Investigation Of Pull-out Strength On A Part Concentrated Screw Designation In Human Spine And Finite Element Analysis Model.

BackgroundThe pedicle screw and rod instrumentation system is one of the most commonly used and rapidly growing forms of stabilization for spinal fusion. Problems associated with pedicle screws include loss of fixation, fatigue, bending failure and improper placement. Bending or breakage is the most common type of hardware failure, which is strongly associated with the purchase strength of screws. Although pure pullout is not the mode of failure in most clinical situations, pullout testing is thought to be a good predictor of pedicle screw fixation strength. Biomechanical studies show that bone mineral density (BMD), pedicle morphology and screw thread area all affect pedicle screw pullout. Trabecular bone at the corticocancellous interface in pedicle is stronger than normal unsupported cancellous bone in the vertebral body. Therefore the pedicle part accounts for more than 80% of the axial purchase strength and more than 60% of the pull-out strength of the whole screw. However, the CCD screw currently used in clinic has the same thread pitch in pedicle as that in vertebral body. Based on these characteristics and the pedicle anatomy features of Chinese Han people, a new set of part concentrated screw (PCS) was designed with concentrated threads in the pedicle part, which was believed to gain more purchase strength with increased thread area in the pedicle of Chinese patients.Objective The purpose of the present research was to test the difference in pull-out strength between the two types of PCS screw (PCSI and PCSII) and CCD screw in human spine. Three dimensional finite element analysis (FEA) was employed to study the contribution of the threads in the pedicle part to the pull-out strength of the whole screw.MethodsFifteen fresh lumbar vertebrae (L1-L5) from three adult Han male cadavers were harvested and stripped of all soft tissues. The BMD in vertebral body and pedicle of all specimens was measured by dual energy X-ray absorptiometry (DEXA). Each individual vertebra was thawed and prepared for pedicle screw placement. Then the 30 pedicles were randomly assigned to three groups of ten pedicles each. The PCSI,PCSII and CCD screws were inserted by hand using a custom driver without tapering. The screw entry point was selected in accordance with the method of Magrel. Each screw was inserted until the hub of the screw abutted the lamina and all threads were fully contained within the bone. The maximum insertion torque was measured by a torque measuring spanner. The vertebrae, mounted in the testing jig, were placed a variable axis frame attached to the base plate. The hydraulic arm of the CSS-22100 machine was attached to the screw head by a universal joint and aligned so that pullout would be co-axial to the screw. Axial pullout was performed at 2.00 mm/minute displacement. Pullout loads and yield displacement were recorded at 0.1-second intervals until failure occurred. The three dimensional geometries of the screw and the bone with a threaded hole was separately created using the UG software. Then the models were imported into ANSYS and combined to generate the final model. The FEA procedure was performed to study the effect of concentrated threads in the pedicle part on the screw total pull-out strength.Results1. The BMD of vertebral body and pedicle of specimen measured by DEXA were 0.494±0.058 g/cm2 and 0.783±0.134 g/cm2 separately, with significant difference between the two area (p<0.05)。Test of normality was performed separately in the data of the two areas. Results showed that all data of the two areas came from a single population separately, with no difference among the data in the same area.2. The pullout loads, maximum torsion torque and yield displacement of PCSI and PCSII screws were significantly higher than CCD screw. Student-Newman-Keuls test showed that there was significant difference among the three screws (p<0.05). The pullout loads, maximum torsion torque of PCSI were also significantly higher than PCSII, but there is no difference between the two PCS screws in yield displacement.3. The FEA analysis was successfully performed using UG and ANSYS software. The force taken by each threaded connection in the pedicle was three times more than that in the vertebral body area. Student's t test showed that the thread in the pedicle can resist significantly higher force compared to the thread in the vertebral body (p<0.05). The force taken by the threads in the pedicle accounts for more than 85% of the whole pullout strength of the PCSI and PCSII screw.ConclusionsThe newly designed PCS screws had significantly improved purchased strength compared to the custom CCD screw. The improvement of pullout strength was mainly derived from and directly associated to the density of threads in the pedicle area. The result of current research can serve as foundation for the further development and application of PCS screw.