Reactive oxygen and nitrogen species (RONS)
Reactive oxygen and nitrogen species (RONS) are free radicals and reactive molecules derived from molecular oxygen and nitrogen species, both as intercellular as well as intracellular messengers. As can be seen in Table 1, RONS can be found in lasers (i.e. LLLT-low-level laser therapy), photosensitizers, bleaching agents, cold plasma, and resin cement as a by-product from dental applications . At low or moderate concentration of RONS, it has beneficial effects which results in the angiogenesis (formation of new blood vessels), proliferation and re-epithelialisation of cells in the gingival and other tissues of the body, and vascular endothelial growth factor (VEGF) induced cell migration . However, at very high levels of reactive oxygen species (ROS), adverse effects may rise causing peri-implant inflammation, carcinogenesis & mutagenesis, mitochondrial dysfunction and cell death [19, 20].
A large amount of the Ca contamination was found in the sodium hydroxide (NaOH) reagent . Kizuki et al.  verified that treatment of Ti with NaOH reagent and heat treatments induced apatite formation with bone-bonding ability with Ti metal. Unfortunately, with increasing volume of NaOH reagent, the apatite formation was decreased due to Ca contamination found in the NaOH reagent. The Ca inhibited apatite formation on the Ti metal in SBF (Synthetic/Simulated Body Fluid) by suppressing Na ion release from the sodium titanate into the surrounding fluid. Even a Ca contamination level of 0.0005% of the NaOH reagent was sufficient to inhibit the apatite formation . Exposure of Ti to simulated physiological solutions (i.e. Ringer’s solution and saline, which contains calcium and phosphate ions) leads to adsorption of calcium phosphate on the surface of the oxide layer, spontaneously . Positively charged Ca ions attached to negatively charged (PO4)3− and (CO3)2− acting as nucleation sites for apatite and improving bone to implant contact, thus resulting in good osseointegration .
Chemical composition of the surface of the Ti dental implants plays an important role creating a surface where the bone cells can attach well thus allowing osseointegration to occur. A study had been conducted to examine the surface phosphorus contents of anodized medical-grade Ti samples . The Ti samples were anodised in phosphoric acid solution at different voltages (10 V, 20 V, 30 V or 40 V) and created TiO2 layers on the surfaces. Anodisation in phosphoric acid solution increases the phosphor content of the surface may promote osseointegration and lead to secondary stability for the dental implants .
Furthermore, dental implant surfaces treated with 37% phosphoric acid modulates cytokine production by blood mononuclear cells, establishing a balance between proteins with anti and pro-inflammatory activity, thus promoting the success of dental implants . A Ti surface coating based on calcium phosphate showed high hydrophilicity and high osseointegration, promoting stem cell differentiation, increasing osteoblast production and bone formation, thus resulting in increasing bone formation in a shorter time .
One study characterized the surface of Ti healing abutments before and after clinical placement to investigate the effects of the oral environment on device surfaces . The researchers found a thick white residue containing C, N, O, Ca and P completely obstructing the Ti surface. They suggested that the presence of P contaminant came from biological residue of the oral cavity.
Hydrochloric acid (HCl) was used to clean the Ti surface. However, a minimal amount of Cl was detected on the implant surfaces. Fortunately, small amount of Cl did not weaken the Ti surfaces as the Cl formed Ti-Cl complex and soluble in water . Another potential contamination by Cl was from sonicated solution of low-frequency ultrasound (used to treat chronically infected wounds). The sonication solution from the ultrasound treatment was able to alter the Ti surface chemistry, depositing Cl as well as Ca, aluminium (Al), Si, Na and K on the implant surface .
Saliva contains K, Na, N, chloride, bio-actonate products and proteins. However, during crevice corrosion, the concentration of chloride ions increases and reduces the pH value of saliva creating an acidic environment. The chloride ions attack the oxidation layer of dental implants leading to a corroded implant-abutment connection . Hence, sterile saline can be used to reduce the minimal traces of chloride on implant surface . However, Cl can be completely removed from the Ti implant surfaces either by rinsing or ultra-sonication, both in ultra-pure water .
Sulphur (S) compounds as well as Na, K, Ca, PO4, CO2 and mucin can be found in the mouth . Traces of sulphates along with fluorides, magnesium oxides, silicates, and calcium oxides are found as a result of the sandblasting and etching process of the implant surfaces . Hydrochloric acid (HCl) and sulphuric acids (H2SO4) are frequently used to pre-treated Ti surfaces. S from the residual S2O82− or SO42− was detected from the samples treated with either Sodium persulfate (Na2S2O8) or H2SO4. However, the Ti-acid complexes (titanium sulfate) was less dissolved in water, thus not suitable for decontamination of Ti surfaces as it can disturb the chemical modification of Ti surface . Giner et al. demonstrated that a double acid etching treatment using hydrofluoric acid followed by sulfuric acid produced a dual roughness Ti surface which improved osteoblast adhesion, proliferation and differentiation thus enhancing osseointegration. S can be completely removed from the Ti samples by the non-thermal plasma treatment but not by UV treatment .
Traces of Na have also been reported on implant surfaces which have been treated with sodium-containing solutions such as saline and sodium hypochlorite, with sodium hypochlorite causing a tenfold higher amount of trace Na than saline . NaOH has been used in alkaline treatment to create a sodium titanate layer by incorporating Na ions onto the Ti surface. The nanoporous hydroxyapatite/sodium titanate bilayer has been reported to improve in-vivo osteoconduction and osteointegration . Moreover, the treatment of hydrophilicity of Ti discs using NaOH tend to enhance the early stages of cell adhesion, proliferation, and differentiation . In one study, SBF solution has been used during a coating procedure for Ti implants, causing precipitation of many minerals (e.g. Na, Ca, Mg, P) presented in the solution, which leads to a higher wear resistance of the implant surface . A study done by Shibli et al. revealed traces of Na contaminant along with carbon, O, N, Ca, Al, and O on the Ti surface of the failed implants. The influence of the contaminants block the sites for the oxygen cathodic reaction thus preventing foreign ions such as iron or chromium to catalyse the oxygen reaction. Hence, causing an increase in the dissolution rate of Ti implants and preventing re-osseointegration .
Surface analysis of Ti implants using X-ray Photoelectron Spectroscopy (XPS) measurements revealed the presence of Al and fluoride which were deposited during the sand-blasting and acid etching process . The oxidized state of Al, (alumina) is considered to be stable in physiological fluids with very minor tissue reaction. Therefore, it has been used as a coating material to enhance the corrosion resistance characteristics of dental implants . In addition, favourable cell reactions were observed for a rough Ti surface enriched with Al, Ca and P ions, when incorporated into the Ti surface appears to improve viability of osteoblasts .
Some Ti dental implants may contain surface contaminants that may cause a problem during the osseointegration process. A study done by Semez et al.  showed that the amount of Al in a dental implant called MYIMPLANT (Nobel Biocare, India) was 12-fold higher than that found in Ti alloys typically used for dental implants (between 0 and about 0.06) . Furthermore, another study suggested that a high concentration of residual aluminium oxide (AlO2) may interfere negatively with the osseointegration process .
Si was detected on the failed implants along with P, Ca, Na, S, Cl, Zn and copper (Cu) on the Ti surface. It has been suggested that the surface contaminants may enhance the inflammatory response, altering the healing process which leads to alteration of the oxide layer surface and failure in reosseointegration. The presence of Si is possibly due to the passivation process where the Si was used as a coating or in treating Ti surfaces [36, 40]. Other than the passivation process, Si may come from ion dissolution from the glass storage vials or probably (less likely) from rubber gloves. It may also originate from the fabrication process, cleaning and sterilization process, the handling environment and storage (glass vials) and analysis preparation procedures .
Nevertheless, Si plays an essential element in bone metabolism including promoting osteoblast differentiation, stimulation of collagen type I synthesis, allowing human cell adherence and mineralization of human tissue [41, 42]. As such, Si has been used as a coating on Ti dental implants forming a Si sol–gel coating Ti. A study done by Martınez-Ibanez et al.  showed that the incorporation of tetraethyl orthosilicate (TEOS) to the sol–gel Si caused hydrolytic degradation that leads to releasing of Si compound to the media. This resulted in an increase in the effect of osteoinductive properties allowing for direct contact between new bone and the Ti implant . Silicon-based coatings have properties in preventing bacterial infection post-implantation and therefore improved patient outcomes .
Dental implants made of Zn were reported to cause dental metal allergy in Japan . Some of the traces of Zn ion can be found as this metal is added to toothpaste and mouthwash solutions as anti-plaque agents. This activity is believed to be due to retention in ‘oral micro reservoirs’ such as soft oral tissues, tooth surfaces and bacterial plaque . Nevertheless, Zn has been recognized as an important trace element in increasing the cell proliferation in osteoblasts, bone formation and biomineralization. In addition, Zn has antibacterial properties therefore, attracting researchers to incorporate the Zn into Ti surfaces in dental implants to enhance bioactivity. Co-implanted Zn and Mg ions into Ti implants showed good osteoinductivity, pro-angiogenic and bacterial effects which can enhance rapid osseointegration [1, 46].
Traces of F and S can be found during the acid-etching process . Fluoride ions (up to 0.1 wt%) can also be found in commercial toothpaste, mouthwash solutions and prophylactic gels. Its functions are to prevent development of dental caries and to alleviate dental sensitivity. However, high concentrations of fluoride ions exhibit negative effects on the protective oxide layer of Ti and its alloys, triggering localized corrosive degradation. The degree of corrosion of Ti and its alloys are depends on the concentration of fluoride ions and the pH of the fluoride-containing environments [14, 17, 47]. Besides, at a concentrations of 3 ppm of fluoride ions, Ti alloy becomes discoloured and at a concentration above 20 ppm, the protective oxide layer becomes degraded . Discoloration of Ti implants can be observed after undergoing autoclaving due to F contamination .
An acid etching technique is popularly used by manufacturers to texture the surface of dental implants. Combination of acids such as hydrofluoric acid-nitric acid are often used to remove the oxide layer of Ti surfaces. In the hydrofluoric acid pretreatment of Ti surfaces, the former attacks the oxide layer and reacts with Ti to form soluble Ti fluorides and H. When the free H is saturated, titanium hydride is formed. The titanium hydride can dramatically affect the mechanical properties of Ti which cause embrittlement of the surface layer. However, by adding nitric acid, it can reduce free H formation [13, 49].
A study on the fracture surface of retrieved Ti screw threads revealed a high amount of H absorption from biological environment of oral cavity to cause delayed fracture of a Ti implant . A synergistic role of Si and H coating improves their interaction with osteoblasts. A study done by Mussano et al.  revealed that hydrogen-rich films increased keratinocytes adhesion and viability thus enhancing osseointegration.