Influence of Curing Tip Distance on Composite Knoop Hardness Values


Lourenço CORRER SOBRINHO1
Adriano de Almeida de LIMA1
Simonides CONSANI1
Mario Alexandre Coelho SINHORETI1
Jonathan C. KNOWLES2

1Área Materiais Dentários, Faculdade de Odontologia de Piracicaba, UNICAMP, Piracicaba, SP, Brasil
2Department of Biomaterials, Eastman Dental Institute, University College London, London, UK


Braz Dent J (2000) 11(1): 11-17  ISSN 0103-6440

Introduction | Material and Methods | Results | Duscussion | Conclusions | Resumo | References


The purpose of this paper was to study the influence of curing tip distance on Knoop hardness values, at different depths, of two composites, Z100 and Silux Plus. Specimens (5 mm in diameter and 2.5 mm in height) were prepared in a copper mold, covered with mylar strip and polymerized for 40 s, at 3 tip-to-composite surface distances: 0 mm (surface contact), 6 and 12 mm, utilizing an XL 3000 curing unit, with 750 mW/cm2 power. The specimens were then stored at 37°C for 24 h. Knoop hardness values were measured using a microhardness tester, with a load of 50 g for 30 s for each indentation. Four specimens were made for each distance and composite and eighteen indentations were made of each specimen. The results were submitted to analysis of variance and Tukey test at 5% significance level. The results indicated that 1) composite Z100: the larger the curing tip distance in relation to the composite, the lower the Knoop hardness values; 2) Silux Plus: increasing the curing tip distance did not produce a statistically significant difference in the Knoop hardness values; however, at 6 and 12 mm, the deeper layers showed lower Knoop hardness values in relation to the surface; 3) Z100: statistically superior in relation to Silux Plus at all three curing tip distances and at all depths (P<0.05).


Key Words: composite resin, Knoop hardness, polymerization, depth, distance.


Introduction

Light-cured resin composites have found widespread use in modern dentistry. However, an adequate light with the correct wavelength range must reach all areas of a light activated restoration to ensure suitable polymerization and long-term clinical success. The depth of cure is limited by and is dependent on several variables such as material, exposure time, color, location of light source and quality of the light source (DeWald and Ferracane, 1987; Pires et al., 1993; Phillips, 1996; Marais et al., 1997; Pilo et al., 1999).

Polymerization depth is directly related to thickness of the material and influenced by light intensity. The top surface gives higher hardness values compared to the bottom surface and hardness may be improved by increasing exposure time (Cook, 1986; Baharav

As expected, light intensity decreases at the composite surface as the curing tip is moved away from the composite. The ideal curing tip position is as close to the composite as possible; however, this is not possible in all situations. In deep cavities, the restoration must be built up in increments. Each increment should not be greater than 2.0-2.5 mm and must be properly cured before insertion of the next increment. Although this appears to be limited, a significant portion of the polymerization shrinkage can be compensated for as the cavity is being filled and cured (Phillips, 1996).

The purpose of this paper was to study the influence of curing tip distance on Knoop hardness values, at different depths, for two light cured composite resins.


Material and Methods

Two resins were tested: Z100, a microfilled composite and Silux Plus, a small-particle filled composite. Both resins were supplied by 3M, Dental Products Division (St. Paul, MN, USA). The resin was placed in the cavity (5 mm in diameter by 2.5 mm in height) of a split copper mold and covered with a mylar strip. It was polymerized for 40 s at one of three distances: 0 mm (surface contact), 6 mm, and 12 mm from the composite surface, utilizing a XL 3000 curing unit (3M, Dental Products Division), with 750 mW/cm2. The distances were standardized by adding copper spacers to a positioning jig. Four specimens were made for each composite resin at each distance.

After the polymerization procedure, the specimens were removed from the copper mold and stored a 37°C and 95 ± 5 % relative humidity for 24 h. The specimens were then placed in a vertical position and embedded in polyester resin (Resapol T208, São Paulo, SP, Brazil). The cured resin was ground and polished to the center of each specimen, using 320, 600 and 1000 grit sandpaper (Norton S.A., São Paulo, SP, Brazil) and polished using chromium oxide (0.03 µm) and alumina (0.05 µm) aqueous solution on an automated polisher (Metalserv-Rotary Pregrinder, London, England).

Knoop hardness values were measured across the section of the composite resin using a microhardness Tester FM (Future Tech Corp., Tokyo, Japan) at automatic procedure with a load of 50 g applied for 30 s. Three measurement positions (A, B and C) were made, each with six indentations (P1 to P6) from the top to the bottom of the specimen section, giving a total of eighteen measurements for each specimen. The layout is shown in Figure 1. The results were submitted to analysis of variance and Tukey test at the 5% significance level.


Results

Table 1 shows the mean Knoop hardness values for the composite resin Z100 at the different depths and cured from the three distances. The hardness values for a curing tip distance of 0 mm was significantly higher than 6 mm distance for depth P1 to P4 and than 12 mm distance for all six depths (p<0.05). For all three tip to surface distances, the composite resin at depth P1 was significantly stronger than depth P3 to P6 (p<0.05).

The Knoop hardness values for the composite resin Silux Plus at different depths and cured from the three distances are shown in Table 2. The increase of the curing tip distance (0 mm contact surface) to 6 and 12 mm did not produce statistically significant differences in the Knoop hardness values for the six depths P1 to P6. For the curing tip to composite distances of 6 and 12 mm, the composite resin cured at P1 depth was significantly stronger than P5 and P6 depth (p<0.05). No statistical difference at any depth was seen when the curing tip to resin distance was 0 mm.

Tables 3, 4 and 5 shows mean Knoop hardness values (KHN) of Z100 and Silux Plus at different depths cured at the three distances 0, 6 and 12 mm, respectively. The composite resin Z100 was statistically superior in relation to Silux Plus at the three curing tip to resin distances and at all depths (p<0.05).


Discussion

Curing lights with very high intensity are recommended almost universally (Lutz et al., 1992), based on curing depths and physical properties of the composites. Doubling the intensity has been reported to increase the curing depth by approximately 15% (Ruyter and Oysaed, 1982; Fan et al., 1993). However, these recommendations fail to consider the possible negative influence of high intensity lights on the development of stress (Unterbrink and Muessner, 1995).

Table 1 shows that the larger the curing distance in relation to the Z100 composite the lower the Knoop hardness values. A reduction in hardness was also obtained with increased depth of the composite. Table 2 shows that increasing the curing tip distance for Silux Plus had no effect. No reduction in hardness with increasing depth for the composite was seen. The hardness values obtained in the present study indicate that the degree of polymerization within the specimen decreases at increasing distance from the surface of the resin in accordance with Pires et al. (1993), Swartz et al. (1993) and Prati et al. (1999). The intensity of light emitted from a curing unit is reduced by distance and by the composite resin material. In our study, it was seen that composite resin polymerization depends greatly on distance from the curing tip. Rueggeberg and Jordan (1992) obtained the same results using infrared spectroscopy to measure monomer conversion.

A reduction in cure leads to a reduction in physical properties of the resin. Light-cured resins with lower hardness have lower transverse strength than those with higher hardness values (Thirta et al., 1982; Burtschep et al., 1997; Marais et al., 1997). Also related are changes in water sorption, stiffness, toughness and color stability when approximation of the curing tip in a class II restoration is impossible (Swartz et al., 1993). From a biological standpoint, an incomplete cure of the resin would be accompanied by a higher residual monomer that would indicate a potential increase in the irritant properties of the resin (Swartz et al., 1993). Matsumoto et al. (1986) report that the curing times recommended by manufacturers for composites should be extended when light intensity is reduced by distance.

In our study, the hardness values of composite Z100 were statistically superior to Silux Plus at the three distances of the curing tip and at all depths. It is apparent that depth of cure is directly related to filler particle size in dental composite resins, as has been shown previously (Ferracane, 1985; Li et al., 1985). Light-scattering within the composite is increased as the particle size of the fillers approaches the wavelength of the activating light. This scattering will reduce the amount of light that is transmitted through the composite. Therefore, the larger particle composite had the greatest depth of cure, since it was least affected by light-scattering (DeWald and Ferracane, 1987). The lower depth of cure for Silux Plus can be explained by the very small particle size. Ruyter and Oysaed (1982) have suggested that scattering is maximized when the effective particle size is one-half the wavelength of the activating light or approximately 0.25 µm. An optimal particle size would be 0.125 µm. However, the colloidal silica particles are approximately 0.04 µm, thus whilst not optimal, significant scattering may occur due to the very high surface area of the very fine colloidal silica, giving a significant level of scattering and reduced depth of cure (DeWald and Ferrracane, 1987).

Therefore, the depth of cure is limited by and is dependent on various factors, principally the location of the light source and the filler particle size of the composite
resin.


Conclusions

1) For the composite Z100, the larger the curing tip distance in relation to the composite, the lower the Knoop hardness values.

2) The increase of the curing tip distance did not produce statistically significant differences in the Knoop hardness values on the surface for the Silux Plus composite. With a curing tip distance of 6 and 12 mm, the deeper layers showed lower Knoop hardness values in relation to the surface.

3) The composite Z100 was statistically superior to Silux Plus at all three curing tip distances and at all depths (p<0.05).


Resumo

Correr Sobrinho L, de Lima AA, Consani S, Sinhoreti MAC, Knowles JC: Influência da distância da ponta polimerizadora na dureza Knoop de compósitos. Braz Dent J 11(1): 11-17, 2000.

O propósito deste estudo foi verificar a influência da distância da ponta polimerizadora, sobre a dureza Knoop em diferentes profundidades, dos compósitos Z100 e Silux Plus. Os corpos-de-prova medindo 5 mm de diâmetro por 2,5 mm de altura foram preparados em matriz metálica, cobertas com tira de celulose, com a ponta polimerizadora colocada nas seguintes distâncias: 0, 6 e 12 mm, utilizando o aparelho XL 3000, com potência de 750 mW/cm2. Após a armazenagem a 37°C por 24 horas, a dureza dos corpos-de-prova foi medida com microdurômetro calibrado com carga de 50 g por 30 segundos. Os resultados foram submetidos à análise de variância e ao teste de Tukey (5%), indicando que: 1) No compósito Z100, quanto maior a distância da ponta polimerizadora, menor os valores de dureza Knoop; 2) No Silux Plus, o aumento da distância da ponta polimerizadora não mostrou diferênça estatística significativa nos valores de dureza Knoop; entretanto, nas distâncias 6 e 12 mm as camadas mais profundas mostraram menores dureza Knoop quando comparadas com aquelas da superfície; 3) O produto Z100 foi estatisticamente superior ao Silux Plus nas 3 distâncias da ponta polimerizadora em todas as profundidades.


Unitermos: resina composta, dureza Knoop, profundidade de polimerização, distância da ponta polimerizadora.


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Correspondence: Dr. Lourenço Correr Sobrinho, Área Materiais Dentários, Faculdade de Odontologia de Piracicaba, UNICAMP, Av. Limeira, 901, 13414-900 Piracicaba, SP, Brasil. E-mail: sobrinho@fop.unicamp.br


Accepted March 30, 2000
Eletronic Publication  july, 2000


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