Finite Element Analysis of Reverse Shoulder Joint Prosthesis

Rotator cuff tear is one of the most common cases among patients that cause severe pain and reduced performance in shoulder joint. Recently, in order to relieve pain and restore stability and function of shoulder, shoulder replacement is commonly performed. However, when normal shoulder replacement is not sufficient to restore the joint function, the reverse shoulder replacement is performed. In reverse replacement unlike the traditional replacement system, which is same to the normal shoulders, the ball component is positioned to the glenoid and the socket is placed to the proximal humerus. Reason of this altered anatomy is to provide a greater lever arm for the deltoid muscle to regain active shoulder elevation. Some complications after reverse shoulder replacement such as loosening in glenohumeral joint and failure of prosthesis at the glenoid attachment area have been reported. Aim of this thesis is to recognize the probable failures at the glenoid prosthesis and artificial glenohumeral joint. In this thesis a 3D models of reverse shoulder implants were created in order to perform finite element analysis (FEA). Therefore FEA is carried out in this study to find out whether or not the stress distribution on implants and micromotion between bone and implant at glenoid part cause the implant failure. It is hypothesized that the ROM of shoulder joint is altered with reverse shoulder implant. The abnormality of ROM of the implanted reverse shoulder joint is examined for abduction movement. Then it is investigated, if contact stress is high enough to cause wear of the humeral cup in glenohumeral joint or not. In order to simplify the analysis only abduction movement of shoulder joint is considered and the duration of the analysis was kept low in 4 seconds. The analysis proves that the peak stress generated of the humeral cup, which is made of polyethylene, can be as high as 25 MPa that exceeds the polyethylene yield strength. Polyethylene wear can be the result of this high contact stress. In addition to the permanent deformation and destruction of the component, one of the reasons for loosening of reverse glenohumeral joint is small particles from the polyethylene wear. Bone ingrowth can provide the long-term attachment between baseplate, which is attached to scapula, and bone after shoulder replacement, when the stable interface is maintained between bone and baseplate; and displacement of baseplate does not exceed 150 !", which is a threshold value to allow bony ingrowth. The result also shows the parallel motion to the glenoid between baseplate component and scapula bone with 104 !" as maximum value. The micromotion does not exceed the limit value but as the obtained result is close to the threshold value to allow bony ingrowth, the probability of failure may arise under more sophisticated modeling conditions. Therefore, this knowledge will enable researchers, engineers and clinicians to improve the design of the reverse shoulder prosthesis. Keywords: Reversed Shoulder Arthroplasty, Complications, Finite Element, Glenohumeral Joint, Stress Analysis