Exploring age-related differences in asymptomatic male shoulder kinematics using four-dimensional computed tomography.

Background: Understanding age-related differences in shoulder glenohumeral and scapulothoracic motion has implications for the understanding, treatment, and management of shoulder injuries and diseases. Previous studies have investigated age-related differences, although statically and in single-plane motion. The shoulder, however, is a complex joint capable of a wide range of motion (ROM) and involves coordinated and synchronous movements from both the glenohumeral and scapulothoracic joints. Therefore, the objectives of this study were to measure age-related differences in kinematics of the glenohumeral and scapulothoracic joints during motion, as well as differences in the neutral positioning of the scapula and humerus. Methods: Thirty-one male participants comprised 2 cohorts based on age (<45 and ≥ 45 years). Participants performed 2 motions, forward elevation (FE) and internal rotation (IR) to the back, with 4-dimensional computed tomography scanning to dynamically track the bones. The kinematics of the humerus and scapula were calculated with 6 degrees of freedom. The neutral position of the scapula and humerus was also calculated based on a static computed tomography scan. Results: During the FE motion, IR of the humerus was significantly greater (13°, P = .033) in the older cohort while the scapula had significantly more anterior translation (9 mm, P = .022) and medial rotation (9°, P = .021) in the younger cohort. Anterior translation of the humeral head during IR was significantly greater (1 mm, P = .05) in the younger cohort, while scapular superior translation was significantly greater (8 mm, P = .006) in the older cohort. When measuring humeral translation as a percentage of glenoid width rather than absolute translation, the older cohort exhibited significantly less translational ROM during FE (6%, P < .001), (5%, P = .036), (5%, P = .009) and IR (6%, P < .001), (6%, P < .001), (4%, P < .001) in the respective anterior, superior, and lateral directions. Furthermore, compared to the younger cohort, the older cohort had a neutral pose with significantly more superior translation (17 mm, P = .05), lateral rotation (8°, P = .009) and posterior tilting (10°, P < .001) of the scapula, and a more anteriorly positioned humerus (2 mm, P = .007). Conclusion: Overall, this study found age-related differences in kinematics and neutral positioning of the scapula and humerus, which may help improve understanding of age-related differences in subluxation, diseases, injuries, and ROM.