The effect of rotation moment on the stability of immediately loaded orthodontic miniscrews: a pilot study

The aim of this study was to evaluate the effect of the direction and magnitude of the rotation moment to loaded orthodontic miniscrews on stability. In six adult male beagle dogs (12 months old), 36 orthodontic miniscrews were inserted into the mandibular buccal alveolar bone without drilling. Immediately after insertion, 24 miniscrews were loaded with 1- and 2-Ncm nickel titanium (NiTi) coil springs with either a clockwise or counterclockwise rotation moment while 12 miniscrews were left unloaded. Following an observation period of 3 or 12 weeks, the animals were killed and the miniscrews with the surrounding bone were prepared for histomorphometric evaluation. Bone-to-implant contact (BIC) and the ratio of bone volume/total volume (BV/TV) were measured. BIC and BV/TV were statistically compared using the Kruskal-Wallis H and Mann-Whitney U tests. Three of the miniscrews loaded with 2 Ncm counterclockwise rotation moment were lost within 3 weeks. At 12 weeks after insertion, the counterclockwise group showed a statistically significantly lower BIC in comparison with the clockwise group. The results suggest that a rotation moment to loaded orthodontic miniscrews, as well as the direction and magnitude of the rotation moment, can influence miniscrew stability. Counterclockwise rotational moments can be a risk factor impairing miniscrew stability.