| Background: It has been shown that more than 5% of total knee arthroplasty patients undergo revision surgery within 10 years after surgery,and that the lifespan of the prosthesis is shorter than its design.The reasons for this may be associated with abnormal kinematic patterns due to the wear of the insert and uneven stress distribution on the surface of the prosthesis.Therefore,in addition to the continuous development of new materials with better wear resistance,precise force line recovery has always been a goal that clinicians strive to achieve in order to effectively extend the service lifespan of the prosthesis.There have been numerous studies on the effects of coronal and rotational alignment on functional recovery and prosthetic service lifespan after knee arthroplasty,but relatively few reports have focused on sagittal prosthesis position,and among those that have been published,more attention has been paid to the tibial side,neglecting the effect of poor femoral alignment on the mechanical response between prosthetic components.Because of the obvious individual differences in the distal femoral flexion angle,it is impossible to simply follow a uniform standard for osteotomy assembly,and most commercially available joint prostheses are designed and manufactured according to the anatomical data of European and American population samples,which often do not match perfectly with the national joint size,so that in the actual surgical process it is often required to select the closest prosthesis model depending on the operator’s judgment,thus making it impossible to strictly follow the standard surgical procedure and relatively difficult to restore the sagittal force line of the femoral prosthesis.Theoretically,placement of the femoral prosthesis in the flexion position increases the mobility of the artificial knee joint,but also increases the risk of flexion contracture,while placement in the hyperextension position may lead directly to anterior femoral notching.Although these findings have been corroborated by individual observational clinical studies,an experimental model thatcan be interpreted from a biomechanical perspective remain elusive.Among the existing biomechanical research methods,finite element analysis is playing an increasingly important role in the design and mechanical evaluation of orthopedic implants due to its advantages of accurate simulation of complex mechanical environments and non-invasive as well as reproducible operation.Based on this,we propose to establish a 3D finite element model of the knee joint of a local population sample,verify the validity of the model,simulate total knee arthroplasty with simulated osteotomy and prosthesis assembly,developing a 3D geometric model of the artificial knee with different femoral prosthesis flexion angles,and investigating the effect of the abnormal sagittal angle of the femoral prosthesis on the biomechanical characteristics of the artificial joint(including the tibiofemoral joint and patellofemoral joint)with the help of finite element analysis,in order to provide a theoretical basis for the reasonable placement of the femoral prosthesis during surgery.Part I.Establishment and validation of 3D finite element model of natural kneeObjective: To establish a 3D finite element model of the human normal knee joint and verify the validity of the model to lay the foundation for the subsequent biomechanical study of the artificial knee joint.Methods: A healthy adult volunteer was randomly selected to obtain the image data of the lower extremity by CT scan,and the geometric anatomical model of the natural knee was reconstructed by using image processing software such as Mimics,3-matic,and Geomagic studio,followed by meshing,material assignment,and defining contact of the model by using ANSYS Workbench to finally obtain the the 3D finite element model of the natural knee.The same boundary conditions and loading methods as in previous studies were used to compare the mechanical response of the model with previous experimental results to verify the validity of our model.Results: A 3D finite element model of the normal knee joint was successfullyestablished based on CT image data and computer-aided design software,which is consistent with the human anatomy and can realistically reproduce the real structure of the knee joint,and some mechanical parameters of the model were calculated by loading conditions,and the results showed that the experimental model is accurate and effective when compared with the experimental data in previous literature.Conclusion: The established 3D finite element model of the natural knee is suitable for subsequent biomechanical studies.Part II.Finite element analysis of the effect of sagittal plane placement angle of the femoral prosthesis on the biomechanics of a total knee replacement modelObjective: To develop a total knee replacement model with different femoral prosthesis flexion angles and to investigate the effect of femoral prosthesis placement at abnormal sagittal angles on the biomechanical characteristics of the artificial knee joint by means of finite element analysis.Methods: A 3D finite element model of the artificial knee with the femoral prosthesis flexed-2°,2.3°,5° and 7° was established by adjusting the distal osteotomy surface on the previously established natural knee geometry model using 3-matic,ANSYS Workbench and other software to simulate total knee arthroplasty.Subsequently,the working conditions under upright and flexed knee conditions were simulated,and the trends of contact pressure in the tibiofemoral and patellofemoral joints of the artificial knee under different femoral prosthesis flexion angles were analyzed statically,using the peak von-Mises stress as the observation index,respectively.Results: The peak von-Mises stress of the femoral prosthesis in the upright positiongradually increased with the increase of the femoral prosthesis flexion angle,from 4.523 MPa to 7.148 MPa,and the stress was concentrated in the circular-like region of contact between the femoral prosthesis and polyethylene,which shifted forward with the deepening of the prosthesis flexion;The peak von-Mises stress on the upper surface of the polyethylene liner also increases with the increase of the prosthesis flexion angle,up to 13.622 MPa when the prosthesis is flexed at 7°,located at the front of the liner column,and the trend of the stress concentration area is consistent with that of the femoral prosthesis.When the knee joint is flexed at 30°,the stress is mainly concentrated on the medial patellofemoral joint surface;as the knee flexion angle increases to 60°,the stress concentration area shifts upward,and the stress on the medial and lateral patellofemoral joint surfaces increases.At the same knee flexion angle,when the prosthesis was flexed greater than 0°,the peak von-Mises stress in the patellar cartilage gradually increased with the increase in the degree of prosthesis flexion,and the peak stress in the-2° prosthesis flexion group was significantly higher than the other groups.Conclusion: The placement angle of femoral prosthesis in sagittal plane is an important factor affecting the biomechanics of knee prosthesis.In the PS-TKA model,the contact pressure of patellofemoral joint will increase regardless of the overflexion or extension of the prosthesis,while the peak stress of the insert surface is greatly affected by the overflexion of the prosthesis.In clinical practice,when there is a mismatch between the prosthesis type and the patient’s joint size,mild flexion of the femoral prosthesis can be considered,but overflexion and extension of the prosthesis should be avoided. |
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