| ObjectiveThe purpose of this study was to explore the cross-sectional characteristics of the femoral neck isthmus(FNI)by measuring the femoral neck torsion angle(FNTA)and the morphological parameters related to the FNI in the transverse plane.The optimal space configuration of the three parallel-arranged screws for the internal fixation of the femoral neck fracture was evaluated and established based on the FNI cross-section.And the ideal insertion position of the femoral cortical screw(FCS)was determined.The mechanical properties of the novel spatial configuration were compared with the traditional inverted triangle configuration through finite element analysis and biomechanical analysis.This study provides a novel configuration for the treatment of femoral neck fractures using the three parallel cannulated compression screw(CCS),aiming to avoid the complications after the surgical treatment of femoral neck fracture.And It also gives perspectives for the screw position of the proximal femoral neck plate.Methods1.The femoral neck extracted from CT scans was reconstructed and the cross-section of the femoral neck isthmus was resliced using Mimics(V22.0,Materialise Mimics Innovation Suite).The FNTA and morphological parameters related to the FNI cross-sections were assessed,and the influential factors on the FNTA and parameters were detected.In this study,the morphology of the FNI cross-section was proved approximately ellipse,which was demonstrated by comparing the formula-predicted values and actual measured values of the circumference and area of the cross-section.2.According to the parameters of the FNI cross-section in the first part,a torsional ellipse model was established using the Computer-Aided Design(CAD)technics to fit the contour line of the FNI cross-section.The oblique triangle configuration(OTC)formed by three parallel screws was determined according to the maximal inscribed triangle within the ellipse.The area,perimeter,and proportion of the oblique triangle(OT)in the ellipse were measured and compared with the traditional inverse equilateral triangle(IET)and the maximum area of the inverse isosceles triangle(MA-IIT).3.The CT datasets from healthy volunteers were used to reconstruct the femur model.The computer-stimulated static pressure test using finite element analysis(FEA)was applied on the model of Pauwel’s Ⅲ femoral neck fracture fixed with the three screws OTC method and tested by assigning ten different material properties.The maximum stress,strain,and relative displacement of the fractured end of the OTC configurations under different loads(350N,700N,1400N,2100N)were compared with the traditional IET configurations(IETC).4.The fourth generation of Sawbone artificial bone was used to establish the model of Pauwel’s type Ⅲ unstable femoral neck fracture.The biomechanical properties of the internal fixation of femoral neck fracture with the three screws OTC method were evaluated by using static pressure test,cycle test,and torsion test,and compared with the FEA results.Results1.There was a significant difference between the torsion angle of the FNI and the base of the femoral neck(30.58°±8.90° vs 23.79°±3.98°,P<0.001).The average cross-sectional area of FNI was 732·83±126/74 mm2,the average ellipsoidal eccentricity,semi-major axis and semi-minor axis of the cross-section was 0.60±0.07,17.43±1.63 mm and 13.81±1.29 mm.All the parameters showed significant differences between the female and male(P<0.05),while there was no significant difference between the left and right sides(P>0.05).There was no statistical difference between the formula calculated value and the actual measured value of the FNI cross-sectional area(P>0.05).The correlation results showed that the height had the greatest influence on the FNTA and the cross-section elliptic eccentricity and cross-sectional area of FNI(r=0.255,r=0.159,r=0.703).2.In the torsion ellipse model,the OT’s area ratio(41.35%)was 4.56%,4.48%higher than the IET and the MA-IIT,and its perimeter ratio(82.46%)was 5.41%,5.23%higher respectively.As the position of FCS changes,the proportion of area and perimeter of OT in the ellipse did not change,while the proportion of area and perimeter of IET and MA-IIT in the entire ellipse gradually decreases with the position of FCS changes.It was also found that when the FCS was positioned at the midline of the femoral neck under the lateral fluoroscope,the posterior upper screw was most likely to avoid the posterior upper region of the femoral neck.In the established univariate model,the area of OTC was not affected by any of the three factors in the torsional ellipse and remained constant(41.35%).Its perimeter decreases with the increase of eccentricity but was not affected by the torsion angle and area of the ellipse.The area and perimeter of IET and MA-IIT decreased with the increase of the torsion angle and eccentricity of the ellipse,and there was no significant difference among them.3.The results of the finite element analysis show that the OTC model was superior to the IETC model in terms of the degree of stress dispersion around the screw hole on the femoral side fracture section under 2100N.Regarding the stress and displacement peak of the femur and internal fixation,the OTC model was lesser than the IETC model in different loadings((350N,700N,1400N,2100N)).In terms of strain peak of the femoral head,the OTC model was larger than the IETC model,while lesser than the IETC model around the screw hole on the femoral side fracture section.In terms of the relative displacement of the fracture end,the OTC model was significantly lesser than the IETC model(P<0.001),while it has shown no statistical difference in the displacement of the upper and lower parts of the fracture end.The peak values of stress,strain,and displacement of the femur and screw and the relative displacement of the fracture end were accordingly multiplied in both models as the static load increasing.4.The biomechanical test showed that in the static pressure test,the vertical stiffness,ultimate load,and 5mm failure load in the OTC group were higher than those in the IETC group,and the vertical stiffness and ultimate load were statistically different(738.64±32.88 N/mm vs 620.74±72.74 N/mm P<0.05,2891.33 ± 183.96 N vs 2589.70±237.88 N P<0.05,2957.61±149.09 N vs 2643.06±238.05 N,P>0.05).The OTC group was significantly lesser than the IETC group in terms of shear displacement,the displacement between two points and the opening angle of the fracture end(4.67±0.41 mm vs 5.39±0.36 mm P<0.05,1.41±0.19 mm vs 1.93±0.19 mm P<0.05,4.02°± 0.55° vs 5.55°±0.56°P<0.05).In the cyclic tests,the OTC group was lesser than the IETC group in terms of the femoral head displacement after 10,000 cycles of 700N,1400N,and 2100N(0.34±0.03 mm vs 0.47±0.05 mm,0.77±0.13 mm vs 0.91 ±0.08 mm,1.21±0.15 mm vs 1.59±0.39 mm),as well as total displacement of the femoral head after 700-1400N and 700-2100N cycles(1.70±0.09 mm vs 1.96±0.20 mm,3.91±0.83 mm vs 5.68±1.83 mm),while the difference was significant in groups of the 700N and 700-1400N cycles exclusively(P<0.05).In the static torsion test,the OTC group was lesser than the IETC group in terms of stiffness(1.31 ±0.23 N/mm vs 1.72±0.36 N/mm),and larger than the IETC group in terms of maximum torque and torsion angle(70.36±9.61 Nm vs 67.46±18.21 Nm,35.46°±1.66° vs 34.04°±2.26°),while there was no statistical significance(P>0.05).In terms of the torsion angle and the corresponding torque at the time of the femoral neck cortex splitting,the OTC group was significantly larger than the IETC group(26.01°±7.32°vs 9.05°±1.08° P<0.001,27.24±2.80 Nm vs 17.89±2.22 Nm P<0.001).Conclusion1.This study provides a three-dimensional and reproducible method in assessing the FNTA and FNI cross-sectional morphology.This study found that FNTA might gradually increase from the base of the femoral neck to FNI,and the size of FNTA is positively correlated with height and weight to a certain extent.In consideration of the FNTA,this study has proved that the cross-section of FNI was an approximately torsional ellipsoid.2.The area ratio of OT in the torsional ellipse model is the largest and remains constant using mathematical analysis.The OTC may be an ideal spatial configuration for three parallel screws when fixing femoral neck fractures,with better mechanical stability.When the FCS was positioned in the midline of the femoral neck under the lateral fluoroscope,the placement of the posterior screw gets safer.The area of the triangle formed by the three screws on the FNI cross-section was used to evaluate the dispersion degree of the screws,and it might be more convenient to evaluate the mechanical properties of the spatial configuration of the parallel screws.3.The finite element analysis and the biomechanical test showed the superior results of the OTC of three screws parallel in fixing Pauwel’s Ⅲ type femoral neck fracture than the traditional IETC.These findings suggested that the OTC may be an ideal spatial configuration for fixation of femoral neck fractures with three parallel screws,the type of fixation can provide more stable mechanical properties at the fracture end and avoid related complications.And the findings also provided evidence for the placement of proximal femoral plate screws. |
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