Rat Model for Studying Tissue Changes Induced by the Mechanical Environment Surrounding Loaded Titanium Implants

Purpose: This study presents a new rat oral implant model for assessing histologic changes in the mechanical environment surrounding loaded and unloaded dental implants. Materials and Methods: The maxillary left first molar from retired breeder rats was extracted, and the site was allowed to heal for 1 month. A titanium miniscrew implant was then placed into the site and allowed to heal for 21 days. The mandibular left first molars in one group of rats were extracted to create an unloaded condition; in a second group of rats the mandibular left first molars were left in occlusion with the opposing screw head to simulate loading. Radiographs were taken on the day of placement and again at 7 days, 14 days, and 21 days after placement and were used to estimate the bone-implant contact ratio. The rats were sacrificed after 21 days. Peri-implant tissue samples from day 21 were processed for histology and immunohistochemistry with antibodies to osteocalcin and matrix metalloproteinase 13 (MMP- 13). Two-dimensional finite element models were created from images of the histologic sections and immunohistochemical samples to observe tissue changes. Results: Areas of high shear stress adjacent to the helical threads of loaded implants were associated with osteocalcin localization and bone formation but only minimal localization of MMP-13. Bone adjacent to unloaded implants showed fibrous tissue and extensive MMP-13 localization surrounding the apical two-thirds of each implant. These results agree with estimated bone-implant contact ratios, which showed a steady decrease in contact ratio for the unloaded implant group but a significantly higher contact ratio in the loaded group between 14 and 21 days. Conclusion: The rat oral implant model is useful for studies of the mechanical and physiologic environment affecting osseointegration in loaded and unloaded implants. Keywords: animal study, finite element analysis, implant, osseointegration, rat model