## 3 Results  
We constructed the proposed method and successfully simulated it, as shown in Figure 6. If the displacement boundary condition is relaxed by a truss element, the excessive element destruction at the soft tissue can be prevented at the point where the binding force of truss and the reaction from the supraspinatus tendon and scapula reach a balance between one another.  
Figure 6a shows the result of simulation, where the relaxation control of the displacement boundary condition is applied to the glenohumeral joint. The peak contact stresses were measured at the final position and at a peak point where the contact stress on the labrum was highest, as shown in Figure 6b. Figure 6c presents a peak stress change curve between the supraspinatus tendon and the labrum according to humeral motion. The peak contact stresses measured at final motion, with spring constants of 100, 200, and 300 N/mm, were 9.49, 13.47, and 23.13 MPa, respectively. As a result, the stress distribution is concentrated at the point where the postrosuperior labrum impinges on the supraspinatus tendon, and the measurements of peak stress showed that contact stress increases with the increase in the spring constant.  
Figure 7 shows the comparison of the boundary condition (the guidance) and the humeral motion (the follower) by the proposed method: The boundary condition is the humeral motion of the 6-DOF (degree of freedom), including the translation (posterior, superior, lateral one) and rotation (elevation, horizontal extension, external rotation). The humeral motion of the 6-DOF was represented with reference to the joint coordinate system of ISB recommendation [32].  
Figure 6: The finite element simulation result by the proposed method: (**a**) Screenshot of the finite element simulation result; (**b**) The point for peak contact stress measurement on the labrum; (**c**) Stress change curve between supraspinatus and labrum according to humeral angle.  
The performance of the proposed method was examined from the differential displacement. The differential displacement means that distance between the displacement of humeral head centers (the guidance and follower). We defined the humeral head center as the center of a sphere that fits the humeral head, as shown in Figure 8a. Figure 8b shows the change in the distance between the humeral head centers of the guidance and follower according to humeral angles. When the humeral angle is small, the distance between the humeral head centers is close to zero. However, after a certain degree of rotation is generated, the distance between the guidance and follower becomes clear because of the surrounding reaction. The distances between the centers measured at final motion, with spring constants of 100, 200, and 300 N/mm, were 4.68, 3.67, and 3.28 mm, respectively. This means that the proposed method allows for FE simulations of shoulder motion with a differential displacement of 3.28 mm.  
Figure 7: The motion comparison of guidance and follower by the proposed method: (**a**) Posterior translation; (**b**) Superior translation; (**c**) Lateral translation; (**d**) Elevation; (**e**) Horizontal extension; (**f**) External rotation.