Analysis Of Tibial Prosthesis Alignment And Stability In Total Knee Arthroplasty Based On Three-dimensional Modeling And Finite Element Analysis Technique

During total knee arthroplasty(TKA),the proper placement of tibial prosthesis is of great significance on the survival rate of prosthesis and functional recovery of patients after replacement.In clinical practice,the position of tibial prosthesis is determined primarily according to the tibial coronal alignment and axial rotation alignment in the process of TKA.In this regard,an accurate determination of the coronal alignment and axial rotation alignment has become the key of the success of TKA.At present,multiple studies have been reported concerning the influence of the proposed two factors misalignment on the stability of tibial prosthesis.However,there are few studies emphasizing on the relationship between tibial rotation and tibial alignment.The major way to determine the tibial coronal alignment is still based on measuring the full-length X-ray film of both lower limbs currently.In addition,some hospitals are unable to obtain enough length of plain film due to limited practical conditions.Therefore,it is necessary to develop a possible approach to predict the tibial coronal alignment by X-ray film of proximal tibiofibula merely.As supported by accumulated clinical work experiments,the author found the persisted existence of sclerotic bone in the medial proximal tibia of osteoarthritis(OA)patients with knee varus deformity after tibial cut in TKA.Meanwhile,there is a lack of study on whether the sclerotic bone influence the stability of prosthesis after TKA.Furthermore,it has been recognized clinically by some orthopaedic surgeons that the tibial prosthesis may contribute to the increased stability of the prosthesis by adding an extension stem.However,there is no evidence to prove the biomechanical effect of tibial prosthesis stem length so far.Simultaneously,there is no accurate analysis on the relation between the length of extension stem and the stability of prosthesis.With respect to the above,the present study was carried out to demonstrate the effect of tibia self-rotation on the alignment after TKA by three-dimensional reconstruction.We tried to analyse a new marking line of tibiofibula that can be drawn in the proximal tibia X-ray plain film by the anatomic mark of the proximal tibiofibula.It showed relatively good reliability to predict tibial coronal alignment even under the interference of different rotation conditions of tibiofibula in the posteroanterior plain film of proximal tibia.By using a finite element biomechanical model,the influence of different thickness of proximal tibial sclerotic bone on the stability of tibial prosthesis was studied under different alignment conditions.Meanwhile,an accurate analysis was made focusing on relationship between the length of tibial prosthesis stem and the stability of prosthesis.It is expected to provide experimental theoretical basis for the selection of tibial prosthesis,the determination of better prosthesis position and the prediction of tibial alignment in TKA.Besides,the research findings may also lay potential biomechanical theoretical foundation for preventing the tibial plateau prosthesis from loosening and improving the survival rate of prosthesis in TKA.The present study is divided into the following five parts:Part I.The establishment and verification of three-dimensional geometric model of tibia and finite element biomechanical model after TKA.In this part,volunteers who were planned to receive TKA due to knee OA were provided with both lower limbs thin-layer CT in our department.The obtained images of tibiofibula were screened by using Mimics software,followed by three-dimensional processing,modification to obtain the three-dimensional model of tibiofibula.Based on the three-dimensional model,3-matics software was then used to simulate tibial cut in TKA and prosthesis installation.Furthermore,the postoperative model of TKA was imported into Hyper Mesh software for meshing process and add material assignment,boundary condition and load.Afterwards,the finite element biomechanical model was obtained by post-processing.In addition,the stress analysis results of the model were compared with the constructed biomechanical models in other similar studies,which in turn verified the accuracy and reliability of the established three-dimensional model and biomechanical model of our experiments.Collectively,it may lay research foundation for the subsequent three-dimensional model measurement, biomechanical experiment and analysis.Part 2.Analysis of the effect of tibial torsion on tibial osteotomy in knee arthroplasty using a three-dimensional computed tomography-based modelling techniqueObjectives: Extramedullary systems are commonly used in knee arthroplasty,with the stem location being determined from the tibial torsion line during surgery.The traditional method for tibial torsion measurement is not in accordance with clinical practice.This study aimed to evaluate proximal and distal tibial torsion using 3-dimensional(3D)computed technology to establish a new evaluation method,as well as to investigate the association between tibial torsion and postoperative alignment deviation.Methods: Fifty-five osteoarthritis tibias with >10°varus preoperatively were divided into valgus,neutral,and varus groups based on their postoperative alignment deviation.A new method based on clinical practice was built using a 3D tibial model.Proximal and distal tibial torsions were measured by both the new and traditional methods.In addition,tibial osteotomy that followed the intramedullary osteotomy system was simulated on the 3D model in the varus and valgus groups to investigate the association between tibial torsion and alignment deviation.Results: Proximal tibial torsion was smaller and distal torsion was greater in the valgus group than the other two groups,according to the new method(P= 0.03 and P= 0.02,respectively).No significant difference was found when comparing these torsions by the traditional method(P= 0.782 and P= 0.753,respectively).In the valgus group,the postoperative alignment deviation improved after simulated osteotomy guided by the intramedullary system,while no significant improvement was found in the varus group.Conclusion: According to this new tibial-rotation evaluation method,valgus deviation in knee arthroplasty was identified as the main cause for knees in which the proximal tibial internal torsion is too small and the distal external torsion is too great.The use of an intramedullary system may help reduce this deviation.Part 3.The Fibular assessment line of the proximal fibula as a reliable landmark for the tibial mechanical axis in total knee arthroplastyObjectives: This study aimed to compare the accuracy of our newly defined fibular assessment line(FA)with other fibular marker lines analyzed in previous studies in predicting the tibial mechanical axis,and to investigate the influence of tibiofibular rotation on the reliability of each fibular marker line.Methods: 30 3D tibiofibular models reconstructed from the computed tomography(CT)images were collected.Each model was rotated along the tibial mechanical axis to simulate the 0° neutral position,5° and 10° internal rotation position,5°and 10° external rotation position,and the projections of the models in the coronal plane of the coordinate system were used to simulate the anteroposterior tibiofibular radiograph at different rotational angles.We measured the angles between the tibial mechanical axis and four different fibular marker lines at each tibiofibular rotation angle to compare the predicting reliability.Results: Except for the 10° external rotation,the angles between the FAA and the three other fibular markers were statistically different(p < 0.001-p < 0.049),and there were no statistical differences in FAAs measured on projections simulating the anteroposterior radiographic view at different rotational angles of the tibia and fibula(p > 0.05).Except for the 10° external rotation,the FA has the highest reliability rates among the four fibular marker lines.Conclusions: The fibular assessment line can predict the tibial mechanical axis more accurately.When filming with the tibiofibula in the neutral or moderate internal rotation,the fibular assessment line can also predict the tibial mechanical axis with good accuracy.Part 4: Finite-element analysis of the proximal tibial sclerotic bone and different alignment in total knee arthroplastyObjectives: This study aimed to determine:(1)the distribution range of tibial sclerotic bone in patients with severe genu varum using three-dimensional measurements,(2)the effect of the proximal tibial sclerotic bone thickness on prosthesis stability according to finite-element modelling of TKA with kinematic alignment(KA),mechanical alignment(MA),and 3° valgus alignment,and(3)the effect of short extension stem augment utilization on prosthesis stability.Methods: The sclerotic bone in the medial tibial plateau of 116 patients with severe genu varum was measured and classified according to its position and thickness.Based on these cases,finite-element models were established to simulate 3 different tibial cut alignments with 4 different thicknesses of the sclerotic bone to measure the stress distribution of the tibia and tibial prosthesis,the relative micromotion beneath the stem,and the influence of the short extension stem on stability.Results: The distribution range of proximal tibial sclerotic bone was at the anteromedial tibial plateau.The models were divided into four types according to the thickness of the sclerotic bone: 15 mm,10 mm,5 mm,and 0 mm.The relative micromotion under maximum stress was smallest after MA with no sclerotic bone and largest after KA with 15 mm sclerotic bone.Relative micromotion was largest with KA and smallest with MA in sclerotic models with the same thickness.Relative micromotion increased as thickness of the sclerotic bone increased with KA and MA(R = 0.937,P = 0.03 and R = 0.756,P = 0.07,respectively).Relative micromotion decreased with short extension stem augment in the KA model when there was proximal tibial sclerotic bone.Conclusions: The influence of proximal tibial sclerotic bone on prosthesis’ s stability is significant,especially with KA tibial cut.Tibial component’s short extension stem augment can improve stability.Part 5: Finite element analysis of the effect of tibial prosthesis extension stem length in total knee arthroplasty in the Chinese populationObjectives: This study aimed to determine the longest usable range of tibial prosthesis extension stems in Chinese patients undergoing primary total knee arthroplasty and to analyze the effect of different stem lengths on prosthesis stability within this range.Methods: We conducted three-dimensional modeling and simulated surgery in patients with genu varum to measure the longest usable range of tibial prothesis stems,identify impinged cortices under tibial posterior slope cut of 0° and 3°,and analyze factors influencing the longest stem length.We built finite element models according to the longest usable range of extension stems to simulate tibial prostheses with different stem lengths,measure the stress distribution of tibias and prostheses and the relative displacement of distal ends of prostheses,and investigate the effect of different stem extension lengths on prosthesis stability.Results: We simulated osteotomy with a tibial posterior slope cut of 0° and 3°,under which the maximum tibial prosthesis stem length was 83 mm(79±24 mm).The simulated tibial cut with a tibial posterior slope of 3° indicated the maximum tibial prosthesis stem length to be 83 mm(83±20 mm).According to the longest usable range of extension stems,we defined five groups for finite element analysis with 40-mm,50-mm,60-mm,70-mm,and 80-mm stem lengths and analyzed each group for posterior slopes of 0° and 3°.The 80-mm stem length models showed minimum relative displacement of the distal end of tibial prosthesis(0°: 2.63,1.61±0.05 μm;3°: 1.48,1.44±0.09 μm),whereas the 40-mm stem length models showed maximum relative displacement(0°: 3.16,3.19±0.12 μm;3°: 1.84,1.81±0.07 μm).As the length of tibial prosthesis stems increased from 40 to 70 mm,the relative displacement of the distal end of prosthesis decreased for both posterior slopes but was insignificant when stem lengths increased to 70–80 mm.Conclusions: Based on the results,we suggest that using the longest tibial stem is not always necessarily a better option to increase stability,as the prosthesis shows greater stability in only a specific range of increased length but shows insignificant change when the length is greatly increased.