July 1, 2003
by Keith Titley, BDS, MScD, FRCD(C), Gajanan Kulkarni, BDS, MSc, PhD, FRCD(C)
After many decades of scientific and clinical controversy new and innovative restorative materials are gaining acceptance as viable alternatives to the traditional use of silver amalgam and stainless steel crowns for the restoration of primary molar teeth. These new generations of materials bond directly to tooth material by mechanical or chemical means or by a combination of the two mechanisms. The materials include resin composites, glass ionomer cements (GIC), resin modified glass ionomer cements (RMGIC) and polyacid modified composite resins (PAMCR). The latter are commonly referred to as ‘compomers’.1
Resin composites achieve a mechanical bond to both enamel and dentin by the use of acidic conditioning agents that remove calcium phosphate thereby providing micro-porosities through which adhesives can permeate and penetrate and when polymerized form a mechanical bond with the substrate.2 GIC is by definition a material that contains a basic glass and acid polymer that in the presence of water sets by means of an acid base reaction and also forms an ionic or chemical bond with the underlying substrate. GICs are very brittle and as a result they do not perform well as restorative materials in load bearing areas. RMGICs retain a significant acid base reaction as part of their curing process and are used as lining and luting cements, and some formulations are used as restorative materials.
They appear to form both mechanical and chemical bonds with their underlying substrates.3 PAMCRs (compomers) are one of the newer generations of restorative materials. Since they are predominantly resin composites their principal mode of adhesion to the underlying substrate is by mechanical means.2 The advantages and disadvantages of these materials for use in restorative dentistry are summarized in Table 1.4
Reviews of the literature reveal that there is a paucity of in vivo clinical data relative to the long-term success of these new generations of materials.5 In all probability the reason for this is that investigators are finding it generally difficult to conduct clinical studies because of informed consent issues and the general mobility of our population. This latter phenomenon means that modern clinical studies require large numbers of patient participants because of the drop-out factor in order to achieve statistically significant results. As a result clinical studies are very expensive to conduct. Similarly the continual changes in product formulations carried out by the manufacturers usually means that by the time the results of these clinical trials are published the product may no longer be marketed in its tested form.5
The net result of this is that the majority of the information on the properties of adhesive and composite materials is derived from in vitro laboratory investigations. There is no question that these investigations provide valuable insights on how a material might behave in the oral environment provided that the products are tested over a period of months rather than days.5 The object of this paper is to report on the relevance of in vitro investigations carried out in our laboratory that examine the bonds of resin containing materials to both enamel and dentin and the relevance to their clinical placement and how the latter may affect the longevity of the restoration. A case report of a failed resin composite restoration will initially be presented in order to provide some background of how clinical failure may relate to evidence provided by in vitro investigations. While it is recognized that the evidence supplied by one case is not only speculative and limited in its scope it is the authors’ opinion that the case selected illustrates how aberrations in clinical technique may affect the longevity of a restoration. This case, however, is representative of numerous similar failures that we have analyzed.
A failed composite restoration
The patient presented with a discharging fistula on the vestibular aspect of a maxillary right first primary molar tooth. Clinical and radiographic examination indicated the presence of a Class II composite restoration and furcation involvement of the supporting bone and breaching the follicle of the succedaneous tooth. These findings resulted in a decision to extract the tooth. Examination of the extracted tooth revealed that there was recurrent caries sub-gingival to the gingival seat and that it extended along the buccal and lingual axial walls (Fig. 1a).
There was also evidence that the matrix band had been incorrectly applied and that composite had been extruded subgingivally. Although there was evidence that there were occlusal deficiencies at the enamel-resin composite interface attempts made to dislodge the restoration from the occlusal aspect met with no success (Fig. 1b). As a result of these findings several reasons may be postulated for the failure of this restoration. They include:
An unrecognized exposure of the pulp during preparation of the cavity.
Incomplete caries removal that resulted in inflammation of the pulp and necrosis.
Faulty matrix application and inter-dental wedge placement resulting in contamination of the restorative field with blood and crevicular fluid
This contamination resulted in micro-leakage leading to recurrent caries.
Bulk filling of the cavity resulting in marked polymerization shrinkage away from the floor of the cavity preparation.
The one positive finding for the restoration indicated that from the occlusal aspect there was, despite the deficiencies at the margins, a strong bond to enamel since it could not be dislodged when force was applied to it. The authors concluded that the most probable cause for failure of this restoration was due to contamination of the operating field with blood and crevicular fluid as a result of poor matrix placement and incorrect application of the inter-dental wedge. Composite materials are sensitive to placement techniques (Table 1) and the evidence that the resin composite in this case had been extruded sub-gingivally was an indication of this with resultant failure.
Bonding to Enamel
Bonding of resin containing materials to acid conditioned enamel was introduced to the profession of dentistry in 1955.6 Later studies showed that acid conditioning could change the surface of enamel and in so doing make it more receptive to adhesion and that the formation of tag-like extensions into the micro-porosities created by the acid resulted in a mechanical bond.7,8 In general ortho-phosphoric acid in concentrations ranging from 10-37 percent has been used to condition enamel.
The etching times may vary from 15-90 seconds and this appears not to have an adverse effect on bond strength.9 There is evidence, however, that etching for 120 seconds with 37 percent ortho-phosphoric acid results in a significant reduction in bond strength because the etched surface collapses upon itself.10 This is illustrated by SEM images in which the enamel of two separate human third molar teeth has been etched for 20 and 120 seconds. They illustrate that the enamel surface collapses over the longer etching time (Figs, 2a & b). Polyacrylic acid is recommended for GIC’s.9
Studies in our laboratory have been focused upon the durability of the bond between orthodontic bracket bases and enamel when cemented with resin composite or resin reinforced GIC’s.10-14 Enamel surfaces were conditioned with 37% ortho-phosphoric acid for resin composite cements and with 10% polyacrylic acid for resin reinforced GIC. The latter cements were placed in a moist environment whereas successful bonding of resin composites to the enamel surface is predicated upon it being dry.
Specimens were shear tested to failure after periods of storage in distilled water for 24 hours and 7, and in later studies 180 days. In general the results indicated that there were some changes in shear bond strength from 24 hours to 7 days and it was concluded that these were, in all probability, due to continuing polymerization and hydration of the cement. Over the 180-day period,
however, there were no statistically significant changes between any of the 7 and 180-day groups. These results indicated that orthodontic appliances bonded to enamel with these cements could reasonably be expected to remain in situ over the active life of the appliance. Similar results were obtained in our laboratory with bonded cylinders of the compomers Dyract (Dentsply/De Trey, Konstanz, Germany) and F2000 (3M Co., St. Paul, MN, USA) to both human primary and bovine primary enamel.2
As a result of these findings we concluded that, provided that these materials are used in accordance with the manufacturers’ instructions, their bonds with enamel are durable and reliable over time. This observation is confirmed by the failure to physically dislodge the failed restoration from the extracted tooth cited above.
Bonding to Dentin
In comparison with enamel dentin is a wet substrate being composed of 70 percent inorganic material (hydroxyapatite), 20 percent organic material (type 1 collagen) and 10% water by weight.15 It is also a permeable tissue because of its fluid filled dentinal tubules that allow a bi-directional flow of fluid between the oral cavity and the pulp.16 When instrumented during cavity preparation a smear layer is formed which is composed of hydroxyapatite crystals, denatured collagen, cariogenic bacteria and blood (Fig. 3).17-19 It is partially porous, decreases dentin permeability by plugging the orifices of the dentinal tubules and is weakly attached to the underlying dentin.18-20
The advantages that are gained by its formation is that it acts as an iatrogenic cavity liner in that it reduces dentin permeability and, in so doing, protects the pulp.20 Its disadvantage is that it interferes with bonding to the underlying intact dentin and acts as a depot for microorganisms.20 Dentin bonding systems have been developed using agents that can interact with an intact or partially modified smear layer; remove the smear layer in its entirety; bond to a modified treated smear layer; or demineralize and infiltrate dentin.19-22 In general the dentin bonding systems that are in current use remove the smear layer by acid conditioning resulting in the demineralization of the intertubular and peritubular dentin causing funneling of the dentinal tubules (Fig. 4). They then rely on hydrophilic primers to penetrate the decalcified dentin surface that is kept deliberately moist 23 to prevent the collapse of the underlying collagen thereby allowing the adhesive to interact with it and also to penetrate and plug the dentinal tubules. The resultant adhesive collagen complex is referred to as the hybrid layer.21
The overall clinical requirement of the bond formed between dentin and the adhesive is that it seals the dentinal tubules, is non-permeable and, at the same time, is both long lasting and durable.21 In order to fulfill these requirements adhesives must penetrate the demineralized zone to the normal dentin interface since failure to do so may result in an unprotected, uninfiltrated zone of weakened exposed collagen.24 The demineralized zone has also been referred to as the resin-dentin interdiffusion zone. It has been hypothesized that the diffusion of resin monomers diminishes with depth so that demineralization with more dilute acids may result in a more complete diffusion of adhesive and higher bond strengths.25 SEM studies carried out in our laboratory, however, showed that conditioning with orthophosphoric acid in a concentration as low as 1 percent results in the adhesive still failing to completely infiltrate to the unetched dentin interface.26
Interestingly shear bond strength testing resulted in significantly higher values than when dentin was etched with 5 percent and 10 percent orthophosphoric acid compared with 37 percent.22 SEM examination of the surface of the dentin and the cylinder of detached resin following shear bond testing to failure revealed uninfiltrated collagen adhering to the surfaces of both (Figs. 5a & b). These findings support the premise that adhesive resin fails to completely infiltrate the resin-dentin inter-diffusion zone.
Other studies in our laboratory have examined the long-term strength of resin dentin bonds. These studies showed that over periods of storage for up to 270 days there was a significant deterioration in shear bond strength when dentin was etched with 37 percent orthophosphoric acid.22, 27 This was true for both multiple component and single bottle adhesive systems with the single bottle systems deteriorating to a greater degree. Other studies have confirmed these findings.28,29
Similar results were also recorded in our laboratory with the compomers Dyract (Dentsply/De Trey, Konstanz, Germany) and F2000 (3M Co., St. Paul, MN) bonded to human dentin where shear bond strength dropped over a 180-day period.2 A recently reported in vitro study reported that resin bonded to enamel protected the resin-dentin bond against degradation while direct exposure of the bond to water over four years resulted in its deterioration.30 These results are similar to those recorded in our laboratory that report that as storage time increases dentin bond strength decreases.22,27
These findings indicate dentin bonds are not as reliable or as durable as those to enamel particularly since they appear to deteriorate over time. The failure of adhesive to fully penetrate the demineralized layer resulting in incomplete hybridization leaves the collagen in these bonds vulnerable and subject to hydrolytic degeneration.30 It would appear, therefore, that in the clinical situation dentin bonds would be more reliable and durable if all of the cavity margins were located in enamel.
THE CLINICAL RELEVANCE OF THESE FINDINGS
A survey of the current literature indicates that although lower shear bond strengths may be expected with human primary teeth when compared to permanent human teeth they may be expected to perform adequately under clinical conditions provided that their technique sensitivity during placement is acknowledged.2
The evidence supplied by the failed clinically placed restoration and the results of the in vitro studies carried out in our laboratory suggest that bonds to enamel are both reliable and durable. On the other hand bonds to dentin have been shown to deteriorate over time. In addition they have the potential for micro-leakage and hydrolytic degradation because of the failure of adhesives to fully penetrate the demineralized layer and completely hybridize it. A recent study presents evidence that the application of a resin-modified glass ionomer to all of the cut dentin in Class II composite restorations significantly reduces micro-leakage along their axial walls.31
The authors conclude that the use of glass ionomer in this way provides better results than direct bonding. Since the results obtained in our laboratory strongly suggest that enamel bonds are more durable and reliable than dentin bonds it would appear that, unless all cavity margins can be located in enamel, the ‘glass ionomer open sandwich technique’ would significantly contribute to the clinical success of the new generation of restorative materials provided that matrix application, a clean dry operating field and careful adherence to the manufacturers instructions are adhered to.32 It is also recommended that because of technique sensitivity in their placement these esthetic bonded materials should be reserved for small one or two surface restorations in primary molar teeth.33-37
In cases where carious involvement is extensive other more durable restorative materials such as stainless steel crowns should be considered.38,39 Given the stringent clinical conditions that must be met, the time required to meet those conditions, and the difficulty of achieving these in difficult to manage children, other treatment options must be considered in restoring primary teeth until such time as properties of composite materials are robust enough to meet the additional challenges posed by the pediatric patient.
The authors acknowledge the expertise and contributions of Mr. R
obert Chernecky, senior laboratory technician, in obtaining the Scanning Electron Microscope images.
Dr. Titley is professor, Department of Paediatric Dentistry, University of Toronto and is Oral Health’s editorial board member for Paediatrics.
Dr. Kulkarni is associate professor, Department of Paediatric Dentistry, University of Toronto.
Oral Health welcomes this original article.
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Background: Alternative materials to silver amalgam and stainless steel crowns are increasingly being used for the restoration of primary molar teeth. The placement of these materials is technique sensitive and there are very few clinical studies that report on their longevity and durability.
Objective: To explore and highlight some of the limitations of the use of these materials in clinical practice with the evidence provided by in vitro laboratory investigations.
Methods: Through an examination of the literature and investigations carried out in the materials sciences laboratories at the Faculty of Dentistry, University of Toronto the durability of enamel and dentin bonds over time were investigated. SEM investigations of failed dentin bonds were also carried out.
Results: In general bonds to enamel were shown to be both reliable and durable over time periods of 180 days. On the other hand bonds to dentin were shown to deteriorate over time periods of 180 and 270 days and adhesives were shown to incompletely infiltrate the demineralized zone of collagen raising the possibility of micro-leakage occurring in the bond.
Clinical Significance: Bonded restorations have a place in the restoration of carious primary molar teeth provided that the clinician is aware of their limitations and technique sensitivity of their placement.
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