Yoshiki Oshida

Microanalytical Characterization and Surface Modification of TiNi Orthodontic Archwires

Orthodontic archwires (equiatomic TiNi alloy) of both used (4 weeks) and unused conditions were microanalyzed by optical and scanning electron microscopes, energy dispersive X-ray spectroscopy, and electron diffraction to characterize the surface layers. They were also subjected to immersion and polarization corrosion tests in a 0.9% NaCl aqueous solution. Based on results obtained from these analytical and experimental studies, surfaces of TiNi archwires were further electrochemically treated to etch away nickel selectively and reform the surface morphology to uniform and porous surface layers. Main conclusions were: (a) surface layers of used archwires were covered contaminants causing the discoloration, and the contaminants were identified as mainly KCl crystals, (b) surfaces of both used and unused wires were observed to be irregular features characterized by lengthy island-like structures, where nickel was selectively dissolved, (c) corrosion tests in a 0.9% NaCl aqueous solution in immersion and polarization methods indicated that by increasing temperature from 3 degrees to 60 degrees C and acidity from pH 11 to pH 3, calculated corrosion rates increased, and (d) surface layers of TiNi archwires can be electrochemically modified to selectively etch nickel away, leaving a Ti-enriched surface layer and forming a uniformly distributed porous surface that may reduce the coefficient of friction against the orthodontic brackets.

Microtensile Bond Strength of Glass Ionomer Cements to Artificially Created Carious Dentin

In this laboratory study, the microtensile bond strengths of a conventional glass ionomer cement (GIC) and a resin modified glass ionomer cement (CRMGIC) to artificially created carious dentin and sound dentin were compared, and the ultrastructural morphology of the fractured interface was examined with a low-vacuum scanning electron microscope (SEM). The specimens were divided into 4 groups: 1) a conventional GIC (Ketac-Fil Plus Aplicap) placed on sound dentin; 2) a conventional GIC placed on artificially created carious dentin; 3) an RMGIC (Photac-Fil Aplicap) placed on sound dentin and 4) an RMGIC placed on artificially created carious dentin. Artificial carious lesions were created using a chemical demineralizing solution of 0.1 M/L lactic acid and 0.2% carbopol. GIC buildups were made on the dentin surfaces according to the manufacturers directions. After storage in distilled water at 37 degrees C for 24 hours, the teeth were sectioned vertically into 1 x 1 x 8-mm beams for the microtensile bond strength test. The microtensile bond strength of each specimen was measured, and failure mode was determined using an optical microscope (40x). The fractured surfaces were further examined with SEM. Two-way analysis of variance showed that the mean microtensile bond strengths of a GIC and an RMGIC to carious dentin were significantly lower than those to sound dentin, and the mean microtensile bond strengths of Photac-Fil to both sound and carious dentin were significantly higher than those of Ketac-Fil Plus. Chi-square tests indicated that there was a significant difference in failure mode between the sound dentin and carious dentin groups. In sound dentin groups, cohesive failure in GIC was pre- dominant; whereas, mixed failure was predominant in carious dentin groups. SEM examination showed that the specimens determined to be cohesive failures under light microscopy in the Photac-Fil/Sound Dentin group were actually mixed failures under high magnification of SEM.

Oxidation and Oxides

This chapter discusses the formation of titanium (Ti) oxides, its influences on the biological process. The crystal structures of Ti oxides are also discussed. Ti is a highly reactive metal and reacts within microseconds to form an oxide layer when exposed to the atmosphere. Due to strong chemical affinity to oxygen, it easily produces a compact oxide film, ensuring high corrosion resistance of the metal. The TiO 2 oxide forms readily as it has one of the highest heats of reaction known. Adhesion and adhesive strength of Ti oxide to substrates are controlled by oxidation temperature and thickness of the oxide layer as well as the significant influence of nitrogen on oxidation in air. The excellent corrosion resistance of Ti materials is due to the formation of a dense, protective, and strongly adhered film—called a passive film. Such a surface situation is referred to as passivity or a passivation state. The “natural” oxide film on Ti ranges in thickness from 2 to 7 nm, depending on parameters such as—the composition of the metal and surrounding medium, the maximum temperature reached during the working of the metal, and the surface finish.

3 – Chemical and Electrochemical Reactions

Ti biomaterials and devices/instruments made of these materials will be subject to hostile environment, causing chemical and/or electrochemical reactions. Some reaction products might be toxic. All related reactions (discoloration, corrosion, metal ion release, dissolution, galvanic corrosion under coupling with dissimilar material, and microbiology-induced corrosion) will be discussed in this chapter.

Implant-Related Biological Reactions

This chapter discusses the important points related to successful implantology. Some of these points include: bone healing, hemocompatibility, cell adhesion, adsorption, spreading, proliferation, and cell growth. The dental implants, usually made from titanium materials, are biocompatible metal anchors surgically positioned in the jawbone underneath the gums to support an artificial crown where the natural teeth are missing. There are three main types of dental implants: the root form implants, the plate form implants, and the subperiosteal implants. The four major factors influencing the success rates of placed implants include: correct indication and favorable anatomic conditions (bone and mucosa), good operative technique, patient cooperation (oral hygiene), and adequate superstructure. For both dental and orthopedic implant surgeries, bone healing is the major step immediately after implant placements. There are principle steps of peri-implant bone healing. The first healing phase, osteoconduction, relies on the recruitment and migration of the osteogenic cells to the implant surface, through the residue of the peri-implant blood clot.

Advanced Materials, Technologies, and Processes

There are still many materials and technologies as well as design concepts that can potentially be employed in the medical/dental field in the near future. The typical areas are tissue engineering and its related materials and structure in nanoscale dimensions, and functionally graded material and structure. In summary, the design and concept of the bioengineering and biomaterials integrated implant system will be introduced and discussed.

Mechanical and Tribological Behaviors

This chapter reviews fatigue, fracture, and biotribological actions, such as friction, wear, and wear debris toxicity. Materials in bioengineering range widely from very small units, such as a virus or bacteria, to very large and complex units, such as a healthy human body. They also cover the various products manufactured by using the methodologies in connection with biological disciplines. Fatigue is the loss of strength and other important properties as a result of stressing over a quite long period of time. It is very important, as it is the single largest cause of failures in metals, estimated as about 90% of the metallic failures. Polymers and ceramics are also susceptible to this type of failure. The mechanical environments related to fatigue are divided into two distinct groups: strain-controlled and stress-controlled. Clasps can undergo permanent deformation to cause fatigue fracture under repeated flexures during the denture insertion and removal, and masticatory actions. Permanent deformation and fatigue fracture are caused by the stress created in the clasp.