Finite Element Analysis Of Stress Distribution Of Acetabulum With Different Fiaxtion Methods In Total Hip Arthroplasty

Although total hip replacement has long been the most efficient way of treating osteoarthritis of late stage and other diseases of hip joint,problems still exist with the acetabular components,one of them is late loosening of the component,which is mostly due to the insufficiency of the initial stability of the cup.At the same time,the implantation of either cemented or cementless acetabular components will inevitably bring in alteration of in vivo biomechanical environment.The change of load on the host bone results in stress concentration or stress shielding,both will have effects on the prosthetic stability postoperatively.Therefore,theoretically the removal or retention of the acetabular subchondral plate during acetabular preparation might influence the stress at the bone-component interface.On the other hand,to achieve the so-called press-fit effect is very important in implanting the acetabular component;sometimes screw fixation is needed for additional stability.The use of screw will also affect the stress distribution at the bone-component interface and the number and placement of screws are also issues to be concerned.Finite element analysis evolved in engineering in order to predict the behavior of structures,and so reduce the need for costly prototyping.Essentially, an engineering system is broken down into a large but finite number of small elements,the behavior of which can be understood under a variety of loading conditions.The current study employed finite analysis method,aiming to construct three-dimensional models,and so to investigate into the stress distribution and stress concentration after removal and retention of subchondral bone,and the influence of screw fixation on stress at the bone-component interface in cementless press-fit total hip replacement,in an attempt to explore ways to reduce wear and loosening,lengthen in vivo survivorship of the prosthesis. PARTⅠDevelopment of three-dimentional finite element model of the hip and assessment of stress distribution of acetabulum after retention and removal of the subcondral plate in acetabular preparationObjective:To set up a 3-D finite element model of the hip and acetabular component, and to assess the change of acetabular stress distribution after retention and removal of the subchondral plate during acetabular preparation in total hip replacement.Methods:The pelvis of a middle aged man was scanned by computer tomography to obtain a set of slices.The geometry and associated mesh of the acetabulum was reconstructed with use of software of ANSYS9.0.A 3-D finite element model of acetabular component was also set up,including a hemispherical cup shell,ultra-high molecular weight polyethylene insert or polyethylene cemented cup,4mm layer of cement mantle and 28mm diameter metal head.The model was executed for both conditions:retention of subchondral bone and removal of the plate.Results:The results of Von Mises stresses were demonstrated at the bone-cement interface.Retention of subchondral bone of the acetabulum resulted in reduced peak stresses at the interface,while removal of the plate increased the peak stresses of acetabular shell,bone cement mantle and acetabular bone surface.Conclusion:The simplified model showed that in cemented total hip replacement retention of subchondral bone could reduce the peak stresses at the bone-cement interface and the cup itself so as to improve load sharing,avoid stress concentrationand decrease creep and wear.PARTⅡStress concentration at the acetabular bone-component interface after retention of subcondral bone objective:To assess the effect of removal of subcondral bone on acetabular bone after insertion of a press-fit metal-backed acetabular cup.Methods:A press-fit model was constructed by heating the metal shell in the direction of insertion of the prosthesis,simulating the press-fit effect along the inner surface of the socket.After loading,the stress distribution was measured with the ANSYS software and that of retention and removal of the subchondral plate were compared with each other.Results:The peak stress lowered at the bone-shell interface with retention of the subchondral plate.The peak stress located in the supralateral part of the acetabulum, in the same direction of the body contacting force.Conclusions:with no screw press-fit fixation of the metal-backed acetabular cup, retaention of subchondrai plate resulted in decrease of the stresses at the acetabular bone-shell interface and that of polyethylene insert,hence decrease the stress concentration and generation of excessive wear,increase the stability of the press-fit cup.PARTⅢThe influence of multiple screw fixations for additional initial stability of the cementless cup on stress distribution at the bone-prosthesis interfaceObjective:To investigate the influence of number of screws and sites of the screws on the stress distribution at the bone-prosthesis interface.Methods:Finite element analysis was performed using commercial package of ANSYS.The acetabular components consisted of a femoral head,liner and hemispherical cup with 3 screw holes.The cup was implanted with 45°of abduction and 15°anteversion at the acetabular socket of the pelvis.The size of the shell was 1mm less than that of the socket in order to achieve the press-fit effect.Six types of screw fixations were evaluated in the study.Results:With one-screw fixation the stress of the acetabular bone did not change much at either hole.With two-screw fixation and three-screw fixation the stress decreased gradually,however the stress on liner did not change accordingly.Conclusions:In use of screw fixation for additional initial stability of the press-fit cementless cup,multiple-screw fixation was needed to lower the peak stresses at the bone-prosthesis interface and the liner,avoiding stress concentration and creep of the polyethylene liner.PARTⅣMeasurement of wear of acetabular omponent and its clinical significancesObjectives:To measure the linear wear of the ultra high molecular weight polyethylene liner of the acetabular component and its direction,and investigate the correlation between wear direction and cup abduction angle,as well as the extent of wear and its clinical relevance.Methods:To measure the linear polyethylene wear,migration in the femoral head center relative to the cup shell center was calculated on digitized consecutive radiographs with special computer software program,the degree and direction of wear were determined as such.Patients were followed up routinely.A total of 38 hips were evaluated clinically and radiographically.Multivariate regressive analysis was used to find out correlations of wear with patients' age,body weight,cup size,activity level and Harris Scoring,et al.Another cohort of 32 hips was followed for 4.2 years on the average.Cup abduction angle was measured,as well as the magnitude and direction of the wear.The results were statistically analyzed,with a P less than 0.05 indicating significant difference.Results:The average linear wear was 1.22mm,wear rate was 0.11mm/year.The wear was associated with osteolysis on the acetabular side(P＜0.05). Multivariate regressive analysis failed to reveal any correlations of wear with patients' age,body weight,cup size,activity level and Harris Scoring,et al.The degree of wear in the first two postoperative years accounted for nearly 40%of the total wear at the end of the study.The migration of femoral head at an average period of 3.4 moriths after operation accounted for 56%of the amount of wear in the first two years.Wear rate decreased gradually with time and stabilized after the 4th year.There were no evidences of component loosening and clinical failure among there 32 hips at a latest follow-up.The average amount of linear wear was 0.81mm,and wear rate was 0.18mm/year;average cup abduction angle was 54.5 degrees.Wear direction was -6.93 degrees superolaterally.Wear direction was associated with cup abduction angle. The direction of wear turned from superomedial to superolateral gradually,as cup abduction angle increased.However,there was no correlation of linear wear amount with wear direction or cup abduction angle.Conclusions:Both creep and wear contributed to the femoral penetration into the polyethylene liner.The influence of creep cannot be ruled out especially in the early period after operation.Polyethylene wear is a multifactoriai process and the study of individual wear patterns might be useful in identifying patients who are at risk of late failure of THA.Wear direction and cup abduction angle was negatively correlated.The wear direction pointed mostly superolaterally with cup abduction angle over 55 degrees,which might result in uneven stress distribution and greater stress concentration superolaterally, consequently increase of wear;meanwhile,if the cup is too vertical,the risk of impingement and dislocation increases.The study showed that acetabular components with cup abduction angle over 55 degrees should be avoided in order to maintain hip stability and desirable range of motion.