- Email: [email protected]
Cuspal movement and microleakage in premolar teeth restored with resin-based filling materials cured using a ‘soft-start’ polymerisation protocol
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 637–643
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Cuspal movement and microleakage in premolar teeth restored with resin-based filling materials cured using a ‘soft-start’ polymerisation protocol G.J.P. Fleming a,∗ , R.R. Cara b , W.M. Palin b , F.J.T. Burke c a
Materials Science Unit, Division of Oral Biosciences, Dublin Dental School & Hospital, Trinity College Dublin, Dublin 2, Ireland Department of Dental Materials, University of Medicine and Pharmacy, 400006, Clujnapoca, Cluj, Romania c Primary Dental Care Research Unit, University of Birmingham School of Dentistry, St. Chad’s Queensway, Birmingham B4 6NN, United Kingdom b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. To investigate the effect of polymerisation shrinkage strain of four posterior filling
Received 5 July 2005
materials on cuspal movement, degree of conversion (DC) and cervical gingival microleak-
Received in revised form
age of mesio-occlusal-distal (MOD) restorations placed incrementally in maxillary premolar
23 February 2006
teeth using a ‘soft-start’ polymerisation protocol.
Accepted 6 June 2006
Methods. Forty sound extracted upper premolar teeth were subjected to standardised preparation of a large MOD cavity before restoration. A ‘soft-start’ polymerisation curing regimen was used and each posterior filling material was placed in eight incre-
Keywords:
ments with the appropriate bonding system. A twin channel deflection measuring gauge
Cuspal movement
allowed a measurement of individual cusp deflections at each stage of polymerisa-
Gingival microleakage
tion. Restored teeth were thermocycled before immersion in a 0.2% basic fuchsin dye
Resin-based composite
for 24 h. After sagittal sectioning of the restored teeth in a mesio-distal plane, the
Ormocer
sectioned restorations were examined to assess cervical microleakage. The DC was
Soft-start polymerisation
also assessed using a diffuse-reflectance accessory on a Fourier transform infra-red spectrophotometer. Results. A significant increase in cuspal movement recorded for Z100 (20.06 ± 4.71) compared with Filtek Z250TM (16.52 ± 3.26), P60 (14.23 ± 3.71) and Admira (11.11 ± 2.47). No significant reduction in cuspal movement was identified when compared with a previous study [6] where a full-intensity standard polymerisation protocol was employed. No significant differences were also identified between the materials when the cervical gingival microleakage scores or DC were examined for the ‘soft-start’ compared with the standard polymerisation protocol. Conclusions. Although material type remained a significant factor, the use of a ‘soft-start’ polymerization compared with a standard curing regime did not offer any significant reduction in associated cuspal movement, DC or gingival microleakage at the cervical dentine cavosurface margin of the cavities restored with the resin-based filling materials. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +353 1 612 7371; fax: +353 1 612 7297. E-mail address: garry.fl[email protected] (G.J.P. Fleming). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.06.002
638
1.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 637–643
Introduction
The patenting of a novel resin-based composite (RBC) based on a highly viscous dimethacrylate monomer, synthesised from the reaction of bisphenol-A and glycidyl methacrylate (BisGMA) revolutionised resin technology in the late 1950s [1,2]. The addition of a co-monomer triethyleneglycol dimethacrylate (TEGDMA) was necessary to decrease the viscosity of the mixture and aid incorporation of filler particles [2]. At present there are a number of methacrylate-based materials with varying monomeric formulations and filler contents available to general dental practitioners. However, the freeradical polymerisation of dimethacrylate monomers during light irradiation involves bulk contraction [3] and as a result polymerisation shrinkage manifested as shrinkage stress [4]. Cuspal movement on polymerisation [5–10] may compromise the synergism at the restoration–tooth interface [3] possibly leading to bacterial micro-leakage [11–13] and ultimately to marginal staining, pulpal inflammation or necrosis and secondary caries [11]. Manufacturers tend to address the issues of excessive contraction on setting by introducing relevant material constraints including the replacement of the diluent TEGDMA from the monomeric formulations. Increasing the filler content was also reported to be associated with a decrease in shrinkage and therefore a decreased cuspal movement on polymerisation [6]. Fleming et al. [6] confirmed these findings by highlighting that a reduction in the reported volumetric polymerisation shrinkage or an increase in the filler content (for given resin constituents) resulted in a reduction in the associated cuspal strain on mesio-occlusal-distal (MOD) cavities. In addition to varying the monomeric formulation and filler content of methacrylate and non-methacrylate-based filling materials it has been proposed that the associated shrinkage stress can be controlled by varying the curing conditions. Several reports have suggested that pre-polymerisation at lower intensity followed by post-cure at full intensity results in RBC restorations with improved marginal integrity [12–14], reduced post-gel contraction [15] and superior physical properties [13,14]. However, other studies have reported the ‘softstart’ light curing phenomenon resulted in RBC restorations with no improvement in marginal integrity [16] and reduced physical properties [17] compared with standard curing protocols. The conflicting data is a result of the curing conditions namely, exposure time, intensity and distance of the curing light from the samples. Laboratory tests are also generally performed using stainless steel or black-nylon moulds which may fail to incorporate light reflectance properties of enamel, dentine or the matrix band associated with the restoration of these materials in vivo [15]. As a result rather than employing indirect materials science techniques to evaluate the magnitude of the polymerisation shrinkage stress, the authors have employed a direct clinically relevant protocol on large MOD cavities such that the geometry resulted in a high C-factor with high shrinkage stress. The RBC materials chosen in the current study were Z100, Filtek Z250TM and P60 (3M ESPE, St. Paul, MN, USA). The principal monomers in Z100 are BisGMA and TEGDMA so that Z100 has an increased concentration of carbon-to-carbon dou-
ble bonds (C C) compared with Filtek Z250TM and P60 where TEGDMA is replaced with urethane dimethacrylate (UDMA) and derivatives of BisGMA, such as bisphenol-A ethoxylated dimethacrylate (Bis-EMA). The manufacturers of Filtek Z250TM report that it has a decreased filler loading compared with P60 and has reduced polymerisation shrinkage. Admira (Voco, GmbH, Cuxhaven, Germany) is an Ormocer (organically modified ceramic) and contains methacrylate groups that photochemically induce organic polymerisation following light irradiation [18]. The aims of the study were to assess cuspal deflection at each stage of ‘soft-start’ polymerisation for the incremental restoration of standardised large MOD cavities with four posterior filling materials using a twin channel deflection measuring guage. The DC was assessed using a Fourier transform infra-red (FTIR) spectroscopy technique to determine the conversion of the methacrylate species. The performance of the restorations was further investigated by assessing the cervical dentine cavosurface margin for gingival microleakage following thermocycling and immersion in 0.2% basic fuchsin dye prior to sagittal sectioning. The hypothesis proposed the ‘sortstart’ curing regime would not influence cuspal deflection, DC or gingival microleakage compared with when the toothcoloured materials were light cured in accordance with the conventional curing protocol advocated by the manufacturers.
2.
Materials and methods
This section is similar to those reported previously [5–7] by the authors.
2.1.
Tooth selection and cavity preparation
Extracted teeth were stored in buffered formal saline for 24 h post-extraction and then in water at room temperature (23 ± 1 ◦ C) except when aspects of the experimental procedure require isolation from moisture. Forty sound extracted upper premolar teeth had surface deposits carefully removed using a hand scaler and were selected if, following visual examination, it was identified they were free from hypoplastic defects and cracks. Into a cubic stainless steel mould (15 mm3 ) which had a central cylindrical hole of 12 mm diameter, each tooth was fixed crown uppermost and long axis vertical using a chemically cured resin (De Trey Formatray: DeTrey, Dentsply, Weybridge, UK). The teeth were fixed so that the resin extended to within 2 mm of the amelocemental junction (ACJ). The maximum bucco-lingual width (BLW) for each tooth was measured with a micrometer screw guage (Moore and Wright, Sheffield, England) accurate to 10 m. The dimensions were used to distribute the specimens into four groups (A, B, C and D), each group consisting of 10 teeth, so that the mean BLW of each group of teeth differed by not more than 5% (Table 1). Each tooth was subjected to preparation of a large MOD cavity, with the BLW of the approximal boxes of the cavity being prepared to two-thirds of the BLW of the tooth and the occlusal isthmus being prepared to half the BLW. The cavity depth at the occlusal isthmus was standardised (3.5 mm from the tip of the palatal cusp and to 1 mm above the ACJ at
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 637–643
Table 1 – Dimensions of the premolar teeth (m) highlighting no statistical differences between the groups restored using soft-start polymerisation Mean (S.D.) Z100 (group A) Filtek Z250TM (group B) P60 (group C) Admira (group D)
9.16 (0.31) 9.09 (0.29) 9.17 (0.32) 9.21 (0.34)
the cervical aspect of the approximal boxes). To ensure consistency in cavity preparation the facial and lingual walls of the cavity were prepared parallel [5].
2.2.
Cuspal deflection
Groups A–C were restored with Z100, Filtek Z250TM and P60, respectively, in conjunction with the associated bonding system (Scotch Bond 1; 3M ESPE St. Paul, MN, USA). In accordance with the manufacturer’s guidelines, the tooth surfaces were dried following the MOD cavity preparation and 37% phosphoric acid etching gel was applied for 15 s, before rinsing with water for 10 s. The cavity surfaces were briefly dried with compressed air and damp cotton wool pledgets before two consecutive coats of the adhesive were applied to the etched surfaces with a fully saturated brush tip [19]. The tooth surfaces were lightly dried with compressed air for 2–5 s. Groups A–C were light cured for 10 s with an Optilux 501 halogen light (Kerr Mfg. Co., Orange, CA, USA) operating at a light intensity of 740 ± 29 mW cm−2 and the restored teeth were isolated to avoid moisture contamination. Group D was restored using the Ormocer material (Admira) using its associated bonding agent (Admira bond: VOCO GmbH, Germany) in accordance with the manufacturer’s instructions [20]. The tooth surfaces were again dried, etched with the supplied gel (Vococid TM Phosphoric acid) for 15 s rinsed with water and air dried with compressed air. The cavity surfaces were coated with Admira bond, for 30 s before being dispersed with air. The bonding agent was then light cured continuously for 20 s with the halogen light and the teeth were again isolated to avoid moisture contamination. The buccal and lingual cusps of the extracted teeth were approximated to the receptors of a Twin Channel Deflection Measuring Gauge (Twin Channel Analogue Gauge Unit: Thomas Mercer Ltd., St. Alban’s, UK) following the standardised cavity preparation. To standardise placement, the teeth were fixed with the palatal measuring gauge placed approximately 2.5 mm from the palatal cusp tip [5–7]. A baseline measurement was recorded and tooth restoration started with the mesial approximal box, packing the appropriate composite in increments against a stainless steel sectional matrix (3M sectional matrix system: 3M ESPE, St. Paul, MN, USA) wedged firmly against the approximal aspects of the teeth. The increment sequence involved the placement of eight nominally triangular-shaped increments, three for each approximal box and two for the occlusal surface, with each increment touching only one cavity wall [5–7]. Each increment was cured using the halogen light with an 11 mm light-curing tip exit diameter, for the manufacturers recommended light exposure of 40 s for Z100 and 20 s for FiltekTM Z250, P60 and Admira. The
639
light-curing tip exit diameter was placed level with the upper surface of each cusp to ensure a standard curing distance from each increment (as each cavity depth was standardised to 3.5 mm from the palatal cusp tip to the ACJ) and irradiated in ‘one-hit’ for each increment. An initial 10 s ‘soft-start’ cure, from 0 to 740 ± 29 mW cm−2 at a rate of 68 ± 7 mW cm−2 s−1 , was employed for each material plus the remainder of the recommended cure time (30 s for Z100, and 10 s for FiltekTM Z250, P60 and Admira) in standard mode (740 ± 29 mW cm−2 ). Following each stage of polymerisation, measurement of cuspal deflection was recorded after 3 min, to allow time for stress relaxation, resulting in eight measurements for each individual/tooth cusp. The data for the buccal and palatal cusp deflections were also combined to give the total cuspal deflection assuming no cuspal recoil for each increment of composite. The results were subjected to statistical analysis by ANOVA techniques and by Scheffe’s multiple post hoc comparison procedure test.
2.3.
Microleakage
The restored teeth were finished, the root apices sealed with sticky wax and all tooth surfaces were sealed with nail varnish, with the exception of a 1 mm band around the margins of each restoration, and the teeth replaced in water when the varnish dried [5–7]. Thermocycling was carried out between two water-baths maintained at 65 ± 1 and 4 ± 1 ◦ C, respectively [21,22], so that the restored teeth were submerged for 10 s with a 25 s transfer from water-bath to water-bath for the time equivalent of 500 cycles. Following immersion in 0.2% basic fuchsin dye for 24 h the teeth were sectioned mid-sagitally in a mesio-distal plane using a diamond cutting saw (Struers, Glasgow, Scotland) with a ceramic disc operating at a speed of 125 rpm with an applied load of 100 g. Sectioned restorations were examined under a stereo-microscope (Wild M3C, Heerburg, Switzerland) at 25× magnification and the extent of the cervical microleakage was recorded. Accordingly [5–7] the degree of cervical margin microleakage was scored: • 0 = no evidence of dye penetration. • 1 = superficial penetration not beyond the amelodentinal junction (ADJ). • 2 = penetration beyond the ADJ but not the cervico-axial line angle. • 3 = penetration along the axial wall. • 4 = penetration into the pulp chamber. The resultant microleakage data was analysed using a non-parametric one-way ANOVA (Kruskal–Wallis) followed by paired group comparisons using Mann–Whitney U-tests at a 95% significance level.
2.4.
Degree of conversion
The DC of uncured and cured samples of Z100, FiltekTM Z250, P60 and Admira were analysed on a FTIR spectrometer (Nicolet 520, Nicolet Instrument Corp., Madison, WI) with a diffusereflectance accessory (Nicolet Instrument Corp., Madison, WI) operating with 64 scans at a resolution of 4 cm−1 within a wavelength of 550–2000 cm−3 . Five specimens of each mate-
640
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 637–643
rial were employed to calculate the DC following 0.1 h postirradiation in accordance with the procedure outlined previously by the authors [7,23]. The uncured paste of the materials under investigation was packed into a stainless steel mould (5 mm diameter, 2 mm depth). The samples were irradiated according to the ‘soft-start’ cure identical to the procedure outlined above. To identify any differences in the DC between specimens irradiated in ‘soft-start’ compared with the standard mode [6], DC was also measured for specimens cured in standard mode in accordance with the manufacturers’ instructions. The DC was calculated using the twofrequency baseline method so that the ratio of peak intensities of aliphatic C C to aromatic C C (1635 and 1608 cm−1 , respectively) were evaluated before and after irradiation to determine the percentage of unsaturated aliphatic C C bonds remaining in the test material from the FTIR spectra obtained. Absorption of the aromatic C C stretching band remains constant during polymerisation and served as the internal standard where the DC of each specimen was equal to 100% minus (%C C) (Eq. (1)): (%C C) =
(aliphatic [C C]/aromatic [C C])polymer (aliphatic [C C]/aromatic [C C])monomer
× 100
(1)
Multiple comparisons of the material group means for the ‘soft-start’ and standard [6] curing regimens, incremental and
total cuspal deflection and DC were made utilising a two-way analysis of variance (ANOVA) and Tukey’s multiple range test. The critical level of significance was set at P = 0.05.
3.
Results
3.1.
Cuspal deflection
The dimensions of the teeth, namely the means and standard deviations, used for the control groups for the ‘soft-start’ polymerisation studies did not vary significantly between the four specimen test groups (Table 1). When the materials employed in the current study were assessed previously by the authors [6] using a standard curing methodology the dimensions of the teeth for the control groups were also not significantly different. The palatal and buccal cuspal deflection data were single dependent variables so that the overall mean palatal or mean buccal cusp deflection per cure increment for each material investigated was assessed. Individual cuspal deflections for each tooth/increment were combined for data analysis (Table 2) since the combined data of all individual incremental deflections with “cusp (buccal or palatal)” as the independent variable revealed no significant difference (P = 0.31). The two-way factorial ANOVA (with product, and increment number as the independent variables) of the cusp strain data
Table 2 – Mean (S.D.) cuspal deflection and degree of conversion measurements for each material examined in the current study compared with data reported previously [6] Product
Z100 FiltekTM Z250 P60 Admira
Cuspal deflection (m)
Degree of conversion (%)
Soft-start
Standard [6]
Soft-start
Standard
20.06 (4.71) a 16.52 (3.26) b 14.23 (3.71) b 11.11 (2.47) b
20.03 (2.92) a 12.34 (2.18) b 13.41 (4.43) b 11.20 (2.58) b
59.1 (4.8) 55.7 (6.0) 57.0 (4.4) 50.4 (4.6)
56.1 (3.4) 58.9 (2.4) 59.9 (4.1) 56.5 (3.8)
For cuspal deflection data, the same letters within columns and rows represent no significant difference. There were no significant differences for degree of conversion between material (rows) or cure type (columns).
Table 3 – The results of a two-way factorial ANOVA for cuspal deflection (Cd) and degree of conversion (DC) for the materials and ‘soft-start’ curing regimens utilised in the current investigation and the standard curing protocol employed previously [6] Dependent variable
d.f.
Material type
Cd DC
3 3
Curing regimen
Cd DC
Interaction
SS
MS
F
P
189 115
63 38
5.90 2.16
0.004 0.119
1 1
25 43
25 43
2.32 2.42
0.141 0.133
Cd DC
3 3
24 87
8 29
0.75 1.63
0.534 0.210
Error
Cd DC
24 24
256 426
11 18
Total
Cd DC
32 32
6914 103500
d.f.: degrees of freedom; SS: sum of squares; MS: mean squares; F: test of significance; P: probability. Bold typeface is indicative of a significant difference.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 637–643
3.3.
641
Degree of conversion
The DC for specimens of Z100, FiltekTM Z250, P60 and Admira following soft-start and standard curing protocols are displayed in Table 2. A two-way ANOVA revealed no significant differences between material and curing methodology employed (P = 0.119; Table 3).
4.
Fig. 1 – A box and whisker plot of the microleakage scores following cuspal deflection and thermocycling of the MOD cavities restored with each RBC polymerised with the soft-start curing regimen in the current investigation and standard curing protocols reported previously [6]. The plot illustrates a summary of the microleakage scores based on the median, quartiles, and extreme values. The box represents the interquartile range which contains the 50% of values, the whiskers represent the highest and lowest microleakage values and the bold black line across the box indicates the median microleakage score.
revealed that the “product type” and “restoration increment” were significant (P < 0.001) although no significant interaction was revealed. Two-way ANOVA followed by the Tukey’s HSD post hoc paired group comparison procedure of total cuspal deflection following ‘soft-start’ polymerisation revealed a significant increase for Z100 compared with the other materials tested in the current study (P = 0.004; Table 3). No significant differences in the cuspal strain data were evident when the results obtained using the ‘soft-start’ light irradiation technique were compared (using two-way ANOVA; Table 3) with those published previously [6] for a standard light irradiation with the Optilux 501 halogen light operating in standard mode at a light intensity of 740 ± 29 mW cm−2 .
3.2.
Microleakage
The Kruskal–Wallis non-parametric one-way ANOVA revealed no significant difference between groups A–D (P > 0.05) (Fig. 1). In addition, no statistically significant differences in microleakage were identified between the teeth that had been incrementally restored when the two light irradiation methodologies, namely soft-start and standard mode [6], were employed (Fig. 1).
Discussion
The cavities utilised in the current investigation were large MOD cavities designed to most closely represent amalgam replacement cavities since many such restorations are currently placed clinically since the advent of improved matrix and bonding systems have made the use of these restorations more viable. The geometry of the cavitity preparations resulted in a high C-factor with the preparations being designed to weaken the remaining tooth structure, to favour cuspal movement during restoration and provide a realistic in vitro simulation of the clinical situation. Considerable care and attention was taken in the present work to ensure that all teeth were placed consistently vertically in a precisely standardised orientation to the experimental apparatus. Previous studies by the authors utilising the cuspal deflection technique successfully identified deficencies in the plasma-arc curing lights for bulk curing RBCs [5], variations in cuspal movement with monomeric formulation [6], filler content [6] and assessed the potential of novel filling materials that do not employ methacrylate-based resin formulations [7]. Previous studies in the dental literature employed either bulk filling [9] or the application of horizontal layers of material of 1–2 mm thickness across the base of the cavities [8,10,24] which minimised the deflection by constraining both the platal and buccal cusps resulting in reduced cuspal deflection and therefore an underestimation of the situation routinely expected clinically. In addition previous studies standardised the cavity design but failed to standardise the size of the teeth resulting in variation of buccal and palatal cusp dimensions [8–10,24]. Interestingly the cuspal movement was greatest with group A (Z100) and least with group D (Admira) although there was no significant differences between groups B–D (Filtek Z250TM , P60 and Admira) at the 95% significance level. Total cuspal movement was greatest with the dimethacrylate-based system that utilised TEGDMA (Z100) as a diluent. The addition of TEGDMA increases polymerisation shrinkage of the composite due to an increased concentration of carbon-to-carbon double bonds (C C) [25]. It was suggested that the total cuspal deflection was reduced by replacement of the TEDGMA with UDMA (Filtek Z250TM , P60 and Admira) which resulted in a significant decrease in polymerisation shrinkage strain compared with Z100. The ‘soft-start’ polymerisation process resulted in no groups producing significantly reduced gingival microleakage at the cervical dentine cavosurface margin when the tooth sections were examined following thermocycling (Fig. 1). Groups A–D experienced severe (code 3) levels of microleakage (Table 4), which would appear to indicate that the curing regimes utilised for these groups and/or the subsequent in vitro thermal stressing regime employed caused the bond to
642
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 637–643
Table 4 – The gingival microleakage scores for 10 MOD cavities restored using ‘soft-start’ polymerisation with each RBC in the current investigation Tooth
1 2 3 4 5 6 7 8 9 10
Degree of microleakage Z100
Z250
P60
1 1 0 0 0 3 1 3 3 2
2 0 0 3 2 3 2 1 2 0
0 3 3 1 0 1 1 2 0 2
Admira 1 3 0 3 0 1 1 3 3 2
fail. Interesingly no significant differences were identified in gingival microleakage at the cervical dentine cavosurface margin between the different curing methodologies (‘soft-start’ or standard) utilised. It is proposed that whilst a reduction in polymerisation shrinkage in tooth coloured filling materials is advantageous and can be readily achieved by modification of the monomer constituents or filler loading any effective reduction in polymerisation shrinkage would appear not to influence the degree of microleakage seen at the cervical dentine cavosurface margin across all groups. The magnitude of polymerisation shrinkage stress generated at the tooth and restoration interface is a multifactorial process, whereby DC, rate of change of DC, the onset of elastic modulus of the material and cavity design will all contribute to the success of the adhesive layer. The DC of the tooth-coloured filling materials examined following ‘soft-start’ or standard curing protocols revealed no significant differences between material (P = 0.119) and curing methodology (P = 0.113) employed. The results are in agreement with the studies of Hofmann et al. [14] and Silikas et al. [26] who identified that DC was not influenced following ‘soft-start’ or standard curing protocols. The conventional wisdom regarding shrinkage strain generated with ‘soft-start’ or standard curing protocols is more difficult to assess. Sakaguchi and Berge [15] suggested a slower polymerisation curing regime may allow for stress relaxation to take place during polymerisation while Miyazaki et al. [17] reported slower polymerisation resulted in longer molecular chains with high flow characteristics. In addition Sakaguchi and Berge [15] proposed that once stiffness development had occurred the application of a higher intensity post-cure would not be expected to increase the contraction strain. However, by employing the well-established bonded-disk technique Watts and Al Hindi [27] highlighted that an ‘intinsic soft-start’ polymerisation did not lead to any reduction in the final equilibrium shrinkage compared with standard curing protocols for the acrylate-base resin composite materials investigated although a reduction in the early shrinkage kinetics was proposed as an ‘intinsic soft-start’ phenomenon. Watts and Al Hindi [27] postulated that during clinical placement the reduction in the early shrinkage kinetics may lower the rate of strain on the adhesive bond. Consequently, the initial lower shrinkage strain generated with ‘soft-start’ compared with the standard curing
protocol would be expected to produce less gingival microleakage at the cervical dentine cavosurface margin of the restored teeth when the tooth sections were examined due to the proposed increase in the synergism and integrity of the adhesive bond. However, the results of the current study suggest that the reduction in the early shrinkage kinetics for ‘soft-start’ polymerisation was not sufficiently low to prevent detrimental strain on the adhesive bond and resulted in no significant reducion in gingival microleakage at the cervical dentine cavosurface margin when the tooth sections were examined. The results therefore suggest that there was no benefit to the operator in terms of cuspal deflection, DC or synergism of the adhesive bond, namely gingival microleakage at the cervical dentine cavosurface margin, when a ‘soft-start’ polymerisation protocol was employed as opposed to a standard polymerisation protocol.
5.
Conclusions
It would appear that using ‘soft-start’ polymerisation compared with a standard curing regime did not offer any significant reduction in associated cuspal movement. No group was identified as having increased DC or producing less gingival microleakage at the cervical dentine cavosurface margin following ‘soft-start’ compared with the standard polymerisation protocols.
references
[1] Bowen RL. Synthesis of a silica–resin direct filling material: progress report. J Dent Res 1958;37:90. [2] Bowen RL. Dental filling materials comprising of vinyl-silane treated fused silica and binder consisting of the reaction product of bisphenol-A and glycidyl methacrylate. US Patent 3,066,112 (1962). [3] Davidson CL, DeGee AJ, Feilzer AJ. The competition between the composite-dentin bond strength and the polymerisation contraction stress. J Dent Res 1984;63: 1396–9. [4] Davidson CL, Feilzer AJ. Polymerisation shrinkage and polymerisation shrinkage stress in polymer-based restoratives. J Dentistry 1997;25:435–40. [5] Abbas G, Fleming GJP, Harrington E, Shortall ACC, Burke FJT. Cuspal movement in premolar teeth restored with a packable composite cured in bulk or incrementally. J Dentistry 2003;31:437–44. [6] Fleming GJP, Hall D, Shortall ACC, Burke FJT. Cuspal movement and microleakage in premolar teeth restored with posterior filling materials of varying reported volumetric shrinkage values. J Dentistry 2005;33:139–46. [7] Palin WM, Fleming GJP, Nathwani H, Burke FJT, Randall RC. In vitro cuspal deflection and microleakage of maxillary premolars restored with novel low-shrink dental composites. Dent Mater 2005;21:324–35. [8] Gonzalez-Lopez S, Lucena-Martin C, de Haro-Gasquet F, Vilchez-Diaz MA, Haro-Munoz C. Influence of different composite restoration techniques on cuspal deflection: an in vitro study. Operative Dentistry 2004;29:600–56. [9] Sulimann AA, Boyer DB, Lakes RS. Cusp movemnet in premolars resulting from composite polymerization shrinkage. Dent Mater 1993;9:6–10.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 637–643
[10] Causton BE, Miller B, Sefton J. The deformation of cusps by bonded posterior composite restorations: an in vitro study. Br Dent J 1985;159:397–400. [11] Lutz F, Kreici I, Barbakow F. Quality and durability of marginal adaptation in bonded composite restorations. Dent Mater 1991;7:107–13. [12] Uno S, Asmussen E. Marginal adaptation of a restorative resin polymerized at reduced rate. Scand J Dent Res 1991;99:440–4. [13] Mehl A, Hickel R, Kunzelmann K-H. Physical properties and gap formation of light-cured composites with and without ‘soft-start polymerization’. J Dentistry 1997;25:321–30. [14] Hofmann N, Denner W, Hugo B, Klaiber B. The influence of plasma arc vs. halogen standard or soft-start irradiation on polymerization shrinkage kinetics of polymer matrix composites. J Dentistry 2003;31:383–93. [15] Sakaguchi RL, Berge HX. Reduced light energy density decreases post-gel contraction while maintaining degree of conversion in composites. J Dentistry 1998;26:695–700. [16] Sahafi A, Peutzfeldt A, Asmussen E. Soft-start polymerisation and marginal gap formation in vitro. Am J Dentistry 2001;24:145–7. [17] Miyazaki M, Oshida Y, Moore BK, Onose H. Effects of light exposures on fracture toughness and flexural strength of light-cured composites. Dent Mater 1996;12:328–32. [18] Wolter H, Storch W, Ott H. Dental filling materials (posterior composites) based on inorganic/organic copolymers (ORMOCERs). Macro Akron 1994:503.
643
[19] Data Sheet for Scotch bond 1 supplied by 3M ESPE (St. Paul, MN, USA). [20] Data Sheet for Admira supplied by VOCO (GmbH, Germany). [21] Palmer DS, Barco MT, Billy EJ. Temperature extremes produced orally by hot and cold liquids. J Prosthetic Dentistry 1992;67:325–7. [22] Spierings TM, Peters MB, Bosman F, Plasschaert AM. Verification of theoretical modelling of heat transmission in teeth by in vivo experiments. J Dent Res 1987;66: 1336–9. [23] Palin WM, Fleming GJP, Burke FJT, Marquis PM, Randall RC. Monomer conversion versus flexural strength of a novel dental composite. J Dentistry 2003;31:341–51. [24] Cerutti A, Flocchini P, Madini L, Mangani F, Putignano A, Docchio F. Effects of bonded composite vs. amalgam on resistance to cuspal deflection for endodontically-treated premolar teeth. Am J Dentistry 2004;17: 295–300. [25] Asmussen E, Peutzfeldt A. Influence of UEDMA BisGMA and TEGDMA on selected mechanical properties of experimental resin composites. Dent Mater 1998;14: 51–6. [26] Silikas N, Eliades G, Watts DC. Light intensity effects on resin-composite degree of conversion and shrinkage strain. Dent Mater 2000;16:292–6. [27] Watts DC, Al Hindi A. Intrinsic ‘soft-start’ polymerisation shrinkage-kinetics in an acrylate-based resin-composite. Dent Mater 1999;15:39–45.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Cuspal movement and microleakage in premolar teeth restored with resin-based filling materials cured using a ‘soft-start’ polymerisation protocol G.J.P. Fleming a,∗ , R.R. Cara b , W.M. Palin b , F.J.T. Burke c a
Materials Science Unit, Division of Oral Biosciences, Dublin Dental School & Hospital, Trinity College Dublin, Dublin 2, Ireland Department of Dental Materials, University of Medicine and Pharmacy, 400006, Clujnapoca, Cluj, Romania c Primary Dental Care Research Unit, University of Birmingham School of Dentistry, St. Chad’s Queensway, Birmingham B4 6NN, United Kingdom b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. To investigate the effect of polymerisation shrinkage strain of four posterior filling
Received 5 July 2005
materials on cuspal movement, degree of conversion (DC) and cervical gingival microleak-
Received in revised form
age of mesio-occlusal-distal (MOD) restorations placed incrementally in maxillary premolar
23 February 2006
teeth using a ‘soft-start’ polymerisation protocol.
Accepted 6 June 2006
Methods. Forty sound extracted upper premolar teeth were subjected to standardised preparation of a large MOD cavity before restoration. A ‘soft-start’ polymerisation curing regimen was used and each posterior filling material was placed in eight incre-
Keywords:
ments with the appropriate bonding system. A twin channel deflection measuring gauge
Cuspal movement
allowed a measurement of individual cusp deflections at each stage of polymerisa-
Gingival microleakage
tion. Restored teeth were thermocycled before immersion in a 0.2% basic fuchsin dye
Resin-based composite
for 24 h. After sagittal sectioning of the restored teeth in a mesio-distal plane, the
Ormocer
sectioned restorations were examined to assess cervical microleakage. The DC was
Soft-start polymerisation
also assessed using a diffuse-reflectance accessory on a Fourier transform infra-red spectrophotometer. Results. A significant increase in cuspal movement recorded for Z100 (20.06 ± 4.71) compared with Filtek Z250TM (16.52 ± 3.26), P60 (14.23 ± 3.71) and Admira (11.11 ± 2.47). No significant reduction in cuspal movement was identified when compared with a previous study [6] where a full-intensity standard polymerisation protocol was employed. No significant differences were also identified between the materials when the cervical gingival microleakage scores or DC were examined for the ‘soft-start’ compared with the standard polymerisation protocol. Conclusions. Although material type remained a significant factor, the use of a ‘soft-start’ polymerization compared with a standard curing regime did not offer any significant reduction in associated cuspal movement, DC or gingival microleakage at the cervical dentine cavosurface margin of the cavities restored with the resin-based filling materials. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +353 1 612 7371; fax: +353 1 612 7297. E-mail address: garry.fl[email protected] (G.J.P. Fleming). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.06.002
638
1.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 637–643
Introduction
The patenting of a novel resin-based composite (RBC) based on a highly viscous dimethacrylate monomer, synthesised from the reaction of bisphenol-A and glycidyl methacrylate (BisGMA) revolutionised resin technology in the late 1950s [1,2]. The addition of a co-monomer triethyleneglycol dimethacrylate (TEGDMA) was necessary to decrease the viscosity of the mixture and aid incorporation of filler particles [2]. At present there are a number of methacrylate-based materials with varying monomeric formulations and filler contents available to general dental practitioners. However, the freeradical polymerisation of dimethacrylate monomers during light irradiation involves bulk contraction [3] and as a result polymerisation shrinkage manifested as shrinkage stress [4]. Cuspal movement on polymerisation [5–10] may compromise the synergism at the restoration–tooth interface [3] possibly leading to bacterial micro-leakage [11–13] and ultimately to marginal staining, pulpal inflammation or necrosis and secondary caries [11]. Manufacturers tend to address the issues of excessive contraction on setting by introducing relevant material constraints including the replacement of the diluent TEGDMA from the monomeric formulations. Increasing the filler content was also reported to be associated with a decrease in shrinkage and therefore a decreased cuspal movement on polymerisation [6]. Fleming et al. [6] confirmed these findings by highlighting that a reduction in the reported volumetric polymerisation shrinkage or an increase in the filler content (for given resin constituents) resulted in a reduction in the associated cuspal strain on mesio-occlusal-distal (MOD) cavities. In addition to varying the monomeric formulation and filler content of methacrylate and non-methacrylate-based filling materials it has been proposed that the associated shrinkage stress can be controlled by varying the curing conditions. Several reports have suggested that pre-polymerisation at lower intensity followed by post-cure at full intensity results in RBC restorations with improved marginal integrity [12–14], reduced post-gel contraction [15] and superior physical properties [13,14]. However, other studies have reported the ‘softstart’ light curing phenomenon resulted in RBC restorations with no improvement in marginal integrity [16] and reduced physical properties [17] compared with standard curing protocols. The conflicting data is a result of the curing conditions namely, exposure time, intensity and distance of the curing light from the samples. Laboratory tests are also generally performed using stainless steel or black-nylon moulds which may fail to incorporate light reflectance properties of enamel, dentine or the matrix band associated with the restoration of these materials in vivo [15]. As a result rather than employing indirect materials science techniques to evaluate the magnitude of the polymerisation shrinkage stress, the authors have employed a direct clinically relevant protocol on large MOD cavities such that the geometry resulted in a high C-factor with high shrinkage stress. The RBC materials chosen in the current study were Z100, Filtek Z250TM and P60 (3M ESPE, St. Paul, MN, USA). The principal monomers in Z100 are BisGMA and TEGDMA so that Z100 has an increased concentration of carbon-to-carbon dou-
ble bonds (C C) compared with Filtek Z250TM and P60 where TEGDMA is replaced with urethane dimethacrylate (UDMA) and derivatives of BisGMA, such as bisphenol-A ethoxylated dimethacrylate (Bis-EMA). The manufacturers of Filtek Z250TM report that it has a decreased filler loading compared with P60 and has reduced polymerisation shrinkage. Admira (Voco, GmbH, Cuxhaven, Germany) is an Ormocer (organically modified ceramic) and contains methacrylate groups that photochemically induce organic polymerisation following light irradiation [18]. The aims of the study were to assess cuspal deflection at each stage of ‘soft-start’ polymerisation for the incremental restoration of standardised large MOD cavities with four posterior filling materials using a twin channel deflection measuring guage. The DC was assessed using a Fourier transform infra-red (FTIR) spectroscopy technique to determine the conversion of the methacrylate species. The performance of the restorations was further investigated by assessing the cervical dentine cavosurface margin for gingival microleakage following thermocycling and immersion in 0.2% basic fuchsin dye prior to sagittal sectioning. The hypothesis proposed the ‘sortstart’ curing regime would not influence cuspal deflection, DC or gingival microleakage compared with when the toothcoloured materials were light cured in accordance with the conventional curing protocol advocated by the manufacturers.
2.
Materials and methods
This section is similar to those reported previously [5–7] by the authors.
2.1.
Tooth selection and cavity preparation
Extracted teeth were stored in buffered formal saline for 24 h post-extraction and then in water at room temperature (23 ± 1 ◦ C) except when aspects of the experimental procedure require isolation from moisture. Forty sound extracted upper premolar teeth had surface deposits carefully removed using a hand scaler and were selected if, following visual examination, it was identified they were free from hypoplastic defects and cracks. Into a cubic stainless steel mould (15 mm3 ) which had a central cylindrical hole of 12 mm diameter, each tooth was fixed crown uppermost and long axis vertical using a chemically cured resin (De Trey Formatray: DeTrey, Dentsply, Weybridge, UK). The teeth were fixed so that the resin extended to within 2 mm of the amelocemental junction (ACJ). The maximum bucco-lingual width (BLW) for each tooth was measured with a micrometer screw guage (Moore and Wright, Sheffield, England) accurate to 10 m. The dimensions were used to distribute the specimens into four groups (A, B, C and D), each group consisting of 10 teeth, so that the mean BLW of each group of teeth differed by not more than 5% (Table 1). Each tooth was subjected to preparation of a large MOD cavity, with the BLW of the approximal boxes of the cavity being prepared to two-thirds of the BLW of the tooth and the occlusal isthmus being prepared to half the BLW. The cavity depth at the occlusal isthmus was standardised (3.5 mm from the tip of the palatal cusp and to 1 mm above the ACJ at
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 637–643
Table 1 – Dimensions of the premolar teeth (m) highlighting no statistical differences between the groups restored using soft-start polymerisation Mean (S.D.) Z100 (group A) Filtek Z250TM (group B) P60 (group C) Admira (group D)
9.16 (0.31) 9.09 (0.29) 9.17 (0.32) 9.21 (0.34)
the cervical aspect of the approximal boxes). To ensure consistency in cavity preparation the facial and lingual walls of the cavity were prepared parallel [5].
2.2.
Cuspal deflection
Groups A–C were restored with Z100, Filtek Z250TM and P60, respectively, in conjunction with the associated bonding system (Scotch Bond 1; 3M ESPE St. Paul, MN, USA). In accordance with the manufacturer’s guidelines, the tooth surfaces were dried following the MOD cavity preparation and 37% phosphoric acid etching gel was applied for 15 s, before rinsing with water for 10 s. The cavity surfaces were briefly dried with compressed air and damp cotton wool pledgets before two consecutive coats of the adhesive were applied to the etched surfaces with a fully saturated brush tip [19]. The tooth surfaces were lightly dried with compressed air for 2–5 s. Groups A–C were light cured for 10 s with an Optilux 501 halogen light (Kerr Mfg. Co., Orange, CA, USA) operating at a light intensity of 740 ± 29 mW cm−2 and the restored teeth were isolated to avoid moisture contamination. Group D was restored using the Ormocer material (Admira) using its associated bonding agent (Admira bond: VOCO GmbH, Germany) in accordance with the manufacturer’s instructions [20]. The tooth surfaces were again dried, etched with the supplied gel (Vococid TM Phosphoric acid) for 15 s rinsed with water and air dried with compressed air. The cavity surfaces were coated with Admira bond, for 30 s before being dispersed with air. The bonding agent was then light cured continuously for 20 s with the halogen light and the teeth were again isolated to avoid moisture contamination. The buccal and lingual cusps of the extracted teeth were approximated to the receptors of a Twin Channel Deflection Measuring Gauge (Twin Channel Analogue Gauge Unit: Thomas Mercer Ltd., St. Alban’s, UK) following the standardised cavity preparation. To standardise placement, the teeth were fixed with the palatal measuring gauge placed approximately 2.5 mm from the palatal cusp tip [5–7]. A baseline measurement was recorded and tooth restoration started with the mesial approximal box, packing the appropriate composite in increments against a stainless steel sectional matrix (3M sectional matrix system: 3M ESPE, St. Paul, MN, USA) wedged firmly against the approximal aspects of the teeth. The increment sequence involved the placement of eight nominally triangular-shaped increments, three for each approximal box and two for the occlusal surface, with each increment touching only one cavity wall [5–7]. Each increment was cured using the halogen light with an 11 mm light-curing tip exit diameter, for the manufacturers recommended light exposure of 40 s for Z100 and 20 s for FiltekTM Z250, P60 and Admira. The
639
light-curing tip exit diameter was placed level with the upper surface of each cusp to ensure a standard curing distance from each increment (as each cavity depth was standardised to 3.5 mm from the palatal cusp tip to the ACJ) and irradiated in ‘one-hit’ for each increment. An initial 10 s ‘soft-start’ cure, from 0 to 740 ± 29 mW cm−2 at a rate of 68 ± 7 mW cm−2 s−1 , was employed for each material plus the remainder of the recommended cure time (30 s for Z100, and 10 s for FiltekTM Z250, P60 and Admira) in standard mode (740 ± 29 mW cm−2 ). Following each stage of polymerisation, measurement of cuspal deflection was recorded after 3 min, to allow time for stress relaxation, resulting in eight measurements for each individual/tooth cusp. The data for the buccal and palatal cusp deflections were also combined to give the total cuspal deflection assuming no cuspal recoil for each increment of composite. The results were subjected to statistical analysis by ANOVA techniques and by Scheffe’s multiple post hoc comparison procedure test.
2.3.
Microleakage
The restored teeth were finished, the root apices sealed with sticky wax and all tooth surfaces were sealed with nail varnish, with the exception of a 1 mm band around the margins of each restoration, and the teeth replaced in water when the varnish dried [5–7]. Thermocycling was carried out between two water-baths maintained at 65 ± 1 and 4 ± 1 ◦ C, respectively [21,22], so that the restored teeth were submerged for 10 s with a 25 s transfer from water-bath to water-bath for the time equivalent of 500 cycles. Following immersion in 0.2% basic fuchsin dye for 24 h the teeth were sectioned mid-sagitally in a mesio-distal plane using a diamond cutting saw (Struers, Glasgow, Scotland) with a ceramic disc operating at a speed of 125 rpm with an applied load of 100 g. Sectioned restorations were examined under a stereo-microscope (Wild M3C, Heerburg, Switzerland) at 25× magnification and the extent of the cervical microleakage was recorded. Accordingly [5–7] the degree of cervical margin microleakage was scored: • 0 = no evidence of dye penetration. • 1 = superficial penetration not beyond the amelodentinal junction (ADJ). • 2 = penetration beyond the ADJ but not the cervico-axial line angle. • 3 = penetration along the axial wall. • 4 = penetration into the pulp chamber. The resultant microleakage data was analysed using a non-parametric one-way ANOVA (Kruskal–Wallis) followed by paired group comparisons using Mann–Whitney U-tests at a 95% significance level.
2.4.
Degree of conversion
The DC of uncured and cured samples of Z100, FiltekTM Z250, P60 and Admira were analysed on a FTIR spectrometer (Nicolet 520, Nicolet Instrument Corp., Madison, WI) with a diffusereflectance accessory (Nicolet Instrument Corp., Madison, WI) operating with 64 scans at a resolution of 4 cm−1 within a wavelength of 550–2000 cm−3 . Five specimens of each mate-
640
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 637–643
rial were employed to calculate the DC following 0.1 h postirradiation in accordance with the procedure outlined previously by the authors [7,23]. The uncured paste of the materials under investigation was packed into a stainless steel mould (5 mm diameter, 2 mm depth). The samples were irradiated according to the ‘soft-start’ cure identical to the procedure outlined above. To identify any differences in the DC between specimens irradiated in ‘soft-start’ compared with the standard mode [6], DC was also measured for specimens cured in standard mode in accordance with the manufacturers’ instructions. The DC was calculated using the twofrequency baseline method so that the ratio of peak intensities of aliphatic C C to aromatic C C (1635 and 1608 cm−1 , respectively) were evaluated before and after irradiation to determine the percentage of unsaturated aliphatic C C bonds remaining in the test material from the FTIR spectra obtained. Absorption of the aromatic C C stretching band remains constant during polymerisation and served as the internal standard where the DC of each specimen was equal to 100% minus (%C C) (Eq. (1)): (%C C) =
(aliphatic [C C]/aromatic [C C])polymer (aliphatic [C C]/aromatic [C C])monomer
× 100
(1)
Multiple comparisons of the material group means for the ‘soft-start’ and standard [6] curing regimens, incremental and
total cuspal deflection and DC were made utilising a two-way analysis of variance (ANOVA) and Tukey’s multiple range test. The critical level of significance was set at P = 0.05.
3.
Results
3.1.
Cuspal deflection
The dimensions of the teeth, namely the means and standard deviations, used for the control groups for the ‘soft-start’ polymerisation studies did not vary significantly between the four specimen test groups (Table 1). When the materials employed in the current study were assessed previously by the authors [6] using a standard curing methodology the dimensions of the teeth for the control groups were also not significantly different. The palatal and buccal cuspal deflection data were single dependent variables so that the overall mean palatal or mean buccal cusp deflection per cure increment for each material investigated was assessed. Individual cuspal deflections for each tooth/increment were combined for data analysis (Table 2) since the combined data of all individual incremental deflections with “cusp (buccal or palatal)” as the independent variable revealed no significant difference (P = 0.31). The two-way factorial ANOVA (with product, and increment number as the independent variables) of the cusp strain data
Table 2 – Mean (S.D.) cuspal deflection and degree of conversion measurements for each material examined in the current study compared with data reported previously [6] Product
Z100 FiltekTM Z250 P60 Admira
Cuspal deflection (m)
Degree of conversion (%)
Soft-start
Standard [6]
Soft-start
Standard
20.06 (4.71) a 16.52 (3.26) b 14.23 (3.71) b 11.11 (2.47) b
20.03 (2.92) a 12.34 (2.18) b 13.41 (4.43) b 11.20 (2.58) b
59.1 (4.8) 55.7 (6.0) 57.0 (4.4) 50.4 (4.6)
56.1 (3.4) 58.9 (2.4) 59.9 (4.1) 56.5 (3.8)
For cuspal deflection data, the same letters within columns and rows represent no significant difference. There were no significant differences for degree of conversion between material (rows) or cure type (columns).
Table 3 – The results of a two-way factorial ANOVA for cuspal deflection (Cd) and degree of conversion (DC) for the materials and ‘soft-start’ curing regimens utilised in the current investigation and the standard curing protocol employed previously [6] Dependent variable
d.f.
Material type
Cd DC
3 3
Curing regimen
Cd DC
Interaction
SS
MS
F
P
189 115
63 38
5.90 2.16
0.004 0.119
1 1
25 43
25 43
2.32 2.42
0.141 0.133
Cd DC
3 3
24 87
8 29
0.75 1.63
0.534 0.210
Error
Cd DC
24 24
256 426
11 18
Total
Cd DC
32 32
6914 103500
d.f.: degrees of freedom; SS: sum of squares; MS: mean squares; F: test of significance; P: probability. Bold typeface is indicative of a significant difference.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 637–643
3.3.
641
Degree of conversion
The DC for specimens of Z100, FiltekTM Z250, P60 and Admira following soft-start and standard curing protocols are displayed in Table 2. A two-way ANOVA revealed no significant differences between material and curing methodology employed (P = 0.119; Table 3).
4.
Fig. 1 – A box and whisker plot of the microleakage scores following cuspal deflection and thermocycling of the MOD cavities restored with each RBC polymerised with the soft-start curing regimen in the current investigation and standard curing protocols reported previously [6]. The plot illustrates a summary of the microleakage scores based on the median, quartiles, and extreme values. The box represents the interquartile range which contains the 50% of values, the whiskers represent the highest and lowest microleakage values and the bold black line across the box indicates the median microleakage score.
revealed that the “product type” and “restoration increment” were significant (P < 0.001) although no significant interaction was revealed. Two-way ANOVA followed by the Tukey’s HSD post hoc paired group comparison procedure of total cuspal deflection following ‘soft-start’ polymerisation revealed a significant increase for Z100 compared with the other materials tested in the current study (P = 0.004; Table 3). No significant differences in the cuspal strain data were evident when the results obtained using the ‘soft-start’ light irradiation technique were compared (using two-way ANOVA; Table 3) with those published previously [6] for a standard light irradiation with the Optilux 501 halogen light operating in standard mode at a light intensity of 740 ± 29 mW cm−2 .
3.2.
Microleakage
The Kruskal–Wallis non-parametric one-way ANOVA revealed no significant difference between groups A–D (P > 0.05) (Fig. 1). In addition, no statistically significant differences in microleakage were identified between the teeth that had been incrementally restored when the two light irradiation methodologies, namely soft-start and standard mode [6], were employed (Fig. 1).
Discussion
The cavities utilised in the current investigation were large MOD cavities designed to most closely represent amalgam replacement cavities since many such restorations are currently placed clinically since the advent of improved matrix and bonding systems have made the use of these restorations more viable. The geometry of the cavitity preparations resulted in a high C-factor with the preparations being designed to weaken the remaining tooth structure, to favour cuspal movement during restoration and provide a realistic in vitro simulation of the clinical situation. Considerable care and attention was taken in the present work to ensure that all teeth were placed consistently vertically in a precisely standardised orientation to the experimental apparatus. Previous studies by the authors utilising the cuspal deflection technique successfully identified deficencies in the plasma-arc curing lights for bulk curing RBCs [5], variations in cuspal movement with monomeric formulation [6], filler content [6] and assessed the potential of novel filling materials that do not employ methacrylate-based resin formulations [7]. Previous studies in the dental literature employed either bulk filling [9] or the application of horizontal layers of material of 1–2 mm thickness across the base of the cavities [8,10,24] which minimised the deflection by constraining both the platal and buccal cusps resulting in reduced cuspal deflection and therefore an underestimation of the situation routinely expected clinically. In addition previous studies standardised the cavity design but failed to standardise the size of the teeth resulting in variation of buccal and palatal cusp dimensions [8–10,24]. Interestingly the cuspal movement was greatest with group A (Z100) and least with group D (Admira) although there was no significant differences between groups B–D (Filtek Z250TM , P60 and Admira) at the 95% significance level. Total cuspal movement was greatest with the dimethacrylate-based system that utilised TEGDMA (Z100) as a diluent. The addition of TEGDMA increases polymerisation shrinkage of the composite due to an increased concentration of carbon-to-carbon double bonds (C C) [25]. It was suggested that the total cuspal deflection was reduced by replacement of the TEDGMA with UDMA (Filtek Z250TM , P60 and Admira) which resulted in a significant decrease in polymerisation shrinkage strain compared with Z100. The ‘soft-start’ polymerisation process resulted in no groups producing significantly reduced gingival microleakage at the cervical dentine cavosurface margin when the tooth sections were examined following thermocycling (Fig. 1). Groups A–D experienced severe (code 3) levels of microleakage (Table 4), which would appear to indicate that the curing regimes utilised for these groups and/or the subsequent in vitro thermal stressing regime employed caused the bond to
642
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 637–643
Table 4 – The gingival microleakage scores for 10 MOD cavities restored using ‘soft-start’ polymerisation with each RBC in the current investigation Tooth
1 2 3 4 5 6 7 8 9 10
Degree of microleakage Z100
Z250
P60
1 1 0 0 0 3 1 3 3 2
2 0 0 3 2 3 2 1 2 0
0 3 3 1 0 1 1 2 0 2
Admira 1 3 0 3 0 1 1 3 3 2
fail. Interesingly no significant differences were identified in gingival microleakage at the cervical dentine cavosurface margin between the different curing methodologies (‘soft-start’ or standard) utilised. It is proposed that whilst a reduction in polymerisation shrinkage in tooth coloured filling materials is advantageous and can be readily achieved by modification of the monomer constituents or filler loading any effective reduction in polymerisation shrinkage would appear not to influence the degree of microleakage seen at the cervical dentine cavosurface margin across all groups. The magnitude of polymerisation shrinkage stress generated at the tooth and restoration interface is a multifactorial process, whereby DC, rate of change of DC, the onset of elastic modulus of the material and cavity design will all contribute to the success of the adhesive layer. The DC of the tooth-coloured filling materials examined following ‘soft-start’ or standard curing protocols revealed no significant differences between material (P = 0.119) and curing methodology (P = 0.113) employed. The results are in agreement with the studies of Hofmann et al. [14] and Silikas et al. [26] who identified that DC was not influenced following ‘soft-start’ or standard curing protocols. The conventional wisdom regarding shrinkage strain generated with ‘soft-start’ or standard curing protocols is more difficult to assess. Sakaguchi and Berge [15] suggested a slower polymerisation curing regime may allow for stress relaxation to take place during polymerisation while Miyazaki et al. [17] reported slower polymerisation resulted in longer molecular chains with high flow characteristics. In addition Sakaguchi and Berge [15] proposed that once stiffness development had occurred the application of a higher intensity post-cure would not be expected to increase the contraction strain. However, by employing the well-established bonded-disk technique Watts and Al Hindi [27] highlighted that an ‘intinsic soft-start’ polymerisation did not lead to any reduction in the final equilibrium shrinkage compared with standard curing protocols for the acrylate-base resin composite materials investigated although a reduction in the early shrinkage kinetics was proposed as an ‘intinsic soft-start’ phenomenon. Watts and Al Hindi [27] postulated that during clinical placement the reduction in the early shrinkage kinetics may lower the rate of strain on the adhesive bond. Consequently, the initial lower shrinkage strain generated with ‘soft-start’ compared with the standard curing
protocol would be expected to produce less gingival microleakage at the cervical dentine cavosurface margin of the restored teeth when the tooth sections were examined due to the proposed increase in the synergism and integrity of the adhesive bond. However, the results of the current study suggest that the reduction in the early shrinkage kinetics for ‘soft-start’ polymerisation was not sufficiently low to prevent detrimental strain on the adhesive bond and resulted in no significant reducion in gingival microleakage at the cervical dentine cavosurface margin when the tooth sections were examined. The results therefore suggest that there was no benefit to the operator in terms of cuspal deflection, DC or synergism of the adhesive bond, namely gingival microleakage at the cervical dentine cavosurface margin, when a ‘soft-start’ polymerisation protocol was employed as opposed to a standard polymerisation protocol.
5.
Conclusions
It would appear that using ‘soft-start’ polymerisation compared with a standard curing regime did not offer any significant reduction in associated cuspal movement. No group was identified as having increased DC or producing less gingival microleakage at the cervical dentine cavosurface margin following ‘soft-start’ compared with the standard polymerisation protocols.
references
[1] Bowen RL. Synthesis of a silica–resin direct filling material: progress report. J Dent Res 1958;37:90. [2] Bowen RL. Dental filling materials comprising of vinyl-silane treated fused silica and binder consisting of the reaction product of bisphenol-A and glycidyl methacrylate. US Patent 3,066,112 (1962). [3] Davidson CL, DeGee AJ, Feilzer AJ. The competition between the composite-dentin bond strength and the polymerisation contraction stress. J Dent Res 1984;63: 1396–9. [4] Davidson CL, Feilzer AJ. Polymerisation shrinkage and polymerisation shrinkage stress in polymer-based restoratives. J Dentistry 1997;25:435–40. [5] Abbas G, Fleming GJP, Harrington E, Shortall ACC, Burke FJT. Cuspal movement in premolar teeth restored with a packable composite cured in bulk or incrementally. J Dentistry 2003;31:437–44. [6] Fleming GJP, Hall D, Shortall ACC, Burke FJT. Cuspal movement and microleakage in premolar teeth restored with posterior filling materials of varying reported volumetric shrinkage values. J Dentistry 2005;33:139–46. [7] Palin WM, Fleming GJP, Nathwani H, Burke FJT, Randall RC. In vitro cuspal deflection and microleakage of maxillary premolars restored with novel low-shrink dental composites. Dent Mater 2005;21:324–35. [8] Gonzalez-Lopez S, Lucena-Martin C, de Haro-Gasquet F, Vilchez-Diaz MA, Haro-Munoz C. Influence of different composite restoration techniques on cuspal deflection: an in vitro study. Operative Dentistry 2004;29:600–56. [9] Sulimann AA, Boyer DB, Lakes RS. Cusp movemnet in premolars resulting from composite polymerization shrinkage. Dent Mater 1993;9:6–10.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 637–643
[10] Causton BE, Miller B, Sefton J. The deformation of cusps by bonded posterior composite restorations: an in vitro study. Br Dent J 1985;159:397–400. [11] Lutz F, Kreici I, Barbakow F. Quality and durability of marginal adaptation in bonded composite restorations. Dent Mater 1991;7:107–13. [12] Uno S, Asmussen E. Marginal adaptation of a restorative resin polymerized at reduced rate. Scand J Dent Res 1991;99:440–4. [13] Mehl A, Hickel R, Kunzelmann K-H. Physical properties and gap formation of light-cured composites with and without ‘soft-start polymerization’. J Dentistry 1997;25:321–30. [14] Hofmann N, Denner W, Hugo B, Klaiber B. The influence of plasma arc vs. halogen standard or soft-start irradiation on polymerization shrinkage kinetics of polymer matrix composites. J Dentistry 2003;31:383–93. [15] Sakaguchi RL, Berge HX. Reduced light energy density decreases post-gel contraction while maintaining degree of conversion in composites. J Dentistry 1998;26:695–700. [16] Sahafi A, Peutzfeldt A, Asmussen E. Soft-start polymerisation and marginal gap formation in vitro. Am J Dentistry 2001;24:145–7. [17] Miyazaki M, Oshida Y, Moore BK, Onose H. Effects of light exposures on fracture toughness and flexural strength of light-cured composites. Dent Mater 1996;12:328–32. [18] Wolter H, Storch W, Ott H. Dental filling materials (posterior composites) based on inorganic/organic copolymers (ORMOCERs). Macro Akron 1994:503.
643
[19] Data Sheet for Scotch bond 1 supplied by 3M ESPE (St. Paul, MN, USA). [20] Data Sheet for Admira supplied by VOCO (GmbH, Germany). [21] Palmer DS, Barco MT, Billy EJ. Temperature extremes produced orally by hot and cold liquids. J Prosthetic Dentistry 1992;67:325–7. [22] Spierings TM, Peters MB, Bosman F, Plasschaert AM. Verification of theoretical modelling of heat transmission in teeth by in vivo experiments. J Dent Res 1987;66: 1336–9. [23] Palin WM, Fleming GJP, Burke FJT, Marquis PM, Randall RC. Monomer conversion versus flexural strength of a novel dental composite. J Dentistry 2003;31:341–51. [24] Cerutti A, Flocchini P, Madini L, Mangani F, Putignano A, Docchio F. Effects of bonded composite vs. amalgam on resistance to cuspal deflection for endodontically-treated premolar teeth. Am J Dentistry 2004;17: 295–300. [25] Asmussen E, Peutzfeldt A. Influence of UEDMA BisGMA and TEGDMA on selected mechanical properties of experimental resin composites. Dent Mater 1998;14: 51–6. [26] Silikas N, Eliades G, Watts DC. Light intensity effects on resin-composite degree of conversion and shrinkage strain. Dent Mater 2000;16:292–6. [27] Watts DC, Al Hindi A. Intrinsic ‘soft-start’ polymerisation shrinkage-kinetics in an acrylate-based resin-composite. Dent Mater 1999;15:39–45.