A Comparative Study On The Effects Of Polyaxial-headed Axial Direction On The Pullout Strength Of Polyaxial Pedicle Screw

【Objective】Pedicle screw instrumentation systems have been persistently improved in the past several years, but the substantial improvement in the design of the pedicle screw has emerged recently , scilicet polyaxial pedicle screw. Because of the polyaxial-head join in the pedicle screw, it will definitely affect the biomechanical characteristics of the pedicle screw. Biomechanical studies, for the most part, have conducted failure testing in a direction parallel to the axis of the screw. For posterior fixation, this direction of loading does not represent the complex cantilever loads experienced by pedicle screws in posterior spinal instrumentation. Our experiments aim at these studies :(1)the change of pullout strength of pedicle screws affected by the angle of polyaxial-head;(2)the pullout strength affected by the change of the angle direction, such as up and down ,inside and outside, etc;(3)The relation between the Bone mineral density and the pullout strength along the polyaxial-head axial.【Materials & Methods】Sixty, fresh-frozen, lumbar vertebrae were harvested from calf ,2~3years old. These vertebrae were harvested before the processing of the animals. The lumbar vertebrae from L2 to L6 were dissected free of all soft tissue and frozen immediately.1.measureUse the dual energy X-ray absorptiometry(Lunar Corp. Madison, WI,USA) to measure the BMD of lumbar calf spines, and record it(BMD g·cm-2). Measure the narrowest horizontal diameter of each lumbar pedicle and the distance between the screw and the anterior vertebrae border with a vernier caliper.2.place the polyaxial pedicle screwIn euthyphoria,according to the Magerl method, each body had two polyaxial 5.5-40mm screws placed. The screws are parallel to the end plate and the"e"angle is 15°.The depth of the pedicle screw is 80% of the lumbar vertebrae .Take the X-ray picture of every vertebrae X-axis and Y-axis to test the accuracy and symmetry of channels.3.measure pullout strength in the polyaxial-head axial directionAfter bundling and stabling the lumbar vertebrae by the dental base acrylic resin powder, the withdrawal force experiment is conducted in the pedicle screws polyaxial-head axial direction at a invariable speed of 18mm/min in the Autograph Universal Testing Machine. The Automatic Recorder accounts the pullout distance of pedicle screws, the change curve, and the maximum polyaxial-head axial direction pullout strength. Then, we use statistics to analyze the pullout strength of the angle of 0°,15°and 30°.【Results】1. Every numerical data is firstly examined by the normality test and analysis of variance (ANOVA). The numerical data corresponds to normal distribution and homoscedasticity. The six groups BMD are examined and analyzed by the ANOVA, without significant difference.(F=0.441,P=0.818)2. Match pairs in each group by t-test. groupA: statistics significance (t=5.291,P=0.000), group B: statistics significance (t=12.584,P=0.000), group C: without significant difference (t=0.304,P=0.768), group D: without significant difference (t=1.821,P=0.102), group E:statistics significance (t=3.097, P=0.013), group F: without significant difference (t=0.665, P=0.523).3.Between the group A and the group B, two groups show no statistics significance in Independent-Sample t-test of 0°angle pullout strength, (t=0.177,P=0.861),15°angle and 30°angle pullout strength have statistics significance(t=4.123,P=0.001).4. Between the group C and the group E, 15°angle pullout strength is examined and analyzed by One-Way ANOVA, no statistics significance(F=0.339,P=0.797). Between the group D and the group F, 30°angle pullout strength is examined by One-Way ANOVA, no statistics significance too .(F=0.442,P=0.738)5. 15°angle pullout strength is positive with BMD ,statistics significance(r=0.439,P=0.005).6. 30°angle pullout strength is positive with BMD statistics significance(r=0.340,P=0.032).【Conclusion】1. When the polyaxial-head axial direction forms an angle with the screw axis, the polyaxial-head axial direction pullout strength exceeds the screw axis pullout strength.2. When the angle of the polyaxial pedicle screw increases, the polyaxial-head axial direction pullout strength also increases.3. The direction of polyaxial-head axis can affect the pullout strength of polyaxial pedicle screw, but no statistics significance.4. The polyaxial-head axial direction pullout has positive relation to BMD.