Lasers Med Sci (2015) 30:869–874 DOI 10.1007/s10103-013-1482-3

ORIGINAL ARTICLE

CO2 laser debonding of a ceramic bracket bonded with orthodontic adhesive containing thermal expansion microcapsules Ayano Saito & Yasuhiro Namura & Keitaro Isokawa & Noriyoshi Shimizu

Received: 12 August 2013 / Accepted: 23 October 2013 / Published online: 13 November 2013 # Springer-Verlag London 2013

Abstract We have been studying an easy bracket debonding method using heating of an orthodontic adhesive containing thermal expansion microcapsules. However, heating with a high-temperature heater brings obvious risks of burns around the oral cavity. Thus, we examined safer and more effective bracket debonding methods. The purpose of this in vitro study was to examine the reduction in debonding strength and the time taken using a bracket bonded with an orthodontic adhesive containing thermal expansion microcapsules and a CO2 laser as the heating method while maintaining safety. Ceramic brackets were bonded to bovine permanent mandibular incisors using bonding materials containing various microcapsule contents (0, 30, and 40 wt%), and the bond strengths were measured after laser irradiation for 4, 5, and 6 s and compared with nonlaser-treated groups. Subsequently, the temperature in the pulp chamber during laser irradiation was measured. After laser irradiation for 5 or 6 s, the bond strengths of the A. Saito Nihon University Graduate School of Dentistry, 1-8-13 Kandasurugadai, Chiyoda-ku, Tokyo 101-8310, Japan Y. Namura : N. Shimizu (*) Department of Orthodontics, Nihon University School of Dentistry, 1-8-13 Kandasurugadai, Chiyoda-ku, Tokyo 101-8310, Japan e-mail: [email protected] Y. Namura : N. Shimizu Division of Clinical Research, Dental Research Center, Nihon University School of Dentistry, 1-8-13 Kandasurugadai, Chiyoda-ku, Tokyo 101-8310, Japan K. Isokawa Department of Anatomy, Nihon University School of Dentistry, 1-8-13 Kandasurugadai, Chiyoda-ku, Tokyo 101-8310, Japan K. Isokawa Division of Functional Morphology, Dental Research Center, Nihon University School of Dentistry, 1-8-13 Kandasurugadai, Chiyoda-ku, Tokyo 101-8310, Japan

adhesive containing 40 wt% microcapsules were significantly decreased to ∼0.40-0.48-fold (4.6–5.5 MPa) compared with the nonlaser groups. The mean temperature rise of the pulp chamber was 4.3 °C with laser irradiation for 6 s, which was less than that required to induce pulp damage. Based on these results, we conclude that the combined use of a CO2 laser and an orthodontic adhesive containing thermal expansion microcapsules can be effective and safe for debonding ceramic brackets with less enamel damage or tooth pain. Keywords Debonding . CO2 laser . Orthodontic adhesive containing thermal expansion microcapsules . Bond strength

Introduction In conventional removal methods for orthodontic brackets, a strong load must be applied physically to the bracket and tooth; this can sometimes cause fractures of the enamel and intense tooth pain. In particular, ceramic brackets offer excellent esthetic results compared with metal brackets, but they are brittle, and it is time consuming to remove the bracket base when the bracket wings are broken. Although it has been reported that enamel fractures occurred with a bond strength as low as 9.7 MPa [1], those of the commercially available orthodontic adhesives are much higher [2–5]. To reduce side effects during debonding, such as enamel fracture, tooth pain, and bracket breakage, several methods have been proposed [6–11]. Tsuruoka et al. [12] reported using an orthodontic adhesive (SuperBond) containing thermal expansion microcapsules, which expand 70-fold at 80 °C, that the bond strength of metal brackets bonded with the adhesive containing 30 or 40 wt% thermal expansion microcapsules decreased to 1/3 (7 MPa) or 1/5 (3 MPa) with heating versus the unheated groups. Using a ceramic bracket bonded with adhesive containing 40 wt% thermal expansion

870

microcapsules, Ryu et al. [13] reported that bond strength decreased to approximately 1/3 (4.5 MPa) versus the unheated groups. In these studies, a heater was used to expand the microcapsules. However, the use of the heater at up to 300 °C is accompanied by the risk of burns around the mouth, and so a safer method should be developed. Recently, various laser products have come to be widely used by dentists for purposes such as a surgical knife for soft and hard tissues, caries removal, pain relief, and bioactivation of tissues [14]. Of the lasers used, CO2 lasers are absorbed well by ceramic brackets and have been used in laser debonding [15–17]. Although some studies of debonding using CO2 lasers have been conducted, problems such as long irradiation time [18] and inadequate decreases in bond strength [18, 19] were reported. Because the energy of laser irradiation is concentrated only around the focus point, the bracket can be heated more effectively than with a conventional heater. Thus, we sought to address the conventional problems in debonding using laser irradiation together with thermal expansion microcapsules. Specifically, we examined the use of laser irradiation as a heating source for debonding ceramic brackets bonded with orthodontic adhesive containing thermal expansion microcapsules (ATEM). The purpose of this in vitro study was to examine the reduction in debonding strength and time taken for a ceramic bracket bonded with orthodontic adhesive containing thermal expansion microcapsules using a CO2 laser as the heating method while maintaining safety.

Materials and methods Preparation of orthodontic ATEM We experimentally produced 4-META/MMA-TBB resin orthodontic adhesives (Orthomite SuperBond, Sun Medical, Moriyama, Japan) containing 30 and 40 wt% thermal expansion microcapsules, which expanded by 70-fold upon heating to 80 °C (Matsumoto Microsphere F-36D, Matsumoto YushiSeiyaku, Osaka, Japan) in the polymer powder, according to our previous studies [12, 13].

Lasers Med Sci (2015) 30:869–874

Tooth specimens and bracket bonding In total, 96 freshly extracted bovine permanent mandibular incisors were collected from a slaughterhouse. The criteria for tooth selection were intact labial enamel with no cracks caused by the extraction forceps and the absence of caries. The teeth were divided randomly into 12 groups of eight specimens each, corresponding to the number of variables tested. Soft tissues were removed from the teeth. After separating the crown from the root, the pulp was extirpated, and the crown was stored in distilled water until further use. Then, the crown was embedded in self-curing acrylic resin (Tray Resin, Shofu, Kyoto, Japan) to facilitate its placement in the testing machine. The labial surface of each crown was flattened to facilitate the application of shearing force and polished with waterproof #400 and #600 silicon-carbide papers. The enamel surfaces were rinsed with water and dried in an oil-free airstream. One operator bonded 96 zirconium ceramic maxillary central incisor brackets (Coby, Biodent, Tokyo, Japan) with a 0.018-in. slot as follows. The enamel surface was etched for 30 s, rinsed, and dried according to the manufacturer's instructions. After placing ATEM (with microcapsules at 0, 30, and 40 wt%) on the bracket base and positioning the bracket at the center of the treated enamel surface, the bracket was firmly bonded. Excess adhesive was then removed with a dental probe without disturbing the bracket. After polymerization for 10 min, the specimens were immersed in distilled water at 37 °C for 24 h. The bracket-base surface area was 12.4 mm2. Shear bond strength measurement The bond strengths of 96 specimens irradiated with a CO2 laser for an irradiation time such that the temperature of the bracket base exceeded 80 °C were measured using a universal testing machine (5567, Instron, Norwood, MA, USA) in shear mode. Tests were performed 10 min following irradiation. Specimens were secured in the lower jaw of the machine so that the bracket base of the sample paralleled the direction of the shear force. The specimens were stressed in an occlusogingival direction at a cross-head speed of 1 mm/ min. The maximum loading was determined as the bond strength.

Bracket-base temperature upon CO2 laser irradiation Measurement of the temperature increase in the pulp chamber We first evaluated bracket-base temperatures. Briefly, the tip of the CO2 laser (Opelaser Pro, Yoshida, Tokyo, Japan) head contacted the labial face of a bracket, and the increase in the bracket-base temperature was measured using a K-type thermocouple sensor (Okazaki Manufacturing, Kobe, Japan) head (0.5 mm diameter) in contact with the bracket base, with an irradiation output of 3 W for a few seconds.

To evaluate the temperature increase in the pulp chamber during and after heating, we used five fresh human permanent first premolars that had been extracted in orthodontic treatments. The protocol for this experiment was reviewed and approved by the Nihon University Department of Dentistry Ethics Committee. The extracted teeth were stored in distilled

Lasers Med Sci (2015) 30:869–874

871

water at 4 °C until use. Brackets were bonded to the teeth with bonding ATEM containing 40 wt% microcapsules. Each tooth was drilled with an air turbine from the lingual cementoenamel junction towards the labial bonded bracket. The 0.5-mm-diameter sensor head of a K-type thermocouple was placed in contact with the inner surface of the pulpal wall, facing the labial surface where the bracket was bonded. The average distance from the labial surface to the pulpal wall was 3.4±0.5 mm. The temperature of the inner surface of the pulp wall was measured at a room temperature of 23±1 °C, and the measurement was performed five times for each of three heating times. Assessment of residual adhesive After the measurement of shear bond strength, each specimen was examined under an optical microscope (SZ-3003; As one, Osaka, Japan) at ×15 magnification to identify the fracture pattern of the bonded surface. The residual adhesive remnant index (ARI) was scored as follows: 0, no adhesive remaining; 1,

CO2 laser debonding of a ceramic bracket bonded with orthodontic adhesive containing thermal expansion microcapsules.

We have been studying an easy bracket debonding method using heating of an orthodontic adhesive containing thermal expansion microcapsules. However, h...
301KB Sizes 0 Downloads 0 Views