Internal impingement-or entrapment of the undersurface of the rotator cuff tendon against the glenoid during overhead activities-is believed to contribute to articular-sided tears. However, little is known about internal impingement outside athletic populations. Therefore, the objectives of this study were to (1) describe glenoid-to-footprint distances and proximity centers during dynamic, in vivo motion in asymptomatic individuals, and (2) determine the extent to which these measures differed between individuals with and without a rotator cuff tear. Shoulder kinematics were assessed in 37 asymptomatic individuals during scapular plane abduction using a high-speed biplane radiographic system. Glenoid-to-footprint distances and proximity center locations were calculated by combining the kinematics with computerized tomography-derived bone models. Glenoid-to-footprint contact was presumed to occur when the minimum distance was less than the estimated labral thickness. The condition of the supraspinatus tendon (intact, torn) was assessed using ultrasound. Minimum distances and proximity centers were compared over humerothoracic elevation angles (90°, 110°, 130°, 150°) and between supraspinatus pathology groups using two-factor mixed model analysis of variances. Glenoid-to-footprint minimum distances decreased consistently across elevation angles (p 0.01) without a significant difference between groups. Contact was estimated to occur in all participants. Proximity centers were generally located on the anterior half of the rotator cuff footprint and on the posterosuperior glenoid. Statement of Clinical Significance: Internal impingement during overhead motions may be a prevalent mechanism of rotator cuff pathology as contact appears to be common and involves the region of the rotator cuff footprint where degenerative rotator cuff tears are thought to originate.
