Factors effecting primary stability of mini-implants in vitro

Objectives: Mini-implants (MIs) are now routinely used in orthodontic treatments; however, compared to conventional implants, MIs suffer higher failure. Achieving primary stability (PS) appears to be the most important factor predicting success of MIs. Factors such as implant diameter, length, and bone quality are known to influence PS in conventional implants; however, little is known of the effects of these factors on PS in MIs. Therefore, the aim of this study was to investigate the effect of MI’s diameter, length and the presence of cortical bone in PS. Methods: AbsoAnchor TAD design from Dentos with 1.5,1.7, and 2mm diameters; 6,8, and 10mm length were placed in Polyurethane bone blocks with densities of cancellous bone (GP- 20). MIs were also placed in blocks were sandwiched with 1 or 2mm polyurethane sheets, which simulated cortical bone density. Four MIs in each group were placed using recommended procedures. PS of MIs was measured with Periotest and Osstell by three testers. The Cronbach Alpha inter-examiners reliability test was used to evaluate agreement among the testers. PS data were analyzed with multifactorial ANOVA to detect the significant influence of each factor in MIs’ PS (a = 0.05). Results: Both Osstell and Periotest indicated significant increase in PS (p<0.05) when cortical bone sheet of 1 or 2 mm thickness were in contact with MIs. MIs’ diameter had significant influence in PS, indicating that MIs with wider diameter (1.7-2 mm) had significantly higher PS (p < 0.05) in both cancellous and cortical sandwiched models. There was no significant difference in PS when different lengths of MIs were used in cortical sandwiched models; however, an increase in MIs’ length appeared to increase PS only in soft bone blocks, which simulated the hardness of the cancellous bone. Conclusions: Important factors in achieving PS in MIs appear to be bone type and implant diameter in heterogeneous bone (combined cancellous & cortical bone) often found in vivo. An increase in the length of the MIs only improves PS in homogeneous soft bone (cancellous bone). Recognizing factors improving PS would expect to decrease unnecessary trauma and failure rate in children and adolescence.Dentistry, Faculty ofGraduat