The Biomechanics Study And Finite Element Modeling Of Kinematically Aligned Total Knee Arthroplasty

BackgroundThe knee is the largest joint,even the most complicated and functional demanding joint in human body.The complex anatomical structure and mechanical environment of the knee joint make it vulnerable to trauma and diseases such as osteoarthritis,so the incidence of knee joint diseases is the highest one among joint diseases.Total knee arthroplasty（TKA）,also known as total knee replacement（TKR）,is a surgical procedure to replace the damaged weight-bearing surface of the knee joint to the knee prosthesis.It is one of the most important surgical progresses in 20 th century.TKA is able to restore proper alignment of lower limb,relieve pain and disability,greatly improved the patients’ quality of life.TKA is not only a bony but also a soft-tissue surgery.Accurate reconstruction of lower limb alignment and correct position of prosthetic components are essential for the success of TKA.As always,much more attention has been given to the alignment of the knee prosthetic components.Recently,in order to achieve neutral mechanical alignment,a large number of medical and healthcare resources and enormous energy have been devoted to the development and use of computer-aided surgery navigation and patient specific instruments.The classic alignment is based on the traditional concepts that favorable postoperative alignment of the lower limb should be within 3° of a neutral mechanical axis（MA）.Several recent studies have reported that there was no significant difference in survivorship when a traditionally long-held safe zone of 0°±3° was used to define well aligned and malaligned TKAs.With modern surgery techniques and TKA implants,some surgeons and researchers have debated describing alignment as aligned or malaligned according to a mechanical axis（MA）of 0°±3°.Howell et al have supported the concept of a kinematic alignment（KA）and Kinematically Aligned TKA（KA-TKA）.Kinematic alignment（KA）is the restoration of the pre-arthritic or normal joint line and rotational axis of the lower limb.Kinematically Aligned TKA（KA-TKA）aligns the femoral and tibial prosthetic components to the “normal” or prearthritic joint line of the knee in order to improve the motor function of the patients’ affected lower extremity.The popularity of KA-TKA reflect concern with the high rate of dissatisfaction after MA-TKA（mechanically aligned TKA）.While lower limb and prosthetic components alignment certainly may contribute to these dissatisfactions,there are many other factors such as patient,surgeon,and environmental factors that play a significant factor,but as yet undefined.From a conceptual standpoint,KA-TKA is certainly intriguing to reproduce the anatomy of knee.However,the clinical and basic research data on which such an approach is based,is not enough.Part 1.Human knee joint and TKA prosthetic components geometric and finite element modelingObjective: To develop a simulation geometric model and finite element model of human knee joint and knee prosthetic components.Methods:A healthy adult volunteer’s left lower limb and knee had been scanned by CT and MRI.After the medical image data was obtained,a simulation geometric model of human knee joint was developed by softwares such as Mimics15.0 and UG NX 7.0.On the basis of this geometric model,the finite element model of knee joint was established by using finite element software ABAQUS 6.8.Reverse engineering technology and software Geomagic Studio 9.0 were used to reconstruct the geometric model of the knee prosthetic components（Genesis II total knee system,posterior stabilized version,Smith & Nephew Inc.）.The prosthetic components models and the knee model were assembled to obtain the geometric model and finite element model of the knee joint after TKAResults:Anatomic simulation geometric model and finite element model of human knee joint and artificial knee which were suitable for biomechanical analysis were obtained.Conclusions:A simulation geometric model and finite element model of human knee joint and artificial knee which based on CT and MRI images and reverse engineering technology can reproduce the anatomical structures and appearance of human knee and TKA prosthesis.The models can be used to TKA procedure simulation and biomechanical analysis.Part 2.The Biomechanical Influence on Knee Prosthesis in Kinematically Aligned TKA and Mechanically Aligned TKAObjective:To study the biomechanics of the knee prosthesis of kinematically aligned TKA and mechanically aligned TKA.To evaluate the contact stress and strain of artificial knee in kinematic and mechanical alignment.Methods:Randomly selected 12 healthy adult volunteers,developed the left knee joint simulation geometry models and finite element models by the method described in part1.According to kinematic alignment（KA）and mechanical alignment（MA）respectively,assembled the suitable size of knee prosthesis to the knee joint finite element models.Simulated the mechanical condition of stand by single foot.Evaluated the contact stress and strain of artificial knee in kinematic and mechanical alignment by finite element analysis software ABAQUS.Results:In the MA-TKA models,the maximum stress on the femoral prosthesis contact surface was 38.25±2.66 MPa,and the maximum stress on the surface of the polyethylene tibial components was 12.07±1.67MPa;however,in the KA-TKA models,the maximum stress decreased respectively to 30.37±2.76 MPa and 8.14±2.49 MPa.In the MA-TKA models,the maximum strain of the femoral prosthesis surface was 1.51x10^-4±1.09x10^-5mm,and that of the polyethylene tibial components was 3.43 x x10^-3±5.12x10^-4mm;While in the KA-TKA models,the maximum strain reduced to 8.18x10^-5±1.05x10^-5mm and 2.10 x10^-3±3.86x10^-4mm.The difference was statistically significant（P<0.05）Conclusions:Compared with mechanical alignment,kinematic alignment in TKA can decrease the maximum stress and strain of the TKA prosthesis,the stress distribution on the artificial knee contact surface is more homogeneous,thereby reduce the wear and tear of artificial knee joint and improve the durability following TKA.