## 4 Discussion
The numerical model used in this research for biomechanical analysis, with realistic muscle forces, was well-validated with experimental tests with a single traction force condition. The results showed that the stiffness of the construct was increased significantly at high abduction angles and low horizontal flexion angles, and not changed for any positions of the flexion movements. Of the three shoulder rehabilitation exercises investigated, horizontal flexion displayed the most unfavorable characteristics. The null hypothesis was rejected, since rehabilitation activities showed different biomechanical behavior in the shoulder joint with GT fracture fixation with lockingplates. The displacements at the fracture site, as well as bone stress, bone plate stress, and average bone screw stress were higher in horizontal flexion than abduction and flexion movements. These results suggested that GT-fracture-treated shoulders using the locking plate technique should be not or minimally mobilized with horizontal flexion during rehabilitation activities, since it may increase the possibility of implant failure and bone breakage at the early stage. Therefore, horizontal flexion activities may be recommended in the later stage of shoulder joint rehabilitation with high caution. An artificial humerus with a more consistent geometry and material properties to the human humerus was selected as the testing material. Researchers found more realistic experimental outcomes with cadaveric humerus fracture models in their studies. Cadaveric bone models, however, may not be the best choice as they may show variations in bone anatomy, bone density, or fracture patterns [19; 21]. These variations may surpass the biomechanical effects when experimenting the shoulder rehabilitation activities at different positions. Also, availability of cadaveric humeri is sometimes limited and difficult to obtain [20]. In the present study, artificial humeri were used because of the above-noted problems with cadaveric humerus. There are some limitations in this study. Firstly, linear elastic isotropic material properties were assumed for the GT fractured shoulder joint models in this study. The numerical analysis produced only the relative outcomes for all situations. Secondly, the loading of shoulder joint construct was simplified to mimic only the supraspinatus, infraspinatus, and subscapularis muscle load. The shoulder is generally controlled by a more complex combination of dynamic and static stabilizers that our experimental model does not consider. Thirdly, motion in the current model was limited to along the longitudinal axis of the bone. Although it was sufficient for in vitro results, other motions such as internal and external rotation may result in more accurate biomechanical analysis that warrant further study. Finally, this study considered only two-part greater tuberosity fracture for the biomechanical investigation of locking plate systems for proximal humerus fracture fixations during shoulder rehabilitation activities using rotator cuff muscle forces. Other humerus fracture models also need to be considered in future research, such as the three-part nondisplaced proximal humerus fracture, three-part displaced proximal humerus fracture, and four-part humerus fracture.