Effect of Stress Distribution in Designing Reverse Shoulder Prosthesis: A Finite Element Analysis

One of the extreme diseases among patients are Rotator cuff tear and degenerative shoulder join, which result in stark pain and shrink performance in shoulder joint. Nowadays, a substitute shoulder is the best way to relieve pain and reestablish stability. A reversed spare shoulder is needed whenever, the substitute shoulder isn’t efficient enough to refurbish the joint. The only difference between those two components is that the reverse replacement is similar to the normal shoulders. For instance, the ball component is positioned to the glenoid and the socket is placed to the proximal humerus. The main reason of the altered anatomy is to provide a greater lever arm for the deltoid muscle to regain active shoulder elevation. Identically to other inventions, reversed replacement has inconvenient, such as loosening in glenohumeral joint and failure of prosthesis at the glenoid attachment area. The main purpose of this thesis is to recognize the probable failures at any of the implant`s components like the glenoid and glenohumeral joint. 3D model of reverse shoulder implant was created using the software SolidWorks in order to perform finite element analysis (FEA). The finite element (FE) analysis has been carried out in this study via ANSYS software to obtain the maximum values for Von Mises stress on each component, in order to evaluate the values and see if the designed component would sustain during the three analyzed movements (abduction, flexion and rotation) for a movement span of 4 seconds. It is hypothesized that the range of motion (ROM) of the shoulder joint is altered with reverse shoulder implant. An investigation is carried out concerning the extent of contact stress to cause wear of the humeral cup in glenohumeral joint. The results show that the maximum stress of the polyethylene made humeral cup happens during abduction, and it can get as high as 26 MPa that exceeds the polyethylene yield strength. This high value of stress Polyethylene would probably wear which can lead to joint loosening of reverse glenohumeral joint. Also according to the obtained results the two screws used in the implementation of the implant (Inferior screw and Superior screw) are the componets with the maximum Von Mises Stress, especially in flexion movement (maximum stress of 134 MPa for superior screw). Almost in all three movements these screws are the most critical component however their maximum stress does not become critical since it does not exceed 15% of the titanium yield strength (neither compressive or tensile). Hence the titanium alloy parts of the implant would not become critical for the design. On an overall conclusion the results shows that in the design of humeral cup, the abduction movement is the key movement since it has the most stress impact on this component, and similarly the flexion movement is the key movement in the design of the baseplate and connection screws. Keywords: Shoulder arthroplasty, Finite Element, Shoulder 3D modeling, Von Mises stress equivalent