| Background:Pertrochanteric femoral fracture is common for the elder people.Internal fixation with an intramedullary nail is popularly used for unstable pertrochanteric femoral fractures because of its biomechanical merits,with the mechanical axis much closer to the central axis of the femoral bone,and a shorter force arm.The AO/OTA type 31-A2 fracture of 2018 version is the most common unstable pertrochanteric femoral fracture.This type of fracture is challenging due to both the medial and lateral wall of the proximal femoral are unstable.In the prerequisite of the bone quality and fracture type are unchangable,the operational effect of the PFNA-Ⅱ fixation for type 31-A2 fractures can only be optimized by continuously exploring more effective fracture reduction and internal fixation methods.The finite element analysis has been widely used in the research related to pertrochanteric femoral fractures,and the reliability of the research results has been certificated.Three-dimensional models of the fracture and implant can be made using the finite element technology.And the method of biomechanical analysis can be used to conduct in-depth research on the stability of different fracture fixation patterns of type 31-A2 pertrochanteric femoral fractures by the PFNA-Ⅱ.The related research could provide a reference for optimizing clinical treatment plans.Purpose:(1)To explore the reasons for the postoperative fixation failure of type 31-A2 femoral pertrochanteric fractures treated with PFNA-Ⅱ.(2)Based on the adult femoral CT scanning data,according to the characteristics of the type 31-A2 pertrochanteric femoral fracture,design and establish a corresponding three-dimensional finite element model.(3)Use the three-dimensional finite element model to simulate different fracture fixation patterns using the PFNA-Ⅱ with a different length.Compare the biomechanical stability of the fracture models in different groups under the circumstance of single-leg-standing weight bearing.To provide a reference for optimizing the fixation method using PFNA-Ⅱ during the operation.(4)Use the three-dimensional finite element model to simulate different fracture fixation manners under different anteromedial cortical support reduction patterns.Compare the biomechanical stability of the fracture models in different groups under the circumstance of single-leg-standing weight bearing.To optimize the fracture reduction method through this study.Methods:(1)A retrospective analysis of 149 cases with type 31-A2 femoral pertrochanteric fractures with PFNA-Ⅱ fixation from January 2016 to December 2020 in the Department of Trauma Surgery in East Hospital Affiliated to Tongji University was performed.There were 52 males and 97 females,aged 43-96 years [(77.9±11.1)years].The types and number of internal fixation failures were observed and documented.(2)A 65-year-old healthy male volunteer was recruited with no hip and systemic diseases.High-resolution CT scanning images of his right femur were collected.These images were imported into Mimics 19.0.The 3D model of the right femur was obtained and incorporated into Geomagic 12.0 for smoothing,meshing and fitting surface processing.Subsequently we constructed the proximal femur model in Creo 2.0 to mesh and simulate the AO/OTA type 31-A2 pertrochanteric femoral fracture.The PFNA-Ⅱ with the length of 170 mm,200 mm,240 mm,and 380 mm were used,including a helical blade,an intramedullary nail and distal locking screws,which were fabricated with Creo 2.0.According to the length of the main nail,6 groups were defined,which include ultra-short group(170 mm),short group(200 mm),standard group(240 mm),lengthened group A(380 mm,1 screw positioned in the proximal static locking hole),lengthened group B(380 mm,1 screw positioned in the distal static locking hole),and lengthened group C(380 mm,2 screws positioned both in the proximal and distal locking hole).According to the material properity,boundary conditions,and loading conditions in the literature,the hip loading of an individual standing on one leg with the body weight of 700 N was simulated.The models were incorporated into ANSYS 15.0 for analysis.The von Mises Stress(VMS)distribution and peak VMS of the PFNA-Ⅱ,femoral shaft and head-neck fragment in different models was also measured and diagramed by nephogram.The deformation displacement of different models was measured to evaluate the system stability.The relative displacement of the head and shaft fragments between the fracture ends was calculated by measuring the displacement in each direction of XYZ axis to evaluate the support effects.These parameters were used to capture the mechanical factors involved in the fixation stability and fracture healing.(3)Using the established fracture model,the PFNA-Ⅱ with the length of 240 mm was chosen for the next analysis.According to the concept of different patterns of anteromedial cortical supports,the simulated fracture reduction patterns were divided into 9 groups according to 9 combinations of medial and anterior cortical supports in the AP and lateral view: positive-positive,positive-neutral,positive-negative,neutral-positive,neutral-neutral,neutral-negative,negative-positive,negative-neutral,negative-negative groups.The relative VMS and displacements of different groups were analyzed to compare the fixation stability.Results:(1)Internal fixation failure occurred in 11 cases,with a failure rate of 7.38%.Two cases had helical blade cutting out,1 case had helical blade penetrating out,6 cases had helical blade withdrawal,3 cases had peri-implant fractures,coxa varus occurred in 7 cases and femoral neck shortening occurred in 8 cases.(2)Under the load of one-leg-standing weight bearing,the maximum VMS of the entire fracture fixation model in the six groups was consistent with the maximum VMS of the PFNA-Ⅱ main nail.The stress was mainly concentrated at the junction of the main nail and the helical blade.The peak VMS gradually decreased with the length of the nail increased.The peak VMS of the ultra-short group was the largest(402.29MPa),and the peak VMS of the lengthened group C was the smallest(271.95MPa).The stress concentration area of the femoral shaft was located in the lateral cortex of the middle and lower part of the femur.The peak VMS showed a decrease trend with the increase of the length of the main nail.The peak VMS of the ultra-short group was the largest(62.07MPa),and the peak VMS of the lengthened group A was the smallest(35.04MPa).The stress concentration area of the head-neck fragment was located at the lateral aspect of the junction of the great trochanter and the main nail.The peak VMS of the ultra-small group was the largest(45.59MPa),and the standard group was the smallest(24.73MPa).The maximum displacement gradually decreased with the length of the main nail increased.The largest maximum displacement was shown in the ultra-small group(4.2325mm),and the smallest maximum displacement was in the lengthened group A(3.9016mm).The relative displacement between the bone fragments of the small group was the largest(0.5400mm),and the relative displacement of the lengthened group A was the smallest(0.5274mm).(3)Under the load of one-leg-standing weight bearing,the maximum VMS of the entire fracture fixation model in the 9 groups was consistent with the maximum VMS of the PFNA-Ⅱ main nail.The stress was mainly concentrated at the junction of the main nail and the helical blade.The negative-negative group had the largest peak VMS(442.03Mpa),and the positive-positive group had the smallest peak VMS(356.77MPa).The stress concentration area of the femoral shaft was located in the middle and lower part of the femur.The negative-neutral group had the largest peak VMS(52.55Mpa),and the positive-negative group had the smallest peak VMS(34.09MPa).The stress concentration area of the head-neck fragment was located at the lateral aspect of the junction of the great trochanter and the main nail.The negative-negative group had the largest peak VMS(101.74MPa),and the neutral-negative group had the smallest peak VMS(21.89MPa).The largest maximum displacement(4.2770mm)and the largest relative displacement between the bone fragments(0.6272mm)were found in the negative-negative group.The smallest maximum displacement(4.0878mm)and the smallest relative displacement between the bone fragments(0.4431mm)were found in the positive-positive group.Conclusion:Postoperative stability loss is an important reason for the fixation failure of type 31-A2 femoral pertrochanteric fractures with PFNA-Ⅱ fixation.Postoperative stability can be improved and the rate of fixation failure can be reduced by optimizing fracture reduction and internal fixation implantation,.Under the premise of obtaining a good reduction,both the short and long PFNA-Ⅱ are suitable for fixing the type 31-A2 pertrochanteric femoral fracture.However,the risk of fractures around the implants using the short PFNA-Ⅱ is higher than the long PFNA-Ⅱ.The fixation stability can be enhanced by increasing the length of the main nail so as to reduce the risks of fractures around the plants.From the perspective of biomechanical stability,the optimized method of fixating type 31-A2 pertrochanteric femoral fractures using PFNA-Ⅱ is as follows: choose a longer PFNA-Ⅱ,only one interlock screw is required,and the screw is better implanted into the proximal static hole.It is recommended to choose a standard PFNA-Ⅱ for fixation if a short nail is required.The positive cortical support in both the medical and anterior cortical support reduction patterns tended to exert a better effect on maintaining the stability between the head-neck fragment and femoral shaft fragment.And the positive cortical support reduction seemed to maintain the total stability of the assembly much better.The negative-negative reduction might be more easily to cause loss of fracture reduction.During postoperative rehabilitation,the negative-negative reduction pattern might increase the rate of fixation failure.The present study demonstrated that the positive-positive support reduction produced better mechanical stability for unstable pertrochanteric fractures and should be obtained as much as possible during the operation.The negative-negative support reduction was prone to cause fixation failure and should be avoided as much as possible during the operation. |
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