Movement-Related Pathogenesis of Rotator Cuff Disease in Persons with Shoulder Pain: Effects of Decreased Scapulothoracic Upward Rotation

Abstract

Background: Shoulder pain is a common musculoskeletal complaint that is often associated with rotator cuff injury and abnormal scapular movement. In particular, decreased scapulothoracic upward rotation has been theorized to increase an individual’s risk for subacromial rotator cuff compression. However, the effect of abnormal shoulder motion on mechanisms of rotator cuff injury remains unclear. Further, the ability to accurately and non-invasively quantify shoulder complex kinematics is limited. Objectives: The objectives of this thesis are: 1) Develop and validate a protocol for using single-plane fluoroscopy and 2D/3D shape-matching to quantify shoulder complex kinematics; 2) Determine the impact of decreased scapulothoracic upward rotation on subacromial proximities; and 3) Identify the kinematic mechanisms by which sternoclavicular and acromioclavicular motion contributes to scapulothoracic upward rotation. Methods: A protocol for using single-plane fluoroscopy and 2D/3D shape-matching to quantify shoulder complex kinematics was validated using radiostereometric analysis in four cadaveric specimens. Shoulder complex kinematics were quantified in 60 participants with and without shoulder pain during scapular plane abduction using the validated protocol. Subject-specific 3D bone models reconstructed from MR images were animated with each participant’s glenohumeral kinematics. Subacromial proximities were calculated between the coracoacromial arch and rotator cuff insertion. The effect of decreased scapulothoracic upward rotation on subacromial proximities was assessed. The relative contribution of sternoclavicular and acromioclavicular motion to scapulothoracic upward rotation was calculated using two derived coupling functions. Results: Single-plane fluoroscopy and 2D/3D shape-matching can accurately quantify static shoulder complex kinematics. Subacromial proximities were generally smallest below 90° humerothoracic elevation. The normalized minimum distance for participants in the low scapulothoracic upward rotation group was significantly smaller (35%) than those in the high scapulothoracic upward rotation group at the minimum position. Scapulothoracic upward rotation can be estimated from acromioclavicular upward rotation, sternoclavicular posterior rotation, and sternoclavicular elevation. Conclusions: Decreased scapulothoracic upward rotation shifts the range of motion in which normalized minimum distances are smallest to lower angles. Acromioclavicular upward rotation and sternoclavicular posterior rotation are the predominant component motions of scapulothoracic upward rotation.