Effect Of Change Of Femoral Neck Anteversion Angle On The Mechanical Stability Of An Intertrochanteric Fracture Fixed With PFNA:A Finite Element Analysis

Objective:For comminuted intertrochanteric fractures,it is difficult to achieve anatomical reduction of the fracture fragments during operation,resulting in changes of femoral neck anteversion angle after internal fixation.At present,the relationship between the femoral neck anteversion angle and the stability after intertrochanteric fracture fixed with PFNA is not clear.In this experiment,the three-dimensional finite element analysis method was used to study the effect of the change of femoral anteversion angle on the mechanical stability of an intertrochanteric fracture fixed with PFNA,providing reference for orthopedic surgeon to deal with intertrochanteric fracture.Methods:According to the reverse engineering principle,a three-dimension model of the normal and complete human femur was established accurately by scanning the femur of normal human.Then the type of 31A1.1 intertrochanteric fractures with different femoral neck anteversion angles of 3°,5.5°,8°,10.5°,13°,15.5°,18°,20.5°,23° were created,respectively,which were assembled with PFNA internal fixation device to build the three-dimension finite element models.By using the finite element analysis software,the biomechanical differences of bone-implant models with varying femoral neck anteversion angles were study and compared.Results:The stress of intersection of main nail and helical blade,region around the lesser trochanter,the junction of helical blade and femoral lateral wall,and the intersection of distal screw and femoral shaft,and the max deformation of femoral head and PFNA of different models,overlap area of fracture surface and max gap value were calculated using finite element methods.As the femoral neck anteversion angles increased or decreased,the stresses of intersection of main nail and helical blade,region around the lesser trochanter,the junction of helical blade and femoral lateral wall,and the intersection of distal screw and medial femoral shaft became bigger generally as well as the gap between proximal and distal fracture fragment and the max deformation of femoral head and PFNA,while the overlap area of fracture surface got smaller.Specially,when the anteversion angle increased by 10°,the max stress imposed on medial femoral shaft was 276.02 MPa.And when the the anteversion angle decresed by 10°,the max stress imposed on region below the lesser trochanter reached 360.12 MPa,and the highest stress concentrated on the intersection of distal screw and femoral shaft was 232.7 MPa;when the the anteversion angle decresed by 7.5°,the max stress imposed on the junction of helical blade and femoral lateral wall reached 282.03 MPa,and the highest stress concentrated on the intersection of distal screw and femoral shaft was 230.11 MPa.All of them exceeded or approached the yield strength of cortical bone and thus led to the increase risk of fracture of femoral shaft and medial wall.When the anteversion angle decresed by 5°,the max stress that the intersection of main nail and helical blade suffered was 724.42 MPa,which was approximately equal to the yield strength of medical titanium alloy and demonstrated the potential risk of metallic fracture.And when the the anteversion angle increased or decreased the same degrees,the max stresses of intersection of main nail and helical blade,the region around the lesser trochanter and the junction of helical blade and femoral lateral wall and the max deformation of femoral head and PFNA of the finite element models with increased anteversion angle were generally smaller than that of models with decreased anteversion angle;and the overlap area of fracture surface of models with higher femoral anteversion angle were bigger than that of models with lower anteversion angle.Therefore,it illustrated that the groups with increased anteversion were better in case of mechanical stability.Conclusion:In terms of treating 31-A1.1 type of intertrochanteric fracture with PFNA,it is of great significance to obtain anatomic reduction of femoral neck anteversion angel.Change of biomechanical factors caused by insufficient reduction of anteversion may increase the risk of failure of internal fixation.In the management of intertrochanteric fracture,the femoral neck anteversion angel is supposed to increased by less than 10°and decreased by less than 5°.As for the case when the change of femoral neck anteversion angel is inevitable,an increased femoral neck anteversion might be better.