Microdamage generation by tapered and cylindrical mini-screw implants after pilot drilling

To investigate the relationship between mini-screw implant (MSI) diameter (1.6 vs 2.0 mm) and shape (tapered vs cylindrical) and the amount of microdamage generated during insertion.

Thirty-six cylindrical and 36 tapered MSIs, 6 mm long, were used in this study. Half of each shape was 1.6 mm in diameter, while the other half was 2.0 mm. After pilot drilling, four and five MSIs were inserted, respectively, into fresh cadaveric maxillae and mandibles of dogs. Bone blocks containing the MSIs were sectioned and ground parallel to the MSI axis. Epifluorescent microscopy was used to measure overall cortical thickness, crack length, and crack number adjacent to the MSI. Crack density and total microdamage burden per surface length were calculated. Three-way analysis of variance (ANOVA) was used to test the effects of jaw, and MSI shape and diameter. Pairwise comparisons were made to control the overall significance level at 5%.

The larger (2.0 vs 1.6 mm) cylindrical MSIs increased the numbers, lengths, and densities of microcracks, and the total microdamage burden. The same diameter cylindrical and tapered MSIs generated a similar number of cracks and crack lengths. More total microdamage burden was created by the 2.0-mm cylindrical than the 2.0-mm tapered MSIs. Although higher crack densities were produced by the insertion of 1.6-mm tapered MSIs, there was no difference in total microdamage burden induced by 1.6-mm tapered and 1.6-mm cylindrical MSIs.

Pilot drilling is effective in reducing microdamage during insertion of tapered MSIs. To prevent excessive microdamage, large diameter and cylindrical MSIs should be avoided.