Multimodality imaging of subacromial impingement syndrome

Subacromial impingement syndrome results from irritation of the tendons of the rotator cuff muscles in the subacromial space and may manifest as a range of pathologies. However, subacromial impingement is a dynamic condition for which imaging reveals predisposing factors but no pathognomonic indicators. Also, the usual imaging features of subacromial impingement may be seen in symptomatic and asymptomatic patients. Therefore, imaging is able to detect tears and describe the risk factors of impingement but cannot confirm subacromial impingement. Radiographs allow assessment of the morphology of the acromion and its lateral extension by means of the acromial index and the critical shoulder angle, which may increase in cases of subacromial impingement. Ultrasound is necessary to evaluate a tendon tear and is the only tool that provides dynamic information, which is essential to assessing dynamic conditions. Magnetic resonance imaging (MRI) allows the assessment of associated intraarticular abnormalities, joint effusion, and bone marrow edema. The objective of this article is to provide an overview of the pathophysiology and clinical manifestations of subacromial impingement and discuss recent advances in the imaging of subacromial impingement and the role of radiography, ultrasound, and MRI in differentiating normal from pathologic findings.