COMPARATIVE PULL-OUT STRENGTH OF TAPPED AND UNTAPPED PILOT HOLES FOR BICORTICAL ANTERIOR CERVICAL SCREWS

Study Design. This biomechanical study analyzed the axial pull-out str ength of tapped versus untapped pilot holes for bicortical screws in t he anterior cervical spine. Objective. To determine which pilot hole p reparation method was mechanically better. Summary of Background Data. Tapping pilot holes in the lumbar spine-was previously shown signific antly to reduce pull-out strength of pedicle screws. No study was foun d investigating the effect of tapping on pilot holes for anterior cerv ical bicortical screws. Methods. Twenty-five unembalmed human cadaveri c cervical vertebrae (C3-C7) were tested. Two identical pilot holes we re drilled into each vertebra: one pilot hole was tapped, and the cont rol pilot hole was not tapped. A fully threaded cortical bone screw wa s inserted into each pilot hole. Screw pull-out strength was determine d using a servocontrolled hydraulic materials testing system and an ax ial load cell. Force-deformation end failure curves were obtained. Res ults. There were no statistically significant differences between the axial pull-out strength of tapped and untapped pilot holes at any vert ebral level. Mean force-to-failure was 386 +/- 42 N in the untapped pi lot holes and 397 +/- 48 N in the tapped pilot holes. Conclusions. Tap ping a pilot hole for bicortical screws of the anterior cervical spine neither weakens nor strengthens the axial pull-out strength of fully threaded cortical bone screws. Tapping may be unnecessary; however, it may be desirable in patients with dense bone to cut the thread profil e into the bone or if the screws have dull tips and threads.