| Background:Femoral neck fracture is a high incidence fracture type,accounting for about 3.6% of all fractures,mainly in the elderly,mostly caused by falls and other low energy injuries.Femoral neck fracture is rare in young adults（age < 65 years old）,accounting for about 3% of fractures at this site,mostly caused by high-energy trauma.Pauwels classification is the commonly used cli nical classification of femoral neck fracture,which divides the Angle formed by the fracture line and two iliac crest line into three types: type I,Pauwels Angle < 30°;Type II,Pauwels Angle: 30°-50°;Type Ⅲ,Pauwels Angle >50°（vertical shear fracture）.The theory behind this classification is that the more vertical the fracture line,the greater the vertical shear stress of the femoral neck,and the higher the incidence of internal fixation failure and adverse complications.Although the incidence of femoral neck fracture in young adults is low,it is mostly Pauwels ⅲ fracture caused by high energy.The fracture end is easily displaced,the biomechanical stability is poor,and the incidence of postoperative adverse complications is high.Common treatments for Pauwels Ⅲ femoral neck fractures in young adults include: Cannulated screws（CS）,Dynamic Hip Screw（DHS）,Femur Neck System（FNS）,etc.,through huge vertical shear stress,The incidence of postoperative nonunion,osteonecrosis of femoral head（ONFH）and other adverse complications can reach 30%-60%.Therefore,its prognostic effect is not ideal.In recent years,CS combined with medial support plate（MBP）has become the latest fixation method due to its good biomechanical performance and clinical effect.Buttress Plate（BP）,also known as anti-sliding plate,is an internal fixation method that places the plate on the fracture apex to clamp the fracture,thus resisting shear stress and converting shear stress into compressive stress.The supporting plate is widely used in Plion fracture,tibial plateau fracture and supracondylar fracture.Mir et al put forward the hypothesis of using BP concept to fix Pauwels Ⅲ femoral neck fracture in2015 to improve the stability of internal fixation of Pauwels Ⅲ femoral neck fracture :A plate was placed in the lower inner side of the femoral neck and fixed at the distal end of the fracture.The vertical shear force between the fracture ends could be converted into compressive stress along the axis of the femoral neck by the support of the plate,so as to facilitate fracture healing.This hypothesis lays a foundation for the treatment of Pauwels Ⅲ fracture with MBP.In recent years,a large number of biomechanical studies have shown that MBP can significantly improve the biomechanical properties of CS fixation alone or DHS fixation,and MBP is also being used by more and more orthopedic surgeons.A large number of clinical efficacy analyses have also shown that MBP can effectively reduce the incidence of postoperative adverse complications in patients with Pauwels type Ⅲ femoral neck fracture.The purpose of this study was to compare the biomechanical performance of different fixation methods for Pauwels Ⅲ femoral neck fracture on biomimetic bone,and to clarify the detailed biomechanical characteristics of these fixation methods by strain electrical measurement,so as to provide theoretical basis for the selection of fracture fixation methods.Our hypothesis is that for the choice of proximal screws for medial bracing plates,unicortical short screw provide the same stability as unicortical long screw.In addition,by treating a case of femoral neck fracture with obturator dislocation of the femoral head and retrieving patients with femoral neck fracture with anterior dislocation of the femoral head,we propose the modified Pauwels classification to better classify such injuries and propose theoretical hypotheses for the injury mechanism of such fractures.Materials and methods:In this study,21 artificial biomimetic femurs were used for biomechanical study.A fracture line（60° from horizontal）was drawn at the lower part of the neck of the specimen femur with a protractor to simulate Pauwels Ⅲ fracture.Then,a band saw was used to cut the artificial femur model from the middle of the shaft and cut it horizontally perpendicular to the axis of the neck,so that the final size of the specimen was 200.0mm.Twenty-one femoral models were divided into three groups with three different fixation methods: control group: three CSs;Group A:CSs combined with MBP.The proximal fracture was fixed with a 3.5mm unicortical long screw（length 40mm）;Group B: proximal fracture mass was fixed with 3.5mm unicortical short screw（length 6mm）.Then biomechanical experiments were carried out: 1)strain electric measurement;2)Stiffness test;3)Maximum load test.All statistical calculations were performed using SPSS software.Kolmogorov-Smirnov test was used to detect the normal distribution of data,and independent sample T test was used to analyze the mean between the two groups.If the data do not conform to the normal distribution,nonparametric MannWhitney test is used.*P < 0.05 means there is statistical difference,and**P < 0.01 means there is significant statistical difference.Results:（1）Strain electric measurement results: In the control group,the stress at no.5,6,7 and 8 was larger,and the stress at No.8 was the largest,3.8± 0.32gpa;The minimum stress is 0.69±0.44 Gpa at the measuring point3,9 and 10.There was no significant difference in stress distribution between group A and group B（P < 0.05）.The maximum stress and strain occurred at measuring points 13,14,15 and 16,and the minimum stress and strain occurred at measuring points 3,9 and 10.The stress values at the measuring points（4,5,6,7,8）of the upper lateral femur in group A and group B were significantly lower than those in the control group.Compared with the control group,the stress at the measuring points（13,14,15,16）of the internal neck of femur was significantly increased,and the stress concentration appeared everywhere in the combination of plate and screw.The stress concentration location is consistent with the fracture location of the specimen,as shown in Figure 4.1,and the detailed stress distribution is shown in Figure 4.2.（2）Stiffness test results: the average stiffness of the control group,group A and group B were312.865±36.95N/mm,399.94±91.21N/mm,390.71±78.14N/mm.The stiffness of CS fixation alone was increased by nearly 30%（P < 0.05）with the combined use of medial bracing plates.There was no statistical difference in average stiffness between group A and group B（P > 0.05）.（3）Maximum load test: the average maximum load of the control group,group A and group B were 1534.75±147.08 N,1963.7±227.29 N and1911.86±215.55 N,respectively.Combined with MBP,the maximum load of CS fixed alone was increased by nearly 30%（P < 0.05）.There was no significant difference in average maximum load between the two groups（group A and group B）using MBP enhancement（P < 0.05）.Conclusions:Our study found that:（1）CS combined with MBP fixation can significantly improve the biomechanical performance of CS fixation alone,and the difference is statistically significant;（2）There was no statistically significant difference between unicortical short locking screws and unicortical long locking screws in the axial loading model for the selection of proximal screws with medial support plates for vertical shear fractures of the femoral neck.Fixation of the proximal end of the medial support plate with unicortical short screws may be a viable surgical solution. |
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