Definition of Shoulder Impingement Injury
Shoulder impingement syndrome is a subacromial outlet obstruction resulting in irritation of the supraspinatus tendon, classified into two distinct types: primary impingement (extra-articular rotator cuff pathology common in adults) and secondary impingement (inability of the humeral head to stay centered in the glenoid fossa during arm motion, more common in younger athletes with rotator cuff weakness and ligamentous laxity). 1
Core Pathophysiological Mechanisms
Primary Impingement
- Occurs from compression of the rotator cuff tendons against the anterolateral aspect of the acromion during overhead movements 2, 3
- Represents extra-articular rotator cuff pathology predominantly seen in adult, non-athletic populations 1
- Results from mechanical compression of the supraspinatus tendon beneath the coracoacromial arch 3
Secondary Impingement
- Develops when the humeral head cannot maintain proper centering in the glenoid fossa during arm motion 1, 2
- Caused by the combination of rotator cuff muscle weakness and ligamentous laxity, particularly in younger athletes 1, 2
- Results from atraumatic microinstability rather than structural anatomic abnormalities 4
- More common in overhead athletes and individuals with repetitive overhead activities 1, 2
Clinical Presentation Characteristics
Pain Patterns
- Anterior or anterolateral shoulder pain that worsens with overhead activities (88% sensitivity) 2, 5
- In throwing athletes, pain occurs specifically during arm cocking and acceleration phases 1, 2
- Pain during release, deceleration, and follow-through phases in rotator cuff tendinopathies 1, 5
Physical Examination Findings
- Decreased range of motion during abduction with external or internal rotation 1, 2
- Focal weakness in the affected shoulder (present in 75% of cases) 2, 5
- Positive Neer's test (88% sensitive, 33% specific) and Hawkins' test (92% sensitive, 25% specific) 2, 5
Underlying Biomechanical Factors
Muscular Dysfunction
- Repetitive eccentric stress on the supraspinatus, external rotators, and scapular stabilizers leading to fatigue and injury 1, 2
- Weakened posterior shoulder musculature combined with overdeveloped anterior musculature creates pathologic biomechanics during arm deceleration 1, 4
Scapular Dyskinesis
- Poor coordination of scapular movements during arm elevation contributes to impingement 1, 4
- Normal scapula should rotate upward and tilt posteriorly during arm elevation; failure of this coordination pattern leads to rotator cuff injury 1, 5
Critical Distinction for Treatment Planning
The differentiation between primary and secondary impingement is essential because they require different treatment approaches 3:
- Primary impingement may respond to structural decompression procedures
- Secondary impingement requires rehabilitation focused on rotator cuff strengthening and scapular stabilization rather than surgical decompression 1
Common Pitfall
A major clinical error is using "impingement syndrome" as a vague diagnostic label without distinguishing the specific underlying pathology 6. The term has been criticized as an "umbrella" diagnosis that is no more informative than "anterior shoulder pain" and may lead to inappropriate surgical interventions when the actual problem is secondary impingement requiring conservative management 6, 7.