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 strength of tapped versus untapped pilot holes for bicortical screws in the anterior cervical spine. Objective. To determine which pilot hole preparation method was mechanically better. Summary of Background Data. Tapping pilot holes in the lumbar spine-was previously shown significantly to reduce pull-out strength of pedicle screws. No study was found investigating the effect of tapping on pilot holes for anterior cervical bicortical screws. Methods. Twenty-five unembalmed human cadaveric cervical vertebrae (C3-C7) were tested. Two identical pilot holes were drilled into each vertebra: one pilot hole was tapped, and the control pilot hole was not tapped. A fully threaded cortical bone screw was inserted into each pilot hole. Screw pull-out strength was determined using a servocontrolled hydraulic materials testing system and an axial load cell. Force-deformation end failure curves were obtained. Results. There were no statistically significant differences between the axial pull-out strength of tapped and untapped pilot holes at any vertebral level. Mean force-to-failure was 386 +/- 42 N in the untapped pilot holes and 397 +/- 48 N in the tapped pilot holes. Conclusions. Tapping 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 profile into the bone or if the screws have dull tips and threads.