Cause Analysis Of The Liner Dissociation Of Reverse Shoulder Prosthesis Based On Finite Element Analysis

Objective:Reverse shoulder arthroplasty(RSA)is an innovative solution to compensate for the loss of rotator cuff function.However,the design of a reverse shoulder joint prosthesis often results in impingement between the polyethylene(PE)liner of the scapular component and the humeral component.This impingement is believed to be associated with scapular notching,wear of the PE liner,and loosening of the glenoid implant.In some cases,dislocation of the PE liner has been observed following RSA,which could lead to shoulder joint dislocation that cannot be reduced by closed reduction.However,the exact cause of PE liner dislocation remains unclear.Deformation of the liner’s fixation structure has been reported in the literature and in our case,leading us to suspect a correlation with humeral adduction impingement.To verify this hypothesis,it is crucial to observe the deformation of the fixation structure of the liner during adduction impingement.Nevertheless,due to the limited precision of radiographic systems and the non-visivility of PE liners,capturing this deformation through video radiography proves challenging.Finite element analysis(FEA)is increasingly utilized in the medical field,particularly in orthopedics.This technology enables dynamic simulation of the impingement between the liner and scapula,facilitating accurate observation of the stress distribution and degree of deformation in the PE liner.Consequently,this study employs finite element analysis to investigate whether adduction impingement is a contributing factor to the dissociation of the claw-type fixation structure of the prosthetic liner,and proposes a solution to reduce the incidence of liner dissociation.Methods:Firstly,the CT data of the volunteer’s shoulder joint was utilized to construct a non-homogeneous bone model of the shoulder joint.A 3D scanner was employed to scan the various components of the prosthesis,obtaining point cloud data,which was subsequently used for reverse modeling to generate a prosthesis model.The prosthesis model and bone model were then assembled in a simulated surgery under the guidance of an experienced clinical physician.Material properties were assigned to both the prosthesis and bone,with the bone’s material properties being nonhomogeneously assigned based on CT grayscale values.Dynamic Finite Element Analysis(FEA)was employed to simulate the impingement between the PE liner and scapula during humeral adduction under the influence of gravity.The study measured and analyzed the stress distribution and deformation degree(DOD)of the fixed claw,as well as the stress at the impingement site in three initial humeral positions(neutral,30° flexion,and 30° extension).Additionally,the study investigated the effect of a ten-fold increase or decrease in the elastic modulus of the liner material.Results:The mesh sensitivity analysis indicated that a mesh size of 1.0 mm for the liner and 2.0 mm for the scapula was sufficient to ensure the accuracy of the analysis.At the neutral initial position,the liner impinged on the anterior and posterior ridge of the scapular neck,with the maximum stress on the scapula reaching 120 MPa.There were two impingement points on the liner located on the posterior medial side,with the maximum stress reaching 145.7 MPa.At 30° of flexion,the impingement point of the scapular neck and liner was the anterior ridge and the posterior medial side of the liner.The maximum stresses on the scapula and the liner were 125.4MPa and 152.9 MPa,respectively.At 30° of extension,the impingement point of the scapular neck and the liner was the posterior ridge and the medial side of the liner.The maximum stresses on the scapula and the liner were 160.7 MPa and 211.3 MPa,respectively.When the elastic modulus of the liner was increased tenfold,the maximum stresses at the scapula and liner impingement points were 155.6 MPa and 338.1 MPa,respectively.When the elastic modulus of the liner was decreased tenfold,the maximum stresses at the scapula and the liner impingement points were 37.9 MPa and42.1 MPa,respectively.The maximum principal stress(MPS)in the fixation claws was 7.7MPa.In some areas where the fixation claws were connected to the liner,compressive stresses on the medial side and tensile stresses on the lateral side were observed,indicating that the fixation claws tended to deform towards the center.When the elastic modulus of the liner was increased or decreased tenfold,the maximum values of MPS were 11.9 MPa and 5.5MPa,respectively.The maximum DOD of the three initial humeral postures(neutral,30° flexion,and 30° extension)was 3.6%,2.8%,and3.5%,respectively.When the elastic modulus of the liner was increased or decreased by 10 times,the maximum DOD of the neutral initial humeral posture was 0.51% and 11.4%,respectively.Conclusion:The impingement between the liner and the scapula can cause deformation of the fixation structure of the polyethylene liner,which may be one of the reasons for liner dissociation.Increasing the elastic modulus of the liner material can help reduce the deformation of the fixation structure.