Development And Biomechanical Analysis Of A Novel Extensible Pedicle Fixture

Background:Lumbar spinal stenosis(LSS) is one of the most common clinical spine disease, surgical procedures should be taken when medical ineffectiveness, cauda equine dysfunction, and neurological deterioration occurred, but the effect is often unsatisfactory. We proposed a new method for the treatment of LSS by cutting the pedicle of vertebrae, moving the posterior column backward to enlarge the spinal and foreman. We found the dimension of spinal and foreman increased after extending the pedicle by 2,4 and 6 mm in the previous study.Objective:To design a novel pedicle extensible fixture(NPEF) and explore the practicality, safety, biomechanical stability, so as to supply a reasonable novel screw with good stability for new surgical method of lumbar spinal stenosis. Methods:6 formalin preserved spine specimens included L1 to S1 (mean age, 64 y) were used. The mean bone mineral density was 0.857±0.156g/cm~2. They were randomly divided into 2 groups for A, B with 3 specimens in each group. Group A was tested for L2, L4 pedicles, while group B was tested for L3, L5 pedicles. Initial pedicle length, pedicle length after inserting screws and turning the inner screw for 1.5 cycles, 2 cycles, 2.5 cycles, 3 cycles were measured.Vertebrae from L2 to L5 obtained from 6 formalin preserved cadavers (mean age, 67 y) were used. The mean bone mineral density was 0.794±0.078g /cm~2.They were randomly divided into 2 groups for C, D with 12 vertebrae in each group. In group C, control screw was inserted into one pedicle of each vertebra, and anterior column was embedded by dental base acrylic resin powder, then pullout test was performed at a crosshead speed of 5mm/min. The pullout test was performed again after inserting the novel screw and extending the other pedicle. In group D, control screw and novel screw were separately into bilateral vertebral cancellous bone and pedicle of each vertebra, and then pullout test was performed. 30 novel screws which the inner screw were turned for 0 cycle,1.5 cycles, 2 cycles, 2.5 cycles, 3 cycles (6 screws in each group) and 6 control screws were respectively examined by three-point bend test.Results:Length of L2~L5 Pedicle were 15.52±0.71mm,16.00±0.82mm,16.71±0.74mm,18.42±0.87mm in control groupⅠ,and length were 14.75±0.73mm,15.16±0.74mm,15.90±0.69mm,17.54±0.87mm in control groupⅡ, which were less than L2~L5 Pedicle in control groupⅠ( P<0.05). Length of L2~L5 pedicle increased while turning inner screw more cycles. While turning the inner screw 1.5 cycles, the length of screw extension was less than 2mm. While turning the inner screw 2 cycles, 2.5 cycles, 3 cycles, the length of screw extension was more than 2mm. While turning the inner screw 1.5 cycles and 2 cycles, the length of pedicle extension was less than 2mm. While turning the inner screw 2.5 cycles, 3 cycles, the length of pedicle extension was more than 2mm.The mean maximum force at pullout of vertebrae was 735±119N for the control screw and 609±132N for the novel screw(P<0.05), The mean maximum force at pullout of vertebral cancellous bone was 348±70N for the control screw and 319±77N for the novel screw(P<0.05), The mean maximum force at pullout of pedicle was 426±79N for the control screw and 433±88N for the novel screw(P>0.05).When the novel screw turn the inner screw 0 cycle, 1.5 cycles, 2 cycles, 2.5 cycles, 3cycles,the mean maximum bending load was respectively 2289±173N, 1987±118N, 1952±96N, 1910±88N,1889±53N, compared with 3279±217N of control screw, they all show significant differences(P<0.05). Compared with turning no inner screw, the mean maximum bending load while turning inner screw 1.5 cycles, 2 cycles, 2.5 cycles, 3 cycles show significant differences(P<0.05). The mean maximum bending load of novel screw was no significant difference between these groups which inner screw was turned for 1.5 cycles, 2 cycles, 2.5 cycles, and 3cycles.Conclusions:The manipulation of novel extensible pedicle fixture is simple. It is effective to extend the pedicle. Compare with conical pedicle screw with the same diameter, the three point bending strength, the mean maximum force at pullout of vertebrae and vertebral cancallous bone for novel screw decreased (P<0.05), but the force at pullout of pedicle was no significant difference (P>0.05).