The evaluation of the primary stability of the orthodontic micro-implants in vitro

The aim of this study was to evaluate and compare primary stability of selfdrilling(SD) and self-tapping (ST) orthodontic micro-implants. In addition,possible correlations of factors influencing primary stability were explored. SDand ST (n=72) were placed into bovine iliac bone blocks subsequently tocomputed tomography measurements on cortical bone thickness and bonedensity (Hounsfield units). Insertion torque (IT) of the implants (n=24) weremeasured and the subjective resistance to insertion was simultaneously scoredby the operator. Periotest measurements were undertaken upon placement ofthe implants. Histomorphometic bone-implant contact (BIC %) of each implantwas measured from axial sections. Pull-out and push-in tests were performedon a total of 48 implants. Kolmogorov-Smirnov test was performed at 95%confidence level to determine whether normal distrbution of the data waspresent. Inter-group comparisons were undertaken by Mann-Whitney U,Independent t-test, and Chi-Square tests at 95% confidence level followed bySpearman?s correlation analyses to explore possible correlations between thesefactors. IT values and subjective assessment scores of SD and ST implantswere similar (P=.552 and P=0.324, respectively). The inter-group differencesbetween Periotest measurements was significant only for Periotest 3measurements (P=.013). Inter-group comparisons did not reveal differences inmaximum pull-out and push-in forces (P=.166 and P=.419, respectively). TheBIC of SD group (%87.60) was higher than that of ST group (%80.73) (P=.046).There was a positive correlation between BIC and cortical bone thickness in SDand ST groups (r=0.909, P=.00 and r=0.823, P=.001, respectively). While apositive correlation was detected between pull-out values and cortical bonethickness (r=0.606, P=.037) and cortical bone density (r=0.748, P=.005) in theSD group, there was no correlation between these parameters in the ST group.Likewise, a positive correlation was found between push-in values and corticalbone density (r=0.661, P=.00) in the SD group, but there was no correlationbetween push-in values and computerized tomography parameters in the STgroup. A positive correlation was detected between IT and cortical bonethickness in SD and ST groups (r=0.544, P=.00 and r=0.516, P=.001, respectively). There was also a significant correlation between IT values andcortical bone density in SD and ST groups (r=0.661, P=.00 and r=0.568, P=.00,respectively). There was a negative correlation between IT and Periotestvalues, predominantly in the SD group (P<.05). The differences in subjectivescores was significant in both groups (P<.05). There was a positive correlationbetween pull-out and push-in values with IT (r=0.916, P=.00 and r=0.58,P=.048, respectively) in the SD group, but not in the ST group. There was apositive correlation between BIC and IT values in SD and ST groups (r=0.657,P=.02 and r=0.839, P=.001, respectively). The primary stability of the ST andSD implants used in this study was comparable, although the bone massaround SD implants was higher. A positive correlation was observed betweenBIC, cortical bone thickness and IT values and a negative correlation betweentorque and Periotest values.