Orthodontic Mini-Implant Insertion Variables: Effects on Retention

Introduction: Little is known about the effects of insertion characteristics on retention of orthodontic mini-implants (OMIs). Objective: The purpose of this study was to quantify the influence, if any, of OMI insertion variables on implant retention, including penetration depth, distance from cortical plate and insertion angle. Materials & Methods: Sixty OMIs (Ormco, USA) were placed in synthetic bone replicas (Sawbone, Pacific Research Laboratories) matched for cortical plate and cancellous bone thickness and density of the maxillary premolar region. Both 6 and 8mm implants were inserted to depths of 4 and 6mm, respectively. The same size implants were also inserted to the same depth creating differences in distance from abutment heads to cortical plate. The implants were also placed at 45°, 90°, and 135° angles to the cortical plate as well as a 45° oblique. Clinically relevant forces were applied to the OMIs until retention failure occurred. Statistical comparisons were made to determine variables that impacted implant retention. Results: Six mm penetration depth of the 8mm OMI required a significantly (32.3% at P<0.05) greater failure force (8.81±0.88N) than 4mm penetration depth of the 6mm OMI (5.96±1.06N). No significant difference was found with increased distances from the cortical plate. A significantly greater failure force was required (P < 0.002) for a 90° insertion angle (8.81±0.88N) than 45° (37.6% or 5.50 ± 1.39N) or 135° insertion angles (40.9% or 5.21±1.78N). No significant difference was found between the 45°and 135° angles. A significant difference (45.6% at P<0.0001) was found between a 90° (8.81±0.88N) and a 45° oblique (4.787±1.816N). Conclusions: Insertion variables have a significant impact on retention of the OMI. A 6mm penetration depth of the OMI at 90° to the cortical plate produced the greatest stability, and should be employed whenever possible. (Partial support funded by NIH Grant K25AG022464)