Orthodontic mini-implant stability at different insertion depths

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 × 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^®. 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^® [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^® values were highly correlated (r = −0.907).

Mini-implant stability varies significantly with insertion depth. The RFA and the Periotest^® 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.