Bolla M.(1), Leforestier E. (1), Muller M. (1), Darque-Ceretti E. (2)
1 :
Laboratoire de Biomatériaux Dentaires. UFR Odontologie. Université de Nice Sophia-Antipolis.
France.
2 : Ecole Nationale Supérieure des Mines de Paris. Centre de mise en forme
des matériaux. Sophia-Antipolis. France.
Abstract
There are now many aesthetic materials that dentists wish to bond to enamel and dentine and to each other. Consequently, numerous adhesives have been developed to cope with the diversity of the applications. Such adhesives include composite resins, glass-ionomer cements and dentine bonding agents. In order to appreciate fully and understand the clinical application of adhesive techniques, it is important for the clinician to have a thorough knowledge of the principles of adhesion, the materials used, the dental adhesive systems and how these are applied in the clinical situation.
Key words: adhesion; aesthetic materials; dental enamel; dentine; dental adhesives
Resumo
Existem atualmente vários materiais estéticos que os dentistas desejam colar ao esmalte e dentina e a eles próprios. Consequentemente, muitos adesivos vêm sendo desenvolvidos para enfrentar a diversidade de aplicações. Tais adesivos incluem resinas compostas, cimentos de ionômeros vítreos e agentes de adesão para dentina. Para que a aplicação clínica das técnicas de adesão seja totalmente valorizada e compreendida, é importante que o clínico tenha um conhecimento muito bom dos princípios de adesão, dos materiais usados, dos sistemas de adesão dentários e como eles podem ser usados na situação clínica.
Palavras-chave: adesão; materiais estéticos; esmalte dentário; dentina; adesivos dentários
I - Introduction
The anatomic and functional reconstruction of the tooth is the main goal of preserving and restoring odontology. Since many years, patients' demands for more aesthetic restorations have caused an increase in the use of tooth colored restorative materials and adhesive restorations have become an acceptable part of routine dental restorative treatment (fig 1).
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Figure 1: aesthetic tooth colored restorations
To achieve clinical success with such
restorations, it is of significant importance to ensure good bonding between
the restorations and tooth substance. Adhesive dentistry is continuing to develop
rapidly with the introduction of new adhesive systems. Such bonding agents must
be covered with aesthetic tooth colored restorations.
The resin-based composite restorative materials ('composites' in brief) (fig
2) that are used in dentistry have three major components :
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Figure 2: Use of dental composite resin
The resin forms the matrix of the composite material, binding the individual filler particles together through the coupling agent. The most commonly used monomer for both anterior and posterior resins is Bis-GMA, which is derived from the reaction of bisphenol-A and glycidylmethacrylate. It has a higher molecular weight than methyl methacrylate, which helps to reduce the polymerisation shrinkage. There are also a number of composites that use an urethane-dimethacryate resin rather than Bis-GMA.
It is possible to categorise dental composites into five main groups, according to the nature and the particle size of the filler, but two of them are most clinically used: microfilled and hybrid composite. The first microfilled resins were introduced in the late 1970's, and contain colloidal silica with an average particle size of 0.02 mm.
The small size of the filler particles means that the composite can be polished to a very smooth surface finish, and provides a very large surface area of filler in contact with the resin. The inconvenient is that this type of composite does not present high mechanical properties and can't be used on posterior teeth. Hybrid (or blended) composites (fig 3) contain large filler particles, of an average size of 15-20 mm and also a small amount of colloidal silica, which has a particle size of 0.01-0.05 mm.
It should be noted that virtually all composites now contain small amounts of colloidal silica, but their behaviour is very much determined by the size of the larger filler particles. By ensuring that the aesthetics are not compromised, composites can be used for both anterior and posterior applications. There is a trend towards the use of universal composites (fig 4), rather than specific composites for anterior and posterior use.
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Figure 3: Hybrid
composite resin
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Figure 4 : Micro-hybrid
("universal") composite resin
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The composite restoration should, at all times, be considered as an adhesive restoration. The advantages are manifold, but principally the reliance on adhesion rather than retention helps to conserve tooth structure, to improve the strength of the tooth crown and to provide a barrier to marginal leakage. It is therefore important that these materials are used only in situations where a good quality adhesive bond can be achieved. A dental adhesive should :
Talking about adhesion to tooth substance need to distinguish the differences between enamel (fig 5) and dentin (fig 6). Enamel is composed mostly of hydroxyapatite crystals, containing a small amount of protein and water. To bond to enamel, it is very important to focus on the mineral component (hydroxyapatite) of enamel. Enamel is the most densely calcified tissue of the human body, and is unique in the sense that it is formed extracellularly. It is a heterogeneous structure, with mature human enamel consisting of 96 % mineral, 1% organic material and 3 % water by weight (89 %, 2 % and 9 % by volume respectively). The mineral phase is made up of millions of tiny crystals of hydroxyapatite [Ca10(PO4)6(OH)2], which are packed tightly together in the form of prisms, held together by an organic matrix.
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Figure 5 : Enamel
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Figure 6 : Dentin
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Bonding adhesive resin to phosphoric acid etched enamel is regarded as a reliable procedure and was introduced in 1955. Relatively hydrophobic adhesive resins can flow into the relatively large conditioned enamel prism (5-7 mm diameter) and sometimes into the submicron sized porosities within the etched enamel prisms. A long-lasting enamel bond can be achieved by means of micro-mechanical interlocking between the resin and enamel. The acid-etch modification of the enamel surface allows the formation of an intimate micromechanical bond between enamel and the resin component of the composite.
However, bonding to dentin is far more challenging because dentin is a composite
of apatite crystal "fillers" embedded in a collagen matrix. Dentin
contains approximately twenty percent protein (mainly collagen) and dentinal
tubules which communicate with the pulp, and contain interstitial fluid. Bonding
to dentin is basically both bonding to the mineral phase and organic phases
of dentin. To date, the most important factor for dentin adhesion under clinical
conditions is the permeation of resin into the intertubular dentin, a process
that could be termed intradental permeation. Usually the dentinal surface is
covered with a smear layer that adheres weakly to the underlying intact dentin.
The smear layer consists essentially of a gelatinous surface layer of coagulated
protein, some 0.5-5 mm thick. It is generally highly contaminated with bacteria
from the caries process and contains cutting debris. The problems with dentin
bonding can be summarised as follows:
The mechanism of dentin adhesion, enhanced by hybrid layer formation between the resin and dentin, was proposed in 1982. The formation of the hybrid layer (fig 7) is believed essential to create a strong bond between the resin and dentin. The penetration of adhesive monomers into the superficially demineralised dentin and subsequent polymerisation into large molecular weight resin polymers are indispensable for creating an ideal hybrid layer.
Hybridization of dentin is a process that creates a molecular-level mixture
of adhesive polymers and dental hard tissues. Hybridized dentin is prepared
in the subsurface of acid-etched tissues by the polymerisation of resin monomers
that have impregnated the tissues. It has a gradient structure because it is
prepared by diffusion of monomers placed on the conditioned surface and their
polymerisation in situ. Intact mineralized dentin does not permit much monomer
diffusion in clinically relevant time periods. Therefore, dentin must be suitably
conditioned to create channels between collagen fibrils to allow monomers, which
have good affinity for demineralised dentin, to diffuse into the substrate.
Dentin conditioning involves the removal ore modification of the smear layer
to allow monomer diffusion through it and into the underlying intact dentin.
Figure 7 : Hybrid layer
When dentin is acid etched, the apatite phase of the smear layer and the underlying dentin is solubilized to permit exposure of the underlying collagen fibrils. These exposed collagen fibrils may leave spaces for bonding resin to penetrate (figure 8).
Figure 8: demineralised collagen fibrils
Five generations of adhesives were therefore developed to obtain an effective bond at the dentine level. Modern dentine adhesives form a resin-dentine bond that is both chemical and micro-mechanical by associating three basic components that are combined or applied separately according to the products : an acid conditioning agent that eliminates the dentine deposits (microscopic wastes due to the instrumentation) and superficially de-mineralises the dentine, a hydrophilic primer that penetrates the treated surface and makes the wetted dentine substrate compatible with the third component of the system, that is the fluid bonding resin .
II - Experimental Procedure
One of the main purpose of dental research about bonding is to make a chemico-mechanical characterisation of the dentinal surface. Then some composites resin specimens used in dentistry are bonded on the characterized dentin and a tensile test is performed. Thus, the effect of different characteristic parameters of human dentine surface (age and dentine depth, microstructure, microhardness, etc...) on the adherence of these composite resin specimens used are objectified. In addition to the study of the bond strength, the fracture patterns are observed to determine whether this fracture is interfacial or cohesive and in which medium is located in order to explain the bonding mechanisms.
In a recently study (fig 9), twenty freshly extracted teeth (for medical reasons) were selected four of them extracted from a same mouth. All the teeth were included in a cold epoxy resin (Epoxid Resinâ by Buehler). Considering the micro-structural changes that teeth go through with ageing, it seemed interesting to work on identical substrates for the physico-chemical and traction measurements. So, a tooth slice, located between the enamel and the pulp, was obtained after cutting (Buehler Isomet Low Speed Saw). All dentin surfaces were treated with orthophosphoric acid Total Etch® (Vivadent) for 20 seconds and rinced. The slice of dentin is used for the physico-chemical dentin characterization by Vickers microhardness, adhesive-dentin interactions measurements, image analyses, scanning electron microscopy associated with X Ray dispersive energy analyses. The remaining segment is used for the mechanical study.
Figure 9: summary of the experimental procedure
The radio-opaque composite resin light-cured used in our experiment is a material intended for the restoration of impaired teeth : Z100® (3M). It contains BIS-GMA (bis phenol A + glycidyl methacrylate) and TEGDMA (triethyleneglycol dimethacrylate) resins, 66 % in volume zircon and silica whose diameter ranges from 0.01 to 3.5 microns. The adhesive system is Syntac Sprint® (Vivadent). This adhesive contains MMPAA (Methacrylate Modified PolyAcrylic Acid), 2-Hydroxyethyl Methacrylate (HEMA), de-mineralised water or organic solvent, maleic acid, a fluorinated compound, catalysts and stabilisers. A specimen of light-cured composite was bonded to the dentin substrate. The manufacturer's indications were carefully followed. But, this fifth generation adhesive has a low resin load (polymer in solution) and it only light-cures as long as the non light-cured composite resin is put in contact with it. So, in order to get this unit we had to adapt the bonding protocol. The adherence of the composite specimen on the dentin substrate is then adequate so that the unit can be subjected to the mechanical test chosen : the tensile test. Tensile stress is the ratio between this tensile stress over the contact section (7 mm2) and is characteristic of the adherence between the resin and dentin. An Instron 1341 H machine with 5 kN capacity sensor is used. The traction speed was set at 1mm/min, temperature 23 °C.
III - Results
The characterization and adherence results are summed up in table 1.
Table 1: physico-chemical and mechanical results
Mean and associated standard deviation were calculated for each series of measurements (Table 2). A non parametric statistical analyses involving Spearman Rank coefficient (p value) was used.
Table 2: statistical analysis
All of the fracture patterns, (dentin and composite side), were observed with a magnifying glass, optical microscope with different magnifications, scanning electron microscope. During observation with scanning electronic microscope, analyses by energy dispersive spectrometry were carried out, either on a pinpoint zone (about 1 mm²) or on 50 mm² fields. By comparing the Ca and Si peaks Ka on the same surface, it was possible to determine the type of substance analysed: since the Ca is representative of the dentin and the Si of the composite load.
IV - Discussion
The orders of magnitude found comply with the litterature. The relatively low value of the wetting angle indicates a surface rich in interactions as of the deposit of the drop. It is interesting to note that the presence of four wisdom teeth obtained from the same patient can be used to assess the possible variations of the dentin substrate in the same mouth. Distribution of the data from a small population (under 30) does not correspond to a normal distribution. If p value is less than 0,05 it is considered statistically significative. The correlations in the first column of table 2 were searched. The Stat-View software traces the cloud of points, the associated tendency curve whether a function of the normality or not of data provides the searched for correlation coefficient.
The correlations
between tensile stress with all the physico chemical dentine parameters were
tried. The statistical analysis of the adherence tests did not provide statistically
significant results. But past studies demonstrated that the calcium concentration
seems to be sufficient to characterise the mineralisation of a given dental
surface, that the micro-hardness of the enamel and dentin surfaces have been
proven to increase in a linear manner with the calcium concentration and as
a result, the micro-hardness seems to be a valid index of the mineral content
of the dental surface. These authors also showed that there is a high positive
correlation between the bond strength and the micro-hardness of the tooth surface.
Considering these analyses it is interesting to note that there is in our results
a tendency as expected instead of the bond strength and wettability angle curve
tendance. The wettability explanation is very difficult and to go further, it
is necessary to take into account the resin adhesive viscosity evolution according
to time.
Observation of fracture patterns
reveals heterogeneous fracture patterns. Adjustment of the optical microscope
reveals that they are in relief, the thickness at this level is about 5mm and
representative of the adhesive. Composite resin is found in certain places on
thickness exceeding 10 mm. The fracture is therefore cohesive in the composite
at that place. On an adjacent zone, the dentin appears to be pre-treated with
acid, with widely open tubules where neither adhesive or composite resin have
penetrated and the fracture is interfacial on this zone.
The heterogeneity of the patterns
is also noted on the composite specimen side. Prints of the digitation by the
adhesive of the tubules and scraps of adhesive are observed.The type of fracture
varies according to the zone of tooth substrate observed: adhesive-tooth interface
or cohesive in the adhesive or composite.
In the case of dentine the ability to form a solid bond is often critical. In
the case of Syntac Sprint®, the adherence with dentine is explained by the
physico-chemical interactions: mechanical adhesion with adhesive penetration
into the tubules, but is it really a mechanical adhesion or a multiplication
of chemical linking between adhesive and dentine? The crosslinking between the
polymer (adhesive) and the dental collagen fibrils is also very important. The
adherence with Chemical linking between the dentine Ca++ and ionised acid functions
included in the Syntac Sprint® depends on the dentin and its intrinsic properties.
The hardness is characteristic of a thickness of 10 mm. If the hardness increases
with an increase in calcium, the adherence is better when the surface is calcium
rich without tubules. This will be an adhesive dentin adherence by the formation
of bonds with the calcium ions. The contribution of the different adhesion hypothesis
is difficult to be evaluated, it seems that they all play an important role
in the adhesion.
V
- Conclusion
The originality of this work was to carry out a physico-chemical characterisation and an adherence test on the same substrate. The dentine-adhesive interface is very complex and does not only depend on the superficial parameters of the dentine but also on the physico-chemistry of the resin and the parameters of the adherence test.
Passage from traditional dentistry to modern adhesive dentistry is accompanied by an evolution in concepts as well as in materials, with the occurence of composites of different viscosities and increasingly numerous clinical protocols . The main problem of composite resin fillings lies in the separation of the material from the walls of the cavity following the polymerisation shrinkage of the resin matrix, opening the door for the invasion of bacteria and tooth decay. Although the bonding of restorative materials to the enamel is now a safe technique, the same is not true for the bonding to the dentine, considering its heterogeneity and active character. In addition, composite materials maintain their physico-chemical properties (polymerisation shrinkage, thermal expansion) which affect their interactions with the dental tissue. Understanding the interfacial mechanisms associated with the progress made in the field of dentine adherence has helped considerably to improve the reliability of cares. The knowledge of these mechanisms helps to determine the choice of a surface treatment, of an adhesive, of a bonding agent and of an associated adherence accelerator.
Obs.: This paper was presented as an invited talk at the Simpósio Matéria 2000, Rio de Janeiro, Oct. 23 - 27, 2000.
