In Vivo Evaluation of Subacromial and Internal Impingement Risk in Asymptomatic Individuals

Objective The study aim was to evaluate subacromial and internal impingement risk between shoulders (dominant/nondominant) during dynamic motion using subject-specific anatomy and precise in vivo kinematics. Design In a prospective cross-sectional study, nine subjects underwent bilateral magnetic resonance (N = 18 shoulders) and fluoroscopic imaging during elevation and external rotation at 90 degrees of abduction. Subject-specific bone models were created and distances from footprint to (a) acromion and (b) glenoid were measured to evaluate risk. Results Throughout elevation, subacromial impingement risk was greater in the dominant shoulder (P = 0.0178). Regardless of side, high subacromial impingement risk occurred at 30% (78 degrees), 50% (101 degrees), and 70% (57 degrees) of the elevation cycle (P < 0.0001). High subacromial impingement risk also occurred at 30% (94 degrees), 50% (120 degrees), and 70% (63 degrees) of the external rotation motion cycle (P < 0.0001). Throughout both motions, internal impingement risk was not observed; however, the footprint and glenoid were closest at 50% of the elevation (101 degrees) and external rotation (120 degrees) cycles (P < 0.0001). Conclusions During elevation, subacromial impingement risk is greatest at lower arm positions (30% cycle, 78 degrees) and is greater in the dominant shoulder. High subacromial impingement risk also occurs with external rotation (63-120 degrees). Internal impingement risk does not occur with maximal elevation (101 degrees) or external rotation at 90-degree abduction but is more closely approached with elevation.