Interaction between the dentinal smear layer and composite bond strength

Freshly extracted human molars were used to study the interaction between dentinal smear layer removal with various agents, a dentinal adhesive, and the shear bond strength of a posterior composite resin to dentin. The use of Scotchbond dentinal adhesive in conjunction with the composite P-30, with the smear layer intact, produced the highest shear bond strengths. Removal of the smear layer produced bond strengths similar to those obtained with the smear layer intact. Application of ferric oxalate, and to a lesser degree 17% EDTA, resulted in diminished bond strengths. It was concluded that optimal bond strengths with the adhesive-composite resin systems tested may be obtained with an intact smear layer.     

The smear layer in endodontics

With the cascade of new restorative products being unveiled almost monthly, dentists incorporating endodontics into their practices must be able to evaluate the potential of these products for successful integration into their procedures. This evaluation should be based on a knowledge of how the new products relate to the smear layer formed along the root canal walls. Rather than relying on information supplied by the dental manufacturers, the aware dentist should regularly resort to the most current research reports available in journal or abstract form. With the use of certain products in some clinical situations, other branches of restorative dentistry may suggest retention of the smear layer. Although pulpally infected teeth have been successfully treated for generations in the presence of the smear layer, it has become accepted practice now in endodontics to remove the smear layer. Different quantities and qualities of smear layer can be produced by various techniques of instrumentation. However, they all present a barrier to intimate contact between obturating materials and the canal wall. Various types of solvents will produce different results in smear layer removal. One ideal endodontic irrigant follows the use of the antimicrobial 5.25 per cent sodium hypochlorite solution with the equally antimicrobial 6 per cent citric acid solution or 17 per cent EDTA. Chelating agents are effective in that they remove the smear layer, open the dentinal tubules, and produce a clean surface for closer obturation. Removal of the smear layer encourages the creation of a good apical plug to prevent over filling, post-filling sensitivity, and possible microleakage. The use of glass ionomer cements and unfilled resin as a cementing medium following smear layer removal shows promising results in both strength of cementation and the possibility of reducing post lengths. Controversies will always arise in dentistry with the advent of new information and the discovery of new clinical techniques. But a total awareness of both sides of a controversy will enable the practitioner to find a way through the confusion.     

Lactic acid root canal irrigation for dowel and core treatment: a pilot study

STATEMENT OF PROBLEM: Various solutions used to irrigate root canals and remove the smear layer prior to obturation of a root canal affect surface topography and may influence the sealing ability of endodontic therapy, retention of endodontic dowels, and restoration survival. PURPOSE: This study microscopically evaluated the effects of varying dilutions of lactic acid on the removal of the smear layer created by hand instrumentation in the coronal, middle, and apical thirds of root canals. Lactic acid solution was compared with other solutions deemed to be suitable root canal irrigants. MATERIAL AND METHODS: Thirty-five extracted human single-rooted teeth were manually shaped with files (K-flex) using the step-back technique. ISO size 50 files were used as master apical files. The teeth were equally divided into 7 test groups according to the canal irrigant used. While the teeth were cleaned and shaped, the root canals were irrigated with 3 mL of 1 of the following irrigants between each file size and as a final flush: no irrigant (control), 5% hydrogen peroxide, 5% sodium hypochlorite, a combination of 5% hydrogen peroxide and 5% sodium hypochlorite, 15% ethylenediaminotetraacetic acid (EDTA), 10% lactic acid, or 20% lactic acid. Specimens were critical-point dried and freeze-fractured for scanning electron microscope analysis at 3 sites (cervical, middle, and apical). RESULTS: Scanning electron micrographs showed no differences between the cervical, middle, and apical levels of the specimens tested with any of the irrigants used. Also, no difference was found within individual groups. The surface of the control specimens had a smear layer that contained smear plug material. Hydrogen peroxide and sodium hypochlorite used separately and in combination consistently produced surfaces with a smear layer. Lactic acid solutions and EDTA, however, removed the dentinal smear layer but left remnants of smear plugs in the dentinal tubules. CONCLUSIONS: Within the limitations of this study, differences related to the type of irrigant used were found in the characteristics of the pulpal dentin surface. Although lactic acid effectively cleaned and smoothed the dentin walls of root canals of endodontically prepared teeth, further study is needed before this procedure can be recommended for routine clinical use.     

In vitro assessment of retention of four esthetic dowels to resin core foundation and teeth

STATEMENT OF PROBLEM: Several new esthetic dowel systems are currently available for the restoration of endodontically treated teeth. These dowel systems enhance the esthetic quality of all-ceramic restorations better than metallic dowel systems. PURPOSE: The purpose of this study was to evaluate the retentive strength of composite and ceramic endodontic dowel systems to the tooth and to the core foundation. MATERIAL AND METHODS: The following dowel systems were tested: resin dowels (Fibrekor [FR]; Luscent [LU]; Twin Luscent Anchor [TLU]); ceramic dowels (Cerapost [CR]; Cosmopost [CO]); and a titanium dowel (ParaPost XH [Ti]). In Part I of the study, core retention was tested by forming Bis-Core resin (n=12) cores around dowels followed by separation using a universal testing machine. In Part II, 60 (n=12) extracted human canines were endodontically treated, and dowel spaces were prepared using the corresponding drill for each dowel system. Nine-millimeter resin and ceramic dowels were cemented with C & B resin luting agent. Additionally, 2 groups (n=12) of Ti dowels cemented with C & B resin luting agent and zinc phosphate luting agent served as control groups. Retention was tested using a universal testing machine to separate the dowels from teeth. One-way analysis of variance and Student Newman-Keuls tests were conducted for statistical analysis (alpha=.05). Surface texture of all dowel systems tested was examined using SEM at original magnification x25 and x250. RESULTS: Core retention of Ti was higher than all esthetic dowels tested (alpha<.05), but FR had higher core retention than the other esthetic dowels tested. Resin dowels had better retention to teeth than ceramic dowels (alpha<.05). CONCLUSION: The esthetic dowel systems were less retentive for the resin core material than the titanium control. Resin dowel systems were more retentive in the root than the ceramic dowels but were similar to the titanium control.     

Origination and progression of microleakage in a restoration with a smear layer-mediated dentinal bonding agent.

Several recently marketed dentinal bonding agents modify, but do not remove, the smear layer. Some controversy exists as to whether the smear layer should be left in place or completely removed from preparations to be restored with composite resin systems. This study utilized scanning electron microscopy with energy dispersive x-ray analysis to evaluate the pathway of microleakage in a composite resin restorative material that utilizes a smear layer-mediated dentinal bonding agent. Microleakage occurred at the smear layer-dentin interface and progressed into both the smear layer and dentinal tubules, suggesting that the smear layer acts as a pathway for microleakage.     

Philosophies and practices regarding the management of the endodontic smear layer: results from two surveys

The purpose of this investigation was to ascertain the prevailing attitudes within the endodontic community regarding smear layer removal. The first survey asked the philosophies and techniques currently being taught to predoctoral dental students and postdoctoral endodontic residents in the United States, whereas the second surveyed some of the members of the American Association of Endodontists currently practicing in the United States. Findings from these surveys revealed that more than three-fourths of the dental students and nearly two-thirds of the endodontic residents are not being taught routine smear layer removal. Slightly more than one-half of endodontists responded they practice smear layer removal. These surveys indicate that there is no clear consensus in the endodontic community, either academically or clinically, as to whether the smear layer should be removed or be allowed to remain before obturation of the root canal space.     

In Vitro Evaluation of Five Core Materials

PURPOSE: This in vitro study determined the fracture strength of five core materials supported by two different endodontic dowels. Diametral tensile strength and microhardness of the three resin composite core materials used in this study were also tested. MATERIAL AND METHODS: The fracture strength study used one lanthanide-reinforced flowable resin composite (Ti-Core Auto E), one titanium- and lanthanide-reinforced composite (Ti-Core), one lanthanide-reinforced composite (Ti-Core Natural), and two metal-reinforced glass ionomer core materials (Ketac Silver and GC Miracle Mix). Two types of dowels were used: a multitiered, split-shank threaded dowel with a flange (#1 Flexi-Flange) and one without a flange design (#1 Flexi-Post). The specimens were divided into ten groups. Each tooth/dowel and core specimen was placed in a special jig at 45 degrees and subjected to a load by a universal testing machine. The diametral tensile strength and the microhardness of the three resin composite core materials were measured by a universal testing machine and Barcol hardness tester, respectively. All test groups contained ten specimens. RESULTS: The fracture strength value of the resin composite core materials was significantly larger ( p < 0.0001) than those for the metal-reinforced glass-ionomer core materials. Analysis of variance (ANOVA) also showed that the Flexi-Flange dowel interacted with Ti-Core and Ti-Core Auto E to significantly ( p < 0.0013) increase the fracture strength relative to the Flexi-Post. One-way ANOVA revealed that there were no significant differences between them in terms of diametral tensile strength. The Barcol hardness values of the composite core materials were statistically different ( p < 0.0001), with the Ti-Core the highest, followed by Ti-Core Natural, then Ti-Core Auto E. CONCLUSIONS: Resin composite core material performed better than glass ionomer material in this in vitro study. The flowable composite core material performed about the same in terms of fracture strength and diametral tensile strength compared with nonflowable composites. Combined with certain core materials, the flange design increased the fracture strength of the tooth/dowel and core combination.     

Shear bond strengths of prosthodontic adhesive systems to a nickel-chromium-beryllium alloy.

Panavia, a composite resin luting agent containing phosphate monomers, bonds chemically to air-abraded base metal alloys and is particularly suited for cementing resin-bonded retainers. Another adhesive methodology (Silicoater system) incorporates a pyrolytically applied silica layer to metal substrates to promote bonding of a resin adhesive. The object of this study was to compare prosthodontic adhesive systems that incorporated the Silicoater system and/or Panavia. The results showed that the surface treatment that provided significantly higher shear bond strengths was the Silicoater system and a layer of unfilled resin. A second test evaluated use of Panavia as a metal opaquing material beneath laboratory light-curing resins. Results showed that Panavia Opaque material was displaced peripherally during placement of overlying laboratory composite resin veneers. This resulted in an uneven intermediate layer of Panavia.     

Scanning electron microscopic and energy dispersive x-ray evaluation of two smear layer-mediated dentinal bonding agents

Teeth treated with smear layer-mediated dentinal bonding agents and restored with composite resin were examined at the dentin-restoration interface by means of scanning electron microscopy used in conjunction with energy dispersive x-ray analysis. Results indicated that these dentinal bonding agents actually bonded to the smear layer, and samples demonstrated delamination of the smear layer from underlying dentin following thermocycling. This finding suggests that the stresses developed within the composite resin exceeded the adhesive strength of the smear layer to dentin. Thus, the bond strength for dentinal bonding agents that require the presence of the smear layer cannot exceed the adhesion of the smear layer to dentin.     

Comparison of Two Luting Agents Used for the Retention of Cast Dowel and Cores

PURPOSE: The first purpose of this study was to compare the retentive values of zinc phosphate and Panavia F resin cements when used for luting cast dowel and cores. The second purpose was to determine whether the use of a lubricant when making the resin pattern for a custom dowel and core would have an effect on the final retention of dowels cemented with either zinc phosphate or Panavia F cements. METHODS AND MATERIALS: Sixty-three caries-free extracted single-rooted human teeth were randomly divided into three groups of 21. Root canal preparations were standardized for all 63 teeth. Clinical protocols for fabricating and cementing dowel and core restorations were examined, comparing zinc phosphate and Panavia F resin cements. Direct dowel patterns were fabricated using the Para Post system and cast in a noble metal alloy. Group I dowel spaces were lubricated with GC lubricant prior to dowel pattern fabrication and cleaned with Cavidry solvent before cementing the cast dowel and core with zinc phosphate cement. Group II dowel spaces were rinsed with water only prior to dowel pattern fabrication. The dowels and cores in this group were cemented with Panavia F resin cement. Group III dowel spaces were lubricated with GC lubricant prior to dowel pattern fabrication; the dowel spaces were cleaned with Cavidry solvent before the cast dowel and cores were cemented with Panavia F cement. The tensile force necessary to remove the cast dowel and cores was determined using a universal testing machine. Results were statistically analyzed using one-way ANOVA and Tukey's HSD test. RESULTS: The dowels and cores in Group I had significantly higher retentive values than either of the two Panavia F groups (p< or = 0.001). No difference in retentive values (p > 0.05) was found between dowels luted with either of the lubricating agents in the Panavia F groups. CONCLUSIONS: Zinc phosphate cement had higher retentive values when cementing cast dowel and cores than Panavia F. The type of lubricant used for the resin dowel fabrication (water or GC lubricant that was removed with a solvent) had no effect on the retention of cast dowels cemented with Panavia F.     

The removal of the smear layer using EGTA: a scanning electron microscopic study

The smear layer associated with endodontic instrumentation is currently thought to be a thin layer that occludes the orifices of the dentinal tubules and covers the intertubular dentin of the prepared canal wall. The purpose of this study was to compare the effect of ethylene glycol bis (beta-amino ethyl ether)-N, N, N', N'-tetra acetic acid (EGTA) and ethylene diamine tetra acetic acid (EDTA) on removal of the smear layer through the scanning electron microscopy. Twenty four single rooted teeth were selected, instrumented and irrigated with various solutions and the specimens were processed for scanning electron microscopy. It was found that though both EGTA and EDTA completely removed the smear layer, EDTA caused erosion and conjugation of the tubules, whereas, EGTA effectively removed the smear layer without inducing any erosion. It was thus concluded that EGTA can be effectively used as an alternative chelator for the removal of the smear layer.     

Evaluation of residual dentin after conventional and chemomechanical caries removal using SEM

The purpose of this in vitro study was to analyze the residual dentinal surfaces following caries removal using rotatory instruments and two chemomechanical methods (Papacárie and Carisolv), by scanning electron microscopy (SEM). Thirty primary incisors were divided into three groups, according to the caries removal method used, and their residual dentin was examined under SEM (15). After caries removal, 15 of these teeth were restored with Single Bond (3M) adhesive system and Z100 Filtek composite resin (3M). The tags of the replicas were observed under SEM. The chemomechanical caries removal methods (Papacárie and Carisolv) formed an amorphous layer, similar to the smear layer and few exposed dentinal tubules; the conventional caries removal method produced a smooth and regular dentinal surface, with typical smear layer and exposed dentinal tubules. All groups showed abundant tag formation. Scanning electron microscopy analysis revealed a difference between dentin treated with rotatory instruments and that treated with chemomechanical methods in spite of the occurrence of a similar tag formation in both groups.     

Effect of pH modified EDTA solution to the properties of dentin

The purpose of this study was to evaluate the influence of smear layer removal with 3% EDTA solution (pH of 9.0) on the dentin in terms of the permeability of root canal disinfectants into the dentin, wetting by endodontic sealer, and adhesive strength of the sealer. Three types of disinfectant (formalin cresol, phenol, and calcium hydroxide) and 4 types of endodontic sealers (included eugenol, non-eugenol, polycarboxylic acid, and resin) were used. The contact angle between endodontic sealer solution and dentin decreased in the 3% EDTA group but increased in the 15% EDTA. The adhesive strength of endodontic sealer to dentin increased in the EDTA groups for all types of sealers. The permeability of root canal disinfectants increased to similar degrees in the 3% and the 15% EDTA groups. In comparing these properties, we propose that the 3% EDTA is more useful for clinical applications.     

The use of reinforced composite resin cement as compensation for reduced post length

STATEMENT OF PROBLEM: Cements that yield high retentive values are believed to allow use of shorter posts. PURPOSE: This study investigated the use of reinforced composite resin cement as compensation for reduced dowel length. MATERIAL AND METHODS: The retention values of stainless steel posts (parallel-sided ParaPost and tapered Dentatus in 5-, 8-, and 10-mm lengths) luted with Flexi-Flow titanium-reinforced composite resin and zinc phosphate cements were evaluated. Single-rooted extracted human teeth with crowns (n = 120), removed at the cementoenamel junction, were randomly divided into 4 groups of 30 samples each. Different post lengths were luted with either Flexi-Flow or zinc phosphate. Each sample was placed into a specialized jig and on a tensile testing machine with a crosshead speed of 2 mm/min, applied until failure. The effect of different posts and cements on the force required to dislodge the dowels was evaluated with multiple analyses of variance (ANOVA). One-way ANOVA with Scheffé contrast was applied to determine the effect of different post lengths on the retentive failure of posts luted with the 2 agents. RESULTS: Flexi-Flow reinforced composite resin cement significantly increased retention of ParaPost and Dentatus dowels (P<.001) compared with zinc phosphate. One-way ANOVA revealed no statistically significant difference (P>.05) between mean retention of both dowels luted with Flexi-Flow for all posts length used (5 mm = 8 mm = 10 mm). Mean retention values of the groups luted with zinc phosphate showed a statistically significant difference (P<.001) for the different post lengths (10 > 8 > 5 mm). Parallel-sided ParaPost dowels demonstrated a higher mean retention than tapered Dentatus dowels (P<.001). CONCLUSION: In this study, Flexi-Flow reinforced composite resin cement compensated for the reduced length of shorter parallel-sided ParaPost and tapered Dentatus dowels.     

Comparison of titanium dowel retention using four different luting agents

STATEMENT OF PROBLEM: Luting material, surface properties, and loading conditions affect the retention of prefabricated dowels to varying degrees. PURPOSE: The purpose of this study was to evaluate the effect of roughening of the dentinal walls and artificial aging on the retention of prefabricated tapered titanium dowels, using 4 different luting materials. MATERIAL AND METHODS: One-hundred twenty-eight single-rooted teeth were selected, the coronal aspect of each tooth was removed, and the remaining root received endodontic therapy. All specimens were divided into 4 groups (n=32). Dowel spaces were prepared to a depth of 10 mm using ISO 90 rotary cutting instruments. Tapered titanium dowels were luted with the following luting materials: zinc-phosphate cement (Harvard cement), glass-ionomer cement (Ketac Cem EasyMix), resin composite luting agent (Panavia 21) with autopolymerizing dentin primer (ED-Primer), or a self-adhesive composite luting agent (RelyX Unicem). Both composite luting agents were used without acid etching of the canal dentin. Each luting agent was used under 2 conditions: in 1 subgroup (n=16) the dentinal walls were not roughened, and in the other subgroup (n=16), walls were roughened with a diamond rotary cutting instrument. Eight specimens from each subgroup were stored in water at 37 degrees C for 3 days; the other 8 specimens were stored for 150 days and subjected to simulated aging conditions using 37,500 thermal cycles (5 degrees C/55 degrees C) and 300,000 mechanical loading cycles with 30 N. Dowel retention (N) was measured using a universal testing machine with a crosshead speed of 2 mm/min. Data were analyzed using 2- and 3-way ANOVAs and the Tukey HSD test (alpha=.05). RESULTS: The dislodged dowels were examined microscopically to evaluate mode of failure. The nonroughened dentinal walls showed no significant differences between the different luting agents. Roughening the dentinal walls increased the retention significantly for all groups. This increase was significantly higher for the resin composite groups (P=.0001). Storage for 150 days with thermal cycling and mechanical loading caused a significant decrease in dowel retention (P=.001). The failure mode was purely adhesive at the luting material-dentin interface for all dowels cemented in nonroughened root canals. A mixed failure mode, adhesive at the luting material-dentin interface and cohesive in the luting material, was observed for dowels cemented in roughened root canals. CONCLUSION: Roughening the dentinal walls and the use of resin luting cements provided statistically significant increases in dowel retention values.     

Influence of Er:YAG laser irradiation on apical sealing of four different sealers

The sealing of the root canal system is of fundamental importance for successful endodontic treatment. To obtain an adequate apical seal, many factors must be considered such as the presence of smear layer and the sealer applied. After canal preparation, this layer must be removed because it prevents close contact between the dentinal walls and the sealing material. The goal of this study was to evaluate the sealing ability of four different sealers after smear layer removal with either 17% EDTA-T irrigation or Er:YAG laser irradiation of 46 teeth. The canals were sealed with four different sealers: Sealapex, Ketac Endo, AH Plus and N-Rickert. The method for smear layer removal did not influence apical sealing. AH Plus and N-Rickert allowed less dye leakage when compared to Sealapex and Ketac Endo.     

Diametral tensile strength of four composite resin core materials with and without centered fiber dowels

This study evaluated the diametral tensile strength of composite resin core materials with and without fiber dowels. Eight groups were established (n = 20), four with composite resins and four with fiber dowels. Samples were tested using a universal testing machine and evaluated using scanning electron microscopy. One-way ANOVA and a Tukey B-rank order test (P = 0.05) indicated that the tensile values of two of the four composite resins decreased significantly when their matching fiber dowels were introduced.     

The smear layer in endodontics – a review

Root canal instrumentation produces a layer of organic and inorganic material called the smear layer that may also contain bacteria and their by-products. It can prevent the penetration of intracanal medicaments into dentinal tubules and influence the adaptation of filling materials to canal walls. This article provides an overview of the smear layer, focusing on its relevance to endodontics. The PubMed database was used initially; the reference list for smear layer featured 1277 articles, and for both smear layer dentine and smear layer root canal revealed 1455 publications. Smear layer endodontics disclosed 408 papers. A forward search was undertaken on selected articles and using some author names. Potentially relevant material was also sought in contemporary endodontic texts, whilst older books revealed historic information and primary research not found electronically, such that this paper does not represent a 'classical' review. Data obtained suggests that smear layer removal should enhance canal disinfection. Current methods of smear removal include chemical, ultrasonic and laser techniques – none of which are totally effective throughout the length of all canals or are universally accepted. If smear is to be removed, the method of choice seems to be the alternate use of ethylenediaminetetraacetic acid and sodium hypochlorite solutions. Conflict remains regarding the removal of the smear layer before filling root canals, with investigations required to determine the role of the smear layer in the outcomes of root canal treatment.     

Morphological aspects of the resin-dentin interdiffusion zone with different dentin adhesive systems.

Cross-sections of resin-dentin interfaces were etched with an argon-ion beam to make their substructure detectable by scanning electron microscopy. The dentin adhesive systems were categorized morphologically into three groups, and an attempt was made to clarify their adhesive mechanism. The first group of products removed the smear layer. The argon-ion bombardment clearly disclosed a hybrid or resin-impregnated dentin layer. It is hypothesized that conditioning with acidic or chelating agents demineralized the dentin surface-layer to a certain depth, leaving behind a collagen-rich mesh-work. Hydrophilic monomers are then believed to alter this collagen-fiber arrangement in a way that facilitates penetration of the adhesive resin, resulting in a mechanical, intermingled link between collagen and the adhesive resin. The second group preserved the smear layer. In this case, the dentinal tubules were obliterated with globular particles at their orifices and remained patent underneath these smear plugs. This type of adhesive system aims at the incorporation of the smear layer into the hydrophilic monomers, which have an affinity for the organic and/or inorganic components of the underlying dentin. Finally, a third, small group only partly dissolved the smear layer, creating a thin resin-impregnated dentin layer and a resin-impregnated smear plug. This study clearly showed that the application of recent adhesive systems induced structural changes in the dentin surface morphology, creating a retentive interface, called the inter-diffusion zone, between the deep, untouched dentin layers and the composite filling material. This resin-dentin interdiffusion zone offers bonding sites for copolymerization with the resin composite and, concurrently, might have protective potential for the pulp tissues.     

Vertical root fractures in endodontically treated teeth--Part II: Etiology and prevention

The etiologic factors for vertical fractures in endodontically treated teeth are predisposing factors, such as loss of tooth material, anatomy of the susceptible teeth, moisture loss, previous dentinal cracks, and loss of bone support; and iatrogenic factors, such as excessive removal of radicular dentin as a result of endodontic and prosthetic procedures and improper selection of dowels. Identification of susceptible teeth and roots, proper selection and cementation of dowels, and avoidance of excessive force during condensation of gutta percha and in removal of tooth structure during endodontic and prosthetic procedures, are all measures that can be taken to prevent root fractures.