Scapular Mechanisms of Movement-Related Shoulder Dysfunction

Abstract

Background: Shoulder pain and dysfunction, including rotator cuff tears, are highly prevalent and can limit an individual’s ability to perform basic activities of daily living, work-related tasks, and recreational activities. Repetitive deformation of the rotator cuff tendons is one hypothesized mechanism of tear formation. Shoulder motion, particularly at the scapula, directly affects the position of the rotator cuff tendons with respect to surrounding structures during motion and may impact rotator cuff tendon deformation. To date, research around abnormal scapular motion has generally not investigated impacts of specific patterns of abnormal scapular motion. Additionally, there is a lack of research guiding clinicians with regard to underlying muscle activation patterns within specific movement patterns. The links between scapular movement abnormalities, muscle activation, and rotator cuff deformation remain unclear, limiting the development of precise, individualized treatment of each patient. Objectives: The overall objectives for this project were to (i) identify muscle activation in two specific scapular movement groups (excessive scapular anterior tilt, scapular lateralization), and (ii) identify rotator cuff deformation caused by surrounding structures in the same two scapular movement groups.Methods: Participants with shoulder pain were classified into the two movement patterns based on clinical exam measurements during overhead reaching, and movement patterns were confirmed using 2D/3D shape-matching. Highly accurate shoulder bone motion was obtained from biplane video radiography in three motions: abduction, flexion, and unrestricted overhead reaching. Participant-specific bone and supraspinatus (rotator cuff) tendon models were derived from magnetic resonance imaging. The bone models were projected onto and aligned with the radiography images (2D/3D shape-matching) to output the 3D joint positions throughout motion. Muscle activity was recorded simultaneously with the radiography collections using electromyography. Muscle activity of key scapulothoracic muscles were compared between groups. The proximity of the supraspinatus tendon to the coracoacromial arch and glenoid was calculated throughout motion by recreating each participant’s arm motion using models of their humerus, scapula, and supraspinatus tendon and their specific arm raising kinematics. Results: Significant interactions of group and motion type and group and range of motion interval were found for the anterior deltoid, ratio of lower trapezius/serratus anterior, and ratio of lower trapezius/anterior deltoid. Both groups had increasing anterior deltoid muscle activity as humerothoracic elevation increased, and the anterior tilt group increased across intervals to a greater extent than the lateralization group. In abduction and flexion, the anterior tilt group had a higher ratio of lower trapezius/serratus anterior compared to the lateralization group indicating relatively increased lower trapezius activity or relatively decreased serratus anterior activity in the anterior tilt group. In abduction and flexion, both groups generally had a ratio >1 indicating relatively more lower trapezius activity than serratus anterior. In unrestricted reaching, both groups had less lower trapezius activity compared to serratus anterior (ratio <1), and this occurred to a greater degree in the anterior tilt group. The anterior tilt group had a higher ratio of lower trapezius/anterior deltoid compared to the lateralization group from 31¬–60 of humerothoracic elevation. Groups were similar from 61–90°, then, though both groups had increasing ratios, the lateralization group progressively increased to a greater extent. The minimum distance between the supraspinatus tendon and the coracoacromial arch was significantly smaller in the anterior tilt group than the lateralization group. All participants experienced contact between the tendon and the glenoid regardless of movement pattern grouping. Conclusions: Individuals with a scapular anterior tilt movement pattern have differences in muscle activity of key shoulder muscles compared to individuals with a scapular lateralization pattern. The supraspinatus tendon is in closer proximity to the coracoacromial arch in people with the anterior tilt pattern, indicating that this might be a movement pattern of concern that warrants further investigation.