The Influnce Of Screw Diameter On Stability Of Simple Compression Fracture And Pedicle Cortical Perforation By Internal Fixation

Objective:This study chose sheep’s spinal samples to build thoracolumbar vertebrae’s simple compression fracture models and simple compression fracture with pedicle cortical perforation models; adopted different diameters of screws for fixation; invested the impacts of different screw diameters on stability of pedicle screw internal fixation after spinal vertebrae fractures; and analyzed the impacts of pedicle cortical perforation on stability of internal fixation.Methods: After purchasing the fresh(within 6 hours after slaughter)sheep’s spinal samples, any tumors, fractures or congenital deformity of spine were excluded via naked eyes and X-ray examinations. A total 40 samples with the pedicle diameter 6.5±2mm were selected and then to cut out(T13-L3)the spinal segments. First of all, 40 samples were used to make the single vertebral compression fracture models. Cone holes were punched in the L1 vertebrae to make a simple compression fracture model with pedicle cleavage.The technique used the small size electric cutting and grinding machine to cut and grind the L1’s left pedicle on the compression fracture model. Fissures about 2 mm were made. All fissure width of pedicle cortical perforation models was shown less than 3 mm after X-ray examination. Those samples were divided into four groups: group A, B, C and D, 10 samples in each group,of which simple compression fracture models were included in group A and B while simple compression fractures with pedicle cortical cleavage models were included in group C and D. Group A was control group while group B, C and D were study groups. Finally, all samples were performed the pedicle fixation with 6 screws. All models were inserted needles based on angle of7-15° from sagittal plane through T14, L1 and L2 pedicle. The depth ofneedling was the full length of the screw channel, with vertical sticks,horizontal connections and screw caps installed. T14, L1 and L2 vertebrae of all samples were fixed with the Kirschner wires.The side internal fixation point was based on a horizontal level inserting at junction of middle vertebrae and posterior 1/3 vertebrae. The vertebrae was bilaterally symmetric. The front internal fixation point was placed in the middle, inserting vertically and making the Kirschner wire cut through the whole vertebral. Kirschner wire at T14 end was as a free end while Kirschner wire at L2 end as a marker. Diameter of 6.0 mm screws were imbedded via pedicle in group A and 6.5 mm screws for group B while diameter of 6.5 mm screws were imbedded via injured pedicle in group C and 6.0 mm screws for group D. All sample models have conducted 10000 times of fatigue tests in a biomechanics machine. A total of 40 samples were fixed in the biomechanics machine one by one, with 6N·m torque loaded each time. Anteflexion, rear protraction, left bending and right bending fatigue tests were carried out after completely unloading(0N·m). The withdrawal force of pedicle screws was tested by using the biomechanics machine for axial withdrawal test. After finishing measuring the thoracolumbar sample’s range of motion(ROM), the vertical sticks and horizontal connections for sample fixation were removed as well as L1 vertebrae to ensure L1 vertebrae’s left pedicle internally fixed screw in good condition. Then single L1 vertebrae was fixed in the biomechanics machine, performing the withdrawal force test for the screw along screw’s y-direction upon an offset speed of 10mm/min. In the force value-deformation curve chart, correspondingly reading out the maximum of withdrawal force, namely that peak value before the curve moving down, was the maximum withdrawal force. The size of four directions of range of motion(ROM) including anteflexion, rear protraction, left bending and right bending fatigue tests and the size [1] of withdrawal force of L1 vertebrae screws were carried out after completely unloading in 4 groups of sample models were tested after experiment.Results:Four directions of ROM including anteflexion, rear protraction,left bending and right bending of models in group D were significantly higher than those of group A, B and C,the result were 4.49±0.40 、 3.72±0.52 、6.67±0.6、6.73±0.5. The difference has statistical significance(P<0.01). Four directions of ROM including anteflexion, rear protraction, left bending and right bending of models in group B were significantly lower than those of group A, C and D,the result were1.81±0.14 、 1.68±0.37 、 4.08±0.41 、4.18±0.12.The difference has statistical significance(P<0.01). Compared with each test index about four directions of ROM including anteflexion, rear protraction, left bending and right bending of models in group A and C, the result of A were 3.15±0.37、2.58±0.29、5.46±0.47、5.46±0.47,the result of C were3.04±0.45、2.80±0.24、5.46±0.58、5.82±0.48,the difference has no statistical significance(P>0.05). The maximum screw withdrawal force in group B was 249.55±44.47 N,significantly higher than that of group A, C and D. The difference has statistical significance(P<0.01). The maximum screw withdrawal force in group D was 112.40±7.32 N, significantly lower than that of group A, B and C. The difference has statistical significance(P<0.01).Compared the maximum screw withdrawal force in group A with group C,the results were 148.53±5.72 N and 144.25±7.22 N,the difference has no statistical significance(P>0.05).Conclusion: The closer the pedicle screw diameter to the pedicle diameter, the better stability the pedicle will be. The greater the diameter, the better stability the pedicle will be. The pedicle cortical perforation may decrease the stability of internal fixation if screw is placed via injured vertebrae. If the screw diameter is properly increased when inserting screws through the injured vertebrae, namely making the partial pedicle cortical cleavage when inserting screws, the stability will be much better.