatus tendon insertion contributes to rotator cuff tendinopathy, creating a vicious cycle where impingement causes further tendon degeneration. 1 Repeated impingement of the coracoacromial arch onto the already compromised supraspinatus tendon perpetuates and worsens the injury. 1

Progressive Pathophysiology

The deterioration sequence:

• Initial rotator cuff tendonitis causes muscle weakness and altered biomechanics 1
• Superior humeral head migration begins during overhead activities 1
• Mechanical impingement develops as the tendon repeatedly contacts the coracoacromial arch 1
• Continued impingement causes further tendon degeneration, potentially progressing to partial-thickness tears 2
• If untreated, full-thickness tears develop with muscle atrophy and fatty infiltration over 5-10 years 3, 4

Clinical Manifestation

The impingement becomes most apparent when the humerus is simultaneously abducted and internally rotated, as this position maximally narrows the subacromial space. 1 Pain occurs in the anterior or anterolateral shoulder, worsening with overhead activities, which has 88% sensitivity for impingement. 2

Key physical examination findings:

• Hawkins' test (forcible internal rotation at 90° forward flexion): 92% sensitive for impingement 1, 2
• Neer's test (full forward flexion 70-120°): 88% sensitive for impingement 1, 2
• Both tests have poor specificity (25-33%), reflecting that impingement is the final common pathway for multiple rotator cuff pathologies 1, 2

Critical Clinical Pitfall

Do not assume impingement is the primary problem. In patients over 40 with repetitive overhead work or trauma history, the rotator cuff pathology (tendonitis or tear) is the underlying cause, and the impingement is the consequence. 2, 5 Treatment must address the rotator cuff dysfunction, not just the impingement symptoms, or the condition will progress to irreversible tendon tears with muscle atrophy. 2, 4