Kinematic analysis of damaged capsulolabral structure in patients with anterior shoulder instability using cine-magnetic resonance imaging.

Background: We assessed damaged anterior capsulolabral motion during axial shoulder rotation in patients with anterior shoulder instability. Methods: Twenty-nine shoulders of 28 patients with anterior shoulder instability who underwent cine-magnetic resonance imaging during axial rotation of the adducted arm were included. The motion was captured after an intra-articular injection of saline solution (10-20 mL). During imaging, the shoulder was rotated passively from maximum internal rotation to maximum external rotation in the first 10 s and then back to maximum internal rotation in the subsequent 10 s. We assessed the rotational angles of the damaged labrum during compressing and pulling the humeral head against the glenoid. Evaluation of the rotational angles was performed on a series of axial images through the humeral head center. Results: The mean angles that damaged labrum compressed and pulled off against the glenoid were 12.0 ± 19.1° and 2.8 ± 21.2°, respectively. Additionally, seven of the 29 shoulders showed that the damaged labrum compressed on the glenoid rim before the rotational angle exceeded 0° during external rotation. In 13 shoulders, the damaged labrum could remain repositioned on the glenoid rim over the neutral position during internal rotation. In two shoulders, the damaged labrum was not compressed against the glenoid at the maximum external rotation. The injected saline moved from the posterior to the anterior side of the glenohumeral joint during internal rotation in each shoulder. Conclusion: The damaged labrum could be positioned on the glenoid when the arm was in a traditional internal immobilization.