Primary stability and viscoelastic displacement of mini-implant system under loading

The objectives of the current study were to examine the effects of mini-implant diameters on 1) primary stability before bone properties are changed by active peri-implant bone remodeling and 2) the time-dependent displacement of mini-implant systems in bone under a functional radial loading and their associations with bone mineral density.

Twenty one, 8mm length mini-implants (7 each for 1.4mm, 1.6mm, and 2mm diameters) were placed in mandibular sections of human cadavers (4 males and 3 females, average 69.7 (SD 13.1) years of age). Displacement of the mini-implant in wet bone was assessed during initial and subsequent continuous radial loading of 2N in the mediolateral direction for 2h. Mean, standard deviation and coefficient of variation of peri-implant bone mineral density were obtained using histograms of cone-beam computed tomography attenuation values. The cortical thickness along with the miniscrew site was also measured.

The primary stability and displacement of mini-implants in bone were not significantly different between the 3 diameter groups (p>0.147, n=21). Moderate positive correlations of time-dependent viscoelastic displacement (creep) were found with bone mineral density variability independent of the mini-implant diameters (p>0.11).

The post-implantation displacements of mini-implant suggested that the orthodontic treatment loading can develop micromotion between the mini-implant and surrounding bone leading to reduction of its primary stability. Current findings also provide an insight that peri-implant bone mineral density variability plays an important role in controlling displacement of the mini-implant, which determine its stability during early and prolonged orthodontic treatment loading periods.