The Optimum Design And Mechanical Test Of The Novel Cannulated Pedicle Screw

Objective:To clarify the effect of the diameter of central hole and distribution of radial holes in novel cannulated pedicle screw on the mechanical strength of pedicle screw. And then to explore the biomechanical stability of cannulated pedicle screw in spinal vertebrae with osteoporosis.Methods:Set up the three-dimensional finite element models of simplified solid pedicle screw and cannulated pedicle screw according to the geometric data of general spine system pedicle screw and the design idea of cannulated pedicle screw in computer. Then optimize the design of cannulated pedicle screw through simulating cantilever bending test, tensile test and torsion test. Make the novel cannulated pedicle screws according to the results of simulation experiments. Twenty vertebrae(L1 to L4) obtained from 5 formalin preserved cadavers were used. The mean bone mineral density was 0.644±0.071g/cm2. The novel cannulated pedicle screws were inserted into one pedicle of each vertebrae randomly, then early-dough-time bone cement (Polylnethylmethacrylate, PMMA) 3-～5ml per vertebrae was perfused with the devices which are matched with cannulated pedicle screw. It was the group of PMMA augmented cannulated pedicle screw instrumentation. The other pedicles were divided into the group of solid pedicle screw and the group of PMMA augmented solid pedicle screw randomly. Before inserting the soild pedicle screw, PMMA 3～5ml per vertebrae was poured into the hole in vertebral pedicle in the group of PMMA augmented solid pedicle screw. Pull-out test was performed in the Material testing system for all screws. Eight cannulated pedicle screws and eight solid pedicle screws were respectively examined by cantilever bending test. Results:1. In cantilever bending test, the yielding load of the finite element models of solid pedicle screw and cannulated pedicle screw with the diameter of central hole 1.0 mm,1.5 mm,2.0 mm and 2.5 mm was 994 N,1026 N,1120 N,1083 N,1064 N respectively. The yielding load of cannulated pedicle screw with central hole 1.5 mm was 113% of that of solid pedicle screw.2. The yielding load of the models of cannulated pedicle screws with the diameter of central hole 1.5mm and radial holes 3, 4 or 6 was 1046 N,997 N,1013 N respectively. The maximum tensile force of three models was 4950 N,4510 N,4675 N respectively.The maximum torque was 10.3 N·m,8.7 N·m,9.9 N·m respectively.3. The mean maximum force at pull-out test for cannulated pedicle screws with PMMA augment in osteoporotic vertebrae was more than that of solid pedicle screws, which showed significant difference (P<0.01). There was no significant difference in the mean maximum force between PMMA augmented solid pedicle screws and cannulated pedicle screws (P> 0.05). Compared with the simple fixation of solid pedicle screws, the force improved in PMMA augmented solid pedicle screws, which showed significant difference (P<0.01). There was no significant difference in the mean maximum force between left and right pedicle screws in the group of PMMA augmented cannulated pedicle screws (t=0.234, P>0.05).4. There was no significant difference in the anti-bending strength at cantilever bending test between cannulated pedicle screws and solid pedicle screws (t=1.049,P>0.05).Conclusions:1. The structure of cannulation in pedicle screw improve the mechanical strength of solid pedicle screw, when the diameter of central hole was 1.0mm,1.5mm,2.0mm,2.5mm respectively. The mechanical strength of the model of cannulated pedicle screw declined after radial holes were established in cannulated pedicle screw.2. The mechanical strength of the novel cannulated pedicle screw with the diameter of central hole 1.5mm and radial holes 6 is equivalent to that of solid pedicle screw.3. The novel cannulated pedicle screw could enhance the fixation stability of pedicle screw significantly in osteoporotic vertebrae and prevent the occurrence of cement leakage effectively. It has a favorable perspective in clinical application.