Bone Mineralization Controls Viscoelastic Migration of Mini-implant under Functional Loading

Objectives: Progressive migration of mini-implants during orthodontic treatment likely increases the failure rate of these devices. As bone is a viscoelastic material, time-dependent viscoelastic deformation (creep) of bone surrounding the implant would be responsible for the implant migration under prolonged constant loading during orthodontic treatment. It has been observed that tissue mineral density (TMD) is associated with creep of bone tissue. Thus, the objective of this study was to examine whether TMD is associated with creep of bone tissue around mini-implants under functional orthodontic loading at the early stage of implantation.

Methods: Mandible sections of individual human cadavers (5 males, 7 females, 81±13 years, n=12) were obtained. 1.5x6mm mini-implants were inserted in the mandibles, and an environment chamber was used to maintain moisture. A constant load (2N) was applied in a perpendicular direction to the long-axis of the mini-implant for 2 hours. Creep was assessed as the change in displacement during 2 hours of loading. Maximum displacement was defined as the initial loading displacement plus creep. After removal of the mini-implant, the specimen was scanned by 3D cone-beam computed tomography (CBCT)(200micron voxel, iCAT). CBCT attenuation values of bone voxels, which correlates to TMD, were estimated surrounding the implant site.

Results: Mean value of maximum displacement was 0.052±0.034mm. The mean CBCT gray level had a significant inverse correlation with maximum displacement (r²=0.51,p<0.01).

Conclusions: A substantial amount of maximum displacement was measured. This early deformation likely amplifies bone remodeling around mini-implant interface leading to more mini-implant migration during orthodontic treatment. The CBCT based TMD measures could explain the deformation. The current finding suggested that non-invasive 3D CBCT image based TMD analysis could improve orthodontic treatment plans utilizing mini-implants.