Microleakage with penetrating stain and recurrent caries on a composite restoration. Photo by Dr. Jorge Perdigao.
Recurrent caries adjacent to an amalgam margin on a cervical restoration. Photo by Dr. Jorge Perdigao.
Use of Materials, Technique, and Procedures to Limit Microleakage
There has been an unending search over the last century using a variety of techniques and materials in restorative dentistry to find something that eliminates microleakage. Elaborate methodology in tooth preparation, material handling, and fitting to the tooth has been developed using cast gold, gold foil, amalgam, and more recently, composites and porcelain to prevent microleakage.
Amalgam as a material has for many years depended upon a varnish to limit initial microleakage.76 Since the 1990s, bonding of amalgam has been shown to limit the severity and extent of microleakage, although it is still unable to totally eliminate it.77,78,79,80,81 The key component of limiting microleakage seems to be the use of a dual or chemically cured dentin bonding system prior to the placement of the amalgam. Some studies of bonding amalgam during its placement showed that procedure can significantly eliminate microleakage at the amalgam interface but is not as effective at eliminating leakage at the cementum interface.82,83,84 Other studies indicate a significant reduction in microleakage at both cementum and enamel margins, with no significant leakage difference between the two interfaces.85,86,87
One explanation for the differences cited by the various studies is that different materials have different capabilities with which to resist microleakage.88 Another factor in the differences cited in the literature is the series of comparisons made between products that line the cavity without adhering to the amalgam versus those materials that adhere to the amalgam through micromechanical retention.89 Other studies have confirmed that ultimately the corrosion products of amalgam will diminish microleakage.90,91,92,93 These studies indicate a minimum of six to 12 months is required to form the corrosion products using high copper amalgams. The variance appears to be due to the specific product tested.
Cast gold and porcelain fused to metal crowns have been tested by a variety of methods, and while fit of the crown plays a role in limiting microleakage, one of the most critical elements seems to be the choice of luting material and the treatment of the surfaces to be bonded. Any of the metals used, from non-precious to precious, all require some surface preparation to enhance the bonding of the luting material. The most consistent finding is that sandblasting the metallic surface using 25- to 50-micron particle aluminum oxide improves the bond strength of any of the luting materials.94,95,96,97,98 The use of the sandblasting simply roughens the surface of the metal, creating micromechanical undercuts, increasing the surface area of the luting material/restoration contact. 4-meta is a chemical adhesion booster. It is said to increase the bond strength of metals to other substrates and is used in several materials for that purpose. SuperBond contains 4-meta, which supposedly wets the surface of the metal, lowering surface tension and improving adhesion.99,100,101
Additional bond strength to precious metals can be achieved using a tin-plating solution. Resins have been shown to chemically bond to the tin-plated metal and improve the bond strength. Tin-plating is valuable in increasing the bond strength to precious metals used in porcelain-fused-to-metal crowns,ll type II and III gold alloys. Non-precious metals need not be tin-plated but may be etched using a strong acid to prepare the surface for bonding.102,103 However, while used often for etched-retained bridges, it has also been found that sandblasting the surface of the non-precious metal and using a resin luting material produced much the same result.104
Porcelain and indirect composite restorations can also be bonded to tooth structure, though again these are not able to totally limit the microleakage. Bonding to composite materials, both laboratory-processed and light-cured materials, can also be enhanced by a variety of surface treatments. Laboratory processed composite materials, such as BelleGlass, have remarkably little remaining monomer because they are heat/pressure processed in a nitrogen atmosphere causing the degree of polymerization to increase dramatically over direct composite materials.105,106 While this enhances the dimensional stability and color stability and strength of the material, it also makes subsequent bonding more difficult because there is very little free monomer with which to bond. However, the surfaces of the material can be prepared for bonding first by sandblasting to roughen the surface and create porosities into which subsequent resin can flow and exposing more of the filler particles and by enhancing the bond to the filler particles of the composite by treating it with silane to link the new resin monomer to the filler particles of the cured composite chemically.
Bonding to direct composites is more easily accomplished because of the availability of free monomer. However, as the composite materials age in the mouth, subsequent bonding to them requires at least the same preparation of the surface as does the laboratory-processed composites because the free monomer becomes oxidized with water soluble stains. Hence the bond to aged composites decreases compared to the bond to newly placed composites.107
Bonding to porcelain materials varies somewhat by the nature of the porcelain (feldspathic, luecite-reinforced, lithium di-silicate, slip-cast aluminum oxide, densely sintered aluminum oxide, and zirconia) but generally involves similar steps. The surface must be roughened, either by etching with a hydrofluoric acid or sandblasting or both. The surface of the porcelain at that point has lowered surface tension and presents a surface more amenable to bonding. Application of silane is required for many types of porcelain, so that the silane material can polymerize with the resin luting material and the glass particles of the porcelain material. After silanization, a resin material is added to the surface of the porcelain before bonding to the tooth structure.108
Certain types of all-porcelain crowns, such as luecite-reinforced, lithium di-silicate, slip-cast aluminum oxide, densely sintered aluminum oxide, and zirconia (IPS Empress 2, In-Ceram, ProCera) do not require silane coating prior to cementation, but do require sandblasting and use of a resin luting material.109
The variety of porcelain materials can create some confusion as to the best surface preparations for them. Aluminous porcelain cores (In-Ceram) with highly sintered porcelain require more surface preparation than do the feldspathic porcelains.17 A coating of silica (Rocatec by 3M-ESPE) can improve the bond strength of the material to tooth structure and is preferable to etching. Such a coating will increase the bond strength more than sandblasting with aluminum oxide alone.
Bonding to other types of porcelain, either leucite-reinforced or feldspathic, can be improved by etching the surface of the porcelain with hydrofluoric acid to create micro-porosities into which resin can flow.110,111 This also decreases the surface tension and permits easier flow of the resin into the micro-porosities. Subsequent treatment with silane improves the bond strength by providing a bond between the glass particles of the porcelain and the resin in the luting agent.
Bonding of composite resin materials as restorative material or as a luting material to the tooth complex is the material and technique used universally to limit microleakage. At the heart of the bonding process is the use of a variety of dentin bonding materials utilizing an etchant, dentin primer, and resin monomer either separately or in some combination.
Various dentin bonding materials and systems have been tested extensively in vitro. In one finding, the research indicates that shear bond strengths of 21 MPa lead to less microleakage, especially on cementum margins.112 In another study, a correlation was found that showed the stronger the shear bond strength to dentin, the greater the ability of the bonding agent to wet the dentin, the less microleakage occurred.113 Theoretically, this would be due to a more robust and deeper dentin hybrid layer engaging more collagen.
Generational improvements within the dentin bonding materials clearly show significant reductions in microleakage when comparing — e.g., Scotchbond 2 with Scotchbond MPS.114,115 As further development of dentin bonding materials occurred, the multi-step materials have been compared to the simplified systems in which the primers and adhesives were combined, and in which the etchants, primers, and adhesives were combined. The research on simplified dentin bonding agents shows conflicting results. Some authors found that the simplified materials improved dentin bonding on cementum margins compared to multiple-step bonding systems and were comparable in eliminating microleakage at enamel margins.116,117 Other authors have found the materials to be comparable on enamel and cementum margins.118 Self-etching primers have been shown to be less effective in bonding to enamel than to dentin.119 Regardless of material, studies have shown that it has been impossible to eliminate all microleakage in vitro.120,121,122
Differences in materials, technique sensitivity, timing, and chemistry can explain the diverse results seen with these studies. Tay and Pashley et al discovered in using the single-step adhesives that by delaying the curing of the adhesive there was a significant decrease in bond strength to the single-step adhesive compared to the three-step adhesive systems.123 They looked at the SEM and TEM images of the specimens and found that there were areas of voids, resin globules, and small blisters resembling an over-wet zone found in acetone-based three-step adhesives. They have theorized this is due to permeability through the adhesive layer of dentin fluid, subsequently interfering in developing the strongest bond to the hybridized dentin. Hashimoto et al have discovered that the use of multiple coats of dentin bonding agents dramatically increased the microtensile bond strength up to the sixth coat, and that by doing so, the leakage was dramatically reduced as well because there was greater resin infiltration into the dentin.124
After choosing a dentin bonding system, the manipulation of the materials becomes critical to limiting microleakage. It has been the vogue to test microleakage of Class II cavities where the gingival margin extends to the cementum using a variety of materials and bonding systems. Basing the cavity axial walls with some type of liner, either a flowable composite or resin-glass ionomer, has been tested extensively to see if it limits the polymerization shrinkage of the composite and decreases microleakage.
Some studies indicate that basing or lining the cavity decreases microleakage.125,126 Others suggest that basing is not necessary to limit microleakage.127,128,129,130 Incremental placement rather than bulk placement has been shown to limit microleakage.131,132 Surface roughness of cavity preparations does not affect microleakage,133 nor does using a surface sealant on composite resin restorations.134 The use of rubber dam isolation decreases the microleakage of enamel margins compared to cotton roll isolation in vivo.135 Finishing method and time of finishing, whether immediate or at 24 hours, does not affect the microleakage of hybrid composites, but it does affect microfilled composite restorations. Microfilled composites exhibit decreased microleakage when finished wet and 24 hours after placement.136
Fruits et al shows that in Class V restorations restored with composite resin, the choice of material affects the microleakage and retention of the restoration. They theorized that the forces that concentrate on the cervical aspect of the tooth from occlusal loading would affect retention and microleakage depending on the flexibility of the resin, whether hybrid composite, microfilled composite, or flowable composite resin. They found that the more flexible microfilled and flowable composite resins were better able to resist microleakage after stress testing than the hybrid composite resin.137
There are investigations into which light-curing unit and method might decrease microleakage. Plasma arc curing lights have been found to increase microleakage in Class I cavities compared to conventional halogen light-curing units.138 Soft start or ramped curing units do not seem to decrease microleakage compared to conventional light-curing units, although incremental placement of composite did decrease the microleakage.139,140
As described earlier, bonding to sclerotic dentin is challenging, with frequently lower bond strengths due to the hyper-mineralization of the dentin surface. Use of self-etching primers on sclerotic dentin has been shown to be ineffective at penetrating the hyper-mineralized surface layer of sclerotic dentin, resulting in significantly lower bond strength.141,142 For this reason, it is recommended that either mechanical undercuts be prepared in the cervical sclerotic dentin or the preparation extended to include normal dentin on the peripheral walls.
Summary and Conclusions
Why do some patients treated with the same materials by the same dentist exhibit far worse effects of microleakage than other patients?
The oral environment is the key to understanding the effects on the materials and restorations. A mouth that is constantly acidic or dry will advance the growth of bacteria, causing a greater effect in whatever microleakage exists. The virulence
of the bacteria as well as the oral environment will affect the speed of lesion advancement and the degradation of the restorative materials.
This review shows that while we presently can minimize microleakage, it may be impossible to totally eliminate it. Our history has shown us that we can improve our patients' dental health and increase the longevity of restorations, longevity of teeth, and health of the periodontium by providing the best fitting, best contoured, and smooth restorations.
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*Part One of this article appeared in the January-February 2005 issue of Northwest Dentistry.
*Dr. Larson is Associate Professor, Department of Restorative Sciences, Division of Operative Dentistry, University of Minnesota School of Dentistry, Minneapolis, Minnesota.
E-mail is larso004@umn.edu
