Impact Of Joint Line Malposition On Biomechanics Of Patello-femoral Joint After Total Knee Replacement: A Finite Element Study

Background The complications of patella-femoral are the most common after total knee replacement and half of non-infected revisions are caused by these.The complications include fracture of patella,abnormality of movement,instability of patella-femoral joint (dislocation or semiluxation of patella),fragmentation of quadriceps tendon and patellar ligament.The complications mentioned above are caused by high pressure of patella-femoral joint and abnormality of movement.Except for design,the efficient cause is malalignment after surgery,includes malalignment of rotation and axial direction of tibia-femoraL malpositon of joint line.The literature review shows there are many studies focus on malalignment and research methods and conclusions are mature,but there is few research on impact ofmalposition of joint line on biomechanics of patella-femoral joint.Regardless of primary or revision total knee replacement,the incidence of malposition ofjoint line is high.Many retrospective analysis' found out that step up ofjoint line is relevant to anterior knee pain and limitation of movement.In theory,malposition of joint line changes the normal position of patella-femoral.It means that the contact position and contact areas of patello-femoral joint have been changed.According to these,mechanics of patello-femoral joint,load force of patellar ligament and quadriceps tendon,contact stress of patella-femoral, the movement of patella are all changed,mentioned above are relevant to soft tissue tension in extension and range of motion.Knee is the most complex joint all over the body,it has flexion-extension,internal and external rotation.Condyles of femur can roll over,rotation and sliding on tibial plateau.After malposition of joint line occurred,the movements of knee become more complex and make the related research very difficult.And that is why previous studies are most limited to clinical observation and analysis,few research biomechanics by using cadaver.Limited by cadaver and research methods,they can't simulate real physiological state of the movement and load force.The conclusions are quite different.Thus, the impact of malposition of joint line on biomechanics of patella-femoral joint is still unknown.The fundamental question is alignment of patella-femoral,that is means the contact of patella and femur.The past studies on the subject,whether they use cadaver,mathematical model,two-dimensional model or finite element model,are related to X-rays.In particular circumstances or conditions the research can't carry on in view of these depend on cadaver specimen or X-rays.The conclusions are not comparable even if use the same methods and conditions due to different models.Thus,to establish a standard three-dimensional finite element model is necessary.And software can be used to simulate and analyses biomechanics of patello-femoral joint in particular circumstances.Objective 1.Find out a method is not depending on specimen or X-rays to evaluate alignment of patella-femoral joint.And then confirm the model validation.2.Establish the three-dimensional finite element model according to public data.Then simulate by software to analyses impact of malposition of joint line on quadriceps tendon and patellar ligament force in extension and contact force of patella-femoral joint and so on.Methods 1.The three-dimensional finite element model of intact knee joint and joint with prosthesis were originated from Visible Human Project(VHP),all data from CT scan. Mathematical formulas were used to simulate load force of patella ligament and quadriceps tendon,which material properties was defined as nonlinear cable units.The insertions and the direction of patellar ligament and quadriceps tendon referred to literatures and general anatomy.2.(1) Another three-dimensional finite element model of an intact knee joint was originated from a healthy volunteer,by using CT and MRI scan.After that,the verified formula which used to finite motion of pattern was applied to define motion of this knee. Then the contact areas between patella and femoral trochlea in a defined position can be operating by computer.(2) Bi-plane X-ray images used to locate the position of healthy volunteer's knee.Using three-dimensional finite element model to coincide with X-ray image, the contact areas between patella and femoral trochlca in the defined position can be operating by computer.Finally the two results mentioned above used for comparation.3. Using the universal 3-D finite element model with prosthesis to analyses the load force of extensor,contact force of patello-femoral joint when joint line changed from -4 to+6mm in 2mm increments.Results 1.A universal finite element knee model with or without prosthesis was built using public data.2.The comparison of the two methods used to evaluate contact areas was not statistically significant.3.The impact of malposition of joint line on biomechanics of patella-femoral joint after TKA was significant,i.e.,The quadriceps tendon and patellar ligament tensile force and the patellofemoraljoint force only slightly changed with the decline of the joint line,and significantly increased when the joint line is elevated over 4mm.Conclusion 1.The method we use to establish a 3-D finite element model was proved to be convenient,fast,precise and universal.2.The verified formula used to finite motion of pattern can be used for model analysis.3.More than 4mm elevated of the joint line should be avoild.