Effect of Cavosurface Angle on Dentin Cavity Adaptation of Resin Composites

Transcription

1 Original paper Dental Materials Journal 18 (3): , 1999 Effect of Cavosurface Angle on Dentin Cavity Adaptation of Resin Composites Jian WU, Kazuo ITOH, Hisashi HISAMITSU and Sadao WAKUMOTO Department of Operative Dentistry, Showa University, School of Dentistry, Kitasenzoku, Ohta-ward, Tokyo , Japan Received February 26, 1999/Accepted June 9, 1999 The effect of the cavosurface angle of dentin cavities prepared in extracted human molars on the cavity adaptation of a resin composite was evaluated by measuring the gap width between the resin composite and the dentin cavity wall. Cavities with cavosurface angles of 90, 120, 135, or 150 were pretreated with one of two commercial dentin bonding systems or an experimental dentin bonding system. The contraction gap width was measured at both the cavity margin and the section cavity using a light microscope. Complete cavity adaptation was obtained with pretreatment of the experimental groups regardless of the Cavosurface angle. The contraction gap observed at the cavity margin was prevented with the two commercial dentin bonding systems when the cavosurface angle was increased to 150 degrees. A high correlation was observed between the contraction gap width and the proportion of the free surface to the adhesive surface of the resin composite restoration. Key words: Cavosurface angle, Dentin bonding, Contraction gap INTRODUCTION Marginal adaptation of a resin composite in a dentin cavity is difficult to establish, mainly because of the contraction stress of the resin composite during polymerization1-3). Various methods to improve the marginal integrity of resin composites have been attempted, such as decreasing the voltage of the lamp source to reduce the velocity of the contraction and concomitant relax the contraction stress of the resin composite4) or by reducing the amount of photoinitiator in the base resin matrix5). Furthermore, the quantity of the composite has been reduced by using the incremental filling techniques6). In addition, Feilzer7) proposed that the effect of contraction stress of the resin composite on cavity adaptation was significantly influenced by the proportion of adhesive and free surface of the resin composite restoration. With the development of dentin bonding systems, and especially following the introduction of the dentin primer, contraction gap-free dentin bonding system have recently been reported8-10). In previous reports, the marginal integrity of the resin composite was examined using a cylindrical dentin cavity which had a cavosurface angle of 90 degrees, although the clinical significance of the cavosurface angle was not discussed completely. It has been recommended that cavity preparation should be

2 296 EFFECT OF CAVOSURFACE ANGLE ON DENTIN CAVITY limited to removing only the infected or strongly discolored tooth substances and to preserve as much of the sound tooth structure as possible. Thus, the conventional box-form cavity has not been practiced in the clinic11), and the clinical cavosurface angle for resin composite restoration may, possibly, be larger than 90 degrees. The purpose of the present study was to examine the relationship between the cavosurface angle of the dentin cavity and the adaptation of the resin composite. MATERIALS AND METHODS The proximal enamel of an extracted human molar was flatly eliminated on a wet carborundum paper grit (number 220) and a cavity approximately 3mm in diameter at the surface, 1.5mm or less as determined by cavosurface angle and diameter in depth, and with a cavosurface angle of 90, 120, 135 or 150 degrees was prepared using an experimental fine grain diamond cutting points mounted on a high-speed cutting machine as shown in Fig. 1. The cavity wall was pretreated by one of two commercial dentin bonding systems (Clearfil Liner Bond 2 (LB2), Kuraray, Osaka, Japan or Single Bond (SB), 3M, St. Paul, MN, USA) according to the manufacturers' instructions. Then a light-cured commercial resin composite (Silux Plus, 3M, St. Paul, MN, USA) was filled in the cavity and the free surface of the resin composite was gently and momentarily pressed on a glass plate mediated with a plastic matrix. The composite was then irradiated for 40sec using a lamp unit (Wite Light, Takara Belmond Co., Osaka, Japan). After storing the specimens in water at room temperature of 24 }1 Ž for 10min, the over-filled excess of the resin composite was eliminated on a wet carborundum paper and the composite surface including the surrounding dentin surface was polished on a linen cloth mediated with an alumina slurry, grain size 0.03 m. In the experimental groups, the dentin cavity wall was conditioned with 0.5 mol/l ethylenediamine tetraacetic acid (EDTA) that was neutralized to ph 7.4 with sodium hydroxide for 60sec, followed by rinsing and drying. Then the cavity was primed with a 35 vol% of glyceryl mono-methacrylate (GM) solution for 60sec fol- Fig. 1 Experimental fine grain diamend points with various apical angles. Fig. 2 Sectioned cavities with cavosurface angles of 90, 120, 135 and 150 degrees.

3 WU et al. 297 lowed by air blasting; the dual-cured dentin bonding agent (Clearfil Photo Bond, Kuraray, Osaka, Japan) was applied and irradiated for 10sec prior to placement of the resin composite filling. The marginal integrity was inspected under a light microscope and the width of the possible marginal gap was measured with a screw micrometer (Eyepiece Digital; Leitz, Wetzlar, Germany) mounted on the ocular lens of a microscope (Metaloplan; Leitz, Wetzlar, Germany). The gap width measurement was performed at eight points every 45 degrees along the cavity margin, and the contraction gap value was expressed by sum of the diametrically opposing widths in percentage to the cavity diameter. The contraction gap of the specimen was presented by the maximum of the four contraction gap values. After the marginal gap width measurement, the specimen was sectioned along the long tooth axis through the center of the cavity and the section of the specimen was polished on wet carborundum paper followed by polishing on a linen cloth mediated with an alumina slurry as presented in Fig. 2. The cavity adaptation of the resin composite was inspected under a light microscope, and the maximum gap width at the occlusal, axial and apical dentin cavity walls was measured with a screw micrometer. The placement and polymerization of the resin composite including the gap width measurement were carried out by the same method as for the commercial dentin bonding systems. Ten specimens for each cavosurface angle and dentin bonding system, 120 in total, were prepared. RESULTS The gap widths measured are presented in Tables 1 and 2. Complete adaptation, both in the marginal and sectioned cavity observation, was obtained with the experimental dentin bonding system regardless of the cavosurface angle. In the two commercial dentin bonding systems, complete marginal sealing was observed when the cavosurface angle was increased to 150 degrees. However, observations of the sectioned cavity, revealed that complete cavity adaptation was not obtained even when the cavosurface angle was regulated to 150 degrees. Table 1 Maximum contraction gap width (%) of SP in a concave dentin cavity with various cavosurface angle %, n=10 *Experimental: the dentin cavity wall was conditoned with 0.5mol/L EDTA for 60 sec, primed with 35 vol% of glyceryl mono-methacrylate for 60sec and Clearfil Photo Bond was applied prior to the Silux Plus filling. Values joined by a vertical line were not different by the statistical analysis of Kruskal-Wallis, one way analysis by ranks, or Mann-Whitney U-test (p>0.05). Values given as (n) indicate the number of gap-free specimens.

4 298 EFFECT OF CAVOSURFACE ANGLE ON DENTIN CAVITY Table 2 Maximum contraction gap width (ƒêm) of the SP measured on the sectioned dentin cavity (Occlusal cavity wall) (Axial cavity wall) (Apical cavity wall) n=10 Mean }SD of the gap width; the number of completely gap-free specimens are in (). Values joined by a vertical line were not different by the statistical analysis of Kruskal-Wallis, one way analysis by ranks, or Mann-Whitney U-test (p>0.05). Table 3 Propotion of free surface to adhesive surface or volume of the resin composite calculated The calculated proportions of free surface to volume or adhesive surface are presented in Table 3. The free surface (S), and adhesive surface (S90, S120, S135 and S150) of the resin composite at each cavity were calculated using the follow formulae: S=ƒÎ R2, S90=2ƒÎ R H+ƒÎ R2, S120=ƒÎ(R+r)L+ƒÎ r2, S135=ƒÎ R L=ƒÎ R ã(r2+h2) and S150=ƒÎ R L=ƒÎ R ã(r2+h2). The volume (V90, V120, V135 and V150) of the resin composite in each cavity was calculated as the follow formulae: V90=ƒÎ R2 H, V120=(ƒÎ H/3) (R2+R r+r2), V135=ƒÎ R2 H/3, and V150=ƒÎ R2 h/3. Where the cavity surface diameter (2R) was 3.0mm, and the cavity depth (H)

5 WU et al. 299 Fig. 3 Schematic representation of cavity types with cavosurface angles of 90, 120, 135 and 150 degrees. The proportion of free surface to adhesive surface or volume of each restoration was calculated. Cavity surface diameter (2R) was 3mm, and cavity depth (H) was 1.5mm. Cavity floor radius (r) of 120 degrees and cavity depth (h) of 150 degrees determined by cavosurface angle and surface diameter. Fig. 4 Relationship between the contraction gap width and the proportion of free surface to adhesive surface of restoration. Fig. 5 Relationship between the contraction gap width and the proportion of free surface to volume of restoration.

6 300 EFFECT OF CAVOSURFACE ANGLE ON DENTIN CAVITY was 1.5mm. The cavity floor radius (r) of 120 degrees, the cavity depth (h) of 150 degrees and the length of the cavity wall (L) were determined by the cavosurface angle and diameter (Fig. 3). In addition, the relationship between the marginal gap and the proportion of free surface to adhesive surface or volume of composite is shown in Figs. 4 and 5; as shown in the figures, high correlation was recognized between the contraction gap and these two factors. In particular, the gap width exhibited an extremely high correlation with the proportion of free surface to adhesive surface of the composite; Pearson's coefficient of correlation was for LB2 and for SB, and the probability of correlation in both groups was higher than 99%. DISCUSSION The primary requirement for a dentin bonding system is to maintain a bond between the resin material and the three-dimensional dentin cavity wall until polymerization of the resin composite is complete. However, the efficacy of dentin bonding systems has been widely evaluated by measuring the bond strength of the resin composite to the two-dimensional flat dentin surfaces. The bonding mechanism has been proposed based on observation of the ultra-microstructure at the sectioned resin-dentin adhesive interface. By these measurements and observations, the detailed bonding mechanism of dentin adhesives has been explained by resin monomer impregnation into the interfibrous network of the dentin collagen which is exposed by decalcification of the dentin conditioner, then expanded by dentin priming12-16). However, it has not been possible to evaluate the interaction between the efficacy of a dentin bonding system and the contraction stress of the resin composite or the behavior of the resin composite in the cavity during polymerization shrinkage because the specimens for the above mentioned investigations were conducted using dentin rod coated with dentin adhesives or using a resin composite cylinder bonded to a flat dentin substrate. With respect to bond strength measurement, adhesive fractures in the dentin and cohesive fractures in the resin composite cylinder were frequently experienced in bond strength measurement whereas these two failures are not observed in contraction gap measurement. It should be noted that contraction gaps are observed between the top surface of the dentin and the resin composite despite the formation of a hybrid layer17). It is possible to speculate that monomer diffusion into the etched dentin to form the hybrid layer is not essential for cavity adaptation between the resin composite and the dentin cavity wall. Therefore, it was apparent that bonding efficacy of the dentin bonding system should be evaluated by observation of the marginal integrity of the resin composite in the cavity rather than by measurement of the load required to destroy the two-dimensional bond between the resin composite and the substrate dentin. From a clinical point of view, the most important requirement for a dentin bonding system is maintenance of the bond between the unpolymerized resin composite paste and the dentin cavity wall until polymerization is complete. Clinical failure of the dentin bonding system is detected as the separation of the resin composite from the

7 WU et al. 301 cavity wall just after polymerization of the composite. The gap is frequently detected by the explorer along the cervical margin where enamel the cavity wall is thinnest. The marginal integrity of a resin composite obtained just after irradiation is ensured by the elastic stress from the center of the resin composite toward the cavity wall, which is generated by water absorption of the resin composite. In addition, after the completion of polymerization of the composite, the cavity adaptation of the resin composite should be promptly inspected to eliminate the effects of volumetric expansion by water absorption of the resin composite which might close the possible contraction gap18). In this study, contraction gap formation both at the cavity margin and in the sectioned cavity was prevented completely in only one group, in which the experimental dentin bonding system was applied prior to the resin composite filling even when the cavosurface angle was a minimum of 90 degrees. In two commercial dentin bonding systems, contraction gap formation was minimized when the cavosurface angle was as large as 150 degrees, although gap formation could not be completely prevented as revealed by observation of the sectioned dentin cavity. This improvement of marginal adaptation of the resin composite by an increased cavosurface angle can be explained by the proportion of free surface to adhesive surface or the volume of the resin composite. High correlation was observed between the marginal gap width and the proportion of free resin composite surface to adhesive surface of the restoration in this study. This finding suggests that the marginal sealing of the resin composite restoration was effectively improved by the flow of resin composite from the free surface. In the bond strength measurement, the proportion of free surface to adhesive surface theoretically increased more than 1.0. In addition, it is thought that contraction gaps are never formed between flat dentin and a resin composite. To explain the mechanism of dentin bonding system for resin composite restoration, specimens should be prepared consistently with a proportion of free surface to adhesive surface of not larger than 1.0 because in clinical situations the resin composite is restored into a concave cavity in which the free surface of the resin composite is always smaller than the adhesive surface. As discussed above, the experimental dentin bonding system was more effective than the two commercial dentin bonding systems tested because the proportion of the free surface to the adhesive surface leading to a complete marginal seal of the resin composite was as low as 0.33 whereas that of the commercial systems was as high as In previous reports, the bonding mechanism of the experimental dentin bonding system was explained by the possible interaction between the high Ca-content in the substrate dentin and the functional monomer in the dentin bonding agent. In addition, the high degree of polymerization of the Ca-monomer compound at the adhesive interface was considered to be essential for dentin bonding19,20). EDTA conditioning had the advantage of removing the smear layer slightly decalcifying the sound dentin beneath the smear layer. Chigira et al. speculated that the GM solution exhibited a complete priming effect in the EDTA-conditioned dentin because it maintained a high monomer content at the adhesive interface which was observed as a high density zone

8 302 EFFECT OF CAVOSURFACE ANGLE ON DENTIN CAVITY under a transmission electron microscope21). The two commercial dentin bonding systems were developed to simplify the bonding procedure by the introduction of dentin etching with phosphoric acid or a self-etching dentin primer composed of HEMA, but the dentin cavity adaptation of the resin composite mediated with these dentin bonding systems was not complete22). It is possible that the quantities of Ca-monomer compound at the adhesive interface were decreased with the decalcification of the dentin by the dentin conditioner. The difference in the priming effects between the HEMA and GM might be explained by the HEMA primer promoting the monomer diffusion into the dentin, resulting in a low monomer concentration at the adhesive interface22). Cavity adaptation at the axial cavity wall is extremely difficult to obtain, probably because the contraction stress is concentrated at the cavity floor even when the cavosurface angle is increased23). Our results were similar to those reported by Kinomoto et al.23), who investigated the distribution of internal stress of resin composite restoration using photoelastic analysis with a box-shape cavity and reported that not only at the axial wall but also on the cavity floor the largest normal stress occurred near the internal line angle. The minimum principal stress close to the cavosurface margin might be the smallest due to the flow of composite decreased with increasing distance from the free outer surface. In addition, this study was performed using different cavity forms with a constant free surface, and the volume of resin composite was significantly reduced with increasing cavosurface angle. The improved cavity adaptation with an increase in the proportion of free surface to volume might have been due to the decrease of the contraction stress of the resin composite. To conclude, the contraction gap observed at the cavity margin was minimized with two commercial dentin bonding systems when the cavosurface angle was increased to 150 degrees, although these dentin bonding systems did not completely prevent gap formation. Complete cavity adaptation was obtained in all specimens pretreated with the experimental dentin bonding system, regardless of the cavosurface angle. This study suggested that increasing the cavosurface angle results in a significant improvement of marginal adaptation of resin composites, and the proportion of the free surface to the adhesive surface of the resin composite may be an important standard to evaluate the efficacy of dentin bonding systems. REFERENCES 1) Asmussen, E.: Composite restorative resins. Composition versus wall-to-wall polymerization contraction, Acta Odont Scand 33: , ) Davidson, C.L., Dee Gee, A.J. and Feilzer, A.J.: The competition between the composite-dentin bond strength and the polymerization contraction stress, J Dent Res 63: , ) Feilzer, A.J., Dee Gee, A.J. and Davidson, C.L.: Setting stress in composite for two different curing modes, Dent Mater 9 (1): 2-5, ) Uno, S. and Asmusen, E.: Marginal adaptation of a restorative resin polymerized at reduced rate, Scan J Dent Res 99: , ) Venhoven, B.A.M., De Gee A.J. and Davidson C.L.: Light initiation of dental resins: dynamics of the polymerization, Biomaterials 17: , 1996.

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