Orthodontic mini-implant stability at different insertion depths Sensitivity of three stability measurement methods

The purpose of this work was to evaluate the influence of insertion depth on the stability of orthodontic mini-implants. Sensitivity of three different methods to measure implant stability based on differences in insertion depth were determined. A total of 82 mini-implants (2 x 9 mm) were inserted into pelvic bone of Swabian Hall pigs. Each implant was inserted stepwise to depths of 4, 5, 6, 7, and 8 mm. At each of these depths, three different methods were used to measure implant stability, including maximum insertion torque (MIT), resonance frequency analysis (RFA), and Periotest(A (R)). Differences between the recorded values were statistically analyzed and the methods tested for correlations. Almost linear changes from each insertion depth were measured with the values of RFA [implant stability quotient (ISQ) values range from 1-100], which increased from 6.95 +/- 2.85 ISQ at 4 mm to 34.63 +/- 5.51 ISQ at 8 mm, and with those of Periotest(A (R)) [periotest values (PTV) range from -8 to 50], which decreased from 13.24 +/- 4.03 PTV to -2.89 +/- 1.87 PTV. Both methods were found to record highly significant (p < 0.0001) changes for each additional millimeter of insertion depth. The MIT increased significantly (p < 0.0001) from 153.67 +/- 69.32 Nmm to 261 +/- 103.73 Nmm between 4 and 5 mm of insertion depth but no further significant changes were observed as the implants were driven deeper. The RFA and Periotest(A (R)) values were highly correlated (r = -0.907). Mini-implant stability varies significantly with insertion depth. The RFA and the Periotest(A (R)) yielded a linear relationship between stability and insertion depth. MIT does not appear to be an adequate method to determine implant stability based on insertion depth.