Bracket bond strength dependence on light power density. STAUDT, Christine Bettina, KREJCI, Ivo, MAVROPOULOS, Anestis. Abstract

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1 Article Bracket bond strength dependence on light power density STAUDT, Christine Bettina, KREJCI, Ivo, MAVROPOULOS, Anestis Abstract In order to reduce curing time for bracket bonding with light-cured composites, manufacturers increase the power density (PD) of light sources. The present study aims at investigating the relationship between PD and shear bond strength (SBS) at short exposure time. Reference STAUDT, Christine Bettina, KREJCI, Ivo, MAVROPOULOS, Anestis. Bracket bond strength dependence on light power density. Journal of Dentistry, 2006, vol. 34, no. 7, p DOI : /j.jdent PMID : Available at: Disclaimer: layout of this document may differ from the published version.

2 journal of dentistry 34 (2006) available at journal homepage: " Bracket bond strength dependence on light power density Christine Bettina Staudt a, *, Ivo Krejci b, Anestis Mavropoulos a a Division of Orthodontics, University of Geneva, Switzerland b Division of Cariology and Endodontology, University of Geneva, Switzerland article info Article history: Received 10 October 2005 Received in revised form 21 November 2005 Accepted 21 November 2005 Keywords: Orthodontic brackets Dental bonding Shear strength Light Halogen Light intensity Power density abstract Objectives: In order to reduce curing time for bracket bonding with light-cured composites, manufacturers increase the power density (PD) of light sources. The present study aims at investigating the relationship between PD and shear bond strength (SBS) at short exposure time. Methods: Stainless steel brackets were bonded to bovine incisors using light-cured adhesive. Six groups of 20 incisors each were exposed to 4 s of halogen light with different PD increasing from 500 to 3000 mw/cm 2 in steps of 500 mw/cm 2. Two more groups were exposed to a PD of 3000 mw/cm 2 for 6 s (n = 15) and 8 s (n = 19), thus simulating nonavailable PD of 4500 and 6000 mw/cm 2 for 4 s. A halogen lamp with a PD of 1000 mw/ cm 2 was used for 40 s in the control group (n = 15). After storage for 24 h at 37 8C in water, SBS and adhesive remnant index (ARI) were recorded. Results: SBS was significantly different among groups (ANOVA, p < 0.001). SBS comparable to the control group could only be achieved with a PD of at least 3000 mw/cm 2. An exponential model described the relationship between SBS and PD. The best-fit curve based on this model reached 85% of the highest possible SBS at approximately 4000 mw/cm 2. ARI scores showed that higher PD was associated with better adhesion at the bracket/adhesive interface. Conclusions: Our findings show the SBS dependence on PD, and thus provide a valuable tool for the development of light-curing systems. An exponential model suggests that SBS enters a region of saturation and cannot be improved significantly by further increasing PD. # 2005 Elsevier Ltd. All rights reserved. 1. Introduction Bonding brackets is an important procedure for every orthodontist. Main issues to be considered are the stability of the appliance bonded and the clinical time spending. Manufacturers of light-curing units for bonding of orthodontic brackets aim at reducing exposure time. One possible way is to increase the power density of their devices, 1,2 since it is a well-known physical law that energy (J) is the product of power (W) and time (s) and energy density (J/cm 2 )the product of power density (W/cm 2 ) and exposure time (s). 3 Energy density of the light-curing procedure influences the degree of conversion, depth of cure and mechanical properties of resin composites Power density expresses the rate of delivered photons per unit surface, and thus determines the rate of free radicals generated in the composite, while exposure time, at a given power density, determines the total number of photons, and thus of free radicals. 14 Since free radicals initiate polymerisation, power density and exposure time are responsable for the velocity and degree of polymerisation, and thus among others, for the mechanical properties of a given resin composite. 13,15,16 However, it has * Corresponding author. Present address: Faculté demédecine, SectiondeMédecineDentaire, Division d Orthodontie, 19, ruebarthélemy- Menn, CH-1205 Geneva, Switzerland. Tel.: ; fax: address: christine.staudt@medecine.unige.ch (C.B. Staudt) /$ see front matter # 2005 Elsevier Ltd. All rights reserved. doi: /j.jdent

3 journal of dentistry 34 (2006) been suggested that there may be no benefit to polymerisation of resin composite when power density is increased above a certain value 14 and conversion profiles corroborating this finding have been observed. 10 In a previous study, the dependence of bracket shear bond strength on exposure time was investigated. It was found that exposure time can be reduced substantially when using the maximum up to date available power density. 2 Now we are interested to know what is the minimum necessary power density in order to achieve sufficient shear bond strength at a short exposure time. The degree of conversion of a composite with increasing power density is characterised by a trend towards saturation. 10,14 The strong relationship between the extent of conversion and the mechanical properties of a resin composite is well-documented. 4,13,15,16,20 Therefore, it is hypothesised that bracket shear bond strength improves with increasing power density to a certain extent only. The objective of the present study is to test this hypothesis by investigating the relationship between shear bond strength and power density at a short exposure time. 2. Materials and methods A hundred and sixty-nine deciduous bovine incisors were stored in 0.2% thymol after extraction. The teeth were cleaned with pumice and stainless steel brackets were bonded using a standard light-cured adhesive system (Table 1). The technical characteristics of the curing devices under investigation are described in Table 2, the groups are defined in Table 3. The test groups were bonded with a calibrated, extremely powerful halogen lamp (Swiss Master Light, see Table 2), which allowed for modification of power density up to 3000 mw/cm 2. Six groups of 20 teeth each were formed and exposed to different light power density increasing from 500 to 3000 mw/cm 2 in steps of 500 mw/cm 2. An exposure time of 4 s was used since it was shown to be the critical time for achieving sufficient shear bond strength at high power density. 2 Two more groups of 15 and 19 teeth were formed and exposed to a power density of 3000 mw/cm 2 for an exposure time of 6 and 8 s, respectively, thus simulating non-available power density values of 4500 and 6000 mw/ cm 2 for 4 s. The manufacturer of the powerful halogen light suggested not using more than 4 s of curing time at once. Therefore, the curing time of 6 and 8 s was cut in half: 3 s respectively 4 s of curing were applied first to the mesial side, and then to the distal side of the bracket. A standard halogen lamp (Optilux 501 with turbo light guide, see Table 2) with a power density of about 1000 mw/cm 2,asmeasured with the built-in radiometer, was used for 40 s (20 s on the mesial and 20 s on the distal side of the bracket) in the control group (n = 15). Bracket shear bond strength was measured with a universal testing machine (Model 1114, Instron Corp., Canton, MA, USA). In order to ensure that the bracket base would be parallel to the applied force, a uniform embedding procedure was followed. Each bonded tooth was embedded into a rectangular acrylic block (Technovit 4071, Lot , Ch.-B , Heraeus Kulzer, Wehrheim, Germany). For this purpose, a metallic cubic blade (2 mm 2 mm) was fixed into the vertical slot of the bracket. The blade was laid over the mould in such a way that the bracket would be situated in the middle of the acrylic block and that the bracket base would be parallel to the borders of the block. The acrylic was poured and the hardened block with the bonded tooth was stored for 24 h at 37 8C in water. It then was secured onto the lower part of the testing machine. The shear bond force was applied by a rod. Due to our special embedding procedure the direction of the shear force exerted by the rod was parallel to the tooth bracket interface and the extremity of the rod was parallel to the occlusal border of the bracket base. The samples were stressed in an occlusogingival direction with a crosshead speed of 0.5 mm/min, as recommended by Eliades and Brantley. 17 The force at bracket failure was recorded in Newton (N) and converted into MegaPascal (MPa), by dividing Newtons by the surface area mm 2 of the bracket base. The adhesive remnant index (ARI) was recorded as well Statistical analysis Mean, standard deviation and 95% confidence interval were calculated for the shear bond strength of each group. Differences among the shear bond strength of different power density groups were detected by ANOVA, followed by the posthoc LSD test. The level of significance was set at p < Nonlinear regression was performed in order to analyze the Table 1 Tooth preparation and bonding procedure Procedure Time (s) Material Product and manufacturer Cleaning 15 Fluoride-free pumice Bimsstein Pulver (Prochimie, Avenches, CH) Rinsing 20 Tap-water Drying 5 Oil-free air Etching 30 35% Phosporic acid Transbond XT etching gel, no (3M Unitek, Monrovia, CA, USA) Rinsing 30 Tap-water Drying 5 Oil-free air Bonding Primer Transbond XT light-cured adhesive primer, no (3M Unitek, Monrovia CA, USA) Composite Transbond XT light-cured paste, no (3M Unitek, Monrovia CA, USA) Stainless steel bracket Mini diamond twin for tooth 11, no (Ormco, Orange CA, USA)

4 500 journal of dentistry 34 (2006) Table 2 Technical characteristics of investigated curing devices Curing device Serial number and manufacturer Type Maximum power density (mw/cm 2 ) Wavelength (nm) Tip diameter (mm) Swiss Master light M 00042, EMS (Nyon, CH) Halogen Optilux , Sybron (Danbury CT, USA) Halogen Table 3 Group allocation and descriptive statistics for shear bond strength (MPa) Group Curing device Power density (mw/cm 2 ) Exposure time (s) Mean * S.D. 95% CI Minimum Maximum Lower Upper 1 Swiss Master Swiss Master , Swiss Master , Swiss Master , Swiss Master ,2, Swiss Master , a Swiss Master b Swiss Master Optilux S.D., standard deviation; CI, confidence interval. a Corresponds to a power density of 4500 mw/cm 2, for an exposure time of 4 s. b Corresponds to a power density of 6000 mw/cm 2, for an exposure time of 4 s. * Significant difference ( p < 0.05) of the labeled group to the designated groups 1 9 (in superscript). relationship between shear bond strength and power density. The Kruskal Wallis test was used to detect differences among groups regarding the ARI. In order to test pair-wise differences between groups, the Mann Whitney test was applied and the level of significance was corrected to p < according to Bonferroni. Statistical analysis was performed with SPSS software (Version 11.5, SPSS, Chicago, IL). 3. Results Our findings showed that power density had a significant effect on shear bond strength ( p < 0.001) (Table 3). For an exposure time of 4 s, the shear bond strength obtained with a power density of 3000 mw/cm 2 was significantly higher in comparison to values obtained with 500 ( p < 0.001), 1000 ( p < 0.01), and 1500 ( p < 0.05) mw/cm 2. Shear bond strength comparable to the control group could only be achieved with a power density of at least 3000 mw/cm 2. Non-linear regression showed that a model of exponential association could describe the relationship between power density and shear bond strength. The function of the general model is: fðxþ ¼y max ð1 expð k ðx aþþþ: The coefficients for our data are given in Table 4. The best-fit curve (Fig. 1) based on the model crosses the abscissa at a power density of approximately 150 mw/cm 2.Then,with increasing power density, the shear bond strength increases rapidly. In the upper part of the curve saturation takes place, so that shear bond strength cannot be improved significantly by further increasing power density. The horizontal asymptote of Fig. 1 Shear bond strength as a function of power density. An exponential model (see Table 4) describes the relationship between shear bond strength and power density. The best-fit curve based on this model shows that shear bond strength enters a region of saturation at a power density of approximately 4000 mw/cm 2. Table 4 Exponential regression coefficients (with 95% confidence interval) for shear bond strength as a function of power density Curing device Swiss Master y max (MPa) (16.65, 29.16) k (1/mW/cm 2 ) (8.805e S 005, ) a (mw/cm 2 ) (S407.9, 711.1) R General model: f(x)=y max (1 S exp(sk(x S a))).

5 journal of dentistry 34 (2006) Table 5 Frequency distribution (%) of the adhesive remnant index (ARI) c Group Curing device Power density (mw/cm 2 ) Exposure time (s) Difference * to group Swiss Master Swiss Master Swiss Master Swiss Master Swiss Master Swiss Master a Swiss Master b Swiss Master Optilux a Corresponds to a power density of 4500 mw/cm 2 for an exposure time of 4 s. b Corresponds to a power density of 6000 mw/cm 2 for an exposure time of 4 s. c ARI score 18 : 0, no adhesive left on tooth; 1, less than half of adhesive left on tooth; 2, more than half of adhesive left on tooth; 3, all adhesive left on tooth. * Significant difference ( p < 0.005) of the labeled group to the designated groups 1 9, concerning the ARI. the curve lies at a shear bond strength of approximately 23 MPa. In order to reach 80% of the highest possible shear bond strength, a power density of more than 3000 mw/cm 2 is necessary, in order to reach 85% almost 4000 mw/cm 2,andin order to reach 90% almost 5000 mw/cm 2. The effect of power density on the adhesive remnant index was statistically significant ( p < 0.001). Results concerning differences between respective groups are presented in Table 5. A power density of at least 2500 mw/cm 2 was necessary in order to obtain ARI scores similar to the control. 4. Discussion Our findings show that power density significantly influences shear bond strength at a short exposure time. Power density values of up to 2500 mw/cm 2 did not achieve sufficient shear bond strength. A power density of 3000 mw/cm 2 for 4 s seems to be necessary to bond metallic brackets. In general, higher power density resulted in stronger shear bond strength. Possibly, polymers exposed to higher power density exhibited higher conversion 5,19 due to a combination of both photo and thermal effects. 5 The extent of conversion relates to the mechanical properties of a resin composite, 4,13,15,16,20 for example, the relationship between double bond conversion and final flexural strength has been explained by a linear model. 5 Therefore, possible increase in conversion in our setting may as well have lead to improvement of mechanical properties, and thus to increased shear bond strength. According to the ARI, an increase in power density tends to enhance adhesion at the bracket/composite interface. This result is supported by other studies, although they compensate high power density by a decrease of exposure time. 21,22 The results of the present study suggest that shear bond strength did not increase proportionally with power density. An exponential fit was found to capture their relationship best. In fact, an exponential model is consistent with the process of free radical polymerisation, where the quantity of monomers reacting to polymers decreases with a rate proportional to the quantity of monomers left. This means that the smaller the quantity of monomers becomes in the course of polymerisation, the slower it decreases. Correspondingly, conversion increases slower 5,10 and remains incomplete even at maximum power density since network development restricts the mobility of reacting constituents. 10 The behaviour of shear bond strength in our study resembles the exponential conversion profiles. Dissimilarity exists close to the origin of the coordinate system, where conversion takes place quite immediately, 10 but shear bond strength can only be measured above a power density of 152 mw/cm 2. Thereafter, similar to conversion, shear bond strength increases rapidly until saturation takes place in the upper part of the curve. This means that an increase in power density improves shear bond strength less and less. For an exposure time of 4 s, no substantial increase of shear bond strength should be expected above a power density of approximately 4000 mw/cm 2. This principle agrees well with another study, 14 suggesting that there may be no benefit to mechanical properties of resin composite when power density is increased above the value of approximately 1000 mw/cm 2.In our study, greater power density was required, probably due to the need of light scattering and indirect polymerisation in the presence of a metallic bracket covering the composite. The existence of saturation is an important finding for the development of light-curing systems, since it indicates an upper limit of useful power density so that unneeded thermal challenge of the tooth can be avoided. At a power density of 3000 mw/cm 2 for an exposure time of 4 s, as recommended for bracket bonding by our study, no thermal damage to the pulp should be expected. 23 According to the general law that energy density is the product of power density and exposure time, it is welldocumented that total energy density influences degree of conversion and mechanical properties of resin composites However, it has recently been supported that not only energy density but also the different combinations of power density and exposure time delivering the same energy density play a role. 14,13 Concerning flexural strength, differences related to different combinations of power density and exposure time were found for low but not for high total energy density values. 13 This justifies extrapolation of values in the region of high energy density and allowed us to simulate non-available power density values. For lower energy density values, the influence of different combinations of power density and

6 502 journal of dentistry 34 (2006) exposure time on shear bond strength has to be investigated further before extrapolations will be possible. In addition, it has to be stressed that the present findings are valid for the specific adhesive system used only, which yet is the most popular for bracket bonding. Results may differ for other adhesive systems, since the degree of conversion depends on material characteristics as type and concentration of photoinitiator, monomers and filler as well as composite shade and thickness. 14,19,24,25 Nevertheless, the model applied to our data may be useful for the development of light-curing systems. 5. Conclusions We demonstrated experimentally that power density significantly affects bracket shear bond strength. Our findings show that the maximum up to date available light power density of 3000 mw/cm 2 for 4 s is indeed the critical value for sufficient bond strength of metallic brackets and should not be reduced. An exponential model can be used to describe the dependence of shear bond strength on power density, and thus provides a valuable tool for the development of light-curing systems. This model suggests that shear bond strength enters a region of saturation, where no substantial improvement of shear bond strength should be expected by increasing power density even further. Acknowledgement We would like to thank Dr. M. Cattani-Lorente for her kind help in performing the shear bond strength measurements. references 1. Oesterle LJ, Newman SM, Shellhart WC. Rapid curing of bonding composite with a xenon plasma arc light. American Journal of Orthodontics and Dentofacial Orthopedics 2001;119: Staudt CB, Mavropoulos A, Bouillaguet S, Kiliaridis S, Krejci I. Light-curing time reduction with a new high-power halogen lamp. American Journal of Orthodontics and Dentofacial Orthopedics, in press. 3. Yoon TH, Lee YK, Lim BS, Kim CW. 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