Impact of insertion depth and predrilling diameter on primary stability of orthodontic mini-implants

To test the hypothesis that the impact of the insertion depth and predrilling diameter have no effect on the primary stability of mini-implants.

Twelve ilium bone segments of pigs were embedded in resin. After implant site preparation with different predrilling diameters (1.0, 1.1, 1.2, and 1.3 mm), Dual Top Screws 1.6 x 10 mm (Jeil, Korea) were inserted with three different insertion depths (7.5, 8.5, and 9.5 mm). The insertion torque was recorded to assess primary stability. In each bone, five Dual Top Screws were used as a reference to compensate for the differences of local bone quality.

Both insertion depth and predrilling diameter influenced the measured insertion torques distinctively: the mean insertion torque for the insertion depth of 7.5 mm was 51.62 Nmm (+/-25.22); for insertion depth of 8.5 mm, 65.53 Nmm (+/-29.99); and for the insertion depth of 9.5 mm, 94.38 Nmm (+/-27.61). The mean insertion torque employing the predrill 1.0 mm was 83.50 Nmm (+/-33.56); for predrill 1.1 mm, 77.50 Nmm (+/-27.54); for the predrill 1.2 mm, 61.70 Nmm (+/-28.46); and for the predrill 1.3 mm, 53.10 (+/-32.18). All differences were highly statistically significant (P < .001).

The hypothesis is rejected. Higher insertion depths result in higher insertion torques and thus primary stability. Larger predrilling diameters result in lower insertion torques.