Cuspal deflection and microleakage in premolar teeth restored with resin-based composites with and without an intermediary flowable layer

journal of dentistry 35 (2007) 482–489

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Cuspal deflection and microleakage in premolar teeth restored with resin-based composites with and without an intermediary flowable layer R.R. Cara a, G.J.P. Fleming b,*, W.M. Palin c, A.D. Walmsley c, F.J.T. Burke c a

Department of Dental Materials, University of Medicine and Pharmacy, Cluj-Napoca, Romania Materials Science Unit, Division of Oral Biosciences, Dublin Dental School & Hospital, Trinity College Dublin, Dublin 2, Ireland c University of Birmingham School of Dentistry, St. Chad’s Queensway, Birmingham B4 6NN, United Kingdom b

article info

abstract

Article history:

Objectives: To assess cuspal deflection and cervical enamel microleakage with and without

Received 26 October 2006

an intermediary flowable RBC layer for the incremental restoration of mesio-occluso-distal

Received in revised form

(MOD) cavities with two resin-based composites (RBCs).

4 January 2007

Methods: Forty sound upper premolar teeth had standardised MOD cavities prepared.

Accepted 10 January 2007

Restoration of the teeth involved the placement of the RBCs (FiltekTM P60 or FiltekTM Supreme) in eight increments with the appropriate bonding system with and without an intermediary flowable RBC layer (FiltekTM Flow). Buccal and palatal cusp deflections were

Keywords:

recorded post-irradiation using a twin channel deflection measuring gauge. Following

Cuspal deflection

restoration, the teeth were thermocycled, immersed in a 0.2% basic fuchsin dye for 24 h,

Cervical microleakage

sagittally sectioned and examined for cervical enamel microleakage.

Resin-based composite

Results: A significant reduction in cuspal deflection was evident when both RBC materials

Flowable RBC

were used to restore the cavity by employing an intermediary flowable (P < 0.001) compared with when no intermediary flowable was utilised. No statistically significant differences were identified in microleakage between the teeth restored with FiltekTM P60 or FiltekTM Supreme when an intermediary flowable (FiltekTM Flow) was employed. Conclusions: The results of the current study suggest that there was a benefit to the operator in terms of a reduction in cuspal deflection but not from the maintenance of the synergism of the adhesive bond, namely microleakage at the cervical enamel cavosurface margin, when an intermediate layer of a flowable RBC was used under higher elastic modulus RBCs. # 2007 Elsevier Ltd. All rights reserved.

1.

Introduction

Light irradiated resin-based composites (RBCs) routinely employed in dental practices today are based on dimethacrylate chemistry and following irradiation, polymerisation occurs and mechanical properties are developed.1 A negative effect of the polymerisation of RBCs is shrinkage and the associated stress generated in the dental tissues2 through the

bonded interfaces of the restoration3 manifested clinically as cuspal deflection.4 Tooth deformation is indicative of a combination of stresses in the tooth, in the restoration or across the tooth-restoration interface. Versluis et al.5 observed that larger restorations resulted in lower stress levels in the restorations and tooth-restoration interface, but increased stresses in the tooth.5 The size and configuration (C-factor) of the cavity influence the amount of cuspal deflection and the

* Corresponding author. Tel.: +353 1 612 7371; fax: +353 1 612 7297. E-mail address: [email protected] (G.J.P. Fleming). 0300-5712/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2007.01.005

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journal of dentistry 35 (2007) 482–489

highest deflection values have been recorded for mesiooccluso-distal (MOD) cavities.6 Post-operative sensitivity by fluid flow in exposed dentinal tubules has been associated with cuspal deflection7 due to the formation and/or propagation of enamel cracks8 or by gap formation at the interface between the tooth and the RBC restoration as a result of bending and/or an insufficient bond strength.9 Cuspal deflection may compromise the synergism of the bond at the toothrestoration interface3 possibly leading to bacterial microleakage and ultimately to marginal discolouration, secondary caries and pulpal inflammation or necrosis.10 To reduce the polymerisation shrinkage stress developed in the tooth-restoration complex manufacturers have reduced the carbon-to-carbon double bond (C C) concentration in the monomeric formulations of RBCs,11 optimised the particle size and distribution and maximised the volume content of the reinforcing filler.12 Polymerisation shrinkage stress has also been reported to be modified by altering the light irradiation protocols including ‘soft-start’ polymerisation13 and high intensity light emitting diode (LED) light curing units (LCUs) may possibly allow for the flow necessary to minimise the polymerisation shrinkage effects.14 Alomari et al.15 suggested recently that cuspal deflection could also be reduced following the application of a low elastic modulus RBC layer between the adhesive and the higher modulus RBC restorative. The application of flowable RBCs as an intermediate thin layer is based on the concept of an ‘elastic cavity wall’ suggested for filled adhesives.16,17 According to the ‘elastic cavity wall concept’ the shrinkage stress generated by a subsequent layer of higher modulus RBC can be absorbed by an elastic intermediary layer, thereby reducing the stress at the tooth-restoration interface18 manifested clinically as a reduction in cuspal deflection.15 In the dental literature numerous in vitro studies investigated the sealing ability of flowable RBCs applied as a thin intermediary layer for MOD cavities. A reduction in microleakage19–21 and no change in microleakage22–25 were reported when employing an elastic intermediary layer between the higher modulus RBC restoration and the tooth when the data from these studies were analysed. As a result the current study aimed to assess cuspal deflection at each stage of polymerisation with and without an intermediary flowable RBC layer for the incremental restoration of MOD cavities with two high elastic modulus RBCs using a twin channel deflection measuring gauge. The synergism of the tooth-restoration interface was investigated by assessing the cervical enamel cavosurface margin for microleakage following thermocycling and immersion in 0.2 wt.% basic fuchsin dye prior to mid-sagittal sectioning. The hypotheses proposed that a low modulus flowable would (1) minimise cuspal deflection and (2) improve the marginal integrity of two RBCs compared with when no intermediary flowable RBC layer was employed.

2.

Materials and methods

2.1.

Materials

A ‘packable’ posterior RBC (FiltekTM P60, A3 shade: 3M ESPE, St. Paul, MN, USA) and a universal RBC restorative (FiltekTM

Supreme, A3B shade: 3M ESPE, St. Paul, MN, USA) were used independently and in combination with a flowable RBC (FiltekTM Flow, 3M ESPE, St. Paul, MN, USA). The principal monomers in FiltekTM P60 and FiltekTM Supreme are bisphenol-A diglycidyl ether dimethacrylate (BisGMA) and urethane dimethacrylate (UDMA) and bisphenol-A ethoxylated dimethacrylate (BisEMA) with small amounts of triethyleneglycol dimethacrylate (TEGDMA).26,27 The monomeric constituents of FiltekTM Flow include BisGMA and TEGDMA.28 The manufacturers of FiltekTM P60 and FiltekTM Supreme report similar zirconia/silica filler loading (61 and 59.5 vol.%, respectively) compared with FiltekTM Flow (47 vol.%).26–28

2.2.

Tooth selection

Forty maxillary premolars, extracted for orthodontic purposes, that on visual examination were free from caries, hypoplastic defects or cracks were selected for the current study. The teeth had been stored for a maximum of 4 months in distilled water prior to use. Calculus deposits were removed with a hand-scaler and the maximum bucco-palatal width (BPW) of each tooth was measured with a digital micrometer gauge accurate to 10 mm (Dentaguage1, ErskineDental, Macksville, Australia). The BPW dimensions were used to distribute the teeth into four groups of 10 teeth and the mean BPW of the teeth between groups varied by no more than 5% when a oneway analysis of variance (ANOVA) and a paired Tukey test comparison were analysed (Table 1). Following distribution into groups the 40 teeth were stored in high purity double distilled water at 23  1 8C except when aspects of the experimental procedure required isolation from moisture.

2.3.

Cavity preparation

The teeth were fixed crown uppermost and long axis vertical so that the resin extended to within 2 mm of the amelocemental junction (ACJ) using a chemically cured activated orthodontic resin (Meadway Rapid Repair, Mr Dental Supplies Ltd., Surrey, UK) into a cubic stainless steel mould (15 mm3 dimensions) which had a central cylindrical hole (12 mm diameter). Standardised MOD cavities were prepared (Fig. 1) whereby the BPW of the approximal box (B) of each cavity was prepared to two-thirds of the BPW of the tooth (A) and the occlusal isthmus prepared to half the BPW (C). In addition, the cavity depth at the occlusal isthmus was also standardised to 3.5 mm from the tip of the palatal cusp (D) and 1 mm above the ACJ (E) at the cervical aspect of the approximal boxes. The cavosurface margins were prepared at 908 and all internal line angles were rounded. Further consistency in cavity prepara-

Table 1 – Dimensions of the premolar teeth (mm) used in the current study highlighting no statistical differences between the groups of teeth to be restored. Mean FiltekTM FiltekTM FiltekTM FiltekTM

P60 Supreme P60 + Flow Supreme + Flow

9.41 9.45 9.46 9.46

S.D. 0.38 0.41 0.61 0.86

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journal of dentistry 35 (2007) 482–489

Fig. 1 – Schematic representation of the standardised MOD cavity prepared in the current study where (A) was the maximum bucco-palatal width of the tooth (BPW); (B) was the maximum bucco-palatal width of the approximal boxes; (C) was the occlusal isthmus width; (D) was the cavity depth at the occlusal isthmus and (E) was the cervical aspect of the approximal boxes.

tion was ensured by parallel preparation of the facial and palatal walls of the cavity in accordance with a previous reported procedure.11,29–32

2.4. Restorative procedures and cuspal deflection assessment All four groups of teeth were incrementally restored with RBCs in conjunction with a single-component adhesive (ScotchbondTM 1 Adhesive, 3M ESPE, St. Paul, MN, USA). Following cavity preparation the tooth surfaces were air-dried for 30 s, prior to the application of phosphoric acid etching gel (ScotchbondTM Etchant, 3M ESPE, St. Paul, MN, USA) for 15 s before rinsing for 10 s with water. The surfaces were lightly dried with compressed air for 3 s prior to two consecutive coats of the adhesive being applied with a saturated brush tip and lightly dried with compressed air for 2–5 s.33 The tooth surfaces were light cured for 10 s with an Optilux 501 halogen light (Kerr, Orange, CA, USA) operating in standard mode at a light intensity of 740  36 mW cm 2.

The buccal and palatal cusps of the prepared teeth were approximated to the receptors of a measuring gauge (twin channel analogue gauge unit: Thomas Mercer Ltd., St. Alban’s, UK) with the palatal measuring gauge placed approximately 2.5 mm from the palatal cusp tip.11,29–32 A baseline measurement was recorded and tooth restored initially 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 oblique incremental restoration with FiltekTM P60 and FiltekTM Supreme, respectively consisted of eight triangular-shaped increments of approximately 2 mm thickness, three for each approximal box and two for the occlusal cavity in accordance with the procedures outlined previously by the authors. The cavity was initially restored with the mesial approximal box followed by the distal and occlusal boxes, respectively. Each increment was irradiated for 20 s with an Optilux 501 halogen light at a distance of 2 mm above the cusp tips. Following each irradiation the cuspal deflection measurement was recorded after 180 s, resulting in eight measurements for each individual/tooth cusp. The authors allowed 180 s for stabilisation of the cuspal deflection measurements in accordance with the previously reported protocols.11,29–32 However, the authors do acknowledge that the stresses were being relaxed during this time and oxygen inhibition between each increment may possibly have reduced the interfacial cure or bond. As a result it may have been more clinically relevant to follow an immediate sequential placement protocol when placing the increments of RBC. In addition, prior to the incremental placement of further FiltekTM P60 and FiltekTM Supreme groups, respectively an intermediary layer of FiltekTM Flow was injected on the floor of the MOD cavity and lightly vibrated to standardise the thickness of the flowable layer and to allow adaptation to the dentine surfaces of the cavity walls. The FiltekTM Flow was light irradiated from the occlusal direction for 20 s with the Optilux 501 halogen light. The MOD cavity restoration was then performed with the oblique incremental application of FiltekTM P60 and FiltekTM Supreme, respectively as outlined above. The buccal and palatal cusp deflections were combined to give the total cuspal deflection assuming no cuspal recoil for each increment. A general linear model ANOVA technique and post hoc Tukey HSD were employed at the 5% significance level to assess the significant effects of product type and increment number on both buccal, palatal and total cuspal deflection for all groups.

2.5.

Cervical microleakage assessment

The teeth were finished with Sof-Lex Finishing discs (3M ESPE, St. Paul, MN, USA) and 15 mm grit Composhape finishing diamond burs (Intensiv, Viganello-Lugano, Switzerland) used in conjunction with an air turbine in a slow hand-piece under water.11 The tooth surfaces were sealed with nail varnish with the exception of a 1 mm band around the margins of each restoration surface. After the varnish dried the root apices were sealed with sticky wax and the teeth were replaced in water maintained at 23  1 8C. Thermocycling was carried out between 65  1 and 4  1 8C, respectively34,35 so that the teeth

journal of dentistry 35 (2007) 482–489

485

were submerged for 10 s at each temperature with a 25 s transfer from water-bath to water-bath for the time equivalent of 500 cycles. The restored teeth were immersed in 0.2% basic fuchsin dye for 24 h, before being sectioned mid-sagittally in a mesio-distal plane using a diamond cutting saw (Struers, Glasgow, Scotland) with a ceramic cutting 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 according to the scoring criteria utilised previously by the authors30–32:

 0 = no evidence of dye penetration.  1 = superficial dye penetration not beyond the amelodentinal junction (ADJ).  2 = dye penetration along the gingival floor and up to the axial wall.  3 = dye penetration along the axial wall and across the pulpal floor.  4 = dye penetration into the pulp chamber from the pulpal floor.  4* = dye penetration into the pulp chamber from the axial wall. A non-parametric one-way ANOVA (Kruskal–Wallis) test followed by paired group comparisons using Mann–Whitney U tests at a 5% significance level were used to analyse the microleakage scores statistically.

3.

Results

3.1.

Cuspal deflection

The means and standard deviations of the teeth used did not vary significantly between the four specimen test groups when a one-way ANOVA and paired Tukey test comparison were analysed (Table 1). The individual buccal and palatal cuspal deflections for each tooth/increment were combined for data analysis (Table 2) since the combined data of all individual incremental deflections with ‘‘cusp’’ as the independent variable revealed no significant difference (P > 0.05). The two-way factorial ANOVA (with product, and increment number as the independent variables) of the cusp strain data showed that the ‘‘product type’’ and ‘‘restoration increment’’ were significant (P < 0.001) although no significant interaction was revealed.

Fig. 2 – A box and whisker plot of the microleakage scores following cuspal deflection and thermocycling of the MOD cavities restored. 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 and the whiskers represent the highest and lowest microleakage values (which are not displayed if the interquartile range extends to the highest and lowest microleakage values). Extreme values are indicated by asterisk (*) and represent microleakage scores which extend more than three boxlengths above or below the median. The bold black line across the box indicates the median microleakage score.

A two-way ANOVA followed by the Tukey’s HSD post hoc paired group comparison procedure of the total cuspal deflection following restoration with FiltekTM P60 and FiltekTM Supreme showed no significant differences in the cuspal strain data (P = 0.052) although there is a definite trend towards FiltekTM Supreme showing less cuspal strain. Significant differences in the cuspal strain data were evident when the results from FiltekTM P60 and FiltekTM Supreme were compared with those when the flowable material was employed for both FiltekTM P60 and FiltekTM Supreme groups examined (P < 0.001; Table 2).

3.2. Table 2 – Mean cuspal deflection measurements (mm) for each group examined in the current study Group TM

Filtek FiltekTM FiltekTM FiltekTM

Cuspal deflection (mm) P60 Supreme P60 + Flow Supreme + Flow

14.8 12.0 6.4 7.5

(3.9) (3.0) (1.2) (1.7)

a a b b

Values in parenthesis represent the standard deviations and groups with the same letter do not differ significantly (P > 0.05).

Cervical microleakage

The Kruskal–Wallis non-parametric one-way ANOVA showed no significant differences in microleakage (Fig. 2) between the teeth restored with FiltekTM P60 or FiltekTM Supreme when an intermediary flowable (FiltekTM Flow) was employed or not.

4.

Discussion

In late 1996 the first generation of flowable RBCs were developed and marketed to improve RBC fluid injectability

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journal of dentistry 35 (2007) 482–489

compared with traditional formulations. By reducing the filler content of traditional RBCs and increasing the amount of diluent such as TEGDMA in the monomeric formulation manufacturers produced a material that was injectable and in principle, adaptable to the proximal boxes of class II preparations.36 However, decreased filler content might be expected to be a concern due to inferior mechanical properties and higher polymerisation shrinkage when compared to traditional hybrid composites. Bayne et al.36 suggested that flowable RBCs shrink more than conventional RBCs thereby creating more stress on the bonding agent during curing and also may ‘prematurely allow deflection’ of the overlaying conventional RBC given the difference in flexural modulus due to the changes in filler contents. Bayne et al.36 suggestion is not in agreement with the ‘elastic cavity wall’ concept which advocates that the shrinkage stress generated by a subsequent layer of higher modulus RBC can be absorbed by an elastic intermediary layer.18 Braga et al.37 highlighted that the contraction stress determined using a tensiometer,12,38 was significantly reduced following the application of an initial precured layer of FiltekTM Flow RBC beneath FiltekTM Z250, compared with when a precured layer of FiltekTM Z250 was used although not all flowable composites were effective at reducing stresses in the subsequent composite increment.37 Labella et al.39 identified that the volumetric shrinkage of flowables was significantly increased compared with traditional RBCs in line with the suggestion proposed previously.36 When Labella et al.39 employed the Watts and Cash bondeddisc methodology to assess the polymerisation shrinkage kinetics of a range of flowable and conventional RBCs, the material dependence of the kinetics parameters was highlighted. Alomari et al.15 published a study on class II cavities that investigated the effects of a flowable as an intermediary on cuspal deflection. Alomari highlighted that the cuspal deflection on restoring a class II cavity with Z100 was reduced from 47  8 to 35  5 mm when a flowable intermediary (Revolution, Kerr, Orange, CA, USA) was used. However, the study by Alomari et al.15 neglected to standardise the size of the teeth used in the study resulting in different cusp thicknesses which influences deflection during restoration.6 In addition the restoration procedure applied the RBC as two layers although whether they were placed horizontally, vertically or obliquely was not detailed by the authors. Despite the studies on contraction stress,37 polymerisation shrinkage kinetics39 and cuspal deflection15 of flowable RBCs it remains unclear to date if the application of a flowable as an intermediary layer is beneficial in terms of clinical performance. In the current study, the authors eliminated the problem with different cusp thicknesses,4,15,40–42 by following a standardised cavity preparation methodology, and standardising the size of the teeth used, with the differences in the mean BPW between groups to less than 5%. The cavity preparations used in the current study were standardised large MOD cavities in order to produce obvious cuspal deformation.6 This could be considered to be typical of an amalgam replacement with the indication of an indirect RBC restoration or large direct restoration, which is becoming more commonplace. In addition, the restorative procedure of filling class II cavities in bucco-lingual increments has been reported

to produce a significantly reduced deflection compared with filling with gingivo-occlusal increments or in bulk.43 Versluis et al.44 assessed the developing stress fields for different incremental techniques using a finite element stress analysis method and noted that oblique and horizontal incremental restorative techniques resulted in much higher coronal stresses than a bulk curing technique. Therefore, an oblique incremental method resulted in the most severe stress concentrations at the cavity-restoration interface and was adopted in the current study. The assessment of linear displacement of the cusps in our study was quantified with the use of a direct current differential transformer (DCDT) where a probe was attached to both the buccal and palatal cusps so that an individual record of the movement of each cusp were obtained simultaneously. Jantarat et al.45 compared cuspal deflection measurements using a DCDT and strain gauges and concluded that strain gauges were less sensitive to the three-dimensional orientation of the teeth compared with the DCDT. The reproducible vertical placement of each DCDT probe against the buccal and palatal cusp associated with the three-dimensional orientation of each tooth in the present study allowed for a nominally identical reference point for each deflection measurement, thereby decreasing the variation in the cuspal deflection data.

4.1.

Cuspal deflection

The present study examined the effect of polymerisation contraction stress (manifested as cuspal deflection) during polymerisation of the RBCs investigated and highlighted that there was no significant difference in cuspal deflection between the group restored with FiltekTM P60 and with FiltekTM Supreme although there is a definite trend towards FiltekTM Supreme showing less cuspal strain. The polymerisation contraction stress on the cusps generated by FiltekTM P60 and FiltekTM Supreme could be explained, in part, by the elastic modulus46,47 which is dependent upon the filler concentration.12,47 FiltekTM P60 and FiltekTM Supreme have a similar filler loading (61 and 59.5 vol.%, respectively) that suggests a similar elastic modulus and therefore a similar shrinkage stress given they also have similar resin constituents. However, one would expect mush more light scattering in the nano-filled FiltekTM Supreme formulation which may result in slightly lower strains on the teeth. The cuspal deflection was significantly reduced for the groups 3 and 4 when a precured intermediary layer of FiltekTM Flow was used under incremental restoration with the FiltekTM P60 and FiltekTM Supreme compared with groups 1 and 2, respectively where no intermediary layer was applied. These results demonstrate that a precured intermediary layer (FiltekTM Flow) reduced the associated cuspal deflection resulting from the polymerisation shrinkage of the two RBCs applied incrementally. The fundamental principle for the reduced cuspal deflection is based on the ‘elastic cavity wall concept’ which is in agreement with Braga et al.37 who demonstrated the stress reduction capacity of a precured intermediate layer of FiltekTM Flow when used under a conventional RBC, namely FiltekTM Z250. However, when a further three flowable RBCs were investigated, Braga did not find a significant stress reduction when used under FiltekTM

journal of dentistry 35 (2007) 482–489

Z250 and suggested that the inconsistent strain capacity behaviour of flowables may explain the variety of results observed in studies evaluating the effect of an intermediate layer on microleakage.

4.2.

Cervical microleakage

In a review of the dental literature, Attar et al.48 identified the application of a flowable intermediate layer in class II RBC restorations did not increase microleakage measured at the cervical enamel interface. Some authors reported reduced microleakage when an intermediate layer was employed19,20,49,50 due to improved cavity adaptation and stress absorbing capacity of the flowable despite the increased polymerisation shrinkage compared with conventional RBCs. Interestingly, it has been reported that there was no difference in microleakage between class II cavities restored with or without an intermediary flowable RBC layer.22–25 In the current study, FiltekTM P60 and FiltekTM Supreme restored with or without an intermediary (FiltekTM Flow) exhibited microleakage, whereby dye penetration was observed (Fig. 2). The application of a thin layer of FiltekTM Flow in an MOD cavity, incrementally filled with FiltekTM P60 or FiltekTM Supreme, resulted in no groups producing significantly reduced cervical microleakage at the enamel cavosurface margin when tooth sections were examined following thermocycling. These results are in agreement with the findings of Tredwin et al.51 who observed following thermocycling that no significant differences in microleakage was evident when gingival margins situated in enamel were assessed when restored with FiltekTM P60 with and without an intermediary (FiltekTM Flow) liner. Efes et al.52 determined that the 2-year clinical performance of class II cavities restored with FiltekTM Supreme with and without an intermediary (FiltekTM Flow), using USPHS modified Ryge criteria, highlighted no secondary caries or post-operative sensitivity in any of the restorations. Therefore, the results of the current study suggest that there was a benefit to the operator in terms of a reduction in cuspal deflection but not from the maintenance of the synergism of the adhesive bond, namely microleakage at the cervical enamel cavosurface margin, when an intermediate layer of a flowable RBC was used under a higher elastic modulus RBC. These findings are in agreement with Alomari et al.15 who identified a reduction in cuspal deflection with no associated difference in gap formation with and without an intermediary layer. Chuang et al.53 demonstrated that the use of a flowable liner was beneficial for reducing voids at the tooth-restoration interface of class II composite cavities by infiltrating the surface irregularities, although again there was no significant improvement in marginal sealing. The reduction in cuspal deflection when an intermediary flowable layer was applied would be expected to produce less microleakage when the tooth sections were examined due to the benefits of the elastic layer proposed by Unterbrink and Liebenberg.18 It would appear that competing factors exist which influence the magnitude of shrinkage stress with flowable RBCs, namely reduced elastic modulus (‘stress absorbing’) and increased resin content (therefore increased polymerisation shrinkage). According to Hooke’s Law, stress is determined by the volumetric shrinkage multiplied by the

487

elastic modulus of the material. However, although the higher polymerisation shrinkage of flowable composites could potentially create increased stress at interfacial areas, the lower elastic modulus may in turn produce less stress through the bulk of the material when compared to RBCs with increased elastic modulus. Consequently, a reason for discrepancies within the literature regarding the ability of flowable RBCs to reduce microleakage may be related to differences in test specimen geometry or ex vivo cavity design. Further investigations on the effects of intermediary lowviscous RBC layer thickness and configuration factor are warranted. Braga et al.37 and Ferdianakis54 also proposed that the higher strain capacity of flowable RBCs may be advantageous to protect the restoration from thermal stress that could cause further damage to the tooth-restoration interface. However, Gueders et al.55 showed that the addition of a thin layer of FiltekTM Flow used in conjunction with ScotchBondTM 1 Adhesive provided no significant improvement in microleakage following thermocycling. In the current study all RBC restorations were placed following the appropriate tooth preparation, namely the etch and rinse technique with a one-bottle adhesive (ScotchbondTM 1 Adhesive), as the technique provides the best adhesion strategy in achieving stable and efficient bonding to dental tissues.55,56

5.

Conclusion

Within the limitations of the current study the first hypothesis was accepted, in that, the use of a flowable RBC as an intermediary layer between adhesive and restorative increments significantly reduced cuspal deflection. The second hypothesis was rejected as there was no observed difference in microleakage at the cervical margin of cavities stored with or without the flowable layer.

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