Finite Element Analysis And Biomechanical Test Of Different Treatment Methods For Pauwels Type Ⅲ Femoral Neck Fractures

Background:No consensus has been reached for the treatment of Pauwels type III femoral neck fractures,the rate of complications after the internal fixation of the femoral neck fractures is still high,which is considered as the “unsolved problems”.Recently,two alternative methods were invented to treat Pauwels type III femoral neck fractures,one of which is a new plate with a sliding groove,which was designed as a substitution of the medial buttress locking plate to combine with cannulated compression screws(CCS)for reducing the breakage possibility of the proximal locking screw during the bone healing.Another one is the femoral neck system(FNS),which was believed with biomechanical superiority.This study aims to compare the biomechanics of these two new implants with three previous methods(CCS BDSF CCS+BL)via finite element analysis(FEA)and biomechanical test(BT)to validate whether they are suitable for the treatment of Pauwels type III femoral neck fractures.Methods:1.FEA: Five 70-degree Pauwels type III transcervical femoral neck fractures(AO/OTA 31B2.3r)models were built and fixed by CCS augmented with the newly designed sliding groove buttress plate(CCS+BS)and FNS.For comparison,models fixed by three parallel cannulated compression screws(CCS),biplane double-supported screw fixation(BDSF),CCS augmented with a medial buttress locking plate(CCS+BL)were also built.A 2100 N load was applied along with the mechanical axis.Parameters of the maximal stress as well as the maximal displacement of the implants and bone,the maximal relative displacement of interfragments,and the stiffness,were analyzed to compare the biomechanical characteristics of the five models.2.BT: Fifty identical synthetic femurs(Synbone LD2350,Synbone AG,Switzerland)were used in this biomechanical study.The length of the femur is 460 mm,with a 120° neck-shaft angle,18° anteversion,49 mm head diameter,and 10 mm canal diameter.The femurs were equally divided into four groups and fixed with femoral neck system(FNS),cannulated compression screws in inverted triangle configuration(CCS),biplane double-supported screw fixation(BDSF),and CCS with medial buttress locking plates(CCS+BL),respectively.A customized 3D-printed guide plate with a 70-degree Pauwels angle was designed and applied in the osteotomy to ensure consistency,then we used the guide plate to build models of Pauwels type III transcervical femoral neck fractures(AO/OTA 31-B2.3r).Besides,to make the standard configuration of different implants,we also printed the customized guide plate of the guidewire in each group.The axial stiffness,torsional stiffness and the relative displacement of fracture fragments under the load of 700 N were analyzed.Results:1.FEA: CCS+BS and CCS+BL showed stronger fixation strength with improved stiffness(1012.05N/mm,1092.04N/mm),reduced maximal displacement of the implants(1.976 mm,1.838mm)and bone(2.075 mm,1.923mm),when compared with CCS(925.11N/mm,2.158 mm and 2.270mm)and BDSF(842.36N/mm,2.299 mm and2.493mm).While FNS showed the weakest stiffness(593.22N/mm)and largest maximal displacement of the implants(3.234mm)and bone(3.540mm)among the five models.2.BT: For CCS+BS,the axial stiffness,torsional stiffness,and the relative displacements between fracture fragments were 99.27±4.85 N/mm,0.52±0.09 N·m/(°),1.41±0.33 mm when the load was 700 N,respectively.CCS+BL were 100.95±7.01N/mm,0.57±0.07 N·m/(°)and 1.19±0.28 mm,respectively.The results of CCS+BS and CCS+BL were similar without statistical difference.They were better than CCS(89.48±2.39 N/mm,0.34±0.05 N·m/(°),1.53±0.43mm)and BDSF(87.18±1.01 N/mm,0.38±0.03 N·m/(°),1.66±0.82mm),and the difference was statistically significant.The results of FNS were 84.63±0.86 N/mm,0.22±0.04 N·m/(°)and 2.29±0.44 mm,respectively,which were weaker than the above four fixation methods,and the difference was statistically significant.Conclusions:In the case of this type of femoral neck fracture,CCS+BS has biomechanical properties comparable to CCS+BL and better biomechanical properties than CCS and BDSF,which can prevent the fracture of the proximal locking screw of CCS+BL during fracture healing,providing a new option for the treatment of this type of P3 FNFs.However,the biomechanical stability of FNS is weaker than that of CCS,BDSF,CCS+BS and CCS+BL,so it is not suitable for the fixation treatment of this type of femoral neck fracture.This study provides a reference and a new method for the clinical treatment of femoral neck fracture.