Stability and Three-Dimensional Analysis of Bone Formation in Longitudinally Fluted Miniscrew Implants

Abstract

The purpose of the present study is to evaluate the effects of longitudinal flutes on mini-screw implant (MSI) bone healing and stability. Using 11 skeletally mature New Zealand White rabbits, 33 longitudinally fluted and 33 non-fluted MSIs were placed and immediately loaded with 100g using NiTi coil springs. Insertion torque values were obtained for each MSI that was placed; removal torque values were obtained for 28 MSIs that had been in place for 6 weeks and 20 MSIs that had been in place for 2 weeks. The bone volume fraction surrounding the implant 6-to-24 ?m, 24-to-42 ?m, and 42-60 ?m from the MSI surface using micro-computed tomography with an isotropic resolution of 6 ?m. The success rate was 97%, with both the fluted and non-fluted MSIs each having one failure. Mean insertion torque was slightly higher for fluted MSIs (3.98 N cm ?0.24) compared to non-fluted MSIs (3.95 N cm ?0.24), but the difference was not statistically significant (p=.930). After 6 weeks, removal torque values were significantly (p=.008) higher for the fluted (3.42 N cm ?0.26) than non-fluted (2.49 N cm ?0.20) MSIs. After 2 weeks, removal torque values were higher for fluted (2.87 N cm ?0.22) than non-fluted MSIs (2.75 N cm ?0.22), but the difference was not statistically significant (p=.702). After six weeks, bone volumes of the 6-24 ?m, 24-to- 42 ?m, and 42-to-60 ?m layers were significantly (p<.05) greater for the non-fluted than fluted MSIs. After two weeks, bone volume of three layers were also significantly (p<.05) greater for the non-fluted than fluted MSIs. Fluted and non-fluted 3 mm long MSIs can have very high success rates, even with all maximum insertion torque values being less than 6.2 N cm. Adding longitudinal flutes to 3 mm MSIs increases their removal torque by 37% after 6 weeks, despite the fact that there was less bone surrounding the fluted than non-fluted MSIs.