Comparison of the mechanical properties of two designs of polyaxial pedicle screw

In this study, the mechanical properties of a novel dual-core pedicle screw were compared with a commercially available cylindrical screw. In order to evaluate and compare their mechanical performance, a series of axial pullout, quasi-static and dynamic bend tests were conducted. In the pullout tests, three polyurethane (PU) foams (density: 0.16, 0.32 and 0.64 g/cm3) were used to compare the pullout strength between both screw types. The ultimate static strength of each screw was determined by a series of quasi-static cantilever bend tests. Dynamic tests were performed with a peak forces corresponding to 10%, 30%, 40%, 50%, 65% and 75% of the ultimate static strength of each screw type. Each specimen was subjected to a sinusoidally varying load which continued until the specimen fractured or reached 2.5 million cycles. The results of the pullout force indicated that the dual-core screws had higher pullout strength, in each PU foam, compared to cylindrical screws, however, the differences were not statistically significant. The average stiffness of dual-core screw during pullout from the 0.16 and 0.32 g/cm3 PU foams was significantly higher (p < .05). In quasi-static tests, results of ultimate bending load; bending stiffness and structural stiffness were significantly higher for dual-core screws (p < .05). During the dynamic bending tests, the dual-core screws had longer fatigue lives for all loading levels. It was observed that the fatigue failures for both screw types occurred either at the head-shank junction or between third and fourth thread. In conclusion, the findings of this study indicated that the dual-core screw design has improved mechanical performance compared to the cylindrical design, with the exception of pullout resistance, which showed no significant difference.