In vivo evaluation of rotator cuff internal impingement during scapular plane abduction in asymptomatic individuals

Abstract

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.

Keywords: internal impingement; kinematics; rotator cuff.

Figure 1:

Descriptive statistics (mean ± SE) for glenohumeral kinematics across the trial of unloaded scapular plane abduction. Between 90–150° humerothoracic elevation, participants generally exhibited glenohumeral elevation, anterior plane of elevation, and external rotation. Glenohumeral orientation data are described relative to a scapular anatomical coordinate system defined based on the recommendations of the International Society of Biomechanics with glenohumeral elevation being transformed to positive values to facilitate interpretation. Glenohumeral anterior and superior position data are described relative to a glenoid based coordinate system and are normalized to glenoid width and height, respectively. Abbreviations: S/I = superior/inferior, A/P anterior/posterior.

Figure 2:

The minimum distance between the glenoid and rotator cuff footprint at the humerothoracic elevation angles assessed statistically. Glenoid-to-footprint minimum distances decreased consistently in all participants between 90–150° humerothoracic elevation and fell below the estimated labral thickness in all participants at an average angle of 114.6° ± 1.7° humerothoracic elevation. Data are presented as mean and standard error. The black line represents the mean (±SE) of all subjects, and the grey circles represent data for individual subjects. The dashed line represents an estimated average labral thickness (4.3 mm) to identify incidence of potential glenoid-to-footprint contact.

Figure 3:

The proximity center locations on the glenoid (lateral view) and footprint (superior view) illustrated as A) the average ± standard error across all participants at the angles assessed statistically (90°, 110°, 130°, and 150° humerothoracic elevation), and B) for a representative subject for all frames of the motion trial between 90–150° humerothoracic elevation. In A, the average proximity center location is superimposed over an approximately average-sized glenoid and humeral head to facilitate interpretation. The dashed lines coincide with the anterior/posterior and superior/inferior axes of the glenoid coordinate system, and the anterior/posterior and medial/lateral axes of the humeral coordinate system. In B, the proximity center path location is superimposed over the participant’s actual anatomy.

Figure 4:

The minimum distance between the glenoid and rotator cuff footprint at the humerothoracic elevation angles assessed statistically in a representative participant. The colormap corresponds to the distance between the glenoid and rotator cuff footprint. The dashed line represents the estimated undersurface of the rotator cuff in the free tendon and insertional footprint regions. Abbreviation: HT = humerothoracic.