Influence of Specimen Dimensions on. Nominal Polymerization Contraction Stress of a Dental Composite

Transcription

1 M. F. Witzel R. R. Braga and R. Y. Ballester Dental Materials Department School of Dentistry Uniersity of São Paulo A. Prof. Lineu Prestes, São Paulo, SP. Brazil and R. G. Lima Mechanical Engineering Department Escola Politécnica Uniersity of São Paulol São Paulo, SP. Brazil Influence of Specimen Dimensions on Nominal Polymerization Contraction Stress of a Dental Composite The objectie of the present study was erify the influence of specimen dimensions on polymerization contraction stress of a self-cure dental composite and inestigate the influence of confinement (expressed by the ratio between the bonded and unbonded area of the composite, or C factor ) and olume of the specimen on stress alues. The composite was inserted between the flat surfaces two glass rods attached to a uniersal testing machine. Specimen dimensions were defined using glass rods with different diameters (2.,, or 8 mm) and adjusting the distance between them (.63,.83, 1.2 or 2. mm). An extensometer was used to keep specimen height constant. Force deelopment was monitored for 3 min and the maximum alue was used to calculate nominal stress (MPa). System deformation (compliance) was estimated in order to calculate stress alues on an ideally rigid situation. Data were analyzed by ANOVA/Tukey test (α=.) and regression analysis. The interaction was significant (p<.1). Differences in nominal stress for different heights were erified only for -mm and specimens. In general, lower heights produced higher stress alues. Regression analysis using all the collected data showed a linear correlation between stress and C factor. Howeer, non-linear relationships were found when stress was plotted against C factor or olume selecting specimens with similar same diameter. It was concluded that specimen dimensions influenced test results. Howeer, neither C factor nor olume can be considered reliable predictors of contraction stress alues. Keywords: Contraction stress, dental composite, mechanical testing Introduction Stresses deeloped during the polymerization of dental composites are frequently associated with the failure of the tooth/composite adhesie joint, jeopardizing the clinical longeity of the restoration (Hilton, 22; an Dijken, 23). Seeral authors hae inestigated the different factors affecting stress deelopment (Feilzer et al., 1987; Versluis et al., 1998; Daidson and Feilzer, 1999; Versluis and Tantbirojn, 1999), while others hae been ealuating restoratie strategies with the purpose of reducing stress magnitude (Choi et al., 2; Condon and Ferracane, 22; Braga et al., 23). 1 Contraction stress alues can be accessed by finite elements analysis (Laughlin et al., 22; Barink et al. 23), photoelastic analysis (Ernst et al, 23; Kinomoto et al., 23) and, more frequently, using an experimental set-up known as tensilometer (Feilzer et al., 1987; Alster et al., 1997 a,b; Miguel and de la Macorra, 21; Condon and Ferracane, 22). The use of a tensilometer to determine contraction stress was introduced in Dentistry by Bowen (1967). After a study by Feilzer et al. (1987), this method began to be used more frequently. Briefly, the experimental set-up consists of two metal or glass rods attached to opposite clamps of a uniersal testing machine. The composite is inserted between the opposing flat surfaces of the rods and the axial force generated by its polymerization shrinkage is monitored for a pre-determined time interal. Force alues are diided by the crosssection area of rods in order to obtain nominal stress. The crescent use of this method raised seeral questions and controersy among researchers. One recurrent point of dispute is regarding the influence of the deformation (compliance) of the testing set-up on force deelopment. The experimental set-up is not ideally rigid and the elongation of the components plus the approximation of the opposite rods upon force deelopment could reduce the alues registered by the load cell. Compliance can be minimized by adding a feedback system (e.g., an extensometer) to the assembly. Its primary function is to detect any approximation Paper accepted June, 2. Technical Editor: Atila P. Sila Freire. between the rods during composite contraction and command the cross-head to moe in the opposite direction, maintaining the initial height of the specimen (Feilzer et al. 1987, Alster et al a,b). By doing so, the extensometer minimizes compliance by excluding any deformation that takes place beyond its fixation points. Howeer, some deformation of the rods still occurs within the extensometer attachments that could influence stress alues. Another important question is related to the dimensions of the specimen. So far, there is no standardization or agreement among authors regarding this matter. As with most mechanical tests, specimen dimensions influence results and may preclude an accurate comparison among different materials. A parameter usually reported in contraction stress studies is the ratio between bonded and unbonded surface of the specimen, known as C factor (Feilzer et al., 1987). The results reported by different authors seem to indicate a direct relationship between stress and confinement of the specimen expressed by its C factor (Bowen, 1967; Feilzer et al., 199; Alster et al., 1997a). This index does not take the olume of the sample into consideration. In other words, it is possible to obtain specimens with different olumes and same C factor. The aboementioned study (Feilzer et al., 1987) did not report any relationship between olume and stress magnitude using a low-compliance assembly. Howeer, studies using less rigid set-ups (e.g., without a feedback system) hae shown some relationship between olume and contraction stress (Bouschlicher et al., 1997; Miguel and de la Macorra, 21; Watts et al., 23). Considering the reduced number of studies ealuating the influence of specimen dimensions on contraction stress deelopment, as well as the discrepancies found among authors regarding the influence of confinement and sample olume on stress alues, the objectie of this study was to erify the influence of specimen diameter and height (and its deriaties, namely, olume and C factor ) on composite contraction stress. The intended contribution is to proide guidelines that might help standardize the testing procedure. Nomenclature l a = Adhesie l = Glass J. of the Braz. Soc. of Mech. Sci. & Eng. Copyright 2 by ABCM July-September 2, Vol. XXVII, No. 3 / 283

2 M. F. Witzel et al l c = Composite A c = Cross-section area of the composite mm 2 A V1 = Cross-section area of the machined region of the glass rod mm 2 A V2 = Cross-section area of the non-machined region of the glass rod mm 2 f = Load N C a = Compliance of the adhesie mm/n C = Compliance of the glass mm/n D c = Composite elongation mm/mm E a = Young s modulus of the adhesie MPa E c = Young s modulus of the composite MPa E = Young s modulus of the glass MPa L = Initial length of the assembly mm L 1 = Length of the non-machined segment of the glass rod before L 2 = Length of the machined segment of the glass rod before L 3 = Adhesie layer thickness before L 4 = Height of the composite before contraction mm L 1 = Length of the assembly after composite contraction mm L 4 1 = Length of the composite before L 1 2 = Length of the non-machined segment of the glass rod after L 2 2 = Length of the machined segment of the glass rod after L 3 2 = Adhesie layer thickness after L 4 2 = Length of the composite after T = Nominal contraction stress of composite MPa Materials and Methods Determination of Experimental Stress Contraction The method used for contraction stress ealuation was similar to those used by seeral authors (Feilzer et al., 1987; Alster et al., 1997a,b; Choi et al., 2). Glass rods (Pyrex) with mm length and diameter of 6 or 8 mm were used. The 6-mm diameter rods had one of their flat surfaces machined in a computer-assisted lathe (Compact CNC EMCO, Hallein, Austria) to obtain a cylindrical surface 2 mm in length and 2. or mm in diameter. The flat surface of the machined end and one of the flat surfaces of the 8-mm diameter rods were finished with 18 grit sandpaper, sandblasted with alumina (2 µm), silane-treated (3M ESPE, St. Paul, MN, EUA, lote 9LT) and coated with a layer of Bis-GMA/TEGDMAbased adhesie (Scotchbond Multi-Uso Plus, 3M ESPE, batch 2MU), light-cured for 3 s. After surface preparation, two rods were attached to the opposite clamps of a uniersal testing machine (Instron 6, Canton, Massachusetts, EUA). Specimen height (.63 mm,.83 mm, 1.2 mm or 2. mm) was determined by adjusting the distance between the glass rods. Therefore, 12 experimental groups were defined (n=3). Each testing condition, with the respectie C factor and olume, is shown in Tab. 1. Table 1. C factor and olume (mm 3 ) of experimental conditions. Diameter (mm) C factor Volume C factor Volume C factor Volume A commercial chemically-actiated dental composite was used (Adaptic, Dentsply Ind. e Com., Rio de Janeiro, Brasil, batch 1781). The composite was mixed for 2 s on a paper pad with a plastic spatula, and placed between the adhesie-coated surfaces of the glass rods. An extensometer (Instron), attached to glass rods to detect any approximation between them, commanded the cross-head of the testing machine to restore the initial distance between extensometer clamps, with.1 µm accuracy (Fig. 1). Figure 1. Schematic diagram of the testing system, showing the two imaginary stages of the test considered for calculating the stress alues corrected taking into account material compliance. Contraction stress deelopment was monitored for 3 min, at controlled temperature of 37±1 o C, and the maximum load (N) was registered. During polymerization, shrinkage forces cause elongation of the glass contained within the extensometer clamps and also of the adhesie layer. When specimens with different dimensions are compared, glass elongation represents an important source of experimental error, because it aries according to the specimen dimensions and, consequently, its elongation is also different. The thickness of the adhesie layer, on the other hand, does not ary according to the sample dimensions. Contraction force alue was corrected according to Young s modulus, dimensions of the glass rod, adhesie layer and the composite itself. Though polymerization contraction and specimen elongation triggered by the extensometer occurred continuously, two distinct stages will be assumed for explaining the equations below: 1) Free contraction of the composite in the axial direction and 2) Deformation of the parts commanded by the extensometer. A schematic diagram of these stages is shown in Fig. 1. Contraction stress was computed using the following equations T = T e + T x (1) where T is the corrected contraction stress, T e is the experimental contraction stress and T x is the hypothetic stress necessary to elongate the composite to original dimension, defined as T x = E c D (2) c where E c = 13, MPa is the Young s modulus of the composite (Laughlin et al., 22). Composite deformation, D c, is calculated by L 4 2 L -L (3) D= L c 2 4 can be obtained taking into account glass and adhesie elongation, where [(L +L + l ) 2]+[(L + l ) 2]+L =L (4) a / Vol. XXVII, No. 3, July-September 2 ABCM

3 where L = 1 mm is the initial and final length of the assembly (i.e., the distance between the clamps of the extensometer) and L 3 =.36 mm is the thickness of the adhesie layer before deformation (Choi et al., 2). Glass elongation, l, depends on glass compliance, C, and maximum load, f, according to l=c f () where f is the maximum contraction force and C is the glass compliance (Alster et al., 1997a; Schroeder 23). L1 L C= 2 + A E A E V1 Glass compliance alues (mm/n) are shown in Tab. 2. The adopted Young s modulus of the glass is 64, MPa (Laughlin et al., 22) V2 (6) Table 3. Mean and standard deiation (MPa) of stress alues (alues followed by the same superscripts are not statistically different, p>.). Diameter (mm) ±. cdef 7.±.4 def 6.8±.7 ef.2±. ef 17.8±1.9 b 13.±1.1 bc 9.2±.2 cde 3.2±1.3 f 8 31.±.4 a 16.7±2. b 12.±1.9 bcd 7.6±.8 cdef Regression analyses between stress and C factor are displayed in Fig. 2. Considering the totality of the specimens (N=36), a positie linear correlation was found. Howeer, regression analysis grouped by diameter detected strong non-linear correlations between the two ariables. Regression analysis between stress and olume for the entire set of data showed high dispersion, without tendency for correlation. Fig. 3 shows stress x olume cures grouped by diameter, eidencing strong non-linear relationships between ariables. Table 2. Glass compliance alues (x 1 - conditions. mm/n) for experimental 3 3 R 2 =.9789 Diameter (mm) R 2 =.996 R 2 =.9986 R 2 = mm diameter -mm diameter Linear (entire data) Adhesie elongation l a depends on adhesie compliance C a and maximum tensile force f where the adhesie compliance is l=c f (7) a a L (8) 3 C= a A V1 E a and E a = 4,78 MPa is the Young s modulus of the adhesie (Choi et al., 2). The computed compliance of the adhesie layer (mm/n) was 3.16 x 1-6 mm/n for specimens with diameter of 2. mm,.79 x 1-6 mm/n for specimens with diameter of mm and.31 x 1-6 mm/n for specimens with diameter of 8 mm. Statistical Analysis Stress alues were submitted to two-way ANOVA and Tukey test with a 9% confidence interal. Regression analysis for stress x C factor and stress x olume was performed for the entire sample and grouping the data by diameter. Results ANOVA reealed a significant interaction between the main factors (p<.1). Stress aerages and standard deiations are shown in Tab. 3. No significant differences between heights were detected for the smallest diameter. For the -mm diameter specimens, there were significant differences among specimens with different heights, particularly among.63 mm, 1.2 mm and 2. mm, and also between.83 mm and 2. mm. For the specimens, those with.63 mm deeloped statistically higher stress than the other groups, and those with.83 mm also present higher stress than the 2.-mm height specimens 'C factor' Figure 2. Regression analysis between stress and C factor for different diameters (logarithm function for the 2.-mm diameter groups, polynomial function for the -mm diameter groups and exponential function for the 8- mm diameter groups) and for entire data set (linear function) R 2 =.9962 R 2 =.9227 R 2 = Volume (mm 3 ) 2.-mm diameter -mm diameter Figure 3. Regression analysis between stress and olume for different diameters (power function for the groups and exponential function for 2.- and -mm diameter groups). J. of the Braz. Soc. of Mech. Sci. & Eng. Copyright 2 by ABCM July-September 2, Vol. XXVII, No. 3 / 28

4 M. F. Witzel et al Discussion In the present study, a self-cure composite was used to allow uniform polymerization, regardless of the specimen s dimensions (Alster et al., 1997a). The use of a light-cured material (more frequently employed in restoratie dentistry) would be problematic, because light attenuation through the composite would result in undesirable differences in conersion, particularly with the thicker specimens. Compliance alues calculated in the present study are approximately 1 times higher than alues related by Alster et al. (1997b). This can be explained by the fact that authors used steel rods, that has Young s modulus much higher than glass (27 GPa and 64 GPa, respectiely) (Laughlin et al., 22). Also, those authors did not use a layer of unfilled resin between the composite and the steel rods. In that study, specimens with.3-mm diameter showed stress alues (after corrected for compliance) between 14. and.2 MPa for heights between.6 and 2.7 mm. These alues are similar to those reported in the present study (between 17.8 and 3.2 MPa for heights between.63 and 2. mm, for -mm diameter). Stress alues aried significantly with specimen height for the - and specimens. In general, taller specimens deeloped lower stress alues, eidencing an effect that we will call boundary effect. For large diameters, reductions in height probably hinder the occurrence of polymerization shrinkage transersally to the long axis of the specimen, due to a large bonded area between the glass and the composite. Consequently, a higher fraction of the contraction force tends to manifest longitudinally. In fact, in the present study, a small influence of the boundary effect associated with a relatiely high compliance may explain the lack of statistical differences among 2.-mm diameter specimens. To some extent, the C factor helps the isualization of the boundary effect. According to the authors that proposed the index, a higher confinement reduces the possibility of plastic deformation during the early stages of polymerization, which is eidenced by the regression cure between stress and C factor for the entire data set. Howeer, the fact that a non-linear effect of C factor on contraction stress (with higher R 2 alues than displayed when using the entire data set) was erified when regression analysis was conducted with data grouped by diameter shows that C factor does not fully explain contraction stress deelopment. Moreoer, it can be obsered in Fig. 2 that, for low C factors, the distance between the experimental obseration and the regression cure may represent an error close to %. The influence of composite olume on contraction stress deelopment seems to be less eident than that of the confinement. No regression cure could be fit between olume and stress for the entire data set. Data grouped by diameter reealed non-linear correlations between the two ariables. When contraction stress was plotted against 1/olume as independent ariable, linear functions were found for 2. mm and mm diameters, while for the 8 mm diameter, the best fit was obtained with the use of an exponential function (Fig. 4). The fact that a non-linear correlation was obsered with the largest diameter can be considered another eidence of boundary effect. Howeer, the distance between the cures hinders any prediction based on sample olume R 2 =.9789 R 2 =.9924 R 2 = / Volume (1/mm 3 ) 2.-mm diameter -mm diameter Figure 4. Regression analysis between stress and 1 / olume index for different diameters (linear function for 2. and -mm diameters and exponential function for ). When the tensilometer is used to determine contraction stress of photoactiated composites, specimen dimensions must be defined in order to allow a homogeneous degree of conersion. Therefore, in order to aoid a gradient of conersion through the thickness of the sample, which would increase the system s compliance, specimen height should be kept around 1 mm (Lim et al., 22). Specimens with reduced height are easier to build, which is particularly important when self-cure composites are being tested. Howeer, it must be considered that reduced heights tend to increase the influence of boundary restraints. In the present study, it was eident that specimen dimensions significantly influence contraction stress alues. Also, the results suggest that stress alues cannot be accurately predicted by C factor or sample olume, because of the influence of boundary constraints defined by specimen s diameter. Acknowledgements The authors acknowledge the financial support by FAPESP (/-) and CAPES. 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