Biomechanical Study Of Expansive Spine System (ESS) Pedicle Screws In Human Lumbar Vertebral Bodies.

The pedicle screw technique is a milestone of the development of spine surgery. It is the outcome of spine biomechanical studies, It has been widely used in all kinds of spine surgery field. Especially the posterior thoracolumbar pedicle screw fixation technique of the short segment is the effective measure in treatment, because the method can achieve the anterior, middle and posterior three-dimensional space fixation of the vertebra with correcting function and many planes stability. Because of the heavy burden and a great sphere activities, so it result in the inadequate stability of the conventional pedicle screw insertion and the insufficient anti-pullout force of spine, that are often complicated by pseudarthrosis, screw loosening or dislocation after operation, especially the treatment will be worse for those patients who have osteoporosis. Many people have studied the problem in order to resolve it, the make use of increasing the length or diameter, selecting the material, altering the placement of the screw and augmentation with the bone cement in the screw orientation. They have not achieve the ideal effect though they make much progress on it. The early complication of internal fixation mainly result from its design and biomechanical property. Basing on mechanical theory, we design a new type internal fixation system —expansive spine system, its pedicle screws can offer good purchase ability. It canexpand in the vertebrate body, but its another part in the pedicles can not expand, which make it possible to resist bending, loosening and pullout without pedicle burst. It not only rectifies deformity, but also can produce the instant stability. Meanwhile it could improve the fusion rate of the grafting bone and decrease the postoperative complication, we test the biomechanical in order to testify its property further, so far, there are few reports on this research field.Objective:To measure and compare the insertion torque and pullout strength of newly-designed expansive spine system (ESS) pedicle screws with those of CD- II, GSS and TSRH pedicle screws in normal and osteoporosis human cadaver vertebrate, and to evaluate the screw purchase of ESS pedicle screws. To observe the expansion of transverse width of post screw-implantation in the pedicle and assess the relation between pedicle mini-fracture and expensive rate of pedicle transverse width.Materials and methodsThirty lumbar vertebral bodies obtained from 6 fresh normal male cadavers were classified into A, Ek C 3 groups randomly, 20 pedicles per group. Bone mineral densities were measured. ESS (45 mm, 6.0mm), CD- II (45 mm, 6.0mm) and GSS (45 mm, 6.0mm) pedicle screws were implanted into the pedicle of each group respectively. Another twenty lumbar vertebral bodies obtained from 4 fresh old female cadavers were classified into D, E 2 groups randomly, 20 pedicles per group, BMD were measured too. ESS (45 mm, 6.0mm) pedicle screws were implanted into one pedicle of each group(C and D) randomly, the CD- II (45 mm, 6.0mm) and TSRH (45 mm, 6.0mm) pedicle screws were implanted in the remain pedicles respectively. The 3-4 last screw circles of each pedicle screw were remained. The remained screw circles were implanted by using a material-testing machine (SWD-10, Changchun, China) to measure the insertion. Then each screw was extracted axially from the pedicle at a rate of 5mm/min until failure by using the same machine. Force data were recorded and analyzed using a one-way ANOVA, LSD, t test and x2 test. PO.05 was considered significant.To observe the pedicle mini-fracture, to measure the pedicle transverse width before and after screw-implantation, to evaluate the expansive rate according to the format: the expansive rate=(pedicle transverse width of post screw-implantation— initial transverse width)/ initial transverse width X 100 %.Result1. The bone mineral density (BMD) of A> B, C\ D^ E were 1.05 + 0.116g/cm2 1.01 ± 0.1169g/cm2 1.04 ± 0.11 g/cm2 0.742 ± 0.076g/cm2 0.716 + 0.0753g/cm2 .The torque of group A> B, C were 5.79+ 1.85Nm 5.19 + 0.75Nm 5.56 +1.3INm. The torque of group D> E were 1.59 + 0.32Nnu 1.57±0.29Nm> 1.56 ± 0.3 INhk 1.63 + 0.30Nm respectively. The pull-out strength of Group ABC were (2219.80+ 367.60)N, (1630.65 + 392.58) N (1963.75 +403.68)N, The pullout strength of D > E were: ESS(1176.2 + 213.67)N ^ CD- II (589.8 + 147.87)N,ESS( 1162.6+ 223.97)1^ TSRH(584.18 + 157.53)No2.The maximum axial pullout strength of ESS was greater than that of CD- II > GSS and TSRH screws both in normal and osteoporosis vertebral bodies. The difference was very significant. (pO.Ol)3. The pullout strength was correlated positively with insertion torque and bone mineral density (BMD)4.The higher BMD, the more expensive, the osteoporosis vertebral bodies have lower expansive rate than that of normal vertebral bodies, the difference was significant. (p<0.05);When the expansive rate lower than 10 % ,there are little mini-fracture; When the expansive rate between 11 % -15 % ,it is prone to mini-fracture; When the expansive rate higher than 16 % -20 % ,there are higher mini-fracture possibilities and even pedicle bursting; When the expansive rate higher than 20 %, the pedicle bursting will always be the results.Conclusion1. Both in normal and osteoporosis human vertebral bodies, the expansive spine system pedicle screws have good purchase ability and good biological stability.2. The ESS has a simple but strong structure, it is easy and secure to maneuver with the excellent result, it is an acceptable in instrument in treating spine disease.3. The expansive rate have a close relation with BMD and mini-fracture, these factors should be taken into consideration when pedicle fixation is applied.