Objective: To investigate the desirable healing time of micro-screws by histomorphologic and histomorphometric evaluations of osseointegration after immediate and early loading. Materials and Methods: Fifty-four micro-screws were bilaterally placed in the maxillary premolar regions of nine beagles. Then the micro-screws with various healing time of 0 day (0D group), 2 weeks (2W group), and 4 weeks (4W group) were loaded with an orthodontic force (100 g) for 8 weeks. The direction of the orthodontic force was vertical to the long axis of the micro-screws. Hard tissue sections containing micro-screws were prepared for histomorphologic and histomorphometric evaluations. Results: The survival rate of the micro-screws in this study was 100%. Bone remodeling, close contact bone-implant interface, and endochondral ossification were observed in all osseous specimens. Activated osteoblasts aggregated to the bone-implant interface of the 4W group, and lamellar bone was found in the peri-implant regions. Micro-screws of the 2W group were partially surrounded by collagen fibers; and neonatal lines of bone, woven bone, and osteoclasts were found in the peri-implant regions. Micro-screws of the 0D group were surrounded by more collagen fibers compared with the other two groups. Bone implant contact ratios of the three groups were 43.74% (0D group), 66.26% (2W group), and 73.28% (4W group), respectively and statistical differences were significant (ANOVA, P , .01). Conclusion: All micro-screws in the three groups can provide stable orthodontic anchorage. However, to obtain improved stationary anchorage, a 4-week healing time is recommended before orthodontic loading. (Angle Orthod 2010;80:354-360.)

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Cyclic compressive force is an important mechanical stimulus on periodontal ligament (PDL). The differential expression of genes in PDL cells is thought to be involved in the remodeling of periodontal tissues subjected to mechanical stress. However, little is known about differentially expressed genes in PDL cells under cyclic compressive force. In our study, human PDL cells were subjected to 4000 μ strain compressive stress loading at 0.5 Hz for 2 h. The effect of mechanical stress on PDL cells proliferation was observed by flow cytometry. Microarray analysis was used to investigate the mechano-induced differential gene profile in PDL cells. Differential expression was confirmed by quantitative real-time polymerase chain reaction (RT-PCR) analysis on genes of interest and explored at two more force loading times (6 h, 12 h). After mechanical loading, cell proliferation was repressed. The microarray data showed that 217 out of 35,000 genes were differentially expressed; among the 217 genes, 207 were up-regulated whereas 10 were down-regulated (p < 0.05). Gene ontology analysis suggested that majority of differentially expressed genes were located in the nucleus and functioned as transcription factors involved in a variety of biological processes. Five genes of interest (IL6, IL8, ETS1, KLF10, and DLC1) were found to be closely related to negative regulation of cell proliferation. The PCR results showed increased expression after 2 h loading, then a decline with extended loading time. The signaling pathways involved were also identified. These findings expand understanding of molecular regulation in the mechano-response of PDL cells.

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