The stabilizing potential of the supraspinatus is inhibited in tearassociated scapula shapes but can be modulated by kinematic adjustments

Erin CS Lee ^1* Nathan M. Young ² Ellen Y Li ¹ Rebekah L Lawrence ³ Michael J Rainbow ^1*

• ¹ Department of Mechanical and Materials Engineering, Faculty of Engineering and Applied Science, Queen's University, Kingston, Ontario, Canada
• ² Department of Orthopaedic Surgery, Medical Center, University of California, San Francisco, San Francisco, California, United States
• ³ Program in Physical Therapy, Washington University School of Medicine, St. Louis, United States

Specific scapula shapes are associated with full-thickness tears of the supraspinatus tendon. A primary role of the supraspinatus is to actively stabilize the glenohumeral joint against muscles that generate destabilizing shear forces. Mechanisms that increase the supraspinatus load required to perform this stabilizing function may increase an individual's susceptibility to tears. Previous work has shown that tear-associated scapula shapes increase the destabilizing action of the deltoid during arm-raising, but no work has investigated whether tear-associated shapes inhibit the stabilizing potential of the supraspinatus itself. We combined statistical shape modeling, kinematics-driven simulations of the glenohumeral joint, and a finite element model of the supraspinatus to investigate the interactions among shape, kinematics, and the stabilizing potential of the supraspinatus. Relative to asymptomatic controls, individuals with full-thickness tears possessed a suite of 3D shape differences including narrower supraspinous fossae and anteverted glenoids. For the same abduction path, tear-associated scapula shapes caused supraspinatus fibres to act more anteriorly and less compressively compared to the control shapes, potentially inhibiting the supraspinatus' ability to stabilize the humeral head. When the abduction path of the tear-associated scapula was internally rotated and shifted anteriorly, the supraspinatus line-of-action closely resembled that of the controlassociated scapula; however, the tear-associated shape still possessed a narrower breadth in its supraspinatus line-of-action. Overall, our findings indicate that tear-associated scapula geometry may inhibit the stabilizing potential of the supraspinatus, but this shape-driven change could be partially modulated when the abduction path of the tear-associated shape was perturbed. The magnitude of kinematic perturbations required to modulate function exceeded the magnitude of shape differences, indicating that the perturbations are not correcting for a simple offset, but rather complex changes in muscle geometry that occur due to 3D shape differences. This work presents an approach that enables the analysis of supraspinatus lines-of-action at any 3D rotational pose across any scapula shape. This