What is the etiology of shoulder impingement syndrome?

Etiology of Shoulder Impingement Syndrome

Shoulder impingement syndrome results from a combination of anatomical factors and biomechanical dysfunctions that lead to compression of the rotator cuff tendons and subacromial bursa between the humeral head and structures of the coracoacromial arch. 1

Primary vs. Secondary Impingement

• Primary impingement occurs due to extra-articular rotator cuff pathology, commonly seen in adults but rare in adolescent athletes, resulting in compression of the rotator cuff against the anterolateral aspect of the acromion 2, 3
• Secondary impingement is more common in younger individuals and athletes, characterized by difficulty for the humeral head to stay centered in the glenoid fossa during arm motion due to weakness in rotator cuff muscles combined with ligamentous laxity 2, 4

Anatomical Factors Contributing to Impingement

• Subacromial bony proliferation (present in 68% of cases) creates mechanical narrowing of the subacromial space 5
• Acromioclavicular joint degenerative changes (66% of cases) with inferiorly oriented osteophytes (32% of cases) can reduce the subacromial space 5
• Degenerative changes in the greater tuberosity of the humerus (66% of cases) can alter normal shoulder biomechanics 5
• Os acromiale (unfused acromial growth plate) can contribute to atypical impingement syndrome 3

Biomechanical Factors

• Rotator cuff dysfunction - weakness or fatigue in the rotator cuff muscles fails to maintain proper humeral head positioning during shoulder movement 2, 1
• Scapular dyskinesis - poor coordination of scapular movements during arm elevation contributes to impingement by failing to properly rotate upward and tilt posteriorly 2, 4
• Posterior glenohumeral capsule tightness alters normal glenohumeral kinematics 1
• Repetitive eccentric stress on the supraspinatus, external rotators, and scapular stabilizers leads to fatigue and injury, particularly in throwing athletes 2
• Muscular imbalance - weakened posterior shoulder musculature combined with overdeveloped anterior musculature creates abnormal biomechanics during arm deceleration 2

Sport-Specific Mechanisms

• In throwing athletes, impingement often occurs during specific phases of throwing:
  • Arm cocking phase - excessive external rotation can cause posterior impingement 2
  • Acceleration phase - rotator cuff is subjected to high eccentric loads 2, 4
  • Deceleration phase - posterior shoulder muscles fatigue, leading to abnormal humeral head translation 2

Pathophysiological Progression

• Initial inflammation of the tendons and subacromial bursa 1
• Progression to tendinopathy/tendinosis with degenerative changes in the rotator cuff tendons 4, 6
• Potential development of partial or full-thickness rotator cuff tears if left untreated 2, 4

Modern Understanding

• Current evidence suggests "impingement syndrome" is not a single condition but a complex of conditions involving both intrinsic and extrinsic factors 6
• The term "impingement syndrome" is increasingly viewed as an overly simplified diagnostic label that may not accurately reflect the underlying pathology 6
• Movement-related impairments and mechanical factors should be considered as potentially modifiable mechanisms rather than assuming a fixed structural pathology 6

Clinical Implications

• Accurate identification of the specific impingement mechanism is crucial for appropriate treatment planning 3
• Radiographic findings, while common (subacromial proliferation, AC joint degeneration), do not reliably predict response to treatment 5
• Understanding whether impingement is primary or secondary guides the treatment approach and helps prevent inappropriate interventions 4, 6