# A Brief Review on Micro-Implants and Their Use in Orthodontics and Dentofacial Orthopaedics
## 3 Results
### Microbial Aggregation around Micro-Implants, Biocompatibility  
The possibility of microbial aggregation and biofilm formation on micro-implants has been intensively investigated; surface roughness and chemical composition play an important role in this issue. An in-vitro study [33] which uses X-ray photoelectron spectroscopy detected high-carbon contamination and other inorganic elements like Pb, Zn, P, Cu, Cr, Ca on the oxide surfaces of five different types of micro-implants. Those chemical impurities disappeared after Ar(+) ion sputtering. The surface roughness was greater for titanium micro-implants (182 nm) than for the stainless-steel ones. Through scanning electron microscopy, structural defects were observed [34]. These retention sites favored the biofilm's formation when the micro-implants were immersed in human saliva; the microbial flora of the biofilm was reduced when the micro-implants were pretreated with chlorhexidine and fluoride mouth rinses. Various other methods have been tried to decrease microbial aggregation around micro-implants to prevent their loss. By using silver nanoparticles (AgNPs) were manufactured AgNP-modified titanium micro-implants (Ti-nAg) by coating OMIs with AgNPs or with a AgNP-coated biopolymer (Ti-BP-AgNP) [35]. Comparative data showed that OMIs coated with BP-AgNP had remarkable antibacterial properties by creating inhibition zones for Aggregatibacter actinomycetemcomitans, Streptococcus mutans, and Streptococcus sanguinis, whereas no antibacterial effects were seen on those coated with AgNPs. Scanning electron microscopy showed a silver atomic percent of 1.05 in the group of OMIs coated with regular AgNPs and much greater, 21.2%, in the group coated with BP-AgNP. Ti-BP-AgNP proved to be an excellent implantable biomaterial, with very good antibacterial properties [34]. The heat treatment (APH treatment) increases the hydrophilicity and the roughness of Ti6 Al4V micro-implants subjected to anodization and cyclic calcification. APH treatment created a surface of the nanotubular TiO\({}_{2}\) layer which was covered with a compact apatite-like film. APH-treated micro-implants showed better bioactivity and biocompatibility, better bone regenerative characteristics, higher removable torque, and greater contact with bone, compared with untreated (UT) and anodized and heat-treated (AH) ones [36]. Self-drilling orthodontic micro-implants were surface-treated with acid (etched), resorbable blasting media (RBM), partially resorbable blasting media (hybrid) to increase, study and compare the bone-cutting capacity and osseointegration [37]. The hybrid type gave the most stable self-drilling OMIs, without reduction of bone-cutting capacity. Chitosan modification of the surface of micro-implants might be an approach to enhance the bioactive- and antibacterial properties of orthodontic micro-implants. Chitosan-modified titanium alloy micro-implants showed better biocompatibility with pre-osteoblastic cells which was confirmed by their improved adhesion, proliferation, and cell viability analysis [38]. Biofilm formation of Streptococcus mutans and Streptococcus sobrinus was reduced by 53% and 31%, respectively, on the surface of OMIs. Nanotechnology is the study, production, and controlled manipulation of materials with a grain size less than 100 nm. As grain size decreases, the interaction between OMIs and the surrounding cellular environment increases. Treating the micro-implants surface with nanophase materials improved their osteo-integration due to a more closely match with the architecture of native trabecular bone [39]. Zirconia micro-implants proved excellent biocompatibility; they were tested and showed initial stability and clinical applicability for orthodontic treatments comparable to that of titanium micro-implants under various compressive and tensile forces [40].