Rotator cuff fatigue and glenohumeral kinematics in participants without shoulder dysfunction

Abstract

Context: Researchers have established that superior migration of the humeral head increases after fatigue of the rotator cuff muscles. In these studies, the investigators used imaging techniques to assess migration of the humeral head during statically held shoulder positions. Their results may not represent the amount of superior humeral head migration that occurs during dynamic arm elevation.

Objective: To investigate the effect of rotator cuff fatigue on humeral head migration during dynamic concentric arm elevation (arm at the side [approximately 0 degrees ] to 135 degrees ) in healthy individuals and to determine the test-retest reliability of digital fluoroscopic video for assessing glenohumeral migration.

Design: Test-retest cohort study.

Setting: Research laboratory.

Patients or other participants: Twenty men (age = 27.7 +/- 3.6 years, mass = 81.5 +/- 11.8 kg) without shoulder disorders participated in this study.

Intervention(s): Three digital fluoroscopic videos (2 pre-fatigue and 1 post-fatigue) of arm elevation were collected at 30 Hz. The 2 pre-fatigue arm elevation trials were used to assess test-retest reliability with the arm at the side and at 45 degrees , 90 degrees , and 135 degrees of elevation. The pre-fatigue and post-fatigue digital fluoroscopic videos were used to assess the effects of rotator cuff fatigue on glenohumeral migration. All measurements were taken in the right shoulder.

Main outcome measure(s): The dependent measure was glenohumeral migration (in millimeters). We calculated the intraclass correlation coefficient and standard error of the measurement to assess the test-retest reliability. A 2 x 4 repeated-measures analysis of variance was used to assess the effects of fatigue on arm elevation at the 4 shoulder positions.

Results: The test-retest reliability ranged from good to excellent (.77 to .92). Superior migration of the humeral head increased post-fatigue (P < .001), regardless of angle.

Conclusions: Digital fluoroscopic video assessment of shoulder kinematics provides a reliable tool for studying kinematics during arm elevation. Furthermore, superior migration of the humeral head during arm elevation increases with rotator cuff fatigue in individuals without shoulder dysfunction.

Keywords: biomechanics; humeral head migration; imaging.

Figures

Figure 1. Experimental set-up with the participant in anatomic position and his entire body rotated 30° from the frontal plane with respect to the X-ray beam. The right shoulder was positioned near (less than 2 cm from) the image intensifier to minimize distortion. The height of the C-arm was adjusted to ensure that the glenohumeral joint and the proximal humeral shaft were visible during the entire range of motion. Foot, scapular, and hand markings were used to ensure similar positioning between trials.

Figure 2. Example of the data extraction technique from a single frame from the digital fluoroscopic video. BC1 depicts the best-fit circle representing the humeral head. BL1 depicts a best-fit line, the ends of which represent the superior and inferior borders of the glenoid fossa. Humeral head migration was defined as the distance between the center of the glenoid line (BL1) and the perpendicular projection of the center of the BC1 to BL1. AF1 is the humeral angle. It was determined as the angle between vertical (L1) and a line drawn on the medial border of the humeral shaft (L2).

Figure 3. Radiographic measurement of humeral head migration was adapted based on a technique described by Deutsch et al. Humeral head migration (e) was defined as the distance between the center of the glenoid line (g) and the perpendicular projection of the center of the humeral head (h) to the glenoid line. This value was expressed as positive if the center of the humeral head was superior to the center of the glenoid or negative if it was inferior to the center of the glenoid. In this figure, the center of the humeral head was inferior to the center of the glenoid line. Reprinted from the Journal of Shoulder and Elbow Surgery, volume 8, Chen SK, Simonian PT, Wickiewicz TL, Otis JC, Warren JF, Radiographic evaluation of glenohumeral kinematics: a muscle fatigue model, p 49–52, 1999, with permission from the Journal of Shoulder and Elbow Surgery Board of Trustees.

Figure 4. Mean superior migration of the humeral head with elevation during prefatigue and postfatigue conditions. Mean prefatigue migration with the arm at the side (approximately 0°), 45°, 90°, and 135° was −1.48 ± 1.82 mm, −0.85 ± 1.14 mm, −0.73 ± 2.15 mm, and 0.22 ± 2.14 mm, respectively. Mean postfatigue migration with the arm at the side, 45°, 90°, and 135° was −0.39 ± 1.73 mm, 0.32 ± 1.49 mm, 0.32 ± 1.75 mm, and 0.07 ± 2.58 mm, respectively.