Effects of the diameter and shape of orthodontic mini-implants on microdamage to the cortical bone

Abstract

Introduction: The purpose of this study was to investigate the effects of the diameter and shape of orthodontic mini-implants (OMIs) on microdamage to the cortical bone during implant placement. Methods: Twenty-eight self-drilling OMIs (Biomaterials Korea, Seoul, Korea; length, 6 mm; diameters, 1.5 and 2 mm; cylindrical and tapered shapes; classified as 1.5C, 2C, 1.5T, and 2T) were placed with a surgical device in the tibias of 7 New Zealand white rabbits (mature males; mean age, 6 months; mean weight, 3.1 kg). Four OMIs of each type per rabbit were placed randomly. Maximum insertion torque (MIT) was measured. Immediately after placement of the OMIs, the block of bone with the OMI was harvested. Cortical bone thickness was measured by using microcomputed tomography, and histomorphometric analyses of the number of cracks (NC), accumulated crack length (ACL), maximum radius of the crack (MRC), and longest crack (LC) were performed. Kruskal-Wallis and Mann-Whitney U tests with the Bonferroni adjustment were done for statistical analyses. Results: Increased diameter (1.5C<2C and 1.5T<2T) and tapering (1.5C<2T) resulted in increased values of MIT, NC, and LC (P <0.01, respectively). Similarly, with increased diameters (1.5C and 1.5T<2C and 2T), there were increases of ACL and MRC (P <0.001, respectively). However, there were no differences in the values of MIT, NC, ACL, MRC, and LC between the cylindrical and tapered OMIs with the same diameters (1.5C and 1.5T, 2C and 2T). Conclusions: OMIs with larger diameters and tapered shapes caused greater microdamage to the cortical bone; this might affect bone remodeling and the stability of the OMIs.