Research On Bonding Property And Colorimetry Of Individualized Tooth-colored Zirconia Ceramic Brackets

BackgroundAdults have become a large group of orthodontic patients in modern orthodontic treatment. With the development of economy and social civilized degree, the market potential of adult orthodontics is tremendous. Adults break through the restricted area of orthodontic treatment over the past century, widened the field of orthodontic treatment, but also produced a series of problems. One of the problems is that adults due to reasons such as social life and occupational factor, demand for aesthetic sense not only after orthodontic treatment, but also during the treatment. Therefore, they put forward higher request on the aesthetic of orthodontic appliance. This requires the appliance not only to achieve the purpose of correcting malocclusion, but also to be more beautiful and secluded, conforming to the physiological and psychological needs of most adult patients especially adult women patients.Aesthetic orthodontic systems mainly include labial aesthetic brackets, lingual orthodontic brackets and Invisalign orthodontic system. The labial aesthetic brackets orthodontic system contains ceramic brackets and plastic brackets. The ceramic brackets used in orthodontic clinic are alumina ceramic brackets. But the fracture toughness of alumina ceramic is not high enough, which leads to high probability of brackets fracture during the orthodontic treatment, especially the fracture of the ligature wings. Brackets fracture not only delays the process of the patients’ treatment, but also increase the danger of inhaling the fragment. In clinical applications, the plastic brackets are easy to be colored. The hardness, strength and abrasion resistance of the plastic brackets are poor, so that deformation, damage and so on creep are easy occurred in orthodontic force under load. Based on these shortcomings, plastic brackets have gradually stopped to be used. Lingual appliance system and Invisalign orthodontic system have excellent aesthetic property, but the clinical operation of lingual orthodontic is inconvenience. The lingual brackets stimulation of the tongue body is bigger and affect pronunciation and oral hygiene, etc. The indications of Invisalign orthodontic system are relatively narrow. The outcomes of Invisalign are not as good as fixed appliances, and it also requests the patients have high adherence. At present, all of the orthodontic systems have shortcomings, so it is necessary to develop new orthodontic appliance.Because of the good properties, such as high fracture toughness, strength, hardness, wear resistance, chemical stability and biocompatibility, zirconia ceramic becomes the most development prospects of new structural materials. Compared with alumina ceramic, zirconia ceramic shows higher mechanical parameters (Bending strength is900-1200MPa, the fracture toughness is9-10MPa·m1/2, which is1-2times higher than that of the alumina ceramic). But the dental zirconia ceramic is chalky, which is unable to meet the clinical requirement for color matching. If the color of zirconia ceramic itself can be closed to the color and semipermeability of the natural teeth, the aesthetic of zirconia ceramic will increase because of dispensing the unnecessary masking and reducing the thickness of the veneer porcelain. We can color the zirconia through soaking it in the staining fluid and adding colorant to the zirconia powder before dense sintering, in order to achieve coordinated aesthetic effect with natural teeth.The application of computer aided design and computer aided manufacture (CAD/CAM) system for processing all-ceramic restoration has gradually become the research hotspot of dental ceramic restoration. The CAD/CAM system has been accepted and favored by dentists since it began to be applied to dentistry in1970’s. The CAD/CAM system is mainly used in individualized lingual brackets and Invisalign technique. The application of CAD/CAM system to manufacture zirconia ceramic into individualized tooth-colored ceramic brackets has not reported now.ObjectiveTo manufacture individualized tooth-colored zirconia ceramic specimens combining CAD/CAM technique, high strength zirconia material and individualized color scheme together, and to evaluate their properties of bonding and colorimetry, in order to provide new train of thought for optimizing the processing technic of ceramic brackets, increasing the property of the materials and improving the aesthetics. To provide some references for exploitation and application of the individualized tooth-colored zirconia ceramic brackets.Materials and methodsPart Ⅰ40maxillary first premolars extracted due to orthodontic treatment which had no caries, no restorations and any other enamel defect seen by eyes were collected. The residual soft tissue on the tooth-surfaces was wiped off, and then the teeth were disinfected, washed and dried. The teeth were fixed vertically in silica gel, and scanned with3Shape D700three-dimensional (3D) dental scanner. The data were saved as STL format. The data of teeth were imported into Pro/Engineer software. The specimens were designed with CAD under the environment of Pro/Engineer. The volume of the specimens is4mmx3mmx2mm, and the specimens are located in the center of the labial surface of the clinical crown. The base form of the specimen and the corresponding tooth surface form are consistent. The data were saved as STL format. The data of the specimens were imported into WorkNC Dental software, and manufactured the Upcera zirconia blank with DWX-505-axis numerical control dental carving machine. The specimens were soaked completely into A3color of Upzir zirconia dyeing liquid for5minutes. Sintered after drying, and the temperature of sintering was1480℃, and insulated for2hours. The individualized tooth-colored zirconia ceramic specimens were obtained.Part II20maxillary first premolars extracted due to orthodontic treatment which had no caries, no restorations and any other enamel defect seen by eyes were collected. The teeth were divided into two groups randomly:the Jingjin dental enamel bonding resin group and the3M Transbond XT light curing adhesive group. The two groups of alumina ceramic brackets Fascination(?)2were bonded in accordance with the requirements to the teeth.24hours later, the shear bond force was measured by ElectroPuls E1000electronic dynamic and static universal material testing machine. Calculated the shear bond strength P(MPa)=shear bond force F(N)/bonded area S(mm2), and counted up the adhesive remnant index.Part IIIThe40specimens manufactured in Part Ⅰ were divided into4groups randomly. Group A:untreated+Jingjin dental enamel bonding resin; Group B:untreated+3M Transbond XT light curing adhesive; Group C:sandblasting with110μm alumina particle(the distance is10mm, the air pressure is0.4MPa)+Jingjin dental enamel bonding resin; Group D:sandblasting+3M Transbond XT light curing adhesive. The surface structure of the specimens’ base was observed before and after sandblasting by scanning electron microscopy. The four groups of specimens were bonded in accordance with the requirements to the corresponding teeth.24hours later, the shear bond force was measured by ElectroPuls E1000electronic dynamic and static universal material testing machine. Calculated the shear bond strength P(MPa)=shear bond force F(N)/bonded area S(mm2), and counted up the adhesive remnant index.Part IVSelected ten clinical orthodontic patients, and numbered them from①to⑩. Measured the color of the center of clinical crown on labial surfaces of maxillary incisors, and patients①to⑤were measured left, while patients⑥to⑩right. The results were recorded according to Vita classical16colors system.The maxillary silicone rubber impressions of the patients were taken and poured with high strength dental stone. The plaster models were fixed on the workbench of the3Shape D7003D dental scanner and scanned in all detections. The digital models of the scanning images were reconstructed by computer, and the data were saved as STL format. The data of teeth were imported into Pro/Engineer software. The individualized zirconia ceramic brackets were designed with CAD under the environment of Pro/Engineer. The volume of the central incisor bracket is4mmx4mmx2mm, while the lateral incisor bracket is4mm×3mm×2mm, and the brackets are located in the center of the labial surface of the clinical crown. The data of the brackets were saved as STL format. The data of the brackets were imported into WorkNC Dental software, and manufactured the Upcera zirconia blank with DWX-505-axis numerical control dental carving machine. The brackets were soaked completely into the corresponding color of Upzir zirconia dyeing liquid for5minutes according to the results of colorimetric assay. Sintered after drying, and the temperature of sintering was1530℃, and insulated for2hours. The individualized tooth-colored zirconia ceramic brackets were obtained.The chromatic values of the center of the clinical crown of the ten patients’ maxillary incisors were determined by computer-aided colorimeter. The CIE-1976-Lab chromaticity system was adopted. The zirconia ceramic brackets and the alumina ceramic brackets were temporarily fixed on the corresponding natural teeth. The chromatic values of ceramic brackets were determined. Chromatic aberrations(AE) between natural teeth and ceramic brackets were calculated.Results1. The shear bond strength of A, B, C, D four groups of zirconia ceramic specimens were4.737±1.191MPa,4.272±1.012MPa,9.762±2.039MPa,8.848±1.211MPa. There was no significant difference between the shear bond strength of the two kinds of orthodontic adhesive (P>0.05), and the shear bond strength of the sandblasting groups were significantly higher than the untreated groups(P<0.05), according to the factorial analysis of variance.2. The shear bond strength of alumina ceramic brackets Fascination(?)2and two kinds of orthodontic adhesive were10.142±2.994MPa and13.790±2.889MPa. The3M Transbond XT light curing adhesive group was significantly higher than the Jingjin dental enamel bonding resin group(P<0.05).3. The chromatic aberrations between zirconia ceramic brackets and natural teeth was3.145±1.745NBS, and the chromatic aberrations between alumina ceramic brackets and the natural teeth was6.357±1.718NBS. There was significant difference (P<0.05).Conclusions1. The shear bond strength of the individualized tooth-colored zirconia ceramic specimens can not meet the needs of orthodontic clinical bond without any surface treatment. After sandblasting with Al2O3, the shear bond strength can meet the clinical needs, while bonding with the two kinds of orthodontic adhesive. It is not easy to cause enamel fracture when removed.2. The shear bond strength of the Dentaurum polycrystalline alumina ceramic brackets Fascination(?)2can also meet the clinical needs while bonding with3M Transbond XT light curing adhesive and Jingjin dental enamel bonding resin. But removing the brackets may cause enamel fracture when bonding with3M Transbond XT light curing adhesive.3. The aesthetic effect of individualized dyeing zirconia ceramic brackets is within the clinical acceptable range. But there is still a long way to achieve optimal aesthetic effect. We have to do some deeper research in terms of personalized color matching in the future.