Computed Tomographic Characterization of Mini-implant Placement Pattern and Maximum Anchorage Force in Human Cadavers

Introduction: The purpose of this investigation was to characterize the placement pattern and factors influencing the primary stability of mini-implants in human cadavers. The factors studied were mini-implant length, placement depth, bone density, and bone type.

Methods: Sixty standard mini-implants (6, 8, and 10 mm; 20 of each size) were placed into the maxillas and mandibles of 5 fresh human cadavers. Computed tomography imaging was used to measure the placement pattern, bone density, and thickness surrounding each device. The mini-implants were subsequently subjected to increasing tensile forces (pull-out force) until failure, and the maximum mechanical anchorage force of each was recorded with a dynamometer. A statistical model was realized by using MATLAB version 7.5.0 with Statistics Toolbox 7 (MathWorks, Natick, Mass) including the maximum anchorage force, mini-implant length, bone type, placement depth, and density surrounding each section of the mini-implant.

Results: Placement depth was strongly dependent on mini-implant length: 15% of the 6-mm implants failed to anchor their parallel sections into cortical bone, but 95% of the 10-mm mini-implant parallel sections penetrated beyond the buccal cortical bone; all 20 tips of the 6-mm mini-implants (100%) reached cancellous bone, whereas 75% of the 10-mm implants penetrated both cortical plates, reaching the lingual cortical bone. Longer mini-implants were associated with greater incidences of sinus and bicortical perforations. The correlation coefficients between the initial maximum mechanical anchorage force and the studied factors were as follows: bone density and placement depth combined (r = 0.65, P <0.001), mini-implant length (r = 0.45, P = 0.004), bone density (r = 0.42, P = 0.007), and placement depth (r = 0.29, P = 0.06).

Conclusions: During mini-implant length selection, the clinician should consider the important trade-off between anchorage and risk of placement complications or damage to the tissues. Longer mini-implants enable more anchorage; however, they are associated with a higher risk of damage to neighboring structures. Placement depth and bone density at the site of mini-implant placement are the best predictors of primary stability.