SCANNING VOL. 38, 43–49 (2016) © Wiley Periodicals, Inc.
Fracture Strength of Restorations in Proximal Cavities of Primary Molars ESMA YILDIZ,1 MINE SIMSEK,2 AND ZEYNEP PAMIR2 1 2
Department of Pediatric Dentistry, Akdeniz University, Antalya, Turkey Department of Pediatric Dentistry, Gaziantep University, Gaziantep, Turkey
Summary: This study evaluated the fracture strength of various restorative materials for primary molars in dovetail and box-only class II cavity designs. Eighty extracted noncarious human primary molars were used. The teeth were randomly divided into two groups for either dovetail or box-only preparations. The teeth were then divided into four subgroups for each restorative material: glass ionomer cement (GIC), resin-modified glass ionomer cement (RMGIC), compomer, and composite. The restorations were tested for fracture strength. The loads at fracture and fracture mode were recorded and a scanning electron microscopy analysis was performed to observe the micromorphology of the borders between the teeth and the materials. The nonparametric Kruskal–Wallis and Mann–Whitney U-tests were used. Although there were significant differences between the restorative materials (p < 0.05), there were no differences between the fracture strength of the box-only and the dovetail cavity designs in any of the groups (p > 0.05) except the composite group. The fracture strength of the compomer and composite groups was significantly higher than that of the GIC and RMGIC groups (p < 0.05). A class II cavity could be selected as dovetail or box-only and compomer and composite are more resistant to fracture than GIC and RMGIC. SCANNING 38:43–49, 2016. © 2015 Wiley Periodicals, Inc. Key words: cavity preparation, fracture strength, primary teeth
Introduction Restorative practices of primary tooth have changed with the development of adhesive restorative materials. Address for reprints: Esma Yildiz, Faculty of Dentistry, Department of Pediatric Dentistry, Akdeniz University, Antalya, 07058, Turkey E-mail: [email protected] Received 13 April 2015; Accepted with revision 22 June 2015 DOI: 10.1002/sca.21239 Published online 14 July 2015 in Wiley Online Library (wileyonlinelibrary.com).
In 1924, Black identified the cavity preparation steps for the restoration of caries using amalgam on permanent molars (Black and Black, ’22). These steps have been modified since then. Although with conventional approaches, more material is removed from the tooth tissue to increase the retention; today, minimal tooth tissue removal is preferred in modifying cavities (Ertugrul et al., 2010). On the other hand, cavity design affects the clinical success or failure of the restoration. Improper cavity design directly affects the mechanical retention and tooth resistance to occlusal forces can lead to failure by increasing the negative stress distribution in the interface between the dentin and the restorative material (Hse and Wei, ’97). A modified class II cavity, namely a box-only class II cavity design, is preserved sound tooth tissue. This cavity mode is applied in posterior teeth, not including fissures in the cavity boundary, is only applied to caries formed at the interface, and is a form of minimal preventive preparation (Suwatviroj et al., 2003). Various in vitro and in vivo studies about the success of class II cavities with different restorative materials in permanent teeth were reported (Laegreid et al., 2011; Scholtanus and Huysmans, 2007). Although modified cavity preparations for composites have been shown to be similar to conventional class II preparations in permanent teeth (Castillo, ’99), they cannot be applied to primary molars because of the differences in morphology and the wear behavior of the primary teeth in comparison to that of permanent teeth (Alves dos Santos et al., 2010). The number of studies on box-only and conventional dovetail cavity preparations in primary teeth is limited (Forsten and Karjalainen, ’90; Suwatviroj et al., 2003) and an agreement has not yet been reached as to the ideal cavity preparation in primary molars. At present, the restorative treatment of primary teeth mainly uses glass ionomer cements (GIC), resinmodified glass ionomers (RMGIC), compomers, and composite resins. Composite resin is recommended for patients in a low caries risk group, compomer is recommended with those in a moderate caries risk group, and the GIC is recommended in patients in a high caries risk group (Powers and Wataha, 2007). Because of their advantages, GICs are still regarded as popular;
SCANNING VOL. 38, 1 (2016)
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nonetheless, they also have some noted disadvantages. In recent years, studies have been performed to eliminate the main disadvantages of GICs. Equia Fil (GC), one GIC currently on the market, is reported to be optimistic for use in the posterior region, even in permanent teeth (Gurgan et al., 2015). Some properties, such as surface roughness, surface loss, microhardness, abrasion, and fracture toughness, were reported about the material (Lohbauer et al., 2011; Zanata et al., 2011). However, no research about the fracture strength (FS) of current GICs in class II cavities of primary teeth was observed in the literature. The present study evaluated the fracture strength of a GIC, a RMGIC, a compomer, and a composite in dovetail and box-only class II cavity designs in primary teeth.
Materials and Methods Preparation of Teeth
The present study used 80 exfoliated noncarious or carious on one surface only of human primary second molars. Visual inspections were performed macroscopically under magnification to detect cracks of the enamel. The specimens were initially stored in a 0.1% thymol solution and immersed in distilled water prior to the experimental steps. Teeth were kept moistened in the water continuously before and after the restorative, aging, and test procedures.
Cavity Designs
Each tooth was embedded vertically in a nylon ring with acrylic to just below the contact area at the cemento-enamel junction. The teeth were randomly divided into two groups for either dovetail or box-only preparations. Standardized cavities (measurements as shown in Fig. 1) (Suwatviroj et al., 2003) were prepared in the teeth using a flat-ended cylinder diamond fissure bur. The depths of cavities were 1 mm in both occlusal and aproximal. Cavities were prepared by a single operator. The cavity widths
were measured with a caliper and the depths were measured with a periodontal probe.
Experimental Procedure
The teeth were divided into four groups (ten per subgroup) for each restorative material (Table I). Group I: Glass ionomer (no adhesive system). Group II: Resin-modified glass ionomer (no adhesive system). Group III: Compomer with STAE Single Component Light Cured (Southern Dental Industries, Bayswater, Victoria, Australia) (one-step self-etch adhesive system). Group IV: Composite with Clearfil SE Bond (Kuraray, Osaka, Japan) (two-step self-etch adhesive system). The materials were handled according to the manufacturer’s instructions. A LED curing device (Ultradent Products, Inc., South Jordan, UT) was used to cure resin containing restorative materials for 20 s with an intensity of approximately 1,000 mW/cm2. No further preparatory procedures, such as the use of a conditioner, were applied for GIC and RMGIC restorations. After the GIC and RMGIC restoration, a G-Coat was applied and light cured. An incremental cure technique was not used because the depth of the cavities was 1 mm. To allow for a complete acid–base reaction, all teeth were aged in distilled water at 37˚C for 15 days.
Load to Fracture
Subsequent to aging, the restorations were tested for ultimate load to fracture using a universal mechanical testing machine (UTM Autograph AG-X, Shimadzu Corp., Tokyo, Japan) with a ball ended cylindrical tip 4 mm in diameter to distribute load. The tip was placed at the marginal ridge on the occlusal surface of the restoration parallel to the vertical axis of the tooth (Fig. 2). An increasing load force was applied with a cross-head speed of 0.5 mm/s until the fraction occurred. The loads at fracture and fracture mode were recorded. The fracture mode of each specimen was evaluated and then classified into one of three groups: adhesive, cohesive, or mixed fracture.
Failure Mode
Fig 1.
Outline width of the cavity preparation.
After the fracture test, the test surfaces of the dentine and restorative materials were examined with an optical microscope (Leica Microscopy Systems, Weltzlar, Germany) under 40x. The dentin surfaces and
E. Yildiz et al.: Fracture strength of proximal restorations TABLE I
45
Components of materials used in the study
Material Glass ionomer Liquid: Polyacrylic acid (30–40%), proprietary ingredient (5–15%) Powder: Fluoroalumino silicate glass (amorphous) (90–100%), polyacrylic acid (5–10%) Glass ionomer coat Urethane methacrylate (30–40%), methylmethacrylate (40–50%), camphorquinone ( 0.05) except composite. The mean FS and standard deviations (SD) for the tested groups are shown in Table II.
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TABLE II Fracture strength of various restorative materials in differently designed class II cavities of primary molars Load at fracture (Newton) Restorative materials (n ¼ 10) GIC (box-only) GIC (dovetail) RMGIC (box-only) RMGIC (dovetail) Compomer (box-only) Compomer (dovetail) Composite (box-only) Composite (dovetail)
Median/mean Minimum/maximum SD
the cavity wall, the adaptation of the interface of the GIC and the tooth failed. Many bubbles were observed and the sample showed microcracks in the restoration (Fig. 4a). In the tooth restored with RMGIC, a crack was clearly visible. Bubbles were observed in both material and border line (Fig. 4b). The SEM image of the compomer restoration showed separation in the bonding area (Fig. 4c). In Figure 4d, the fracture of composite was observed without any cracks in the body of the remaining restoration. A buble also was observed in the restoration.
270/247a 255/289a 249/249a 258/264a 462/461b
98/362 141/470 133/339 148/346 336/593
92 119 56 75 81
576/548b,c
408/691
100
466/476b
358/608
84
Discussion
619/624c
516/792
71
In the present in vitro study, the effects of cavity design and material type on the success of the restorations and fracture modes were evaluated in the primary teeth. In the literature, most of the research on the successful restoration of class II cavities focused on composite restorative materials in permanent teeth (Laegreid et al., 2011; Akbarian et al., 2014). Although compomers are widely used, GICs are indispensable materials for pediatric dentists. However, there is not a specific scientific assessment about the most appropriate cavity design in primary teeth for the different restorative materials used in pediatric dentistry. Although the cervical part of a class II restoration is the most technically challenging part of the cavity in a clinical situation, visual inspection, moisture control, insertion, and light curing of the restorative material are all challenging at this stage of the preparation (Laegreid et al., 2011). There is a 33% carie progression rate, which was adjacent to RMGIC and the reported compomer restorations (Qvist et al., 2004a). Therefore, resin-containing materials, such as compomers and RMGICs, have disadvantages with the curing and moisture control during the bonding step; GICs still have advantages, as they are cariostatic and they chemically bond to dentin. Therefore, it is important to compare the success of the materials and to evaluate resistant cavity designs for each restoration. Prabhakar et al. (2008) reported that beveling in class I cavities increased the survival rate of RMGIC in a oneyear period. However, the present study did not employ beveling. To maintain better bond strength, dentin pretreatment is typically suggested for numerous restorative procedures. Hamama et al. (2014) studied conditioning agents for the pretreatment of dentin before RMGIC restorations and concluded that conditioning dentin had no adverse effect on the bonding of RMGIC restorations. Conversely, some studies reported adverse effects of conditioning in the bond of GICs (Tay et al., 2001; Yap et al., 2003). In the present study, dentin was not conditioned in the GIC and RMGIC groups.
Same lowercase letters indicate an insignificant difference (p > 0.05).
When restorative materials were compared, the FS of the compomer groups was significantly higher than that of the GIC and RMGIC groups (p < 0.05). However, the GIC and RMGIC groups were similar (p > 0.05). Although the dovetail designs in all groups were slightly higher than in the box-only preparations, significant differences were observed only in composite group (p < 0.05).
Failure Mode
Specimen failure modes were evaluated. Figure 2 shows the distribution of the failure mode (adhesive, cohesive, and mixed) for the material types and cavity designs. According to results, mixed type fractures were mainly observed for all tested materials. Adhesive failures were observed in RMGIC-box-only and compomer-box-only and composite-box-only.
SEM
Representative SEM images of the intact restorations and the samples after the fracture test are shown in Figures 3 and 4. Before the fracture test, border of the tooth and material was scanned. In the GIC and RMGIC groups, bubbles and cracks were observed. Bubbles were shown to induce cracks (Fig. 3a, b). In the compomer group, border was more regular (Fig. 3c). In the composite group, adaptation of the tooth and the restoration is better than the other groups (Fig. 3d). In some part of the cavity wall of the primary tooth restored with GIC, the dentin surface is still partially sealed with GIC particles. In addition, in some part of
E. Yildiz et al.: Fracture strength of proximal restorations
Fig 3.
SEM images of (a) GIC, (b) RMGIC, (c) compomer, and (d) composite before the fracture test. T, tooth; R, restoration.
Laegreid et al. (2011) used nanocomposite to restore class II cavities of permanent teeth that were horizontally 2 mm and vertically 3 or 4 mm; they reported no significant difference between the two different cavity designs and the intact teeth control group. In addition, they emphasized that the areas of available cervical enamel affect the FS of the restoration (Laegreid et al., 2011). In the present study, all cavities have cervical enamel. Suwatviroj et al. (2003) studied the cavity designs in primary teeth in vitro and reported that the RMGIC group had a significantly lower FS then the composite in
Fig 4.
47
box-only subgroups. They also reported an insignificant difference between the two cavity designs in both the composite and the RMGIC groups (Suwatviroj et al., 2003). In the present study, RMGIC had a significantly lower FS than the compomer and composite for both the dovetail and the box-only subgroups. Different from the Swatviroj et al.’s study, dovetail design of composite group had significantly higher FS than composite-boxonly group. Owais et al. (2013) reported the means of maximum occlusal bite force for the dentition stages (DS) as follows: 240 N in late primary DS, 289 N in early mixed
SEM images of (a) GIC, (b) RMGIC, (c) compomer, and (d) composite after the fracture test.
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SCANNING VOL. 38, 1 (2016)
DS, 433 N in late mixed DS, and 527 N in the permanent DS. According to this, until the late primary dentition, all materials with any cavity design are appropriate. From early mixed DS on, dovetail cavity design in GIC and RMGIC may be beneficial. As the age of the children grow, more resistant material and cavity design are essential. The GICs in the different cavity designs in the primary teeth were examined during a period of 5–14 months; insignificant differences between cavity designs was observed consistent with the present study (Forsten and Karjalainen, ’90). In one of the clinical studies, Qvist et al. (2004a) reported 1.7 years survival time for non-beveled conventional class II RMGIC and 2.2 years for compomer restorations in primary teeth (Qvist et al., 2004a). In addition, RMGIC restorations in all cavity types of the primary teeth were found more successful than GIC (Qvist et al., 2004b). Another clinical study reported that RMGIC restorations were significantly more successful than silverreinforced GIC in box-only cavities in primary teeth (Espelid et al., ’99). In contrast, the compomer and GIC restorations of box-only cavities of primary teeth were found similar in a 1-year period (Marks et al., 2000). In the present in vitro study, GIC restoration in both box-only and dovetail cavities had similar FS with RMGIC. In a novel study, Al-Angari et al. (2014) reported that Equia Fil had the highest fracture toughness among the tested GICs. They guessed that the high fracture toughness of Equia Fil might be due to the resin coat applied to the surface. In the present study, a resin coat was used in the GIC and RMGIC groups. Fracture of restoration was reported as the main reason for the failure of the GIC restorations in class II cavities. This mode of fracture indicates inadequate physical properties of GIC (Qvist et al., 2004a). Nevertheless, in the present study, mixed type failure in all of the groups were observed. SEM images showed bubbles and microcracks in GIC and RMGIC used in the present study. Thus, micromechanical strength of materials were weakened. It was reported that GICs exhibit more wear than composites due to their composition (Al-Angari et al., 2014). Their acid–base reaction results in a matrix comprising an ionically cross-linked polyalkenoate network, which is weaker than the matrix of resins strengthened by fillers and methacrylate polymer chains (Davidson, 2006). The SEM image of composite without any cracks confirmed the result that composite showed more resistance to fracture. A significant increase in microhardness was reported in GIC up to the 180-day period; from that point, the values stabilized (Zanata et al., 2011). In the present study, a longer aging procedure might be affecting the results. However, different setting techniques for the GIC restorations should be studied in future research. As novel posterior restorative materials are brought into the marketplace, the question remains as to the
largest potential width of class II cavities for successful and long life restoration. As different materials are brought into use, the fracture resistance of cavities with different dimensions and different bonding strategies should be examined in further research.
Conclusions 1. The present study showed that resin-based materials, compomer and composite, are more resistant to fracture than GIC and RMGIC. 2. According to the size of the caries, the lesion design of a class II cavity could be selected as dovetail or box-only. In composite restoration, dovetail cavity design is beneficial. 3. As GIC and RMGIC have a similar FS, to be avoided from resins that are sensitive to moisture during bonding, GIC could be preferred in cavities, which are hard to isolate.
References Akbarian G, Ameri H, Chasteen JE, Ghavamnasiri M. 2014. Fracture resistance of premolar teeth restored with siloranebased or dimethacrylate-based composite resins. J Esthet Restor Dent 26:200–207. Al-Angari SS, Hara AT, Chu TM, et al. 2014. Physicomechanical properties of a zinc-reinforced glass ionomer restorative material. J Oral Sci 56:11–16. Alves dos Santos MP, Luiz RR, Maia LC. 2010. Randomised trial of resin-based restorations in Class I and Class II beveled preparations in primary molars: 48-month results. J Dent 38:451–459. Black GV, Black AD. 1922. A work on operative dentistry. Chicago, IL: Medico-Dental Pub. Co. Castillo MD. 1999. Class II composite marginal ridge failure: conventional vs. proximal box only preparation. J Clin Pediatr Dent 23:131–136. Davidson CL. 2006. Advances in glass-ionomer cements. J Appl Oral Sci 14:3–9. € Ertugrul F, Cogulu D, Ozdemir Y, Ersin N. 2010. Comparison of conventional versus colored compomers for Class II restorations in primary molars: a 12-month clinical study. Med Princ Pract 19:148–152. Espelid I, Tveit AB, Tornes KH, Alvheim H. 1999. Clinical behaviour of glass ionomer restorations in primary teeth. J Dent 27:437–442. Forsten L, Karjalainen S. 1990. Glass ionomers in proximal cavities of primary molars. Scand J Dent Res 98:70–73. Gurgan S, Kutuk Z, Ergin E, Oztas S, Cakir F. 2015. Four-year randomized clinical trial to evaluate the clinical performance of a glass ionomer restorative system. Oper Dent 40:1–10. Hamama HH, Burrow MF, Yiu C. 2014. Effect of dentine conditioning on adhesion of resin-modified glass ionomer adhesives. Aust Dental J 59:193–200. Hse KMY, Wei SHY. 1997. Clinical evaluation of compomer in primary teeth: 1-year results. JADA 128:1088–1098. Laegreid T, Gjerdet NR, Vult von Steyern P, Johansson AK. 2011. Class II composite restorations: importance of cervical enamel in vitro. Oper Dent 36:187–195.
E. Yildiz et al.: Fracture strength of proximal restorations Lohbauer U, Kramer N, Siedschlag G, et al. 2011. Strength and wear resistance of a dental glass-ionomer cement with a novel nanofilled resin coating. Am J Dent 24:124–128. Marks LA, van Amerongen WE, Borgmeijer PJ, Groen HJ, Martens LC. 2000. Ketac molar versus dyract class II restorations in primary molars: twelve month clinical results. ASDC J Dent Child 67:37–41. Owais AI, Shaweesh M, Abu Alhaija ES. 2013. Maximum occusal bite force for children in different dentition stages. Eur J Orthod 35:427–433. Prabhakar AR, Raju OS, Kurthukoti AJ, Satish V. 2008. Evaluation of the clinical behaviour of resin modified glass ionomer cement on primary molars: a comparative one year study. J Contemp Dent Pract 9:130–137. Powers JM, Wataha JC. 2007. Dental materials: properties and manipulation. 10th edition. Missouri: Elsevier Health Sciences. Qvist V, Laurberg L, Poulsen A, Teglers PT. 2004a. Class II restorations in primary teeth: 7-year study on three resinmodified glass ionomer cements and a compomer. Eur J Oral Sci 112:188–196.
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Qvist V, Manscher E, Teglers PT. 2004b. Resin-modified and conventional glass ionomer restorations in primary teeth: 8year results. J Dent 32:285–294. Scholtanus JD, Huysmans MC. 2007. Clinical failure of class-II restorations of a highly viscous glass-ionomer material over a 6-year period: a retrospective study. J Dent 35:156–162. Suwatviroj P, Messer LB, Palamara JE. 2003. The effects of cavity preparation and lamination on bond strength and fracture of tooth-colored restorations in primary molars. Pediatr Dent 25:534–540. Tay FR, Smales RJ, Ngo H, Wei SH, Pashley DH. 2001. Effect of different conditioning protocols on adhesion of a GIC to dentin. J Adhes Dent 3:153–167. Yap AU, Tan AC, Goh AT, Goh DC, Chin KC. 2003. Effect of surface treatment and cement maturation on the bond strength of resin-modified glass ionomers to dentin. Oper Dent 28:728–733. Zanata RL, Magalhaes AC, Lauris JR, et al. 2011. Microhardness and chemical analysis of high-viscous glass-ionomer cement after 10 years of clinical service as ART restorations. J Dent 39:834–840.
Fracture Strength of Restorations in Proximal Cavities of Primary Molars ESMA YILDIZ,1 MINE SIMSEK,2 AND ZEYNEP PAMIR2 1 2
Department of Pediatric Dentistry, Akdeniz University, Antalya, Turkey Department of Pediatric Dentistry, Gaziantep University, Gaziantep, Turkey
Summary: This study evaluated the fracture strength of various restorative materials for primary molars in dovetail and box-only class II cavity designs. Eighty extracted noncarious human primary molars were used. The teeth were randomly divided into two groups for either dovetail or box-only preparations. The teeth were then divided into four subgroups for each restorative material: glass ionomer cement (GIC), resin-modified glass ionomer cement (RMGIC), compomer, and composite. The restorations were tested for fracture strength. The loads at fracture and fracture mode were recorded and a scanning electron microscopy analysis was performed to observe the micromorphology of the borders between the teeth and the materials. The nonparametric Kruskal–Wallis and Mann–Whitney U-tests were used. Although there were significant differences between the restorative materials (p < 0.05), there were no differences between the fracture strength of the box-only and the dovetail cavity designs in any of the groups (p > 0.05) except the composite group. The fracture strength of the compomer and composite groups was significantly higher than that of the GIC and RMGIC groups (p < 0.05). A class II cavity could be selected as dovetail or box-only and compomer and composite are more resistant to fracture than GIC and RMGIC. SCANNING 38:43–49, 2016. © 2015 Wiley Periodicals, Inc. Key words: cavity preparation, fracture strength, primary teeth
Introduction Restorative practices of primary tooth have changed with the development of adhesive restorative materials. Address for reprints: Esma Yildiz, Faculty of Dentistry, Department of Pediatric Dentistry, Akdeniz University, Antalya, 07058, Turkey E-mail: [email protected] Received 13 April 2015; Accepted with revision 22 June 2015 DOI: 10.1002/sca.21239 Published online 14 July 2015 in Wiley Online Library (wileyonlinelibrary.com).
In 1924, Black identified the cavity preparation steps for the restoration of caries using amalgam on permanent molars (Black and Black, ’22). These steps have been modified since then. Although with conventional approaches, more material is removed from the tooth tissue to increase the retention; today, minimal tooth tissue removal is preferred in modifying cavities (Ertugrul et al., 2010). On the other hand, cavity design affects the clinical success or failure of the restoration. Improper cavity design directly affects the mechanical retention and tooth resistance to occlusal forces can lead to failure by increasing the negative stress distribution in the interface between the dentin and the restorative material (Hse and Wei, ’97). A modified class II cavity, namely a box-only class II cavity design, is preserved sound tooth tissue. This cavity mode is applied in posterior teeth, not including fissures in the cavity boundary, is only applied to caries formed at the interface, and is a form of minimal preventive preparation (Suwatviroj et al., 2003). Various in vitro and in vivo studies about the success of class II cavities with different restorative materials in permanent teeth were reported (Laegreid et al., 2011; Scholtanus and Huysmans, 2007). Although modified cavity preparations for composites have been shown to be similar to conventional class II preparations in permanent teeth (Castillo, ’99), they cannot be applied to primary molars because of the differences in morphology and the wear behavior of the primary teeth in comparison to that of permanent teeth (Alves dos Santos et al., 2010). The number of studies on box-only and conventional dovetail cavity preparations in primary teeth is limited (Forsten and Karjalainen, ’90; Suwatviroj et al., 2003) and an agreement has not yet been reached as to the ideal cavity preparation in primary molars. At present, the restorative treatment of primary teeth mainly uses glass ionomer cements (GIC), resinmodified glass ionomers (RMGIC), compomers, and composite resins. Composite resin is recommended for patients in a low caries risk group, compomer is recommended with those in a moderate caries risk group, and the GIC is recommended in patients in a high caries risk group (Powers and Wataha, 2007). Because of their advantages, GICs are still regarded as popular;
SCANNING VOL. 38, 1 (2016)
44
nonetheless, they also have some noted disadvantages. In recent years, studies have been performed to eliminate the main disadvantages of GICs. Equia Fil (GC), one GIC currently on the market, is reported to be optimistic for use in the posterior region, even in permanent teeth (Gurgan et al., 2015). Some properties, such as surface roughness, surface loss, microhardness, abrasion, and fracture toughness, were reported about the material (Lohbauer et al., 2011; Zanata et al., 2011). However, no research about the fracture strength (FS) of current GICs in class II cavities of primary teeth was observed in the literature. The present study evaluated the fracture strength of a GIC, a RMGIC, a compomer, and a composite in dovetail and box-only class II cavity designs in primary teeth.
Materials and Methods Preparation of Teeth
The present study used 80 exfoliated noncarious or carious on one surface only of human primary second molars. Visual inspections were performed macroscopically under magnification to detect cracks of the enamel. The specimens were initially stored in a 0.1% thymol solution and immersed in distilled water prior to the experimental steps. Teeth were kept moistened in the water continuously before and after the restorative, aging, and test procedures.
Cavity Designs
Each tooth was embedded vertically in a nylon ring with acrylic to just below the contact area at the cemento-enamel junction. The teeth were randomly divided into two groups for either dovetail or box-only preparations. Standardized cavities (measurements as shown in Fig. 1) (Suwatviroj et al., 2003) were prepared in the teeth using a flat-ended cylinder diamond fissure bur. The depths of cavities were 1 mm in both occlusal and aproximal. Cavities were prepared by a single operator. The cavity widths
were measured with a caliper and the depths were measured with a periodontal probe.
Experimental Procedure
The teeth were divided into four groups (ten per subgroup) for each restorative material (Table I). Group I: Glass ionomer (no adhesive system). Group II: Resin-modified glass ionomer (no adhesive system). Group III: Compomer with STAE Single Component Light Cured (Southern Dental Industries, Bayswater, Victoria, Australia) (one-step self-etch adhesive system). Group IV: Composite with Clearfil SE Bond (Kuraray, Osaka, Japan) (two-step self-etch adhesive system). The materials were handled according to the manufacturer’s instructions. A LED curing device (Ultradent Products, Inc., South Jordan, UT) was used to cure resin containing restorative materials for 20 s with an intensity of approximately 1,000 mW/cm2. No further preparatory procedures, such as the use of a conditioner, were applied for GIC and RMGIC restorations. After the GIC and RMGIC restoration, a G-Coat was applied and light cured. An incremental cure technique was not used because the depth of the cavities was 1 mm. To allow for a complete acid–base reaction, all teeth were aged in distilled water at 37˚C for 15 days.
Load to Fracture
Subsequent to aging, the restorations were tested for ultimate load to fracture using a universal mechanical testing machine (UTM Autograph AG-X, Shimadzu Corp., Tokyo, Japan) with a ball ended cylindrical tip 4 mm in diameter to distribute load. The tip was placed at the marginal ridge on the occlusal surface of the restoration parallel to the vertical axis of the tooth (Fig. 2). An increasing load force was applied with a cross-head speed of 0.5 mm/s until the fraction occurred. The loads at fracture and fracture mode were recorded. The fracture mode of each specimen was evaluated and then classified into one of three groups: adhesive, cohesive, or mixed fracture.
Failure Mode
Fig 1.
Outline width of the cavity preparation.
After the fracture test, the test surfaces of the dentine and restorative materials were examined with an optical microscope (Leica Microscopy Systems, Weltzlar, Germany) under 40x. The dentin surfaces and
E. Yildiz et al.: Fracture strength of proximal restorations TABLE I
45
Components of materials used in the study
Material Glass ionomer Liquid: Polyacrylic acid (30–40%), proprietary ingredient (5–15%) Powder: Fluoroalumino silicate glass (amorphous) (90–100%), polyacrylic acid (5–10%) Glass ionomer coat Urethane methacrylate (30–40%), methylmethacrylate (40–50%), camphorquinone ( 0.05) except composite. The mean FS and standard deviations (SD) for the tested groups are shown in Table II.
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TABLE II Fracture strength of various restorative materials in differently designed class II cavities of primary molars Load at fracture (Newton) Restorative materials (n ¼ 10) GIC (box-only) GIC (dovetail) RMGIC (box-only) RMGIC (dovetail) Compomer (box-only) Compomer (dovetail) Composite (box-only) Composite (dovetail)
Median/mean Minimum/maximum SD
the cavity wall, the adaptation of the interface of the GIC and the tooth failed. Many bubbles were observed and the sample showed microcracks in the restoration (Fig. 4a). In the tooth restored with RMGIC, a crack was clearly visible. Bubbles were observed in both material and border line (Fig. 4b). The SEM image of the compomer restoration showed separation in the bonding area (Fig. 4c). In Figure 4d, the fracture of composite was observed without any cracks in the body of the remaining restoration. A buble also was observed in the restoration.
270/247a 255/289a 249/249a 258/264a 462/461b
98/362 141/470 133/339 148/346 336/593
92 119 56 75 81
576/548b,c
408/691
100
466/476b
358/608
84
Discussion
619/624c
516/792
71
In the present in vitro study, the effects of cavity design and material type on the success of the restorations and fracture modes were evaluated in the primary teeth. In the literature, most of the research on the successful restoration of class II cavities focused on composite restorative materials in permanent teeth (Laegreid et al., 2011; Akbarian et al., 2014). Although compomers are widely used, GICs are indispensable materials for pediatric dentists. However, there is not a specific scientific assessment about the most appropriate cavity design in primary teeth for the different restorative materials used in pediatric dentistry. Although the cervical part of a class II restoration is the most technically challenging part of the cavity in a clinical situation, visual inspection, moisture control, insertion, and light curing of the restorative material are all challenging at this stage of the preparation (Laegreid et al., 2011). There is a 33% carie progression rate, which was adjacent to RMGIC and the reported compomer restorations (Qvist et al., 2004a). Therefore, resin-containing materials, such as compomers and RMGICs, have disadvantages with the curing and moisture control during the bonding step; GICs still have advantages, as they are cariostatic and they chemically bond to dentin. Therefore, it is important to compare the success of the materials and to evaluate resistant cavity designs for each restoration. Prabhakar et al. (2008) reported that beveling in class I cavities increased the survival rate of RMGIC in a oneyear period. However, the present study did not employ beveling. To maintain better bond strength, dentin pretreatment is typically suggested for numerous restorative procedures. Hamama et al. (2014) studied conditioning agents for the pretreatment of dentin before RMGIC restorations and concluded that conditioning dentin had no adverse effect on the bonding of RMGIC restorations. Conversely, some studies reported adverse effects of conditioning in the bond of GICs (Tay et al., 2001; Yap et al., 2003). In the present study, dentin was not conditioned in the GIC and RMGIC groups.
Same lowercase letters indicate an insignificant difference (p > 0.05).
When restorative materials were compared, the FS of the compomer groups was significantly higher than that of the GIC and RMGIC groups (p < 0.05). However, the GIC and RMGIC groups were similar (p > 0.05). Although the dovetail designs in all groups were slightly higher than in the box-only preparations, significant differences were observed only in composite group (p < 0.05).
Failure Mode
Specimen failure modes were evaluated. Figure 2 shows the distribution of the failure mode (adhesive, cohesive, and mixed) for the material types and cavity designs. According to results, mixed type fractures were mainly observed for all tested materials. Adhesive failures were observed in RMGIC-box-only and compomer-box-only and composite-box-only.
SEM
Representative SEM images of the intact restorations and the samples after the fracture test are shown in Figures 3 and 4. Before the fracture test, border of the tooth and material was scanned. In the GIC and RMGIC groups, bubbles and cracks were observed. Bubbles were shown to induce cracks (Fig. 3a, b). In the compomer group, border was more regular (Fig. 3c). In the composite group, adaptation of the tooth and the restoration is better than the other groups (Fig. 3d). In some part of the cavity wall of the primary tooth restored with GIC, the dentin surface is still partially sealed with GIC particles. In addition, in some part of
E. Yildiz et al.: Fracture strength of proximal restorations
Fig 3.
SEM images of (a) GIC, (b) RMGIC, (c) compomer, and (d) composite before the fracture test. T, tooth; R, restoration.
Laegreid et al. (2011) used nanocomposite to restore class II cavities of permanent teeth that were horizontally 2 mm and vertically 3 or 4 mm; they reported no significant difference between the two different cavity designs and the intact teeth control group. In addition, they emphasized that the areas of available cervical enamel affect the FS of the restoration (Laegreid et al., 2011). In the present study, all cavities have cervical enamel. Suwatviroj et al. (2003) studied the cavity designs in primary teeth in vitro and reported that the RMGIC group had a significantly lower FS then the composite in
Fig 4.
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box-only subgroups. They also reported an insignificant difference between the two cavity designs in both the composite and the RMGIC groups (Suwatviroj et al., 2003). In the present study, RMGIC had a significantly lower FS than the compomer and composite for both the dovetail and the box-only subgroups. Different from the Swatviroj et al.’s study, dovetail design of composite group had significantly higher FS than composite-boxonly group. Owais et al. (2013) reported the means of maximum occlusal bite force for the dentition stages (DS) as follows: 240 N in late primary DS, 289 N in early mixed
SEM images of (a) GIC, (b) RMGIC, (c) compomer, and (d) composite after the fracture test.
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DS, 433 N in late mixed DS, and 527 N in the permanent DS. According to this, until the late primary dentition, all materials with any cavity design are appropriate. From early mixed DS on, dovetail cavity design in GIC and RMGIC may be beneficial. As the age of the children grow, more resistant material and cavity design are essential. The GICs in the different cavity designs in the primary teeth were examined during a period of 5–14 months; insignificant differences between cavity designs was observed consistent with the present study (Forsten and Karjalainen, ’90). In one of the clinical studies, Qvist et al. (2004a) reported 1.7 years survival time for non-beveled conventional class II RMGIC and 2.2 years for compomer restorations in primary teeth (Qvist et al., 2004a). In addition, RMGIC restorations in all cavity types of the primary teeth were found more successful than GIC (Qvist et al., 2004b). Another clinical study reported that RMGIC restorations were significantly more successful than silverreinforced GIC in box-only cavities in primary teeth (Espelid et al., ’99). In contrast, the compomer and GIC restorations of box-only cavities of primary teeth were found similar in a 1-year period (Marks et al., 2000). In the present in vitro study, GIC restoration in both box-only and dovetail cavities had similar FS with RMGIC. In a novel study, Al-Angari et al. (2014) reported that Equia Fil had the highest fracture toughness among the tested GICs. They guessed that the high fracture toughness of Equia Fil might be due to the resin coat applied to the surface. In the present study, a resin coat was used in the GIC and RMGIC groups. Fracture of restoration was reported as the main reason for the failure of the GIC restorations in class II cavities. This mode of fracture indicates inadequate physical properties of GIC (Qvist et al., 2004a). Nevertheless, in the present study, mixed type failure in all of the groups were observed. SEM images showed bubbles and microcracks in GIC and RMGIC used in the present study. Thus, micromechanical strength of materials were weakened. It was reported that GICs exhibit more wear than composites due to their composition (Al-Angari et al., 2014). Their acid–base reaction results in a matrix comprising an ionically cross-linked polyalkenoate network, which is weaker than the matrix of resins strengthened by fillers and methacrylate polymer chains (Davidson, 2006). The SEM image of composite without any cracks confirmed the result that composite showed more resistance to fracture. A significant increase in microhardness was reported in GIC up to the 180-day period; from that point, the values stabilized (Zanata et al., 2011). In the present study, a longer aging procedure might be affecting the results. However, different setting techniques for the GIC restorations should be studied in future research. As novel posterior restorative materials are brought into the marketplace, the question remains as to the
largest potential width of class II cavities for successful and long life restoration. As different materials are brought into use, the fracture resistance of cavities with different dimensions and different bonding strategies should be examined in further research.
Conclusions 1. The present study showed that resin-based materials, compomer and composite, are more resistant to fracture than GIC and RMGIC. 2. According to the size of the caries, the lesion design of a class II cavity could be selected as dovetail or box-only. In composite restoration, dovetail cavity design is beneficial. 3. As GIC and RMGIC have a similar FS, to be avoided from resins that are sensitive to moisture during bonding, GIC could be preferred in cavities, which are hard to isolate.
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