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®, 3.5-mm, self-tapping screws were inserted into synthetic bone blocks and divided into 5 groups. Group 1 had screws with their tips inserted 1 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 followsgroup 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 II 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® 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.