Effects of hole preparation on screw pullout resistance and insertional torque: a biomechanical study

Object. The authors conducted a study to assess the effect of a pilot hole preparation on screw pullout resistance and screw insertional torque. Methods. Three different screws were tested: cancellous lateral mass screws , cortical lateral mass screws, and pedicle screws. Synthetic bone blocks w ere used as the host material. Each screw group was separated into two subg roups. The first subgroup of screws was inserted into the test material fol lowing pilot hole preparation. Pilot holes were prepared; a drill bit diame ter size smaller than the core diameter of the screws was used. The second group of screws was inserted into the test material without pilot hole prep aration (a 3- or 4-mm hole drilled for entrance site preparation only). The insertional torque was measured as the screw was advanced into the materia l. The screws were axially extracted from the host material at a constant s peed of 2.5 mm/minute. The pullout resistances and insertional torques for the pilot hole and the nonpilot hole groups were then statistically compare d. The authors found that preparation of a pilot hole caused a significant dec rease in the insertional torque. The screws inserted without a pilot hole s howed greater pullout resistances compared with those inserted following a pilot hole preparation; however, there was no statistically significant dif ference. Conclusions. The optimum screw insertion technique may involve drilling a s hort pilot hole and using a drill bit with a smaller diameter than the scre w core diameter to increase bone-screw purchase. This applies to cancellous and cortical lateral mass screws as well as pedicle screws.