Subacromial impingement

Subacromial impingement is by far the most common form of shoulder impingement, and occurs secondary to attrition between the coracoacromial arch and the supraspinatus tendon or subacromial bursa.

Pathology

Aetiology

• os acromiale
• type III acromion
• acromioclavicular degenerative disease
• thickening of the coracoacromial ligament
• coracoacromial ligament ossification
• shoulder instability
• post traumatic deformity
• supraspinatus over development
• low lying acromion

Radiographic features

Although commonPrimarily, clinical diagnosis of subacromial impingement is difficult to demonstrate at a clinical diagnosis and one should not make a diagnosis  or exclude it solely based on imaging; however imaging has an important role in supporting the diagnosis, finding the possible cause as well as sequelaes of impingement.

Static

• static imaging modalities such as MRI and radiographs occasionally depict reduced subacromial distance as an indirect evidence to support clinical diagnosis: 
  • type III acromion 
  • Os acromiale
  • osteophytes extruding from AC joint inferiorly
  • Subacromial/subdeltoid bursitis
  •  lateral acromial tilt
  • anecdotal experience also suggests that slight contact between the coracoacromial arch and the subacromial bursa can occur in healthy individuals.

    Dynamic

     Yet, significant contact or snapping between these two structures are not common in the absence of symptoms and suggest clinically relevant impingement ⁵
• dynamic ultrasound may provide more reliable evidence by demonstratingdepict abnormal contact between the coracoacromial arch and peritendinous tissue during shoulder abduction. 

  However

  ; however, dynamic diagnosis at US is not free of controversy: although earlier studies have demonstrated thickening of the subacromial bursa following shoulder abduction in symptomatic shoulders,^1-3 a more recent investigation found no significant difference in the degree of bursal gathering in impingement patients compared with healthy volunteers.⁴

  Anecdotal experience also suggests that slight contact between the coracoacromial arch and the

Complication

• rotator cuff tear
• subacromial bursa can occur in healthy individuals.bursitis
• bicipital tendinitis⁵ Yet, significant contact or snapping between these two structures are not common in the absence of symptoms and suggest clinically relevant impingement.⁵⁶ 

Related pathology

Less common types of shoulder impingement include:

• subcoracoid impingement: affects subscapularis
• posterosuperior impingement: involves infraspinatus