SEARCH · PubMed Health
Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
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.
Explore the source record for details and available documents.
Smear layers are generally present on any dentin surface prepared with cutting instruments and are often the only available substrate for bonding. It is commonly reported that acid removes these layers, but to date there has been no chemical evidence to support this observation. Confocal Raman microspectroscopy was used to investigate changes in the composition and molecular structure of acid-treated smear debris and in situ dentin smear layers. The exposed dentin in human molars was abraded with 600-grit silicon carbide sandpaper. Raman spectra were acquired on the smear debris and collected from the sandpaper before and after treatment with 10% citric acid, 35% H3PO4, or 0.5M ethylenediaminetetraacetic acid (EDTA). The resultant smeared dentin samples were treated with one of the aforementioned reagents, and spectra were acquired at 1.0-microm intervals across the interfaces of the smear layers/demineralized dentin/mineralized dentin. Corresponding specimens were morphologically analyzed with scanning electron microscopy (SEM). The results showed that the composition of the smear debris and the in situ smear layers was a mixture of disorganized collagen and mineral. Spectral changes in the smear debris suggested that the disorganized collagen was denatured by acid treatment. The denatured collagen formed a gelatinous matrix around the mineral in the smear layer, thereby shielding it from the acid. The smear layers were not apparent in the SEM micrographs of acid-etched dentin prepared and processed with conventional techniques. The micro-Raman spectroscopic results presented in this study provide the first direct evidence that partially denatured collagen within smear layers is not removed and that the mineral is only partially removed with acids that represent conventional dentin adhesive etchants.
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.
Smear-layer debris was sonicated from dentine surfaces and trapped on microfilters for microscopy, which showed that the layers are composed, in part, of aggregates of globular subunits approx. 0.05-0.1 micron in diameter. Such globules were also seen in smear layers in situ and on fractured dentine surfaces. The composition of smear layers may reflect the substructure of dentine matrix.
When the root canals are instrumented during endodontic therapy, a layer of material composed of dentine, remnants of pulp tissue and odontoblastic processes, and sometimes bacteria, is always formed on the canal walls. This layer has been called the smear layer. It has an amorphous, irregular and granular appearance under the scanning electron microscope. The advantages and disadvantages of the presence of smear layer, and whether it should be removed or not from the instrumented root canals, are still controversial. It has been shown that this layer is not a complete barrier to bacteria and it delays but does not abolish the action of endodontic disinfectants. Endodontic smear layer also acts as a physical barrier interfering with adhesion and penetration of sealers into dentinal tubules. In turn, it may affect the sealing efficiency of root canal obturation. When it is not removed, the durability of the apical and coronal seal should be evaluated over a long period. If smear layer is to be removed, EDTA and NaOCl solutions have been shown to be effective, among various irrigation solutions and techniques, including ultrasonics, that have been tested. Once this layer is removed, it should be borne in mind that there is a risk of reinfecting dentinal tubules if the seal fails. Further studies are needed to establish the clinical importance of the absence or presence of smear layer.
When dentin is planed by endodontic instruments, a smear layer forms. Whether this layer should be removed is unknown and controversial. This study was conducted to assess the effect of the smear layer on retention of bacteria using an in vitro root canal bacterial colonization model. Canals of 26 extracted human canines were step-back prepared using 2.5% NaOCl. Teeth were then randomly divided into two groups based on the type of high volume final flush: 1-20 ml of sterile saline (0.85% wt/vol) or (2-10 ml of 17% EDTA followed by 10 ml of 2.5% NaOCl which removes smear layer. Streptococcus anginosus (milleri) was cultured in trypticase soy broth supplemented with 0.5% yeast extract at 37 degrees C in 5% CO2. Cells were harvested by centrifugation and resuspended in fresh media. Serial dilutions were performed to achieve inocula of 10(6) colony-forming units in a 30-microliters volume. Teeth were inoculated and incubated for 2 h in 5% CO2 at 37 degrees C. Following incubation, teeth were split and processed for microbiological analysis. Numbers of colonizing bacteria were determined by a spiral-plating system. Enumeration of the numbers of bacteria revealed a reproducible, order of magnitude difference (p = 0.0002) between teeth with smear layer (10(4) colony-forming units) versus teeth without smear layer (10(5) colony-forming units). This suggests that smear layer produced during root canal therapy may inhibit bacterial colonization of root canals. One suggested mechanism is that smear layer may block bacterial entry into dentinal tubules.
The smear layer is a direct consequence of instrumentation of the root canal wall. Hand instrumentation as well as ultrasonic instrumentation produce a smear layer on the canal wall. This smear layer is composed of dentine, remnants of pulp tissue and odontoblastic processes and bacteria in an infected canal. Removal of the smear layer is accomplished by the irrigation of root canals with EDTA (17%) followed by NaOCl (5.25%). Permeability of the dentine is increased by the removal of smear layer. In this way the bacteria within the infected tubuli can be more easily destroyed by an intracanal dressing. Whether the removal of smear layer decreases the apical leakage is uncertain. To establish the clinical consequences from removal or preservation of the smear layer, further research is needed.
OBJECTIVES: The goal of this study is to determine if the smear layer affects the passage of bacteria through or around obturating material as evidenced by penetration of bacteria through and out the canal. Specifically, this study focused on determining the effect of the smear layer on the magnitude of bacterial penetration through the apical foramen. METHODS: Thirty extracted, maxillary central or lateral incisors were collected. Teeth were randomly assigned (10 teeth per group) to three groups: (1) smear layer removed, (2) smear layer present, and (3) negative control. Canal preparation and obturation using lateral condensation, gutta percha, and AH 26 sealer was performed on all of the teeth. Removal of the smear layer was accomplished by rinsing with 17% EDTA. The model systems consisted of an upper chamber attached to the cemento-enamel junction and a lower chamber at the apices of the teeth. Standardized bacterial suspensions containing Fusobacterium nucleatum, Campylobacter rectus, and Peptostreptococcus micros were inoculated into the upper chambers. Models were incubated anaerobically at 37 degrees C. At various times over a 60-day period, samples were taken from the lower chamber and spiral-plated on selective-differential media to determine numbers and types of bacteria. RESULTS: Leakage results were as follows: (1) smear layer present-6/10 leaked; (2) smear layer removed-0/10 leaked; (3) negative control-0/10 leaked. Profiles of bacterial leakage were similar among the groups. F. nucleatum was the predominant microorganism. CONCLUSIONS: This study indicated that removal of the smear layer reduced the leakage of bacteria through the root canal system.
The purpose of this in vitro study was to test the effect of Ketac-Endo (KE) and AH 26 on resistance to root fracture and also to evaluate the effect of smear layer. Seventy-two human maxillary canine teeth were randomly divided into six groups (n = 12) and were prepared using six different methods: group 1: KE, without smear layer (smear -); group 2: KE, with smear layer (smear +); group 3: AH 26, smear (-); group 4: AH 26, smear (+); group 5 (negative control): nonprepared; group 6 (positive control): prepared but unfilled. After storing 1 week in 100% humidity at 37 degrees C, the coronal lingual walls and root canal spaces were lowered 2 mm below the buccal walls of the roots. The samples were placed into acrylic resin blocks so that 10 mm of buccal roots were exposed and were placed in a specially designed steel pad at an angle of 15 degrees to the long axis of the root. A slowly increasing force was applied at the junction of the buccal wall and root canal space until fracture occurred. The force required to fracture each tooth was recorded as kg and statistically analyzed using one-way analysis of variance and Duncan tests. The results indicated that instrumentation of the root canals significantly weakened the tooth structure to fracture (p < 0.05). The canals obturated with either sealer were significantly stronger than roots whose canals were instrumented but not obturated (p < 0.05), and there was no difference between the sealers. The presence or absence of the smear layer did not cause any significant effect on the root fracture resistance of the teeth (p > 0.05).
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.
AIM: To evaluate smear layer removal by different irrigating solutions under ultrasonic agitation. METHODOLOGY: Twenty recently extracted mandibular incisors with a single root canal were divided into four equal groups. Three groups were instrumented using the modified double-flared technique, the fourth remained unprepared. Each group was irrigated with either distilled water, 1.0% sodium hypochlorite alone or associated with 15% EDTAC between each file size. The final group was not instrumented but irrigated with 1.0% sodium hypochlorite and 15% EDTAC. A size 15 file energised by ultrasound was used with small amplitude filing movements against the canal walls in all groups. The teeth were split longitudinally and the roots measured to provide three sections of the same size (cervical, middle and apical). Samples were examined under the scanning electron microscope and assessed for the amount of smear layer by three independent and calibrated examiners. The scoring system ranged from 1 (no smear layer) to 4 (all areas covered by smear layer). Due to the non-parametric nature of the data, Friedman's test was used for statistical analysis. RESULTS: Canal walls were covered with smear layer in the group irrigated with 1% sodium hypochlorite alone and the group irrigated with distilled water. Canals irrigated with 1.0% sodium hypochlorite associated with 15% EDTAC had less smear layer throughout the canal (P < 0.001). There were no statistical differences for the amount of smear layer found on the cervical, middle and apical thirds when each group was analysed separately. CONCLUSIONS: Under ultrasonic agitation, sodium hypochlorite associated with EDTAC removed the smear layer from root canal walls, whereas irrigation with distilled water or 1.0% sodium hypochlorite alone did not remove smear layer.
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.
OBJECTIVE: This study examined, with the use of transmission electron microscopy (TEM), the aggressiveness of three self-etching adhesive systems in penetrating dentin smear layers of different thickness. METHODS: Dentin disks were produced from extracted human third molars. For the control group, the middle dentin surface was cryofractured to create a bonding surface that was devoid of a smear layer. The experimental teeth were polished with wet 600 or 60-grit SiC paper to produce bonding surfaces with thin and thick smear layers. They were bonded using one of the three self-etching systems: Clearfil Mega Bond (Kuraray), Non-Rinse Conditioner and Prime&Bond NT (Dentsply DeTrey) and Prompt L-Pop (ESPE). Bonded specimens were then demineralized and embedded in epoxy resin for TEM examination. RESULTS: For Mega Bond, thin authentic hybrid layers between 0.4-0.5 microm were found. Smear layer and smear plugs were retained as part of the hybridized complex. For Non-Rinse Conditioner/Prime&Bond NT, the authentic hybrid layers were between 1.2-2.2 microm thick. Smear layer and smear plugs were completely dissolved in dentin with thin smear layers, but were partially retained as part of the hybridized complex in those with thick smear layers. For Prompt L-Pop, authentic hybrid layers were 2.5-5 microm thick and smear layer and smear plugs were completely dissolved even in dentin with thick smear layers. SIGNIFICANCE: Contemporary self-etching systems may be classified as mild, moderate and aggressive based on their ability to penetrate dentin smear layers and their depth of demineralization into the subsurface dentin. The more aggressive system completely solubilized the smear layer and smear plugs and formed hybrid layers with a thickness approaching those of phosphoric acid conditioned dentin.