Comparison Of Fixed-Bearing And Rotating-Platform In Total Knee Arthroplasty: Finite Element Analysis, Biomechanics In Vitro And Its Clinical Study

BackgroundTotal knee arthroplasty(TKA) has become a very effective way in curing advanced arthritis and other diseases conceming knee joint.However,in terms of implant selection the surgeon has some options concerning the polyethylene insert.TKA designs attempt to comply as closely as possible with normal knee kinematics in order that the implant will respond favourably to the soft-tissue biomechanics.Conceptually this should minimize stresses at the bearing and implant/bone interfaces and in turn minimize wear and loosening,as well as achieve better clinical function.All non-interlinked TKAs experience rotation and translation between the femoral and tibial components directed by the soft tissue and implant positioning.Current total knee prosthesis(TKP) devices can be subdivided into two groups based on different fundamental design principals:①fixed-bearing prosthesis,where the polyethylene tibial insert locked with tibial tray.Absolute rigidity of this interface is not usually achieved.The tibial plate is rarely polished due to the difficulty of combining this with an insert locking mechanism.Therefore,during function there will be relative movement of the two.This is responsible for backside wear which,in addition to wear at the articulating surface,is a source of polyethylene debris linked to osteolysis and loosening.All movements in all directions occur at the femur/tibial insert interface which is therefore subject to multidirectional wear.This necessitates a 'round-on-flat' articulation with limited congruency of the articulation and hence relatively high contact stress.②mobile-bearing designs which facilitate movement of the insert relative to the tray.In rotating-bearing group,some designs allow both anterior-posterior translation and internal-external rotation at the tray-insert interface while others rotation only is facilitated at the tray-insert counterface.Rotating-bearing has greater or total congruency of the femur/tibial insert articulation,which means increased surface area of the articulation,less contact stress and hence in principle reduced polyethylene wear.And Rotating-bearing has a large polished tibial surface to minimize backside wear,but a greater amount of motion than with fixed-bearing.It was also intended that the better approximation with normal knee kinematics(particularly in terms of rotation) would improve performance.In addition, the mobile-bearing knee also solves the kinematic conflict of fixed-bearing knee because a high conforming articular surface can now coexist with free rotation.The posterior cruciate ligament(PCL) may be retained or sacrificed in the fixed-bearing or rotating-bearing type.Although rotating-bearing design has hypothetical advantages over fixed-bearing knee, fixed and rotating-bearing in total knee arthroplasty(TKA) are still discussed controversially.In biomechanical aspect,the rotating-bearing design has proved to provide less tibiofemoral contact stresses under tibiofemoral malalignment conditions.It also provides less wear rate in in-vitro simulator test.Patients with posterior stabilized mobile-bearing knees had more axial tibiofemoral rotation than patients with posterior stabilized fixed-bearing knees during gait as well as in a deep knee-bend activity.However, in clinical aspect,both designs show excellent survival rates of up to 95%in 10-year follow-up,the mid-term or long-term survivorship of rotating-bearing knees has no superiority over that of fixed-bearing knees.The theoretical advantages for rotating-bearing design to provide a long-term durability have not been demonstrated by any outcome studies.There is a risk of rotating-bearing dislocation which,although small, would sway preference towards using fixed-bearing TKAs,given that they appear to perform just as well.Much of the published work on mobile-bearing TKAs is based on the popular LCS rotating-platform TKA which rotates around an axis through the centre of the knee.Normal knee kinematics have now been shown to rotate the knee around an axis passing through the medial condyle.Although there is no experimental work to investigate the significance of this,it may be hypothesized at this stage that this inconsistency may result in increased stress transfer to the implant/bone interface.This disadvantage may counteract the advantage of improved wear rate and hence there is no overall difference in final outcome. A physiologically functioning TKA may be a fruitless pursuit in any case as normal knee kinematics are not consistently present in the arthritic/post-TKA knee.Some movement appears to be random.Also,unless the TKA rotating or sliding mechanisms are situated in the precise anatomical locations(which is not the case with current designs) and the centres of rotation match.Because of different of variance,the chosen patients and methods in lab and experiments clinical researches,it is difficult to determine which one is better.According to the controversy between fixed and rotating-bearing in total knee arthroplasty(TKA),we designed the experiment.Firstly,we established a computer model of human knee to simulate TKA so as to analyze the influence of tibial-femoral joint. Using a computer mode,we can,on one hand,reduce cost,save time and eliminate the difference of specimen,and on the other hand accurately simulate the mechanics of living body using controlled experiment conditions.Secondly,it was considered to be a golden standard to use biological mechanic experiment on.Therefore,we used cadaver specimen to analyze the influence of tibial-femoral joint at different rotate angle of tibial prothesis. Thirdly,retrospective study was also done to compare the clinical outcomes of fixed-bearing and rotating-bearing TKA.Based on this,the study was divided into the following steps:Part One Development and validation of a three-dimensional finite element knee modelObjective:To develop a three-dimensional anatomical configuration model and a three-dimensional finite element model of the human knee and validate the kinematics, biomechanics of tibial-femoral joint and patellofemoral joint.Methods:A healthy volunteer's knee had been scanned by CT and MRI.According to the model building principle from point to line to area to volume of Mimics,Imageware and Ansys software,a three-dimensional anatomical configuration model and a three-dimensional finite element of the knee joint was reconstructed on the basis of the images of CT and MRI.In addition,quadriceps,ligaments and other soft tissues are defined as non-linear fiber.The three-dimensional anatomical configuration model simulatd knee extension activity with input variable of quadriceps forces.The three-dimensional finite element model simulatd knee squatting during dynamic loading conditions.Results:An effective three-dimensional anatomical configuration model and an explicit three-dimensional finite element model of the knee joint,including both the cortical and trabecular bone of the femur,tibia and patella,articular cartilage of the femoral condyles, tibial plateau and patella,both the medial and lateral menisci with their horn attachments, the patellar tendon,the anterior cruciate ligament,the posterior cruciate ligament,the medial collateral ligament,and the lateral collateral ligament.Some kinematics parameters,contact stress and contact area of tibial-femoral joint and patellofemoral joint are calculated based on the knee model.Compared with the published experimental results and vitro experimental results,our model is also available.Conclusion:A three-dimensional anatomical configuration model and a three-dimensional finite element model of the knee joint,which can reflect the real geometry structures of knee joint and can be used to compute some parameters of tibial-femoral joint and patellofemoral joint.The three-dimensional finite element model can be used for future studies which include the artificial knee and portions of the surrounding biological materials to investigate this interaction and can be used to predict implant biomechanical behavior under various static and dynamic loading conditions.Part Two The biomechanical influence on tibial-femoral joint of fixed-bearing and rotating-platform in Total knee arthroplasty: A finite element study Objective:To study the biomechanics of the tibial-femoral joint of fixed-bearing and rotating-platform following total knee replacement.To investigate the difference between two PFC prothesiss on the tibial-femoral contact stress and contact area.Methods:According to the size of femoral and tibial of three-dimensional finite element model of the knee joint,two three-dimensional finite element models of posterior stabilized knee prostheses were constructed,one was fixed-bearing(FB) and another was rotating-platform(RP).The polyethylene insert was modeled with elastoplastic properties. For computational efficiency,we developed rigid body analyses that can reasonably reproduce the contact stress distribution,and contact area of a fully deformable system. Five flexion angles(0°,30°,60°,90°,120°) were analyzed.Appropriate pressure mimicking the virtual magnitude and orientation of load on articular surfaces of upright weight-borne knees was applied to the model.A vertical compressive load of 300N was applied simultaneously.Results:The medial contact stress of tibial-femoral joint was larger than that of the lateral in both two kinds of prothesiss.The RP model had smaller contact stress and larger contact area than the FB model at five flexion angles(0°,30°,60°,90°,120°)(p＜0.05).The RP model had larger contact area than the FB model at four flexion angles(30°,60°,90°,120°) (p＜0.05).We also found during the excessive extension of the knee,stress concentration occurred at the interface of cam-post and the tibia prosthesis anterior area and the peak stress value increased gradually and reached the maximal 25.68Mpa at the extension of excessive-extenion 15°;this was not found in the RP model.Conclusion:Rotating-platform TKA which potentially reduce long-term polyethylene wear and minimize stabilizing post impingement by providing increased implant conformity and reduced polyethylene contact stresses and increase the potential for enhanced polyethylene longevity. Part Three Biomechanics study on rotational Alignment of fixed-bearing and rotating-platform prothesis in total knee arthroplastyObjective:To investigate the effect of tibial rotational malalignment on tibiai-femoral joint contact stress and contact area of fixed-bearing and rotating-platform.Methods:We tested 5 fresh-frozen cadaveric knees using a custom knee jig which permited the simulation of static loading conditions.An electronic resistive stress measuring sensor was used to detect the contact stress and area between the tibial-femoral joint at at five flexion angles(0°,30°,60°,90°,120°).The fixed-bearing and rotating-platform tibial prothesis rotation was first determined by the medial 1/3 of the tibial tubercle and then was internally and externally rotated from 5°to 10°and 15°, respectively.Results:When the knee was at fully extension in fixed-bearing and rotating-platform prothesiss,both the lateral and medial tibial-femoral contact stress was smallest and the contact area was largest when the tibial rotation was determined by the medial 1/3 of the tibial tubercle.The malrotation caused the decrease of the contact area and the increase of the contact stress.During the knee's flexion,the smallest contact stress were not always obtained at the tibial neutral rotational position.When compared at the same flexion and tibial rotational angles,we found the internal rotation had larger influence on the medial tibiofemoral contact stress than the lateral,and the external rotation had larger influence on the lateral tibiofemoral contact stress the medial.But at the same flexion and tibial rotational angles,the rotating-platform had smaller contact stress and larger contact area than the fixed-bearing in the lateral and medial tibial-femoral joint(p＜0.05).Conclusion:The malrotation of fixed-bearing tibial prosthesis will increase the contact stress and decrease the contact area between tibial-femoral joint during different flexion angle.But this phenomenon was not obviously in the rotating-platform,which proved the self-aligning behavior of rotating-platform can maintain large,centrally located surface contact areas at the tibial-femoral joint during both flexion-extension and axial rotation of the tibial prothesis.Part Four Recent clinical comparison of fixed-bearing and rotating-bearing total knee arthroplastyObjective:To compare the preliminary clinical outcomes of fixed-bearing prothesis and rotating-bearing prothesis by literature review and observation of cases.Methods:One hundred knees of 100 patients underwent total knee arthroplasty(TKA) by using cemented and PCL-substitute prosthesis were retrospective reviewed between January 2006 and December 2006.There were 32 males and 68 females.Fifty patients(50 knees) with an average age of 78 years(range 54-80 years) received a FB prosthesis,and 50 patients(50 knees) with an average age of 67.5 years(range 50-77 years) received a RP prosthesis.The diagnosis of all patients were osteoarthritis.All patella was routinely unresurfaced.All cases were followed up at least one year.Make sure statistical comparability between 2 groups by American Knee Society score(AKSS),range of motion(ROM),age,gender,height,weight and radiological study,respectively.Results:All patients were followed up for an average time of 15 months(range 12-20 months).Preoperatively,there were no differences between the two groups,and surgical procedure and postoperative protocol were the same for both.At the latest follow up,no significant differences of RP over FB design could be demonstrated with respect to the American Knee Society score(AKSS),pain score,range of motion(ROM),or complication rates.Radiographs showed no significant difference in prosthetic alignment or evidence of loosening.The percent of the patients' self-satisfaction was 90%in FB group and the percent of the patients' self-satisfaction was 96%in RP group. Conclusion:We conclude that at short-term follow-up there is no evidence to prove the superiority of rotating-bearing over fixed-bearing TKA with regard to the clinical and radiographic results.The reason for preferring to the rotating-bearing still need being demonstrated.