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In these two papers the nature of the smear layer will be considered first. The clinical management will then be discussed, and finally, preparation of the dentine surface to receive the new bonding systems will be considered. A subsequent paper will discuss bonding agents.
The smear layer was first described as a debris layer which is left on all cavity walls following tooth preparation. It is composed of an outer contiguous layer of amorphous instrumentation matrix which covers all cavity walls, and a deeper zone of matrix plugs which obturate the cut tubules. Recent scanning electron microscopic (SEM) studies have characterized the smear layer as mineralized collagen fibers appearing as globules dispersed within an amorphous cutting matrix. Removal of smear plugs increases the outward hydraulic conductance (Lp) of dentinal fluid flow which may lead to dentinal hyperalgesia, bacterial infection and pulp pathosis if left untreated.
The smear layer is a debris-ladden layer, adhering firmly to the hard tissues of the tooth. It plays a major role in the different adhesive systems. An update of the recent knowledge on smear layer will be done in this article. When the carious lesion is removed, a microscopic debris-ladden layer forms at the level on the enamel and the dentine. This layer of materials is extremely adherent to the dental structure, and will come in between the hard dental tissues and the filling materials. In this article different studies were reviewed on the formation, structure and physiology of the smear layer. Consequences of this smear layer's removal on the pulp are discussed. A small study of different adhesive systems is conducted.
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The influence of a smear layer on the adhesion of sealer cements to dentin was assessed in recently extracted human anterior teeth. A total of 120 samples was tested, 40 per sealer; 20 each with and without the smear layer. The teeth were split longitudinally, and the internal surfaces were ground flat. One-half of each tooth was left with the smear layer intact, while the other half had the smear removed by washing for 3 min with 17% EDTA followed by 5.25% NaOCl. Evidence of the ability to remove the smear layer was verified by scanning electron microscopy. Using a specially designed jig, the sealer was placed into a 4-mm wide x 4-mm deep well which was then set onto the tooth at a 90-degree angle and allowed to set for 7 days in 100% humidity at 37 degrees C. This set-up was then placed into a mounting jig which was designed for the Instron Universal Testing Machine so that only a tensile load was applied without shearing or applying preloading forces. The set-up was subjected to a tensile load at a crosshead speed of 1 mm per min. The results show significant differences (p less than 0.001) among AH26, Sultan, and Sealapex, with AH26 being the strongest and Sealapex being the weakest. The only significant difference with regard to the presence or absence of the smear layer was found with AH26, which had a stronger bond when the smear layer was removed.
Smear layers are created on hard tissues whenever they are cut with hand or rotary instruments. This thin (1-2 microns) layer of denatured cutting debris is very tenacious and, in fact, is often the surface to which restorative materials are luted. The solubility characteristics, chemical reactivity and the structure-function relationships of this layer have not yet been well-defined. During creation of the smear layer, cutting debris is forced variable distances into dentinal tubules. These so-called smear plugs, together with the smear layer decrease dentin permeability, dentin sensitivity and surface wetness. Bonding adhesive resins to smear layers appears to limit the theoretical bond strength unless the smear layers are loosened or partially removed. Future research in this area will include the use of surface analytical techniques such as Auger electron spectroscopy and ESCA. These techniques are required because of the thinness of the smear layer. It is clear that the nature of this critical interface between dental materials and cut hard tissues remains largely unknown. This field will provide fertile ground for future research.
The influence of a smear layer on the adhesion of sealer cements to dentin was assessed in recently extracted human anterior teeth. A total of 120 samples was tested, 40 per sealer; 20 each with and without the smear layer. The teeth were split longitudinally, and the internal surfaces were ground flat. One-half of each tooth was left with the smear layer intact, while the other half had the smear removed by washing for 3 min with 17% EDTA followed by 5.25% NaOCI. Evidence of the ability to remove the smear layer was verified by scanning electron microscopy. Using a specially designed jig, the sealer was placed into a 4-mm wide x 4 mm deep well which was then set onto the tooth.
The effect on bond strength of smear layer removal (40% polyacrylic acid or 10% phosphoric acid) versus smear layer conditioning for dentin bonding agents requiring conditioning was compared. Results indicated no difference in shear bond strength for groups in which the smear layer was conditioned or removed with phosphoric acid. Smear layer removal with polyacrylic acid resulted in lower bond strengths between DBAs and dentin than either phosphoric acid removal or conditioning (P less than 0.01). Scanning electron photomicrographs of dentin treated with each dentin bonding agent revealed various surface morphologies.
A smear layer is formed after cavity or root canal preparation. The aim of the present study was to reinforce the dental surface in order to prevent the invasion of foreign irritants, by treating the smear layer with fluoride. Dentinal samples whose surfaces had been washed with water after the formation of a smear layer, and corresponding samples without washing, were examined by the dye penetration test, and the results were compared. Although there was no significant difference between the two groups of samples, dye penetration was suppressed by about 30% in washed samples, whereas the suppression was 20% in unwashed samples. When washed samples were treated with 1.0% SnF2, 10.0% SnF2, 7.5% Na2PO2F, 15% Na2PO2F, APF, 10 mM In(NO3)3, 100 mM TiF3, 50 mM TiF3, or 10 mM TiF3, samples washed after treatment with 1.0% SnF2 showed a dye penetration suppression of about 60% as a whole, in comparison with samples having no smear layer. Hardly any suppression of dye penetration was observed after treatment with other fluorides.
The effect of the removal of the smear layer on coronal leakage of root fillings was studied using two sealer cements. Sixty single-rooted teeth with mature apices were prepared chemomechanically using a modified double-flared technique with non-cutting tipped files and copious irrigation with 2% sodium hypochlorite. The teeth were randomly allocated to four groups of 15 teeth each. Two groups were treated with 40% citric acid to remove the smear layer. Two groups of teeth, one with and one without smear layer, were filled by lateral condensation of gutta-percha with either Tubliseal or a resin-based glass ionomer, Vitrebond. The teeth were stored for 1 week, thermocycled, and the extent of coronal leakage determined for each group after immersion in Indian ink for 90 h. The teeth were demineralized, dehydrated and immersed in methyl salicylate which rendered them transparent. Linear measurement of dye penetration was recorded. The mean depth (+/- SD) of leakage for the groups in which the smear layer was left intact was 4.26 +/- 1.53 mm and 6.83 +/- 1.65 mm for the Vitrebond and Tubliseal, respectively. When the smear layer had been removed, the mean depth of leakage for the Vitrebond group was 1.13 +/- 0.29 mm, and 3.72 +/- 1.23 mm for the Tubliseal group. There was a statistically significant difference in leakage between the groups (P < 0.001). With both sealers, those teeth in which the smear layer had been removed showed less leakage than the specimens in which the smear layer was left intact (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)
In this study, the effect of citric acid and LIV CENERA Liquid (40% poly-acrylic acid) on the smear layer of the dentine surface was investigated. Twelve freshly extracted, non-carious human molar teeth were used and approximately 25 mm2 flat occlusal dentine surfaces were prepared. Citric acid and LIV CENERA Liquid were applied for 10 and 30 seconds to the dentine surfaces. The results of our study indicated that: 1. Conditioning with LIV CENERA Liquid for 10 seconds was not effective in removing the smear layer on the dentinal tubules but application for 30 seconds was partially effective. 2. Although 10 seconds application of citric acid was not effective in removing the smear layer, 30 seconds application removed it completely, but harmful effects were observed on the dentinal tubules. 3. Conditioning with citric acid is more effective than the application of LIV CENERA Liquid in removing the smear layer, but LIV CENERA Liquid in contrast to citric acid application, did not enlarge the dentinal tubules.
Many studies have been made of pulpal reactions to pulp protection (lining) materials have always employed a smear layer between the dentin wall and the lining materials. In this study, the author has attempted to evaluate pulpal response without a smear layer and compared effects of pulp protection (lining) with and without smear layers. Materials and methods. The experiment employed 84 vital human teeth ranging in age from 14 to 54 years. The experimental teeth were divided into 4 groups for the purposes of histopathological studies and scanning electron microscope (SEM) observation. Group C: No EDTA-treatment and lining with gutta-percha temporary material, 10 cases for histopathological study and 4 cases for SEM observation. Group TC: EDTA-treatment and lining with gutta-percha temporary material, 10 cases for histopathological study and 4 cases for SEM observation. Group P: No EDTA-treatment and lining with Protect Cement. 20 cases for histopathological study and 8 cases for SEM observation. Group TP: EDTA-treatment and lining with Protect Cement. 20 cases for histopathological study and 8 cases for SEM observation. After local anesthesia, Black's simple class I cavities were made in the experimental teeth with a diamond instrument mounted on a high-speed air turbine at room temperature with a water spray coolant. In Groups TC and TP only, an aqueous solution of 0.5% EDTA (pH 7.4) 10 ml was applied from cavity floor to dentin for 60 seconds. All cavities were then washed with distilled water, and the dentin was dried with an air stream. The floor of cavities were lined with thin layers of the following materials: Group C and TC, gutta-percha temporary material (G-C Dental Industrial Co., Tokyo, Japan). Group P and TP lining of Protect Cement which is a zinc phosphate cement containing 0.5% paraformaldehyde (Neo Dental Chemical Products, Tokyo, Japan). The restoration of each cavity was then filled with silver amalgam. Experimental teeth were clinically observed for 2 periods: short (about 14 days) and long (about 60 days) after treatment. They were then extracted under local anesthesia and prepared for histopathological study and SEM observation. Results 1. Clinical observations. During the observation period, symptoms occurred as indicated below. Group C: 2 (20%) out of 10 cases Group TC: 3 (30%) out of 10 cases Group P: None out of 20 cases Group TP: 3 (15%) out of 20 cases The only kind of clinical discomfort observed hypersensitivity to cold water. 2. Clinical evaluations Group C: 10 cases (100%) were good.(ABSTRACT TRUNCATED AT 400 WORDS)
The alternating current impedance characteristics of a uni-directional porous glass membrane and serial sections of dentin from unerupted human third permanent molars (with and without smear layers) were measured. The membrane was equilibrated at 20 +/- 1 degrees C in a range of concentrations of unbuffered KCl, and the tooth sections were equilibrated in 0.1 mmol/L unbuffered KCl. Each section's impedance was measured for a 50-mV (rms) alternating potential difference applied over 1 Hz to 65 kHz. By means of complex non-linear least-squares regression analysis, the dc resistance of each specimen was determined. Scanning electron microscopy and subsequent image analysis enabled the cross-sectional area of the pores in the membrane and dentin tubule surface-openings to be measured. The results showed that the measured cross-sectional area of pores in the membrane was in good agreement with that calculated from the measured dc resistance and solution resistivity over the range of electrolyte concentrations. In dentin, the presence of smear layers on the cut surfaces increased the measured resistance. The resistance of the smear layer varied, suggesting that its thickness changed across the surface of the dentin. This was superimposed on regional variations in resistance of smear-free dentin which were found to be related to alterations in the number of tubules present and their cross-sectional area. The resistivities of the dentin tubule contents and smear layer material were generally lower than that of the hydrating electrolyte. It is postulated that this could be due to ions from partial dissolution of the mineral in the dentin.
Endosonic files are prone to constraint; this reduces their efficiency, particularly in the apical third and in curved root canals. Precurving the file may reduce constraint, thereby improving the file's ability to debride. This investigation was undertaken to compare both straight and precurved endosonic files as to debris and smear layer removal. Thirty-six curved root canals were randomly allocated into six groups. Each group was prepared using a step-down technique, then instrumented with either a straight or precurved endosonic file of size 15, 20, or 25. The tooth roots were split and examined under the scanning electron microscope. Blind examinations were made for debris and smear layer removal and evaluation scores were analyzed using a log linear approach. The root canals instrumented with the precurved files had a significantly lower debris score than those prepared with straight files. Precurving did not affect smear layer removal. In conclusion, precurving of files decreased the amount of debris but did not affect smear layer removal.
The morphology and treatment of the smear layer are discussed. Also included are a review of dentin bonding agent classification and chemistry, and a summary of recent bond strength research results. Finally, the current status of dentin bonding agents is discussed.
A Scanning Electron Microscope was used to study the so-called "smear layer" produced in biomechanically prepared root canals. Fine, freshly-extracted incisors were used and the canals instrumented with K-type files and irrigated with 5% NaOCI and 3% H2O2. The structure of the "smear layer" produced was investigated.
According to hydrodynamics, any agent blocking the dentinal tubules reduces the flow of fluids and diminishes hypersensitivity. The properties of the desensitizing agents that sponsor tubular occlusion and the barrier efficiency resulting from the interaction of the smear layer with test materials were examined with the scanning electron microscope and energy-dispersive x-ray microanalysis. Selected dentinal desensitizing was accomplished with burnishing procedures, cavity varnish, calcium hydroxide, and topical fluoride. Subjective evaluations were also recorded clinically after tooth preparation. This investigation indicated that the smear layer did not protect against zinc phosphate cement, and that cavity varnish prevented the formation of the smear plugs. The smear layer and plugs were basically composed of calcium and phosphorus, the major ingredients of dentin.
Exposed dentine may be sensitive to stimuli depending on the patency of the dentinal tubules. Most abrasive elements tend to produce a smear layer which obturates the dentinal tubules. This layer is easily removed by a number of erosive agents, particularly dietary acids. Mouthwashes are increasingly used and largely investigated for possible benefits to dental health. Rarely do studies assess potential detrimental activity. The aim of the present study was to examine effects of mouthwashes on dentine. Smear layers artificially produced on dentine sections were exposed, for time periods ranging from 3 to 300 min, to mouthrinses alone, or with subsequent brushing for 2 min using water or a fluoride toothpaste. Assessments were made by scanning electron microscopy. Of the nine rinses tested, six produced no consistently observable changes compared to water treated controls at any time period with or without brushing. A hexetidine rinse and a fluoride/antiseptic rinse both exposed tubules after exposure times of 2 h and longer, an effect enhanced by post-treatment brushing. A phenolic antiseptic rinse consistently removed the smear layer from specimens, an effect enhanced by brushing such that after 10 min exposure and 2 min brushing many tubules were open at the surface. The results indicate that the use of some mouthrinses could predispose to excessive tooth substance loss and dentine hypersensitivity, particularly if used prior to toothbrushing. There is a need to determine whether the intermittent use of some mouthrinses produces cumulative effects on dentine.