Risk of Reduced Subacromial Space During Activities of Daily Living for Users of Manual Wheelchairs

Background and significance: Users of manual wheelchairs have to perform many movements within the confines of the wheelchair as well as utilize the wheelchair as a means of locomotion, for transferring to adjacent seating, and performing weight relief lifts, among other activities. The shoulder does not normally function as a weight bearing joint; however, users of manual wheelchairs rely heavily on their shoulder joints for these activities and shoulder pain is prevalent. There is general consensus that reduced subacromial space caused by altered glenohumeral kinematic patterns during wheelchair activities exposes the soft tissues at the shoulder, primarily the rotator cuff tendons, to mechanical compression during movement. The purpose of this study was to model the potential impact of angular kinematics during activities of daily living (wheelchair propulsion, weight relief raises, and scapular plane abduction) on the underlying subacromial space and resulting proximity of anatomical structures in a population of manual wheelchair users with reported shoulder pain.;Research Methods: Fifteen spinal cord injured individuals participated in the study. The individuals used manual wheelchairs as their primary means of mobility and reported anterolateral shoulder joint pain presumed to be caused by mechanical impingement. The three-dimensional position and orientation of the subjects' thorax, scapula, and humerus were collected using an electromagnetic tracking system during weight relief, propulsion, and scapular plane abduction. Scapulothoracic (3 rotations), glenohumeral (3 rotations) and thorax flexion/extension angles were determined throughout the tasks. Each subject's glenohumeral rotation values were combined with CT-generated bone models of the scapula and humerus to simulate all three tasks. At each of the time steps, the proximity (distance) mapping and minimum distance from each of three tendon footprints (infraspinatus, supraspinatus, and subscapularis) to the acromion and coracoacromial ligament were determined.;Analysis: Between-task and within-task comparisons of the angular kinematics were performed using repeated-measures ANOVA. Between-task comparisons of the minimum distance were performed using repeated-measures ANOVA, while the proximity maps were qualitatively assessed. Sensitivity of the linear distance values to errors in kinematic values were computed as well as an exploratory regression to predict linear distances from glenohumeral kinematics.;Results: Significant between-task and within task differences were observed in many of the kinematic variables. The weight relief task possessed peak values for scapulothoracic internal rotation and anterior tilt, and glenohumeral internal rotation, and when comparing mean event data, it possessed greater anterior tilt (equal to propulsion) and glenohumeral internal rotation. Scapular plane abduction possessed the least at-risk kinematics, with the smallest event data across tasks for anterior tilt and glenohumeral internal rotation as well as for peak scapular internal rotation, anterior tilt, and glenohumeral internal rotation. Further, significant between-task differences were seen in linear distance values and risk (area between linear distance curves and 5.0mm threshold). In general, linear distances were smaller and risks were higher between the tendon footprints and the acromion (versus the coracoacromial ligament). Further, linear distances were smaller and risks higher during propulsion and scapular plane abduction, than during weight relief. Sensitivity analysis of the linear distance values resulted in sub millimeter changes for ± 3 degree changes in glenohumeral rotations and less than 2.0 mm for ± 6 degree changes. Proximity maps (within 2.0 mm and 5.0 mm thresholds) depicted changes in location of the maps on the tendon footprints and acromion, as well as coverage area, between tasks and muscles. The supraspinatus proximity area within a 2.0 mm threshold was greater than the infraspinatus area for both propulsion and scapular plane abduction, and within the 5.0 mm threshold for scapular plane abduction. Significant differences were only seen between the two tasks for the infraspinatus areas within 2.0 mm and 5.0 mm with larger areas during propulsion.;Discussion and conclusions: When the current study findings were evaluated according to currently-held beliefs about at-risk scapulothoracic kinematics, the weight relief task placed the shoulder at greater risk for reduction in the subacromial space. Findings from the linear distance and risk analysis, however, did not support this result. Scapular plane abduction and propulsion were found to cause substantial risk of impingement using these measures. As subacromial impingement risk is defined based on glenohumeral motion changes, future research should focus on the glenohumeral articulation rather than on the scapula and humeral motions independently to attempt to define risk.