Objectives. This study aimed to evaluate the effect of immediate and delayed loading of orthodontic micro-implants on bone-implant contact. Materials and Methods. Sixty four micro-implants were implanted in dog's jaw bone. The micro-implants were divided into loaded and unloaded (control) groups. The control group had two subgroups: four and eight weeks being implanted. The loaded group had two subgroups of immediate loading and delayed (after four weeks healing) loading. Loaded samples were subjected to 200g load for four weeks. After sacrificing the animals micro-implants and surrounding tissues were observed histologically. Bone-implant contact ratios (BIC) were calculated and different groups' results were compared by three-way ANOVA. Results. Mean survival rate was 96.7% in general. Survival rates were 96.7%, 94.4% and 100% for control, immediate and delayed loaded groups, respectively. BIC values were not significantly different in loaded and control groups, immediate and delayed loading groups, and pressure and tension sides. Mandibular micro-implants had significantly higher BIC than maxillary ones in immediate loading, 4-weeks control, and 8-weeks control groups (P = 0.021, P = 0.009, P = 0.003, resp.). Conclusion Immediate or delayed loading of micro-implants in dog did not cause significant difference in Bone-implant contact which could be concluded that healing time had not significant effect on micro-implant stability. 1. Introduction Anchorage is an important factor in achieving optimum results in fixed orthodontic treatment. The success of conventional anchorage reinforcement techniques depends on patients’ cooperation; however, some undesirable tooth movements may occur even in the best situations. Using implants for anchorage reinforcement could be helpful in solving these problems [1, 2]. Dental implants are large in size and their surgical procedures and space required make them not preferred choice for orthodontic anchorage. Gradually smaller and simpler implants like onplant, mini-implant, and microimplant were introduced which need to have their own researches. For orthodontic uses, some fibrous tissue formation in bone-implant interface would be suitable because it facilitates implant removal at the end of treatment. However abundance of fibrous tissue can cause implant mobility. On the other hand the effect of starting time of loading on bone-implant interface is not clear. Considering the above, stability of immediate loaded implants has been subject to serious investigations in recent years [3–5]. In this regard, some
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