gms | German Medical Science

Objective: Pullout strength of the pedicle screws depend on several factors such as core geometry, size, bone mineral density (BMD). Expandable pedicle screws (EPS) provide superior pullout strength. EPS’s are formed like an inverse conical geometry inside bones. In this study, a novel pedicle screw fixation system was designed and manufactured.

Method: The novel fixation system is designed as a fixing plug and screw assembly. It contains a PEEK outer shell and a Ti6Al4V core component. The outer shell’s expansion mechanism is designed as two-pin plug. Shell’s both inner and outer surfaces are designed as conical structure. The core is also designed in conical structure and core head is assembled with a standard poly-axial screw head. The novel system was tested according to ASTM F543. Polyurethane Grade 20 foams were used.

Results: Three revisions completed on first design. Also randomly selected conical core classical pedicle screw (CPS) is selected from our laboratory database. Over than 40 different screw designs tested according to ASTM F543, F1798 and F1717. Novel design exhibited significantly higher pullout performance than the classical pedicle screw. (P<0,05) On the other hand pullout test results between revision are not drastically higher or lower than each other. P values are 0,05 and 0,048 between Rev3-Rev1 and Rev3-Rev-2, respectively.

Conclusions: The novel modular fixation system was biomechanically evaluated. Tested specimen’s pullout strength was 1196,3 N by performing static pullout test according to ASTM F543. This result is drastically higher than the classical pedicle fixation screws. In addition to this, PEEK shell’s fusion tendency will provide an extra durability to the system. Using a polymer-based shell will provide to absorb micro vibrations which cause micro fractures and accelerate the pullout of screw.