Redesign of reverse shoulder implant and FEM analysis

There are four kind of joints in human shoulder which are called as glenohumeral joint, acromioclavicular joint, scapulothracic joint and sternoclavicular joint. Glenohumeral joint, which is the main joint of the shoulder, helps the human shoulder mobility in daily activities. This joint is also called as shoulder implant in many studies. The nearest component of the glenohumeral joint is scapula bone, which is shaped like concave socket. The other component of this joint is humerus bone, which is shaped like convex head. Glenohumeral joint is mechanically stable with muscles, tendons and ligaments. With these stabilizers, humerus head is compressed through glenoid socket. Many muscles around the shoulder for stabilizing process while they allow the movement of shoulder. These muscles can divide in three main category; axiohumeral muscles, scapulohumeral muscles and the axioscapular muscles. As it can understand from name of muscles, axiohumeral muscles are on the humerus, while axioscapular muscles on scapula. On the other hand, scapulohumeral muscles which includes many main muscles such as deltoid, teres minor, teres major etc. to provide lots of movements and compressions to humeral head. The deltoid muscles provide approximately 50% of the moment which is required to elevate the shoulder. In addition to deltoid muscles, the other important muscles which are called as rotator cuff provides a compressing effect on humeral head to stabilize the shoulder. These muscles are combined from teres minor, subscapularis, suspraspinatus and infraspinatus. If any miscompression occur in humerus head location, the forces which supply the compression can affect the glenohumeral joint socket. Then this effect can create instabilization and lots of tear on muscles. Variety of treatments are developed to solve glenohumeral joint problems. The most applicable treatment of this problem is solved by implants. Implants can be combined in three main versions which are total shoulder implants, partial shoulder implants and reverse total shoulder implants. For all three implants effectively decrease pain and solve the main problems of shoulders. Deltoid muscle load is needed to allow avtive movement and from that force, joint loads magnitude are important drivers for life of shoulder implants. Purpose of this thesis is to improve reverse shoulder implant which are generally used to restore stability and function of the shoulder for advanced rototar cuff tears. Reverse shoulder implant is more reliable than total shoulder implants. In total shoulder implants, polyethlene glenoid component is the most failure part of implants. With the high shear force while elevation of shoulder, polyethlene part is damaged and it creates catastrophic failures such as lots of pains, loss of function and a cluncking noise.Therefore, new kind of implants are researched in many years. In that case, reverse shoulder implant is proposed by Paul M. Grammont. Reverse shoulder implants are assembled to shoulder with inverse of shoulder anatomy and there are four main parts for that implants which are Humeral Stem, Polietilen Insert (Socket), Glenoid Sphere (Ball Head) ve Glenoid Fixation Device. In that implant, there is a ball head in Glenohumeral side, and socket of this ball head and humeral stem are implanted to the humerus bone. Material of ball head and arm of socket are made by titanium (Ti-6AI-4V Alloy) and socket is made by polyethylene (UHMWPE). Since reverse shoulder implant is inverse of shoulder anatomy, it can also create many problems in daily activities. One of the most important problems of this implant is scapular notching which can appear even rest position of arm and shoulder. Scapular notching is occurred between scapula and side of polyethylene implant. As per many research results, scapular notching incidence is reported as range of 44% - 96%. To improve the strength of implants against scapular notching, new improved polyethlene part is designed with this thesis. Main aim is to compare the nominal implant with redesigned implants as per stress analyses and geometrical advantages. First of all, implant parts are measured and modelled as 3-D via Unigraphics programme. All dimensions are measured from sample of implants and sketched for modelled parts. Humeral stem length is an important factor for shoulder implants. Lots of studies are done to optimize of humeral stem length and in that thesis one of the commonly used length of humeral stem is used. Humeral stem angle is taken as 155° and length of stem is modelled as 140 mm for both nominal and redesign cases. Then, polyethyelene part is redesigned for redesing models. In redesign, there is a feature which reduces the scapular notching effect. In nominal geometry, there is a circular end on polyethlen part, but in redesign with new added feauture, contact location has a concave shape. In addition, a large fillet radius is added to concave feature ends to reduce the effective stress and smooth transition between the faces. As per Unigraphics assembly condition, patients will have an additional shoulder movement with this redesign case. This additional shoulder movement is calculated as approximately 11.2° with comparison to nominal case. Therefore, scapular notching which can appear even patient's resting position can be prevented with this new redesign models. To assess for abnormal conditions if patient push his/her shoulder in adduction way too much, stress analysis is performed to show the scapular notching effect on redesign case. After modelling of 3-D parts for nominal and redesign cases, 3-D mesh models are created via HyperMesh programme for stress analyses. In mesh creation, tetra meshes are created for scapula, glenohumeral head, humerus stem and polyethlene parts for both nominal and redesign parts. In that creation, all minimum tetra mesh sizes and angles are selected as same for all parts. Min element size taken as 0.1mm and average of them is taken as 1 mm for all models. Angle between of elements is taken as 8° and with this selections, contact areas have fine meshes with smooth transitions. After all mesh models are created, models are exported to use in Abaqus Solver for stress analysis. Materials of implants are found from previous researhs and selected commonly used Elastic Modulus and Poisson Ratio for stress analysis. Based on historical researchs for material characteristics, humeral stem and ball head are made by Titanium (Ti-6AI-4V alloy). For titanium, elastic modulus are taken as 114 Gpa, and poisson ratio is used as 0.34. Insert implant (Socket) is modeled using the material characteristic of ultra-high-molecular-weight polyethlene (UHMWPE). Elastic modulus of Polyethlene is taken as 1.29 Gpa and poisson ratio of this material is taken as 0.38. In order to defined a young modulus for skapula material, cortical bone characteristics are used. Since cortical bone is a more uniform structures rather than the other bone's locations, young is selected as 12.4 Gpa with 0.27 poisson ratio. For back to back stress analysis comparison between nominal and redesign models, same boundary conditions are applied for both models. There is a frictionless contact between the polyethlene part and ball head part. End of scapula part is holded in x,y and z direction. Additionally, humerus stem is allowed its movement only in x and z direction, because analysis consider to check the only adduction movement on model. Force is applied on humeral stem as 1,000 Newton, and it is calculated by moment equations. Von mises stresses are considered to compare of nominal and redesign solutions. As per stress analysis results, there is 22.4 Kpa at scapula contact location in nominal, and 4.7 Kpa is in redesign case. On the other side of implant, polyethlene part has 36.5 Kpa on contact location in nominal case, but in redesign this stresses also decreased up to 5.5 Kpa. As mentioned before these results are for only back to back stress comparison and applied same boundary conditions on models, therefore stresses are not exact value of human shoulder. As per back to back comparison, stress result shows that there is a stress decreasing on scapula contact location which observs scapular notching. As a result of this thesis, comparison of nominal and redesign motion capability, redesign parts have an additional ~11.2° gap for adduction movement with added feature and fillet, therefore scapular notching effect on resting position of shoulder will bot be issued by new design case. Additional stress analyses are performed to show abnormal condition for adduction movement. As per stress analysis result which are done with same boundary conditions redesign case has less stress levels for scapular notching problem. Therefore, redesign reverse shoulder implant will provide additional adduction movement with less stress effects for scapular notching problem. This improved redesign implants should be used in clinical cases to check the stability and compare the scapular notching effect with previous cases. For further researches, this new model can be combined with other laterilized implants to provide more additional movements to patients.