Primary migration of a mini-implant under a functional orthodontic loading

Objectives: The objective of this study was to examine whether cortical bone thickness and bone mineral density (BMD) can explain the primary migration of a mini-implant under a functional orthodontic tangential loading at the early stage following implantation. Materials and methods: Mini-implants were installed in human mandibular sections. A constant tangential load (2 N) was applied to the mini-implant under hydration. Creep, which is a time-dependent viscoelastic displacement in the bone surrounding the mini-implant, was assessed as the change in displacement during 2 h of loading. The total migration was measured as a maximum displacement that combined an initial elastic displacement and creep. After removal of the mini-implant, all specimens were scanned together by cone beam computed tomography. Cortical bone thickness and BMD were measured for the bone voxels surrounding the implant site. Results: BMD had significant correlations with the displacement parameters (p < 0.019), but the cortical bone thickness did not (p > 0.272). Permanent bone deformation adjacent to the implant was observed to be resulting from substantial creep development under the orthodontic functional loading level. Conclusions: BMD controls the primary migration of the mini-implant system in mandibular bone. Viscoelastic creep can develop at a small constant functional loading level, leading to migration of the mini-implant. Clinical relevance: The current results indicated that mini-implant migration can develop under the small level of functional orthodontic load used in clinic. If the active bone remodeling around the mini-implant accelerates the migration, the risk of causing damage in vital organs next to the mini-implant increases.