Quantitative assessment of orthodontic mini-implant displacement: The effect of initial force application

Objective: Primary stability and micromovement of orthodontic mini-implants depends on a number of factors and influences clinical success or failure. The purpose of this study was to assess the behavior of orthodontic mini-implants upon initial load application. Method and Materials: Orthodontic mini-implants (n = 39) were inserted in the alveolar process of maxillary human cadaver specimens (n = 10). Increasing horizontal forces (up to 2.5 N) were applied, and triggered images were taken in 0.5-N load intervals. Additionally, peri-implant parameters based on micro-CT volume data were recorded. Data were subjected to a two-sided nonparametric Wilcoxon signed rank test, and betweengroup comparisons were assessed with a Mann-Whitney test (α = .05). Results: Initial load application led to displacement beyond elastic recovery of the surrounding bone after force release (P .001). Cortical thickness and insertion depth, despite numeric differences, did not reveal any statistical differences, while displacement of mini-implants was significantly affected by contact to neighboring teeth (P .001). Conclusions: Insertion technique and initial load application on orthodontic mini-implants may be regarded as two crucial factors for success, while repeated application of orthodontic force does not seem to increase screw mobility. Keywords: cortical bone thickness, human cadaver, maxillary bone, mini-implant, optical measurement, primary stability