Subacromial impingement is the most common form of shoulder impingement and occurs secondary to attrition between the coracoacromial arch and the underlying supraspinatus tendon or subacromial bursa, leading to tendinopathy and bursitis respectively.
On this page:
Pathology
Etiology
coracoacromial ligament ossification or thickening
post-traumatic deformity
supraspinatus overdevelopment
chronic bursitis
Radiographic features
Primarily, subacromial impingement is 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 sequelae of impingement.
Static imaging modalities such as MRI and radiographs occasionally depict reduced subacromial distance as indirect evidence:
osteophytes extruding from the acromioclavicular joint inferiorly
reduced acromiohumeral interval
sharpened inferolateral edge of acromion 9 - SLAM sign
Anecdotal experience also suggests that slight contact between the coracoacromial arch and the subacromial bursa can occur in healthy individuals; yet, significant contact or snapping between these two structures are not common in the absence of symptoms and suggest clinically relevant impingement 5.
Ultrasound
Normally, during shoulder abduction there is depression of the humeral head to allow space for the supraspinatus and the subacromial subdeltoid bursa to slide under the acromion.
Dynamic ultrasound may depict abnormal contact between the coracoacromial arch and peritendinous tissue during shoulder abduction; however, dynamic diagnosis at ultrasound is not free of controversy: although earlier studies have demonstrated thickening of the subacromial bursa following shoulder abduction in symptomatic shoulders,1-3 an investigation found no significant difference in the degree of bursal gathering in impingement patients compared with healthy volunteers 4.
Treatment and prognosis
Complications
Related pathology
Less common types of shoulder impingement include:
subcoracoid impingement: affects subscapularis
posterosuperior impingement: involves infraspinatus