Volume 8, Issue 7 (July 2011)
Comparison of Self-Drilling and Self-Tapping Cervical Spine Screws Using ASTM F543-07
ASTM F543-07 is used to evaluate insertion load (Annex A4), driving torque (A2), and axial pullout (A3) characteristics of self-tapping metallic medical bone screws. F543-07 was designed to evaluate screws that utilize a pre-drilled pilot hole and was not intended for self-drilling screws. The objectives of this study were to (1) adapt the F543-07 test methods to allow evaluation of both self-drilling and self-tapping screws and (2) quantify differences in performance between the two screw types. For all three tests (A4, A2, A3) appropriately sized pilot holes were pre-drilled into the surrogate test medium for all the self-tapping screw designs per ASTM F543-07. No pilot holes were created for the self-drilling screws. During the insertion load tests (A4), a constant rotation was applied to the screws under an initial, near-zero axial force state. While monitoring axial load, torque, and displacement, the axial load was gradually increased. The maximum load at which there was a marked increase in torque and displacement on the graph was indicative of the force required to engage the self-drilling or the self-tapping feature. To measure driving torque (A2), a constant rotation was applied. To maintain screw driver–screw contact, a constant-force axial pre-load was required. The axial insertion load, determined during the insertion load testing (A4), was used as the preload for the driving torque testing. Axial pullout testing (A3) measured the axial tensile force required to remove a screw from the test medium. Quantifiable differences in mean insertion load, driving torque, and pullout load were found, demonstrating that these methods are able to discern differences in performance between different screw designs. Finally, this study demonstrated it is possible to adapt the test methods outlined in ASTM F543-07 to evaluate the insertion load, driving torque, and axial pullout performance of self-drilling screws in addition to self-tapping screws.