  Reverse Shoulder Arthroplasty (RSA) has become a very common procedure for
shoulder joint replacement, even for scenarios where an anatomical
reconstruction would traditionally be used. In this study, we investigate joint
reaction forces and scapular kinematics for rotator cuff tears of different
tendons with and without a reverse prosthesis. Available motion capture data
during anterior flexion was input to a finite-element musculoskeletal shoulder
model, and muscle activations were computed using inverse dynamics. The model
was validated with respect to in-vivo glenohumeral joint reaction force (JRF)
measurements, and also compared to existing clinical and biomechanical data.
Simulations were carried out for the intact joint as well as for various
tendons involved in a rotator cuff tear: superior (supraspinatus),
superior-anterior (supraspinatus and subscapularis), and superior-posterior
(supraspinatus, infraspinatus and teres minor). Each rotator cuff tear
condition was repeated after shifting the humerus and the glenohumeral joint
center of rotation to simulate the effect of a reverse prosthesis. Changes in
compressive, shear, and total JRF were analysed, along with scapular upward
rotation. The model compared favourably to in-vivo measurements, and existing
clinical and biomechanical knowledge. Simulated JRF lie in the ranges of
in-vivo JRF measurements and shows a linear increase past 90 degree flexion.
Implanting a reverse prosthesis with a functional rotator cuff or with an
isolated supraspinatus tear led to over 2 times higher compressive force
component than with massive rotator cuff tears (superior-anterior or
superior-posterior). Such higher compression forces might increase the risk of
wear and implant fracture.
