Pullout strength variance among self-tapping screws inserted to different depths

The cortical self-tapping screw (STS) has replaced the non-STS as an aid in fracture fixation. In a recent biomechanical investigation, Berkowitz and colleagues found that STS pullout strength increased with insertion depth up to 1 mm past the far cortex only. In the present study, we wanted to apply a standardized protocol of assessing pullout strength to STSs of different compositions and manufacturers while eliminating the sample-size and block-variance issues that affected the previous investigation. Ninety STSs were randomly divided into 5 groups, each representing a different insertion depth. Peak force was determined with trials ending in screw pullout or failure. A statistically significant difference in pullout strength was identified with insertion depths up to 1 mm past the far cortex. No block variance was detected. These results support the recommendation that STSs be inserted only 1 mm past the far cortex in healthy cortical bone.