Investigation of laser debonding of orthodontic ceramic brackets

Facial esthetics is an important personal and social concern. Orthodontics is a specialized branch of dentistry concerned with the development and management of irregularities and abnormalities of the teeth, jaws and face. Its aim is to produce a healthy, functional bite, creating greater resistance to disease and improving personal appearance. Correct placement of the teeth can create a good looking and healthier mouth. Orthodontic brackets are small attachements used in for orthodontics to fasten an arch wire. Three types of brackets are presently available for orthodontic bracket bonding, such as, plastic, metal and ceramic brackets. Ceramic brackets have been the preffered choice for esthetically conscious clinicians and patiens since their introduction in the 1980s. Most ceramic brackets are made of polycristalline and monocrystalline alumina. Because of their structure, they are not able to bond chemically with acrylic and diacyrlate bonding adhesive materials. Also ceramic brackets are very rigid and brittle. After treatment ceramic brackets need to be debonded from enamel surface. A major clinical concern when using ceramic brackets is the risk of enamel damage at debonding, since ceramic brackets have a high bracket bond strength. Various methods were developed for debonding orthodontic brackets: special pliers for mechanical debonding, ultrasonic debonding, special kinds of burses,electro thermal debonding and laser debonding. All these techniques have their own advatages and disadvantages. Laser application in dentistry has been investigated since generation of the first ruby laser. In dentistry, lasers are used for surgery of the soft tissues in the oral cavity, etching of the enamel surface, bleaching, tooth drilling, removing of hard dental tissues and debonding of ceramic brackets. Laser irradiation minimizes bleeding and bacterial infections. Also, there is no need for sutures and anasthesia. Lasers have been used experimantally to debond orthodontic ceramic brackets since the early 1990s. Various lasers were used in ceramic bracket debonding such as, CO2 (10600 nm), Nd:YAG (1060 nm), KrF(248 nm), XeCl (308 nm), Tm:YAP (1980 nm), GaAlAs (808 nm) and Er:YAG (2940 nm). In literature researches found that not all laser wavelengths can be usefull and effective for bracket debonding. Therefore, new possibilities for debonding are being investigated. If the incident light is reflected from or transmitted through tissue without absorption, it will not cause a thermal effect. On the other hand, if the incident light is absorbed by tissue, it will be converted into heat. In biological tissues, water molecules or macromelecules, such as, proteins and pigments are the absorbing agents. The optical properties of tissues are influenced by the optical properties of its components and concentration and distribution of those subsctances within the tissue. The composition of tooth structure is not homogenous. In the absoption spectrum of Hydroxiapatite (Ca10(PO4)6(OH)2) and enamel, absorption coefficient of every part of the tooth is different. Minimal absorption coefficient of hydroxiapatite was observed in visible and near infra-red region. Laser energy can degrade the adhesive resin as a result of the effects of thermal softening, thermal ablation, and photoablation. Generally accepted mechanism is the thermal softening of the adhesive resin. Laser energy is absorbed and converted into heat in a very thin surface layer of the bracket. The energy absorbed softens the composite resin at the opposite side of the bracket. This type of the interaction depends on the wavelength and power density of the laser source. Despite its advantages, thermal effect can cause undesirable results on dental tissues during laser irradiation. Some studies showed that an increase in intrapulpal temperature was accepted up to 5.5 C as a safety threshold value in order to prevent undesirable results after ortodontic treatment. No histological changes were observed with an intrapulpal temperature increase of 1.8 C. In this study we used a bovine mandibular incisor. Previous studies found that the adhesion to enamel and the superfacial layer of dentin showed no statistically significant differences between human and bovine teeth. Three laser systems were used for the bracket debonding procedure: Diode Laser (980 nm), Diode Laser (1470 nm) and Thulium Fiber Laser (1940 nm). Laser irrardiation was applied Continius Wave (CW) mode. The residual load needed to debond the bracket, debonding time of the orthodontic ceramic bracket and the intrapulpal thermal changes were monitored and evaluated while debonding. Intrapulpal changes were collected by a K-type thermocouple. Shear bond strength (SBS) or tension tests were done by evrensel test cihaz. We recorded maximum value for load at the debonding moment of ceramic bracket from enamel surface and called it "Load at breaking point". A student t-test was performed on results.A statistical significance level was accepted p<0.05 in this study. After the study enamel surfaces and debonding surfaces of the bracket base observed with a stereoscopic microscope, the residual adhesive on the surface of enamel or bracket was evaluated. The adhesive remnants left on the enamel surface were scored and classified using a modified Adhesive Remnant Index (ARI). Score 0 = no adhesive left on tooth surface; score 1 = less than 50% of adhesive left on tooth; 2 = more than 50% of adhesive on tooth; 3 = all adhesive left on tooth. Significantly decreased bond strength was observed when debonding ceramic brackets with a Thulium Fiber Laser (1940 nm) at a power of 3 W and exposure of 2.6 s. No significant differences were found between other power levels. A 3 W power intrapulpal temperature rise did not reach 5.5 C safety threshold. Besides, this energy level reduced the debonding time when compared to control group. At 980 nm laser treatment did not show any significant advantage any power and exposure time setting. At 1470 nm only 5 W exposure caused a decrease in debonding force, that was statistically unsignificant. From microscopic studies and measured debonding forces concluded 980 nm and 1470 nm diode lasers were not effective in laser debonding of ceramic brackets, wheares 1940 nm Thulium Fiber Laser treatment a significantly decreased debonding force and time while including less thermal effect at 3 W.