CÉSAR ALBERTO POMACÓNDOR HERNÁNDEZ
EFEITO DA ADIÇÃO DE ZINCO NAS
PROPRIEDADES FÍSICO-QUÍMICAS DE
ADESIVOS DENTÁRIOS
EFFECT OF ZINC ADDITION ON
PHYSICOCHEMICAL PROPERTIES OF
DENTAL ADHESIVES
PIRACICABA
2015
EFEITO DA ADIÇÃO DE ZINCO NAS PROPRIEDADES
FÍSICO-QUÍMICAS DE ADESIVOS DENTÁRIOS
EFFECT OF ZINC ADDITION ON PHYSICOCHEMICAL
PROPERTIES OF DENTAL ADHESIVES
Tese apresentada à Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas como parte dos requisitos exigidos para a obtenção do Título de Doutor em Materiais Dentários
Thesis presented to the Piracicaba Dental School of the University of Campinas in partial fulfillment of the requirements for the degree of Doctor in Dental Materials
Orientador: Prof. Dr. Simonides Consani
Este exemplar corresponde à versão final da tese defendida por César Alberto Pomacóndor Hernández e orientada pelo Prof. Dr. Simonides Consani.
PIRACICABA
2015
A Deus,
Por ter me acompanhado e guiado sempre, e porque com Ele tudo é possível.
Ao meu pai César (in memoriam),
Porque sei que ele estaria muito feliz por eu ter concluído mais uma etapa importante.
À minha mãe Mariana,
À Universidade Estadual de Campinas – UNICAMP, pela oportunidade de um curso de pós-graduação de ótima qualidade, na pessoa do Reitor José Tadeu Jorge.
À Faculdade de Odontologia de Piracicaba – FOP-UNICAMP, pela formação que tive nesta Instituição. Levarei o nome desta escola com muito orgulho. Agradeço ao diretor Guilherme Elias Pessanha Henriques e diretor associado Francisco Haiter Neto.
Aos Professores da Área de Materiais Dentários da Faculdade de Odontologia de Piracicaba – UNICAMP, pelos ensinamentos transmitidos nesses 6 anos de pós-graduação.
Ao Prof. Dr. Simonides Consani, pela orientação, apoio e disponibilidade durante o doutorado.
etapas deste estudo.
À Associação Universitária Iberoamericana de Pós-graduação – AUIP, pela concessão da bolsa para realizar parte deste estudo na Espanha.
Aos Professores Manuel Toledano, Raquel Osorio, Fátima Aguilera e Inmaculada Cabello da Universidade de Granada – Espanha, pela co-participação na realização de parte deste estudo.
Aos meus colegas da pós-graduação, pelo companheirismo ao longo desses anos.
óxido de zinco (ZnO) ou cloreto de zinco (ZnCl2) nas propriedades físico-químicas de adesivos
dentários. No Capítulo 1, foi verificado o efeito da adição de ZnCl2 a 1 ou 2% (p/p), ou ZnO a 5,
10 ou 20% (p/p) na sorção de água (WS), solubilidade (SO), módulo de elasticidade (ME), resistência máxima à tração (UTS), e microdureza (MH) de dois adesivos dentários: Adper Single Bond Plus (SB - 3M ESPE) e o componente bond do Clearfil SE Bond (SEB - Kuraray). Foram confeccionados espécimes em forma de disco para avaliação da WS, SO e MH, e em forma de halter para mensuração do ME e UTS. Os dados foram submetidos a teste estatístico ANOVA de um fator e teste post hoc Bonferroni (α=5%). Foi observado aumento da WS e SO nos adesivos contendo ZnCl2. Houve redução da WS nos adesivos com ZnO a 10 e 20%, enquanto a SO não foi alterada
nos adesivos com ZnO. Para SB, aumento do ME foi observado apenas com ZnCl2, enquanto para
SEB, o zinco não alterou o ME. Houve diminuição da UTS no SEB com ZnO, e no SB o zinco não modificou a UTS. Apenas a adição de ZnO a 20% aumentou a MH do SB, enquanto todos os compostos contendo zinco aumentaram a MH em SEB. Em conclusão, os adesivos contendo ZnO a 20% reduziram a WS, aumentaram a MH, e não apresentaram efeitos negativos na SO, ME e UTS. No Capítulo 2, foi avaliado o efeito da adição de ZnCl2 a 1, 2 ou 4% (p/p), ou ZnO a 5, 10 ou
20% (p/p) no grau de conversão (DC), resistência à flexão (FS) e módulo de elasticidade (ME) de dois adesivos dentários: Adper Single Bond 2 (SB - 3M ESPE) e Ambar (AM - FGM). As misturas foram fotoativadas diretamente no cristal do FTIR para avaliação do DC, e foram confeccionados espécimes em forma de barra para mensuração da FS e ME. O tempo de fotoativação nas metodologias foi 10 segundos. Os dados foram submetidos a teste estatístico ANOVA de um fator e teste post hoc Student-Newman-Keuls (α=5%). Foi observado para ambos os adesivos que quanto maior a concentração de ZnCl2, menores os valores de DC, FS e ME, exceto para AM contendo
ZnCl2 a 4% que apresentou FS e ME similar ou maior que o AM-controle, respectivamente. As
diferentes concentrações de ZnO não promoveram efeito ou produziram redução do DC no AM e SB, respectivamente. Quando foi incorporado ZnO a 5 e 10% em SB, foi observado maior FS e ME que SB-controle e SB com ZnO a 20%. Para AM, houve redução de FS e ME com a adição de ZnO a 10 e 20%, enquanto AM com ZnO a 5% apresentou similar FS e ME que AM-controle. Em conclusão, as propriedades físico-químicas foram prejudicadas em adesivos contendo ZnCl2. A incorporação de ZnO a 5% não afetou negativamente o DC, FS e ME.
oxide (ZnO) or zinc chloride (ZnCl2) on physicochemical properties of dental adhesives. In Chapter
1, it was assessed the effect of addition of 1 or 2 wt% ZnCl2, or 5, 10, or 20 wt% of ZnO in water
sorption (WS), solubility (SO), modulus of elasticity (ME), ultimate tensile strength (UTS), and microhardness (MH) of two dental adhesives: Adper Single Bond Plus (SB - 3M ESPE) and the bond component of Clearfil SE Bond (SEB - Kuraray). Disk-shaped specimens were prepared for evaluation of WS, SO and MH, and dumbbell-shaped specimens for measuring ME and UTS. Data were submitted to one-way ANOVA and Bonferroni post hoc test (α=5%). It was observed the increase of WS and SO in ZnCl2-doped adhesives. WS values decreased in 10 and 20% ZnO-doped
adhesives, whereas SO was not affected in ZnO-doped adhesives. For SB, ME values increased just with ZnCl2, while for SEB, zinc did not alter the ME. There was reduction of UTS in ZnO-doped
SEB blends, and zinc did not modify UTS in Zn-doped SB blends. Only the addition of 20% ZnO increased MH in SB, whereas all zinc compounds augmented MH in SEB. In conclusion, 20% ZnO-doped adhesives reduced WS, increased MH, and did not present negative effects in SO, ME, and UTS. In Chapter 2, it was evaluated the effect of addition of 1, 2, or 4 wt% ZnCl2, or 5, 10, or 20
wt% of ZnO in degree of conversion (DC), flexural strength (FS) and modulus of elasticity (ME) of two dental adhesives: Adper Single Bond 2 (SB - 3M ESPE) and Ambar (AM - FGM). Blends were fotoactivated directly over the FTIR cell for evaluation of DC, and were prepared bar-shaped specimens for measuring FS and ME. Data were subjected to one-way ANOVA and Student-Newman-Keuls post hoc test (α=5%). It was observed for both adhesives that the higher the ZnCl2
concentration, the lower the DC, FS and ME values, except for 4% ZnCl2-doped AM that presented
FS and ME similar and higher than AM-control, respectively. Different concentrations of ZnO had no effect or reduction in DC for AM and SB, respectively. When 5 and 10% ZnO were incorporated into SB, it was observed higher FS and ME than SB-control and 20%-doped SB blend. For AM, there was reduction of FS and ME with addition of 10 and 20% ZnO, whereas 5%-doped AM presented similar FS and ME than AM-control. In conclusion, physicochemical properties were jeopardized in ZnCl2-doped adhesives. Incorporation of 5% ZnO did not negatively affect DC, FS
and ME.
1 INTRODUÇÃO 11
2 ARTIGOS 14
2.1 Effect of zinc-doping in physicochemical properties of dental adhesives 14
2.2 Evaluation of degree of conversion and flexural properties of zinc-doped dental
adhesives 33
3 DISCUSSÃO 46
4 CONCLUSÃO 47
REFERÊNCIAS 48
1 INTRODUÇÃO
Desde que Michael Buonocore (1955) deu início ao estudo da adesão de materiais odontológicos aos tecidos dentários, a Odontologia Adesiva tem evoluído rápida e enormemente nessas seis décadas. Na atualidade, a eficaz adesão de materiais dentários resinosos sobre os substratos dentais possibilita grande variedade de tratamentos de alta estética nas áreas de Odontologia Restauradora e Prótese Dentária como, por exemplo, restaurações de resina composta, cimentação de pinos intrarradiculares pré-fabricados estéticos, cimentação de próteses fixas de cerâmica pura, facetas e fragmentos cerâmicos, dentre outros.
O sucesso dos tratamentos restauradores adesivos está baseado principalmente na união resistente e duradoura dos materiais resinosos ao tecido dentário. No entanto, apesar da união imediata pode ser obtida eficazmente com a maioria de adesivos dentários disponíveis comercialmente na atualidade, a estabilidade e longevidade da união do material resinoso com a dentina pode ser comprometida por meio de processos degradativos (Breschi et al., 2008). Tal problemática deve-se às características da dentina, substrato aonde a união se torna mais complexa devido ao alto conteúdo de água e presença de enzimas endógenas que participam da degradação das fibrilas colágenas desprotegidas (Breschi et al., 2008).
As atuais estratégias adesivas utilizadas na dentina consistem na desmineralização do substrato deixando as fibrilas colágenas expostas e desprotegidas de hidroxiapatita, seguida da infiltração dos monômeros resinosos dos adesivos dentários e subsequente fotoativação in situ para formar uma estrutura composta por resina e colágeno dentinário chamada de camada híbrida (Nakabayashi et al., 1982). Além disso, as estratégias podem ser classificadas em: 1) condicionamento ácido total, se a desmineralização inicial do substrato for realizada em um passo prévio com a aplicação de ácido fosfórico, e 2) autocondicionante, quando a desmineralização e penetração for conduzida simultaneamente por monômeros ácidos contidos na composição do adesivo, dispensando a utilização prévia de ácido fosfórico (Pashley et al., 2011; Van Meerbeek et al., 2011). Qualquer que seja o tipo de estratégia adesiva utilizada, a hidrofilia dos adesivos dentários e a presença de fibrilas colágenas não infiltradas corretamente por monômeros propiciam processos degradativos tanto no componente resinoso como no colágeno, resultando em união resina-dentina menos estável ao longo do tempo (Tay et al., 2002; Carvalho et al., 2005). A infiltração de monômeros hidrófilos sobre um substrato úmido propicia um processo de polimerização incompleto e o polímero formado na camada híbrida pode apresentar maior permeabilidade e nanoinfiltração o que, consequentemente, favorece a degradação da resina através de sorção de água, solubilidade, plastificação, hidrólise e perda das propriedades
mecânicas (Ito et al., 2005; Hosaka et al., 2010). Por outro lado, enzimas endógenas presentes na dentina como as metaloproteinases da matriz (MMPs) e as cisteína catepsinas hidrolisam as fibrilas colágenas desmineralizadas e desprotegidas de hidroxiapatita que não foram infiltradas pelos monômeros adesivos, favorecendo assim a destruição da camada híbrida (Mazzoni et al., 2015).
Novas estratégias adesivas vêm sendo pesquisadas para superar os problemas da degradação da união resina-dentina (Tjäderhane et al., 2013). Estudos recentes têm demonstrado que o zinco tem a capacidade de inibir a atividade colagenolítica das MMPs presentes na matriz orgânica da dentina (Osorio et al., 2011; Toledano et al., 2012a). Apesar de que o mecanismo de inibição do zinco não ser completamente compreendido, acredita-se que esteja relacionado com a coordenação entre resíduos de ZnOH+ e o zinco contido no sítio catalítico das MMPs. Este modo de inibição foi demonstrado com a carboxipeptidase A, outra metaloenzima que contém zinco (Larsen e Auld, 1989). Com o intuito de conseguir a inibição da degradação enzimática do colágeno na camada híbrida, compostos contendo zinco como cloreto de zinco (ZnCl2) e óxido de zinco
(ZnO) tem sido incorporados experimentalmente na composição de adesivos dentários comerciais, demonstrando preservar os valores de resistência da união e estabilização das propriedades nanomecânicas da união resina-dentina (Toledano et al., 2012b, 2013). Adicionalmente, o zinco tem mostrado ter efeito metabólico nos processos de mineralização e remineralização dos tecidos duros por meio da formação de cristais de fosfato de zinco e da deposição de íons Ca e P (Osorio et al., 2014). Dessa forma, a estratégia de adesivos contendo zinco se apresenta como uma alternativa atrativa para melhorar a longevidade da união resina-dentina.
No entanto, quando materiais resinosos são associados com algum tipo de elemento bioativo como o zinco, deve ser tomada atenção para que a incorporação de tal composto químico nos adesivos dentários não produza alteração nas propriedades físico-químicas do polímero formado prejudicando o desempenho. Polimerização deficiente, menores propriedades mecânicas e maior sorção de água e solubilidade são fenômenos que devem ser evitados em materiais resinosos, e este quesito deve ser considerado na determinação da quantidade mínima de zinco com capacidade de exercer benefício sem prejudicar as propriedades do material. Considerando que não foram encontrados estudos na literatura científica que avaliem a influência do zinco nas propriedades físico-químicas dos adesivos dentários, os objetivos do presente trabalho apresentado em dois Capítulos, foram:
1 – Avaliar o efeito da adição de diferentes concentrações de cloreto de zinco (ZnCl2) na sorção de água, solubilidade, grau de conversão, módulo de elasticidade, resistência
máxima à tração, resistência à flexão e microdureza de adesivos dentários.
2 - Avaliar o efeito da adição de diferentes concentrações de óxido de zinco (ZnO) na sorção de água, solubilidade, grau de conversão, módulo de elasticidade, resistência máxima à tração, resistência à flexão e microdureza de adesivos dentários.
2 ARTIGOS
2.1 Effect of zinc-doping in physicochemical properties of dental adhesives
1ABSTRACT
Zinc addition to resin adhesives may exert an inhibitory effect on MMPs-mediated collagen degradation and promote dentin remineralization. Purpose: To evaluate changes in the physicochemical properties, water sorption (WS), solubility (SO), modulus of elasticity (E), ultimate tensile strength (UTS), and microhardness (MH) tests were undertaken in zinc-doped dental adhesives. Methods: Two bonding resins (Adper Single Bond Plus -SB- and Clearfil SE Bond -SEB-) were zinc-doped by mixing them with 5, 10 or 20wt% of ZnO powder, or with 1 or 2wt% ZnCl2. Resin disks were made of each adhesive blend for the evaluation of WS, SO, and MH,
and dumbbell-shaped specimens were prepared for E and UTS testing. Results: An increase in WS and SO was observed for adhesives doped with ZnCl2. A reduction in WS was observed for the
adhesive blends containing 10% or 20wt% ZnO, while the SO was not altered in any of the ZnO-doped adhesives. An increase in E values was observed only for the SB adhesive ZnO-doped with ZnCl2.
For SEB-blends, the incorporation of zinc compounds did not alter the E values. UTS values decreased when SEB was doped with ZnO. SB-blends doped with 20wt% ZnO significantly increased their MH, and the addition of zinc to the SEB-blends augmented the MH values in all cases.
CLINICAL SIGNIFICANCE: The addition of 20wt% ZnO particles to adhesive blends is preferred as it decreased water sorption and increased microhardness of the tested adhesives. No deleterious effect was encountered on the other tested properties.
INTRODUCTION
The adhesion of contemporary resin-based restorative materials to dentin relies on the creation and stability of a microscopic interfacial structure composed of collagen fibrils reinforced by a resin matrix called the hybrid layer.1 Although the hybrid layer is efficiently obtained using most dental adhesives, its stability can be compromised by the degradation of both of its resin and collagen components.2 It is well-known that endogenous enzymes present in dentin may cause the breakdown of resin-sparse collagen fibrils in resin-dentin bonds.3,4 Matrix metalloproteinases (MMPs) are a family of structurally related calcium- and zinc-dependent endopeptidases that contribute to the organization and mineralization of the dentin matrix, while also playing an important role in pathological processes such as caries progression.5 Degradation of the collagen component of resin-dentin bonds attributed to MMPs is evidenced by the loss of integrity of hybrid layers6-8 and reduction of bond strength in vitro
and in vivo long-term studies.9-11 Thus, the activity of MMPs can importantly affect the
long-term bonding durability of esthetic restorations.
To promote adhesion to dentin, the mineral phase from the substrate has to be removed and the water-filled voids left by removal of mineral should be filled with the adhesive resin that undergoes complete in situ polymerization to form the hybrid layer.1 Two main strategies are employed in creating dentin bonding. The first strategy involves the use of etch-and-rinse adhesives, which requires the treatment of dentin with phosphoric acid to remove the smear layer and to demineralize the underlying dentin, exposing a dense filigree of organic-matrix fibrils, followed by the application of a primer/bonding adhesive to form the hybrid layer.12 The second strategy utilizes self-etch adhesives, which are based on polymerizable acidic monomers that simultaneously condition/prime and bond dentin.13 Even though the self-etch strategy seems to reduce depth of the non-resin covered collagen layer, both bonding strategies unprotected collagen may provide the sites for collagen hydrolysis by endogenous enzymes such as MMPs.14-16
Different approaches for inhibiting the enzymatic activity of MMPs have been studied in attempts to improve the stability of hybrid layers.17 Recently, it has been reported that zinc is a potent inhibitor of MMPs in dentin collagen degradation.18 The incorporation of zinc into an etch-and-rinse adhesive not only exerted a protective effect on MMPs-mediated collagen degradation, but also preserved bond strength, thus representing a promising and novel strategy for stabilizing resin-dentin bonds over time.19,20 Although the mechanism of zinc
coordination of the hydroxide moiety of ZnOH+ to the catalytic zinc ions of MMPs. This mode of inhibition was demonstrated with carboxypeptidase A, a zinc metalloenzyme.21
The addition of zinc compounds (i.e. zinc chloride [ZnCl2] or zinc oxide [ZnO] in different concentrations) into dental adhesives may affect mechanical properties of the resin-dentin interface. Mechanical properties are important for dental adhesives to compensate the stresses generated by the polymerization shrinkage of resin composites.22 Additionally, occlusal loading stresses may deteriorate an adhesive interface with inadequate mechanical properties, leading to the failure of the restoration.22 Properties related to resin degradation such as water sorption and solubility may also be altered in zinc-modified polymers, thereby influencing stabilization of the resin-dentin bond over time.2 It is known that water sorption and solubility of dental adhesives causes both plasticization and hydrolysis of the polymers within the hybrid layer.16 However, when using zinc-doped adhesives, solubility may contribute to the effective
release of zinc ions at the resin-dentin interface, thereby not only exerting an improved protective effect of collagen from MMPs, but also influencing signaling pathways and stimulating a metabolic effect in hard tissue mineralization, permitting nucleation of hydroxyapatite crystallites in collagen fibrils during remineralization.20
Analysis of the limited literature available pertaining to this innovative approach presents no information regarding the effect of zinc on the physicochemical properties of zinc-doped adhesives. Therefore, the aim of the present study was to evaluate water sorption, solubility, modulus of elasticity, ultimate tensile strength, and microhardness of zinc-doped dental adhesives. The null hypothesis tested was that the incorporation of ZnCl2 or ZnO into dental adhesives does not affect their physicochemical properties.
MATERIALS AND METHODS Preparation of adhesive solutions
An etch-and-rinse adhesive system, Adper Single Bond Plus (SB)a, and the adhesive component of the two-step self-etch adhesive Clearfil SE Bond (SEB)b were tested. Both adhesive resins were zinc-doped by mixing them with 5, 10 or 20wt% ZnO microparticlesc (2 µm diameter), or 1 or 2wt% of ZnCl2.d To achieve complete dissolution of ZnCl2 and
dispersion of ZnO particles, adhesive blends were vigorously shaken for 1 min in a tube agitator.e Preparation of the adhesives and zinc concentrations were based upon previous studies.19,20,23 The complete process was performed in a darkroom. Descriptions of the adhesives are provided in Table 1.
Table 1. Description of the adhesive resins used in the study.
Adhesive and manufacturer Basic formulation
Adper Single Bond Plus (SB)a Bis-GMA, HEMA, glycerol 1,3-dimethacrylate, UDMA, ethyl
alcohol, water, silane treated silica, copolymer of acrylic and itaconic acids
Clearfil SE Bond - Bonding component (SEB)b
Bond: MDP, Bis-GMA, HEMA, hydrophobic aliphatic dimethacrylate, dl-camphorquinone, diethanol-p-toluidine, colloidal silica, non-volatile solvents
Bis-GMA, bisphenol A diglycidyl ether methacrylate; HEMA, 2-hydroxyethyl methacrylate; UDMA, diurethane dimethacrylate; MDP, 10-methacryloyloxydecyl dihydrogen phosphate.
Water sorption (WS) and solubility (SO)
The WS and SO were determined according to ISO specification 4049, except for the specimens’ dimensions.24 Resin disks (n = 10) of each adhesive blend were prepared using
a silicon mold (6.0 mm-diameter and 1.0 mm-thick in order to fit the light output guide of the light curing unit). An acetate strip was placed on top of the adhesives, and then covered with a 1.0 mm-thick glass slide. Resin blends were light activated for 10 s with the light tip in contact with the glass slide, using a LED light curing unitf with light irradiance of 750 mW cm-2. The light was tested for light output by means of a commercial radiometer.g The opposite surface of the specimens was submitted to the same light curing procedure. Then, disks were polished using 1200-grit SiC paper to obtain specimens of approximately 6.0 mm in diameter and 0.5 mm in thickness.25 The polymerized specimens were stored at 37ºC in a desiccator containing silica gel. Disks were weighed after a 24 h interval in an analytical balance until a constant mass (m1) was obtained (mass loss of each disk was not greater than 0.1 mg in 24 h). Diameter and
thickness were measured to calculate the exact volume (V) of each specimen. Then, they were individually immersed in one milliliter of distilled water at 37ºC. After 7 days of water storage, disks were wiped with absorbent paper and weighed again (m2). Next, specimens were dried in
a desiccator, as previously described, and weighed daily until a dried constant mass (m3) was
obtained. The WS and SO were calculated individually for each specimen over the 7 days of water immersion using the formula (1). The WS and SO values were expressed in μg/mm3.
(1) WS = (m2–m3)/ V
SO =(m1–m3)/ V
Modulus of elasticity (E)
Adhesive blends were poured into dumbbell-shaped (10 mm-long x 1.0 mm-thick) silicone molds with a gauge length of 5 mm. After 10 min of solvent evaporation, an acetate strip was placed on the top of the adhesive blends and covered with a 1.0 mm-thick glass slide. Adhesive blends were then light activated for 90 s; after that, the opposite surfaces received a similar light curing protocol. The number of specimens per group was 10 according to previous studies.26,27
Following light activation, the dumbbell-shaped specimens were stored for 24 h at 37ºC in dry condition and then subjected to a three-point flexural bending test in order to obtain the E values. The point flexural bending test was performed with a miniature three-point bending aluminum device, consisting of a supporting base with a 5 mm span and a loading piston. Three-point flexure was measured by centrally loading the polymer specimen using a universal testing machineh at a displacement rate of 0.5 mm min-1, sufficient to induce a 1% strain. The compressive force necessary to induce a 1% strain in the polymer specimens was measured using a 50 N load cell. Load-displacement values were converted to stress and strain. The width and thickness of all specimens were measured to ensure accuracy of the test. The E values were calculated as the slope of the linear portion of stress-strain curve using the following formula:
(2) E = FL3/ 4Dbh3
where F is the force (N), L is the span length (5.0 mm), D is the vertical deflection (mm) of the specimen, b is the width of test specimens (1.0 ± 0.1 mm), and h is the thickness (1.0 ± 0.1 mm). The E values were obtained in MPa and converted to GPa.
The strain (ε) in the three-point flexural bending test followed the formula:
(3) ε = 6hd / L2
where h is the thickness of the beam (1.0 ± 0.1 mm), d is displacement of the beam (mm), and L is the span length of the beam between the supports (5 mm).
Ultimate tensile strength (UTS)
Dumbbell-shaped specimens (n = 20) with same dimensions and fabrication method used in three-point flexural test, were attached to the universal testing machine using a cyanoacrylate adhesivei for UTS determination.26,27 A tensile load was applied at a cross-head speed of 0.5 mm min-1 until failure. The width and thickness of the specimens were measured at the fracture site and the UTS of the polymers was calculated with the formula:
(4) UTS = F / A
where F is the tensile force at failure (N) and A is the cross-sectional area of the specimen (mm2). The UTS (N mm-2) was expressed in MPa.
Microhardness (MH)
Five disk-shaped specimens (6.0 mm in diameter; 2.0 mm in height) were prepared with each adhesive blend using a silicon mold. An acetate strip was placed on top of the adhesives, and then covered with a 1.0 mm-thick glass slide. Resin blends were light activated for 20 s and then polished using silicon carbide paper (up to 4000 grit). Microhardness was evaluated on the photoactivated surface by applying 50 g of load for 30 s using a Knoop diamond microindentor.j Ten indentations (0.5 mm of distance between them) were made in each specimen (n = 50). The dimension of each indentation was measured as the length of the longest diagonal of the indentation mark (800X). Lengths were converted to Knoop Hardness Number (KHN) using the following formula:
(5) KHN = 14.229 P / L2
Statistical analysis
Differences in the mean values of WS, SO, UTS, E, and MH were examined for each adhesive system (SB or SEB) using one-way analysis of variance (ANOVA). The Bonferroni test was used for post hoc multiple comparisons. In cases where equal variance tests failed, the data were analyzed using Tamhane's test. The statistic tests were applied at a significance level of 5%, using the software SigmaPlot 12.0.k
RESULTS
The mean values and standard deviations oftested physicochemical properties in all experimental groups are displayed in Tables 2 (SB-based blends) and 3 (SEB-based blends).
Water sorption and solubility
An increase in WS and SO was observed for both SB and SEB when doped with 1% or 2% ZnCl2. WS of the adhesives doped with 5% ZnO were not significantly different
from the control groups. A significant reduction in WS was observed for the 10% and 20% ZnO-doped adhesive blends. The SO was not modified when ZnO was incorporated in the tested adhesives.
Modulus of elasticity
E values increased whenSB was doped with ZnCl2. There were no differences
in E values between SB-control and ZnO-doped SB-blends. For the SEB-blends, the incorporation of zinc compounds did not modify E values.
Ultimate tensile strength
No significant differences were found in UTS values between SB-blends. When SEB adhesive was doped with ZnO, UTS values were decreased.
Microhardness
A significative increase in MH was only observed whenSB was doped with 20wt% ZnO. For SEB-blends, the addition of zinc increased microhardness values in all cases. ZnCl2
-doped SEB-blends attained higher microhardness values than did ZnO--doped SEB ones.
Table 2. Physicochemical properties of Zn-doped Adper Single Bond Plus blends.
Adhesive blends WS (μg/mm3) SO (μg/mm3) E (GPa ) UTS (MPa) MH (KHN) SB 256.5 (13.7)c 84.5 (11.0)a 1.82 (0.48)a 20.68 (3.6)a 22.8 (4.0)a SB+ZnCl2 (1%) 279.3 (14.3)d 116.4 (9.1)b 2.59 (0.32)b 19.23 (3.7)a 24.6 (5.0)ab SB+ZnCl2 (2%) 280.3 (7.3)d 142.3 (5.0)c 2.70 (0.35)b 21.64 (2.4)a 25.9 (6.9)ab SB+ZnO (5%) 250.6 (13.9)bc 84.2 (4.3)a 2.49 (0.57)ab 19.86 (4.2)a 26.0 (6.7)ab SB+ZnO (10%) 227.3 (16.5)ab 77.5 (7.2)a 2.42 (0.43)ab 20.24 (4.4)a 26.3 (5.2)ab SB+ZnO (20%) 213.1 (11.4)a 73.9 (7.2)a 2.26 (0.29)ab 21.34 (4.5)a 28.8 (5.8)b P values < 0.001 < 0.001 0.004 0.411 < 0.001
Values are means and standard deviations. Different superscript lower case letters indicate significant differences within columns (p < 0.05).
WS = water sorption; SO = solubility; E = modulus of elasticity; UTS = ultimate tensile strength; MH = microhardness.
DISCUSSION
Two commercial dental adhesives were doped with two different zinc compounds and the physicochemical properties of the resulting adhesive blends were evaluated. The null hypothesis tested in the present study was rejected because the addition of zinc chloride (ZnCl2)
or zinc oxide (ZnO) to dental adhesives altered some of their physicochemical properties.
Incorporation of zinc compounds yielded similar variations in WS and SO for both adhesives evaluated (Tables 2 and 3). WS is a phenomenon related to adhesives formulation (i.e. the polarity of monomers, chain topology, and amount of solvent) that consists basically of water uptake into the polymer network.28,29 As a result, the polymer is softened by swelling
of the network and reduction of the frictional forces between the polymer chains, a process known as plasticization.30 In this way, the higher the WS of an adhesive system, the larger is
the reduction of mechanical properties when polymers are exposed to water.26,31-32
Table 3. Physicochemical properties of Zn-doped Clearfil SE Bond blends.
Adhesive blends WS (μg/mm3) SO (μg/mm3) E (GPa ) UTS (MPa) MH (KHN) SEB 83.4 (6.5)b 8.8 (5.2)ab 3.00 (0.21)ab 50.45 (7.2)b 27.2 (3.4)a SEB+ZnCl2 1%) 102.8 (3.3)c 22.1 (4.6)c 2.67 (0.33)a 51.65 (4.8)b 37.5 (2.9)d SEB+ZnCl2 (2%) 118.0 (5.8)d 24.1 (6.5)c 2.76 (0.30)ab 52.89 (5.4)b 38.5 (4.5)d SEB+ZnO (5%) 87.9 (2.4)b 16.5 (3.6)bc 2.95 (0.30)ab 47.01 (5.6)a 33.8 (3.6)bc SEB+ZnO (10%) 74.8 (4.0)a 6.1 (2.7)a 2.89 (0.34)ab 45.74 (7.54)a 31.4 (3.5)b SEB+ZnO (20%) 76.2 (2.7)a 4.6 (3.8)a 3.15 (0.30)b 44.80 (6.9)a 34.4 (5.3)c P values < 0.001 < 0.001 0.014 <0.001 < 0.001
Values are means and standard deviations. Different superscript lower case letters indicate significant differences within columns (p < 0.05).
WS = water sorption; SO = solubility; E = modulus of elasticity; UTS = ultimate tensile strength; MH = microhardness.
The SO consists of the elution of some inadequately polymerized monomers and other resin components as photoinitiators, photostabilizers, decomposition products, and resin-sparse inorganic filler particles.33,34 WS, SO, and hydrolysis constitute a simultaneous and dynamic process, where hydrolysis and SO depend primarily on water uptake into the polymer network; SO is also significantly influenced by the existence of low-weight molecules (e.g. previously hydrolyzed or unreacted oligomers/monomers, and resin-sparse fillers) that can be leached out of the polymer network. Therefore, WS generally shows a direct association with SO.25 In the present study, this association is clearly evidenced, since ZnCl2-doped adhesives
produced increased WS and SO values, whereas ZnO-doped adhesives showed a tendency toward reduction of WS and SO with higher concentrations of ZnO. It is known that the presence of filler might provide the adhesives with improved mechanical properties and decreased water sorption.34-36 It can be speculated that ZnO particles were not completely solubilized into the adhesive blends, thereby performing as fillers. As more zinc “fillers” were added, the E and MH values increased (Tables 1 and 2) because there was less resin in the mixture. Similarly, as the resin content fell with increases in zinc oxide, there was less resin to absorb water so WS decreased.
The gradual hydrolysis of the polymerized adhesive and the low-weight hydrolyzed molecules leaching out of the hybrid layer due to WS and SO permit some matrix-bound endogenous enzymes (e.g. MMPs) to enter and cleave unprotected collagen fibrils within the decalcified dentin.19 Furthermore, the polymer degradation may also serve as a pathway for dentinal fluid from the pulp chamber charged with additional MMPs, which may increase the collagen proteolysis over time.16 Although it is well-known that WS and SO processes may reduce the mechanical properties of the resin-bond interface,26,31,32 it is important to note that in the zinc-doped adhesives, SO of the polymer are necessary for release of zinc ions into the hybrid layer and the underlying dentin.19,20 In this way, zinc may not only act as an MMP inhibitor, but may also influence signaling pathways and stimulate a metabolic effect in hard tissue mineralization.37,38 The precipitation of a calcium phosphate layer has been described for other zinc-doped biomaterials, and further formed hydroxyapatite has been shown to be less soluble, being in terms of solubility and quality strictly dependent on.39 Moreover, zinc has also been shown to enhance the occlusion of dentinal tubules by crystal precipitation, and these crystals do not easily dissolve after acid exposure.40 Many zinc containing/releasing materials have been widely employed in Restorative Dentistry with high degrees of clinical success (e.g.
silver amalgam, and a variety of zinc oxide-containing cements used as temporary fillings, cavity liners, and root canal filling materials).41,42
Mechanical properties such as UTS and E are important for dental adhesives to compensate the stresses generated by the polymerization shrinkage of resin composites.22 Also, occlusal loading stresses deteriorate the adhesive interface over time, leading to the failure of the restoration.22 For this reason, it was important to know if the addition of zinc compounds into dental adhesives could affect their mechanical properties. While addition of up to 20wt% ZnO reduced the amount of resin in the adhesive by up to 20%, adding of 2wt% ZnCl2 would
not be expected to produce any changes due to its “filler” effect.
It was observed that the addition of zinc compounds into SB did not modify UTS values. In the case of SEB, a slight reduction of UTS mean values was observed (approx. 10%) when the adhesive was doped with ZnO (Table 3), which may be related to zinc chelation by one of the main components in SEB (10-MDP), by inducing differences in coherence within the polymer matrix. However, when ZnO-doped SEB was bonded to dentin, bond strength was not affected, being similar to that of the zinc-free SEB.19
The expected decrease in the mechanical properties originated by WS and SO processes may be reduced when these Zn-doped adhesives are in contact with dentin, since the release of zinc ions within the hybrid layer inhibits MMPs-mediated collagen degradation and may induce crystals precipitation and hydroxyapatite formation,23 thereby rendering the resin-dentin interface mechanically and chemically more resistant over time.20
E values of SB-blends significantly increased only with the addition of ZnCl2 (Table
2). Atendency towards increased E values with the incorporation of ZnO was also observed, reaching a magnitude significantly similar to theZnCl2-doped SB-blends. The variation in E of
Zn-doped SB-blends when compared to SB-control ranged from 24% to 48% (Table 2). On the other hand, E values of Zn-doped SEB-blends were not significantly different in comparison with zinc-free SEB (with a variation from -11% to 5%) (Table 3). Thus, in the present study it was demonstrated that the addition of zinc compounds to dental adhesives did not negatively affect tested mechanical properties.
Microhardness is usually considered as an indirect measurement of the degree of conversion of polymeric matrices that have the same composition.43 As zinc-doped resin blends used in the present study have different compositions (different percentage of ZnCl2 or ZnO),
the MH test was interpreted only as a mechanical property to confirm the E and UTS results. It was observed that 20% ZnO-doped SB-blends and all ZnO-doped SEB-blends had MH values that were significantly higher than control resins. ZnCl2-doped SEB-blends also increased MH
values to levels that were higher than ZnO-doped SEB-blends. As the effect of zinc incorporation in E and microhardness was adhesive-dependent, it may be speculated that noticeable differences in adhesives formulations are the major reason for this finding. SB contains high rates of solvent (ethanol/water) and copolymer of acrylic acids that provide a hydrophilic nature to this adhesive. On the other hand, the adhesive component of SEB contains non-volatile solvents and has self-etching capacity due to the presence of a functional acidic 10-MDP monomer. Further studies are necessary to clearly understand how zinc might interact with 10-MDP.
In conclusion, addition of 20wt% ZnO to resin adhesives does not decrease their physicochemical properties but instead, reduces WS and increases MH. Evaluation of long-term water aging on the mechanical properties of zinc-doped adhesives is recommended to predict the durability of restorations treated with this novel approach. Other benefits of Zn-doping etch-and-rinse dentin adhesives might also be studied, e.g. possible antimicrobial properties.44-46 Endodontic sealers, luting cements and caries preventive materials may also be benefited by this Zn-doping strategy.
a. 3M ESPE, St. Paul, MN, USA b. Kuraray, Tokyo, Japan
c. Panreac Química, Barcelona, Spain d. Sigma Aldrich, St. Louis, MO, USA
e. Vortex Wizard 51075; Velp Scientifica, Milan, Italy f. Bluephase; Ivoclar Vivadent, Schaan, Liechtenstein
g. Model 100; Demetron Research Corporation, Danbury, CT, USA h. Instron 4411; Instron, Canton, MA, USA
i. Zapit; Dental Venture of America, Corona, CA, USA
j. V-testor 402; Instron Wolper GmbH, Ludwigshafen, Germany k. Systat Software Inc., San Jose, CA, USA
CONFLICT OF INTEREST STATEMENT
The authors have no financial affiliation or involvement with any commercial organization with direct financial interest in the materials discussed in this manuscript. Any other potential conflict of interest is disclosed.
ACKNOWLEDGMENTS
This investigation was supported by grants CICOM/FEDER MAT2011-24551, JA-P08-CTS-3944, CEI Biotic from UGR and AUIP-JA 2011.
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2.2
Evaluation of degree of conversion and flexural properties of zinc-doped
dental adhesives
ABSTRACT
Objective: To evaluate the influence of addition of different concentrations of zinc chloride
(ZnCl2) and zinc oxide (ZnO) on the degree of conversion (DC), flexural strength (FS) and
modulus of elasticity (ME) of dental adhesives.
Methods: Two etch-and-rinse dental adhesives [Adper Single Bond 2 (SB) and Ambar (AM)]
were zinc-doped by mixing them with 5, 10, or 20 wt% of ZnO powder, or with 1, 2, or 4 wt% ZnCl2. The DC of the resulting blends was measured using FT-IR spectroscopy (n = 5), whereas
FS and ME were determined by three-point bending test of bar-shaped specimens (n = 10). Data were subjected to one-way ANOVA and Student-Newman-Keuls post hoc test (α = 0.05).
Results: For both SB-blends and AM-blends, the higher the concentration of ZnCl2 the lower
the DC and flexural properties, except for 4% ZnCl2-doped AM-blend that presented FS similar
than AM-control, and ME statistically higher than AM-control. Different concentrations of ZnO had no effect or very slight reduction of DC in AM-blends and SB-blends, respectively. When 5% and 10% ZnO were incorporated into SB, it was observed higher FS and ME values than SB-control and 20% ZnO-doped SB-blend. For AM-blends, there was reduction of flexural properties with the addition of 10 and 20% ZnO, whereas 5% ZnO-doped AM-blend presented similar FS and ME than AM-control.
Conclusions: Some physicochemical properties were jeopardized when doping simplified
dental adhesives with ZnCl2. Incorporation of 5% ZnO into SB and AM did not negatively
affect DC, FS, and ME.
Clinical significance: 5% ZnO-doped etch-and-rinse dental adhesives might be used as a
promising material for stabilization of resin-dentin bonds over the time according to the physicochemical properties presented.
INTRODUCTION
Novel strategies for creating stronger and more durable resin-dentin bonding interfaces have been widely studied in the last decades (1). The rationale for the improvement in dentin adhesion is because the stability of the hybrid layer created by contemporary commercially available adhesives is compromised by degradation processes of dentin organic matrix and resin polymer (1,2). It is known that independently of the adhesive strategy (i.e. etch-and-rinse or self-etch), unprotected resin-sparse collagen fibrils are left into the hybrid layer, thus acting as susceptible substrate for hydrolytic degradation activated by host-derived enzymes such as matrix metalloproteinases (MMPs) and cysteine cathepsins (3). Furthermore, the infiltration of hydrophilic resin monomers into water-rich dentin results in nanoleakage that is a precedent phenomenon for degradation processes of the resin polymer (4). In that manner, novel adhesive approaches as the inhibition of host-derived enzymes and the progressive water replacement by dentin remineralization may be a suitable strategy for extending the longevity of resin-dentin bonds (5).
Some studies have demonstrated that zinc (Zn) is a potent inhibitor of collagen degradation mediated by dentin MMPs (6,7). When zinc compounds were experimentally incorporated into dental adhesives, microtensile bond strength values and nanomechanical properties of resin-dentin interfaces were stabilized, thus confirming that a protective effect on MMPs-mediated collagen degradation may be attained (8,9). Although the mechanism of zinc inhibition of matrix-bound MMPs is not completely understood, it is possible that it involves coordination of the hydroxide moiety of ZnOH+ to the catalytic zinc ions of MMPs, as was demonstrated with carboxypeptidase A zinc metalloenzyme (10). In addition, zinc also may influence signaling pathways and promote a metabolic effect in hard tissue mineralization and remineralization processes. Toledano et al. (11) reported that a zinc oxide (ZnO)-doped adhesive induces Ca and P deposition and it could facilitate the incorporation of these ions into interfacial bonding zone. Thus, zinc-doped adhesives would represent a promising and novel strategy for stabilizing resin-dentin bonds over time.
An important factor to be considered concerning stability of resin-dentin bonds is the adequate polymerization of adhesive resins. The incomplete polymerization of dental adhesives increases the permeability and nanoleakage of the resin-dentin interface promoting water sorption, plasticization and elution of hydrolyzed molecules (12). These degrading processes may reduce mechanical properties of the polymer over time and also may permit endogenous enzymes (i.e. MMPs and cathepsins) to enter and cleave unprotected collagen
fibrils within the decalcified dentin (13). Furthermore, acceptable mechanical properties of dental adhesives are essential for compensating polymerization shrinkage of resin composite and for supporting masticatory stresses (14). Thus, characterization of flexural properties (strength and modulus) of resinous materials is important clinically because adhesive interface can be subjected to considerable flexural stresses in both anterior and posterior teeth (15).
Many factors may alter the degree of conversion and mechanical properties of a resin dental material as irradiance of light curing unit, curing time, presence and type of solvent, chemical nature of monomers, amount of filler, and type of photoinitiator (12). In that way, the addition of zinc compounds, such as zinc chloride [ZnCl2] or zinc oxide [ZnO] into dental
adhesives may have influence on degree of conversion and mechanical properties. To the best of our knowledge, there are no previous studies evaluating the influence of zinc addition on these properties.
The purpose of this study was to evaluate the influence of addition of 5, 10, or 20% ZnO microparticles, or 1, 2, or 4wt% of ZnCl2 on the degree of conversion, flexural strength
and modulus of elasticity of two etch-and-rinse dental adhesives. The null hypothesis tested would be that the incorporation of ZnO or ZnCl2 into dental adhesives does not affect these
MATERIAL AND METHODS
Preparation of adhesive blends
Two etch-and-rinse dental adhesives (Adper Single Bond 2 - SB; Ambar - AM) were used to prepare the adhesive blends (Table 1). Both adhesive resins were zinc-doped by mixing them with 5, 10, or 20 wt% ZnO microparticles (2 µm diameter) (Dinâmica Química, Diadema, SP, Brazil), or 1, 2, or 4wt% of ZnCl2 (Dinâmica Química, Diadema, SP, Brazil). To
achieve complete dissolution of ZnCl2 and dispersion of ZnO particles, adhesive blends were
vigorously shaken for 1 min in a tube agitator (AP 56; Phoenix, Araraquara, SP, Brazil). Preparation of the adhesive blends and zinc concentrations were based upon previous studies (8,9). The complete process was performed in a darkroom.
Table 1. Description of dental adhesives used in the study
Product information Basic formulation*
Adper Single Bond 2 (SB) Bis-GMA, HEMA, UDMA, glycerol 1,3-dimethacrylate, (3M ESPE, St. Paul, MN, USA) copolymer of acrylic and itaconic acids, diphenyliodonium Batch number: N575521 hexafluorophosphate, EDMAB, silane treated silica, ethyl alcohol,
N576767 water.
Ambar (AM) UDMA, HEMA, acid methacrylated monomers, hydrophilic
(FGM, Joinville, SC, Brazil) methacrylated monomers, silane treated silicon dioxide,
Batch number: 080514 camphorquinone, EDMAB, ethanol.
*Composition provided by manufacturers.
Bis-GMA, bisphenol A diglycidyl ether dimethacrylate; HEMA, 2-hydroxyethyl methacrylate; UDMA, diurethane dimethacrylate; EDMAB, ethyl 4-dimethyl aminobenzoate.
Degree of conversion (DC)
The degree of conversion was measured using a Fourier Transform Infrared (FT-IR) spectrometer (Spectrum 100 Optica; Perkin Elmer, MA, USA), equipped with an attenuated total reflectance (ATR). A constant volume of the adhesive blend (3 µL) was placed on the horizontal face of ATR cell. Previously light activation for 10 s using a LED light curing unit (Valo; Ultradent, South Jordan, UT, USA) with irradiance at 1100 mW/cm2, the solvent of each adhesive blend was volatilized for 10 s with air stream. Absorption spectra (n =5) were obtained by the baseline technique with 16 scans at 4 cm-1 of resolution in region between 1800 and 1400 cm-1. DC was calculated by the ratio (R) between the peak heights of aliphatic carbon double bonds (1637
cm-1) and aromatic group band absorptions (1608 cm-1, used as internal standard) for cured and uncured adhesives, according to formula:
DC (%) = [1 – (R polymer / R monomer)] x 100
Flexural strength (FS) and modulus of elasticity (ME)
Flexural strength and modulus of elasticity of the adhesive blends were measured using a standard mini-flexural three-point bending test method. Bar-shaped specimens (n = 10) were made for each group using polydimethylsiloxane molds (7.0 × 1.5 × 1.0 mm). The adhesive blends were placed into the mold, air-dried for 10 s, and covered with an acetate strip and a 1.0 mm-thick glass slide. Adhesive blends were light activated for 10 s with the light tip in contact with the glass slide. After light activation, the specimens were removed from the mold and stored in 100% relative humidity at 37°C. After 24 h, specimens of each group were measured using a digital caliper, and submitted to a three-point bending test on a universal testing machine (Instron 4411; Instron Corporation, Canton, MA, USA). The load was applied centrally on the bar at a crosshead speed of 0.5 mm min−1 until failure.
Statistical analysis
Data were analyzed by one-way analysis of variance (ANOVA) and Student-Newman-Keuls post hoc test (α = 0.05). The analysis was performed independently for SB-blends and AM-SB-blends using the software SigmaPlot 12.0 (Systat Software Inc., San Jose, CA, USA).
RESULTS
The mean values and standard deviations of degree of conversion and flexural properties in all experimental groups are displayed in Tables 2, 3 and 4.
Degree of conversion
For both SB-blends and AM-blends, the higher the concentration of ZnCl2 the lower the
degree of conversion. ZnO-doped AM-blends had statistically similar DC than AM-control (p > 0.05). ZnO-doped SB blends were not statistically different between them, but were lower than SB-control (p < 0.05).
Table 2. Means of degree of conversion for zinc-doped dental adhesives Degree of conversion (%)
Single Bond 2 Ambar
Control 76.6 (2.5) a 64.7 (1.0) a 1% ZnCl2 69.0 (3.1) b 59.0 (2.4) b 2% Zn Cl2 61.5 (4.0) c 54.1 (2.1) c 4% ZnCl2 54.9 (4.7) d 37.1 (3.2) d 5% ZnO 66.7 (4.4) b 63.2 (1.3) a 10% ZnO 68.8 (3.9) b 64.9 (1.9) a 20% ZnO 70.8 (3.0) b 62.8 (1.0) a
Different letters in each column indicate significant differences by Student-Newman-Keuls’s test (p < 0.05).
Flexural strength
For blends, 5 and 10% ZnO-doped blends had the highest FS values, while SB-control, 1% ZnCl2- and 20% ZnO-doped SB-blends were not statistically different. The 4%
ZnCl2 SB-blend had the lowest FS values.
For AM-blends, 4% ZnCl2-, 5% ZnO-doped and AM-control had the highest FS
values, while 1% ZnCl2- and 10% ZnO-doped AM-blends were not statistically different. The
2% ZnCl2- and 20% ZnO-doped AM-blends had the lowest FS values.
Table 3. Means of flexural strength for zinc-doped dental adhesives. Flexural strength (MPa)
Single Bond 2 Ambar
Control 14.9 (3.4) cd 21.5 (3.6) a 1% ZnCl2 16.5 (3.6) c 17.0 (5.7) b 2% Zn Cl2 11.8 (2.4) de 8.4 (2.1) c 4% ZnCl2 9.3 (3.2) e 23.3 (3.2) a 5% ZnO 34.6 (2.5) a 23.6 (4.1) a 10% ZnO 27.1 (4.7) b 15.8 (3.7) b 20% ZnO 12.5 (3.4) cde 11.0 (3.5) c
Different letters in each column indicate significant differences by Student-Newman-Keuls’s test (p < 0.05).
Modulus of elasticity
For SB-blends, the highest ME values were attained by 5 and 10% ZnO-doped blends. SB-control, 1% ZnCl2- and 20% ZnO-doped blends had statistically similar ME values, while
2 and 4% ZnCl2-doped blends had the lowest values.
For AM-blends, the higher the concentration of ZnCl2 and ZnO, the lower were the
ME values, except for 4% ZnCl2-doped AM-blends that showed the highest values.
Table 4. Means of modulus of elasticity for zinc-doped dental adhesives Modulus of elasticity (MPa)
Single Bond 2 Ambar
Control 290.0 (65.5) b 155.0 (41.4) bc 1% ZnCl2 285.0 (53.5) b 134.3 (48.9) c 2% Zn Cl2 171.4 (52.4) c 72.0 (16.4) d 4% ZnCl2 109.0 (73.7) c 222.0 (21.0) a 5% ZnO 782.0 (183.4) a 181.4 (54.6) b 10% ZnO 786.0 (126.0) a 111.7 (26.4) cd 20% ZnO 201.7 (76.5) bc 63.3 (16.3) d
Different letters in each column indicate significant differences by Student-Newman-Keuls’s test (p < 0.05).
DISCUSSION
Zinc-doped dental adhesives represent a novel strategy for stabilizing resin-dentin bonds over time, as zinc inhibits collagen degradation mediated by dentin MMPs, and also may influence signaling pathways promoting a metabolic effect in hard tissue mineralization and remineralization processes (6,7,11). In this study two zinc compounds (ZnCl2 and ZnO) were
incorporated at different concentrations into two etch-and-rinse dental adhesives to evaluate the effect on DC, FS and ME of resulting adhesives blends. The null hypothesis tested was rejected since physicochemical properties were affected with incorporation of zinc compounds into dental adhesives.
The results demonstrated that ZnCl2 reduced the DC values in both adhesives, and
this reduction was associated with the ZnCl2 concentration. Therefore, the higher the ZnCl2
concentration into adhesives, the lower the DC values (Table 2). Incomplete polymerization of adhesive monomers, especially of simplified adhesives, is associated with nanoleakage and permeability of both hybrid layer and layer of adhesive (12). Water permeation through the resin-dentin interface facilitates degradation processes as water sorption, plasticization, hydrolysis, and elution of some inadequately polymerized monomers and other adhesive components (e.g. previously hydrolyzed or unreacted oligomers/monomers, and resin-sparse fillers), therefore reducing mechanical properties over time. Furthermore, the gradual hydrolysis of the polymerized adhesive and the low-weight hydrolyzed molecules leaching out of the hybrid layer permit some matrix-bound endogenous enzymes to cleave unprotected collagen fibrils within the decalcified dentin (13). The polymer degradation may also serve as a pathway for dentinal fluid from the pulp chamber charged with additional MMPs, which may increase the collagen proteolysis over time (3).
Zinc chloride is a colorless salt considered a deliquescent solid, that is, it is so hygroscopic that absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms a solution (16). Deliquescence occurs when the vapor pressure of the saturated aqueous solution of a substance is less than the vapor pressure of water in the ambient air. This physical property of ZnCl2 is also associated with its high solubility (4320 g/L in water) (16),
as could be corroborated in the present study with its adequate and effortless dissolution in adhesives blends. In this manner, it can be speculated that when adhesive blends were doped with ZnCl2, water molecules might also being incorporated into the mixture producing a slight
dilution and subsequently DC reduction. The water affinity of ZnCl2 and lower DC values may
The incorporation of ZnO particles into dental adhesives did not affect the polymerization process in AM-blends, and slightly reduced the DC values in SB-blends. It was also observed that 5, 10 and 20% ZnO-doped adhesive blends presented no differences in DC between them. These results may be understood because zinc oxide and zinc chloride are both zinc compounds with considerably different physical properties. ZnO is a white powder insoluble in water and it is not efficiently dissolved in dental adhesives. Those insolubilized ZnO particles had no effect on monomers into polymer conversion of adhesive, and after the polymerization process were included into the polymer network thus probably performing as filler particles.
Flexural strength has been shown to be a more accurate test and more sensitive to subtle changes in a material substructure than other tests. Characterization of flexural properties (i.e. strength and modulus of elasticity) of dental adhesives is important as, clinically, resin-dentin bonds are subjected to considerable flexural stresses. Acceptable mechanical properties of dental adhesives are essential for compensating the polymerization shrinkage of resin composite, and for supporting masticatory stresses over time (14). For both SB-blends and AM-blends, it was observed a tendency for reduction of FS and ME with higher concentrations of ZnCl2 (Tables 3 and 4), the same phenomenon observed for DC. Therefore, it may be assumed
that incomplete polymerization is related with lower mechanical properties in ZnCl2-doped
adhesive blends. Interestingly, 4% ZnCl2-doped AM-blend was the exception, as presented FS
similar than AM-control, and ME statistically higher than AM-control. Such result needs to be explain with additional tests, since ZnCl2 might react in a different way with some acid
methacrylated monomers contained in AM (18).
The effect of ZnO on mechanical properties of dental adhesives was dependent of the type of adhesive and ZnO concentration (Table 3 and 4). Although SB and AM are both two-step etch-and-rinse adhesives, differences in composition are responsible for different effects produced by ZnO. As lower concentrations of ZnO are recommended to ensure no reduction of mechanical properties of ZnO-doped adhesives, it seems to be necessary the determination of a minimal concentration of ZnO to exert MMPs inhibition and remineralization in dentin. It is important to remark that curing time used in this study was 10 s as described in manufacturers’ instructions.
In conclusion, some physicochemical properties may be jeopardized when doping simplified dental adhesives with ZnCl2. Incorporation of 5% ZnO into SB and AM does not
reduce DC, FS and ME and could be a good option for stabilization of resin-dentin bonding interfaces over time. The effect of Zn-doping in flexural properties may be influenced by
adhesive composition. Other properties of Zn-doped adhesives could also be studied, e.g. antimicrobial and rheological properties.
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