Pullout strength of self-tapping screws inserted to different depths

Objectives: The purpose of this study was to determine whether the depth of insertion through the far cortex of self-tapping screws significantly affects pullout strength. Design: Fifty Synthes((R)), 3.5-mm, self-tapping screws were inserted into synthetic bone blocks and divided into 5 groups. Group I had screws with their tips inserted I mm short of the far cortex. Group 2 had screws inserted flush with the far cortex. Groups 3, 4, and 5 had screws inserted 1 mm, 2 mm, and 3 mm past the far cortex respectively. Pullout strength was then tested. Setting: Institutional research laboratory. Main Outcome Measurements: Pullout strength (peak force) was measured for each group and analyzed using a single factor analysis of variance-balanced incomplete block design. Results: Peak force values presented as mean +/- SD for the 5 groups were as follows: group 1 (1380 +/- 69 N), group 2 (1566 +/- 137 N), group 3 (1956 +/- 137 N), group 4 (2013 +/- 184 N), group 5 (2044 +/- 174 N). With a P <= 0.05, it was found that groups I and It had statistically different pullout strengths than all other groups. However, there was no significant difference in pullout strength between groups 3, 4, and 5. Conclusions: Synthes((R)) self-tapping screws exhibit their highest pullout strength when inserted 1 mm past the far cortex, and there is no significant increase in pullout strength with deeper insertion depths.