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[Development of static three dimensional screw-plate system and the biomechanic features thereof, a comparative study].

OBJECTIVE
To develop a new internal fixation device and the matching tools for subtrochanteric fracture, investigate the biomechanical features thereof, and compare its effects with those of other instruments.


METHODS
A new internal fixation device, static three dimensional screw-plate system: (STDSP) was developed based on the anatomical parameters and the biomechanical characteristics of proximal femur and combined with the biomechanical advantages of the model of three-dimensional plate and multiple-screws plate fixation. Sixteen paired cadaveric adult femurs were randomly divided into 2 equal groups: fracture healing group undergoing insertion of implant and then taking out thereof with a nail left, and fracture group with a trapezoid defect to mimic subtrochanteric fracture 4 of which were fixed with dynamic hip screw (DHS) and STDSP or Gamma nail and STDSP. All the specimens underwent A-P position four point bending test, axial compression test, torsion test, and destroyable test.


RESULTS
(1) In the fracture group, A-P position four point bending test showed that the stretch displacement of STDSP group was (0.221 +/- 0.089) mm, significantly smaller than that of the Gamma nails group [(0.506 +/- 0.086) mm, t = 38.278 P = 0.000]; Axial compression test showed that the inferior displacement of the STDSP group was (2.804 +/- 0.585) mm, significantly larger than that of the Gamma nail group [(1.874 +/- 0.549) mm, t = -5.454, P = 0.012]; Torsion test showed that the torque values at 50 and 100 of the STDSP group were (3.78 +/- 0.84) J and (2.3 +/- 1.08) J, both significantly larger than those of the Gamma nail groups [(2.3 +/- 1.08) J and (4.35 +/- 1.81) J, t = -4.729 P = 0.018, and t = -3.227 P = 0.048] and those of the DHS group [(7.3 +/- 1.273) J and (4.325 +/- 0.88) J respectively, t = -3.242 P = 0.048]; and torsion destroyable test showed that the torque values at 200 of the STDSP group was (21.88 +/- 2.51) J, significantly larger than those of the Gamma nail and DHS groups, [(14.36 +/- 5.01) J and (15.475 +/- 1.924) J and (12.075 +/- 1.179) J, t = -3.323 P = 0.048, t = - 3.732 P = 0.034]. (2) In the fracture healing specimens, A-P position four point bending test showed that the radial degree displacement of the STDSP group was (1. 050 +/- 0.316) mm, significantly smaller than that of the DHS group [(1.261 +/- 0.364) mm, t = 3.4 P = 0.042].


CONCLUSIONS
More capable of resisting bending than DHS and less capable of resisting axial compression, STDSP is an excellent internal fixator with good biomechanical feature in treating subtrochanteric femoral fracture.