Degradation in the fatigue strength of dentin by cutting, etching and adhesive bonding

Abstract

The processes involved in placing resin composite restorations may degrade the fatigue strength of dentin and increase the likelihood of fractures in restored teeth.

Objective

The objective of this study was to evaluate the relative changes in strength and fatigue behavior of dentin caused by bur preparation, etching and resin bonding procedures using a 3-step system.

Methods

Specimens of dentin were prepared from the crowns of unrestored 3rd molars and subjected to either quasi-static or cyclic flexural loading to failure. Four treated groups were prepared including dentin beams subjected to a bur treatment only with a conventional straight-sided bur, or etching treatment only. An additional treated group received both bur and etching treatments, and the last was treated by bur treatment and etching, followed by application of a commercial resin adhesive. The control group consisted of “as sectioned” dentin specimens.

Results

Under quasi-static loading to failure there was no significant difference between the strength of the control group and treated groups. Dentin beams receiving only etching or bur cutting treatments exhibited fatigue strengths that were significantly lower ( p ≤ 0.0001) than the control; there was no significant difference in the fatigue resistance of these two groups. Similarly, the dentin receiving bur and etching treatments exhibited significantly lower ( p ≤ 0.0001) fatigue strength than that of the control, regardless of whether an adhesive was applied.

Significance

The individual steps involved in the placement of bonded resin composite restorations significantly decrease the fatigue strength of dentin, and application of a bonding agent does not increase the fatigue strength of dentin.

Introduction

Resin composites are now the primary material for tooth cavity restorations . But there is growing concern that bonded composite restorations have higher failure rates than their predecessors . The three most common forms of failure are reportedly secondary caries, marginal degradation and fracture (including either the restorative material, the supporting tooth tissues or both) . Although not the most common form of failure, tooth fracture is potentially the most detrimental as it more commonly results in complete tooth loss. Teeth without restorations generally do not fail by fracture, which raises an important question. Does tooth fracture occur due to an increase in stress within restored teeth, or from defects introduced within the hard tissue foundation by the restorative process and subsequent fatigue?

In comparison to materials of the past, the placement of composite restoratives is complex . As such, there are a number of steps that could inadvertently cause the introduction of defects within the tooth structure. For example, the excavation of demineralized tissue involves material removal, and an interaction between the cutting tools and hard tissue under dynamic conditions. Surface defects introduced during machining/grinding of brittle materials are extremely detrimental, and often lead to a reduction in strength . The introduction of defects within hard tissues could diminish their structural integrity , thereby reducing durability of the restoration and increasing the likelihood of tooth fracture.

Past investigations have evaluated the material removal processes in cutting of hard tissues and the resulting surface integrity . For instance, carbide and diamond abrasive bur preparations were found to introduce cracks during cutting of enamel, whereas the same processes were not found to cause damage while cutting dentin . Similarly, though cracks were not found to result from bur treatments in dentin, Banerjee et al. reported that sono-abrasion and Carisolv gels introduced flaws. One could perceive that the flaws introduced by cutting are small, and that other aspects of the restorative process serve to enlarge the cracks resulting from cutting. Sehy and Drummond introduced Class I or Class II MOD preparations in molars using either coarse diamond burs or an Er:YAG laser. The preparations were followed by placement of a resin composite, bulk curing to maximize interfacial stresses, and then evaluation of the tooth-composite interface via microscopy. Neither of the two cutting processes and subsequent steps resulted in visible microcracks in dentin.

Using measures of strength to assess the presence of damage, Staninec et al. showed that cracks exceeding 100 μm in length were introduced within the dentin by laser preparations under some treatment conditions. That could suggest that flaws introduced with dental burs are too small to see in direct evaluations (i.e., microscopy), but they certainly alter the natural flaw population and distribution within the tissue. As dentin is susceptible to degradation by fatigue small flaws may propagate and facilitate fracture by fatigue crack growth . Indeed, Majd et al. reported that while there was no influence of burs or airjet surface treatments on the strength of dentin under quasi-static loading, both preparations caused a degradation of strength when assessed by cyclic loading. That study did not consider other steps used in the placement of composite restorations (e.g., etching or adhesive bonding), or that flaws introduced by cutting operations may be removed by subsequent etching. Despite the importance of this topic to restored tooth integrity, this area of investigation has received limited attention.

The primary objective of this investigation was to evaluate the reduction in quasi-static strength and fatigue resistance of dentin resulting from the steps involved in preparing cavities and placement of resin-composite restorations. The null-hypothesis to be tested was that etching and application of a resin adhesive in the use of 3-step (etch-and-rinse) bonding systems, has no influence on the fatigue strength of dentin, regardless of whether or not the tissue has been prepared by bur cutting.

Materials and methods

Caries-free third molars were obtained from participating dental practices in Maryland according to a protocol approved by the Institutional Review Board of the University of Maryland Baltimore County (Approval Y04DA23151). All teeth were from donors between 18 ≤ age ≤ 25 years old. The teeth were maintained in Hanks Balanced Salt Solution (HBSS) with 0.2% sodium azide as an antimicrobial agent at 4 °C, then cast in a polyester resin foundation and sectioned using a highspeed grinder (Chevalier Smart-H818II, Chevalier Machinery, Santa Fe Springs, CA, USA) and diamond abrasive slicing wheels (#320 mesh abrasives) with water-based coolant bath. Primary sections were made in the bucco-lingual plane, and secondary sectioning was performed to obtain beams as shown in Fig. 1 (a) . The beams were prepared with width of 1.5 mm and thickness of either 0.5 or 0.65 mm, depending on whether a bur treatment was performed. Each of the beams was inspected; those with pulp horn intrusions, enamel end-caps or other non-uniformities were discarded.

Fig. 1
Specimen preparation and flexure loading of the dentin beams. (a) Location of the crown in which the specimens were obtained and the treatments. The dentin tubules are oriented perpendicular to the beam length (along the y axis). After sectioning, the treated controls were subjected to either bur treatment (BT) or etching (ET). Thereafter, the treated groups were comprised of bur treatment and etching (BT + ET), and bur treatment, etching and application of the primer and adhesive (BT + ET + Ad). All treatments were conducted on the surface facing the DEJ. (b) Nominal specimen geometry and flexure loading configuration for both monotonic and fatigue loading. All dimensions in millimeters. The beams were loaded with the treated surfaces subjected to tension.

Five different groups of beams were prepared including a nominally “flaw-free” control group that was evaluated directly as-sectioned, and a total of four treatment groups. Two of the treated groups received a single surface preparation, and two additional treated groups received a combination of preparations. One of the treated groups was subjected to a bur treatment (i.e., BT) in which cutting was performed using a 6-flute tungsten carbide straight fissure bur (Model FG 57, SS White, Lakewood NJ, USA) as shown in Fig. 1 (b). The cutting was performed using a commercial air turbine (Midwest Quiet Air-L High Speed Handpiece, Dentsply, York, PA, USA) and with water spray irrigation. Material was removed from one surface only ( Fig. 1 (b)) in three equal passes for a total depth of material removal of 0.15 mm; the final beam thickness after material removal was 0.5 mm. The appropriate depth of cut was determined by preliminary tests in which a dentist made finishing passes on beams and controlled the depth of cut by tactile sense. Then the resulting depth of cut was measured, and used in controlled cutting operations performed by attaching the handpiece to a miniature milling machine (Dyna Mechtronics, Model Dynamyte 2400 CNC milling machine, San Jose, CA) that provided controlled feed of the bur across the dentin specimens. That process ensured that a realistic depth of cut was used in the bur preparations and that cutting was performed uniformly, and without development of surface craters that would interfere with the flexure loading. A new bur was used after every 10 specimens to ensure that the bur was sharp. The sequential specimens obtained from each bur (i.e., 1st, 2nd, etc.) were distributed randomly for testing within the low and high cycle regimes of the appropriate treatment groups. The total time involved in treatment (involving preparation and cutting) was less than 5 min. The second treated group received an acid-etching treatment (i.e., ET) after the diamond abrasive sectioning. Etching was performed with a 37.5% phosphoric acid gel (Kerr Gel Etchant, Kerr Co., Lot #4574966). The gel was distributed on one surface of the beam for 15 s, and then removed by rinsing with distilled water for 15 s as recommended by the manufacturer. After etching and rinsing, the beams were lightly dried by gentle air blowing according to the product instructions.

The two additional treated groups of specimens involved a combination of bur and etching treatments. The first consisted of sectioning using the diamond slicing operation, bur treatment of one surface as described and followed by etching of that surface (i.e., BT + ET) according to the aforementioned conditions. The second group consisted of the combination of treatments (BT + ET), followed by application of a commercial primer and adhesive resin, and is referred to herein as BT + ET + Ad. The dentin primer (OptiBond FL Prime, Kerr Co., Lot # 4346590) was applied to the moist dentin surface by a light scrubbing motion for 15 s using a disposal applicator. After gently air-drying for few seconds, resin adhesive (OptiBond FL Adhesive, Kerr Co., Lot # 4346594) was applied to the primed surface. To ensure a thin and even layer of adhesive resin that would not interfere with the flexure testing, the excess resin was wiped off gently with clean tissue paper. Light curing of the resin was then performed using a quartz-tungsten-halogen light-curing unit (Demetron VCL 401, Demetron, CA, USA) with output intensity of 600 mW/cm 2 and with tip diameter wider than 10 mm. A twenty-second light exposure was overlapped at three different locations (center and both sides) to achieve a sufficient degree of polymerization over the entire dentin beam. The prepared beams were then placed into the HBSS bath at room temperature for 24 h before testing.

The average surface roughness ( R a ) and peak to valley height ( R y ) of the prepared surfaces were assessed using contact profilometry (Model T8000, Hommelwerke, Jena, Germany). Profiles were obtained with direction parallel to the long axis of the beams using a 10 μm diameter probe. This profile orientation characterizes the state of the surface topography resulting from each method and provides a means of assessing the potential influence of surface defects to the fatigue response. The surface roughness parameters were calculated according to the standard ANSI B 48.1 using a traverse length and cutoff length of 4.8 mm and 0.8 mm, respectively. The values of R a and R y were compared using a one-way ANOVA with Tukeys HSD and the critical value (alpha) was set at 0.05. Thereafter, the specimens were returned to a bath of HBSS and soaked for a period of at least two hours for rehydration prior to testing. It is important to note that the final thickness of the beams from all five groups was approximately 0.5 mm after completion of the preparations. Nevertheless, the exact cross-section geometry of each beam was measured and recorded prior to testing for accurate estimation of the bending stress that resulted from the flexure loading.

Quasi-static and cyclic four-point flexure testing was conducted at room temperature (22 °C) within a bath of HBSS using a universal testing system (Model 3200, BOSE ElectroForce, Eden Prairie, MN, USA) and routine methods described elsewhere . Quasi-static flexure was performed under displacement control loading at a rate of 0.06 mm/min ( Fig. 1 (b)) according to previous studies . Fifteen dentin specimens were evaluated from each of the five groups. For the four treated groups of dentin specimens, that surface representing the outermost dentin (closest to the dentin enamel junction) received the treatments. The beams were then oriented in the flexure arrangement such that the treated surface was always subjected to tension (in both quasi-static and cyclic loading experiments). The instantaneous load and load-line displacement were monitored at a frequency of 2 Hz to failure, with the average test requiring slightly less than 5 min. The strength (S) was determined using conventional beam theory Popov in terms of the maximum measured load (P) and beam geometry (b, h; Fig. 1 (b)) according to S = 3Pl/bh 2 , where l is the loading span (l = 2 mm). The flexure strengths were compared using a one-way ANOVA with Tukeys HSD and the critical value (alpha) was set at 0.05.

Cyclic loading experiments were conducted with the same loading arrangement used for quasi-static evaluation ( Fig. 1 (b)) with a stress ratio ( R = min load/max load) of 0.1 and frequency of 5 Hz. These conditions are consistent with previous studies . Each beam was subjected to cyclic loading until failure at a cyclic stress that resulted in failures in between 100 cycles and 1200 kcycles. For specimens that withstood 1200 kcycles without failure, the experiment was discontinued as that is near the apparent endurance limit identified in previous studies . The fatigue life distribution of the specimens that underwent fatigue failure in each group was modeled using a Basquin-type model according to

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='σ=A(N)B’>σ=A(N)Bσ=A(N)B
σ = A ( N ) B

where A and B are the fatigue-life coefficient and exponent, respectively, and were obtained from a regression of the fatigue responses plotted on a log-normal scale. For convenience of comparison, the apparent endurance limit was estimated from the models for a fatigue limit defined at 1 × 10 7 cycles.

The fatigue evaluation required a large number of samples and some results were recruited from previous studies. The control specimens ( N = 75) consisted of a combination of previously reported data and additional specimens that were prepared to validate the fatigue behavior. The bur treated (BT) specimens ( N = 41) consisted of previously reported data and one additional specimen. For the ET ( N = 29) and BT + ET ( N = 25) groups, all of the specimens were prepared specifically for this investigation. The same is true for all specimens of the BT + ET + Ad group ( N = 35). The fatigue strength distributions for the five groups were compared over the defined range of cycles to failure using a Mann Whitney U test with the critical value (alpha) set at 0.05.

The treated surfaces resulting from the bur and etching preparations, as well as adhesive bonding, were analyzed using a Scanning Electron Microscope (SEM: JEOL Model JSM 5600, Peabody MA, USA) in secondary electron imaging mode. The fracture surfaces of representative specimens from all groups were examined using the SEM and optical microscopy to identify flaws, the origin of failure or other features that could be important to the mechanical behavior.

Materials and methods

Caries-free third molars were obtained from participating dental practices in Maryland according to a protocol approved by the Institutional Review Board of the University of Maryland Baltimore County (Approval Y04DA23151). All teeth were from donors between 18 ≤ age ≤ 25 years old. The teeth were maintained in Hanks Balanced Salt Solution (HBSS) with 0.2% sodium azide as an antimicrobial agent at 4 °C, then cast in a polyester resin foundation and sectioned using a highspeed grinder (Chevalier Smart-H818II, Chevalier Machinery, Santa Fe Springs, CA, USA) and diamond abrasive slicing wheels (#320 mesh abrasives) with water-based coolant bath. Primary sections were made in the bucco-lingual plane, and secondary sectioning was performed to obtain beams as shown in Fig. 1 (a) . The beams were prepared with width of 1.5 mm and thickness of either 0.5 or 0.65 mm, depending on whether a bur treatment was performed. Each of the beams was inspected; those with pulp horn intrusions, enamel end-caps or other non-uniformities were discarded.

Fig. 1
Specimen preparation and flexure loading of the dentin beams. (a) Location of the crown in which the specimens were obtained and the treatments. The dentin tubules are oriented perpendicular to the beam length (along the y axis). After sectioning, the treated controls were subjected to either bur treatment (BT) or etching (ET). Thereafter, the treated groups were comprised of bur treatment and etching (BT + ET), and bur treatment, etching and application of the primer and adhesive (BT + ET + Ad). All treatments were conducted on the surface facing the DEJ. (b) Nominal specimen geometry and flexure loading configuration for both monotonic and fatigue loading. All dimensions in millimeters. The beams were loaded with the treated surfaces subjected to tension.

Five different groups of beams were prepared including a nominally “flaw-free” control group that was evaluated directly as-sectioned, and a total of four treatment groups. Two of the treated groups received a single surface preparation, and two additional treated groups received a combination of preparations. One of the treated groups was subjected to a bur treatment (i.e., BT) in which cutting was performed using a 6-flute tungsten carbide straight fissure bur (Model FG 57, SS White, Lakewood NJ, USA) as shown in Fig. 1 (b). The cutting was performed using a commercial air turbine (Midwest Quiet Air-L High Speed Handpiece, Dentsply, York, PA, USA) and with water spray irrigation. Material was removed from one surface only ( Fig. 1 (b)) in three equal passes for a total depth of material removal of 0.15 mm; the final beam thickness after material removal was 0.5 mm. The appropriate depth of cut was determined by preliminary tests in which a dentist made finishing passes on beams and controlled the depth of cut by tactile sense. Then the resulting depth of cut was measured, and used in controlled cutting operations performed by attaching the handpiece to a miniature milling machine (Dyna Mechtronics, Model Dynamyte 2400 CNC milling machine, San Jose, CA) that provided controlled feed of the bur across the dentin specimens. That process ensured that a realistic depth of cut was used in the bur preparations and that cutting was performed uniformly, and without development of surface craters that would interfere with the flexure loading. A new bur was used after every 10 specimens to ensure that the bur was sharp. The sequential specimens obtained from each bur (i.e., 1st, 2nd, etc.) were distributed randomly for testing within the low and high cycle regimes of the appropriate treatment groups. The total time involved in treatment (involving preparation and cutting) was less than 5 min. The second treated group received an acid-etching treatment (i.e., ET) after the diamond abrasive sectioning. Etching was performed with a 37.5% phosphoric acid gel (Kerr Gel Etchant, Kerr Co., Lot #4574966). The gel was distributed on one surface of the beam for 15 s, and then removed by rinsing with distilled water for 15 s as recommended by the manufacturer. After etching and rinsing, the beams were lightly dried by gentle air blowing according to the product instructions.

The two additional treated groups of specimens involved a combination of bur and etching treatments. The first consisted of sectioning using the diamond slicing operation, bur treatment of one surface as described and followed by etching of that surface (i.e., BT + ET) according to the aforementioned conditions. The second group consisted of the combination of treatments (BT + ET), followed by application of a commercial primer and adhesive resin, and is referred to herein as BT + ET + Ad. The dentin primer (OptiBond FL Prime, Kerr Co., Lot # 4346590) was applied to the moist dentin surface by a light scrubbing motion for 15 s using a disposal applicator. After gently air-drying for few seconds, resin adhesive (OptiBond FL Adhesive, Kerr Co., Lot # 4346594) was applied to the primed surface. To ensure a thin and even layer of adhesive resin that would not interfere with the flexure testing, the excess resin was wiped off gently with clean tissue paper. Light curing of the resin was then performed using a quartz-tungsten-halogen light-curing unit (Demetron VCL 401, Demetron, CA, USA) with output intensity of 600 mW/cm 2 and with tip diameter wider than 10 mm. A twenty-second light exposure was overlapped at three different locations (center and both sides) to achieve a sufficient degree of polymerization over the entire dentin beam. The prepared beams were then placed into the HBSS bath at room temperature for 24 h before testing.

The average surface roughness ( R a ) and peak to valley height ( R y ) of the prepared surfaces were assessed using contact profilometry (Model T8000, Hommelwerke, Jena, Germany). Profiles were obtained with direction parallel to the long axis of the beams using a 10 μm diameter probe. This profile orientation characterizes the state of the surface topography resulting from each method and provides a means of assessing the potential influence of surface defects to the fatigue response. The surface roughness parameters were calculated according to the standard ANSI B 48.1 using a traverse length and cutoff length of 4.8 mm and 0.8 mm, respectively. The values of R a and R y were compared using a one-way ANOVA with Tukeys HSD and the critical value (alpha) was set at 0.05. Thereafter, the specimens were returned to a bath of HBSS and soaked for a period of at least two hours for rehydration prior to testing. It is important to note that the final thickness of the beams from all five groups was approximately 0.5 mm after completion of the preparations. Nevertheless, the exact cross-section geometry of each beam was measured and recorded prior to testing for accurate estimation of the bending stress that resulted from the flexure loading.

Quasi-static and cyclic four-point flexure testing was conducted at room temperature (22 °C) within a bath of HBSS using a universal testing system (Model 3200, BOSE ElectroForce, Eden Prairie, MN, USA) and routine methods described elsewhere . Quasi-static flexure was performed under displacement control loading at a rate of 0.06 mm/min ( Fig. 1 (b)) according to previous studies . Fifteen dentin specimens were evaluated from each of the five groups. For the four treated groups of dentin specimens, that surface representing the outermost dentin (closest to the dentin enamel junction) received the treatments. The beams were then oriented in the flexure arrangement such that the treated surface was always subjected to tension (in both quasi-static and cyclic loading experiments). The instantaneous load and load-line displacement were monitored at a frequency of 2 Hz to failure, with the average test requiring slightly less than 5 min. The strength (S) was determined using conventional beam theory Popov in terms of the maximum measured load (P) and beam geometry (b, h; Fig. 1 (b)) according to S = 3Pl/bh 2 , where l is the loading span (l = 2 mm). The flexure strengths were compared using a one-way ANOVA with Tukeys HSD and the critical value (alpha) was set at 0.05.

Cyclic loading experiments were conducted with the same loading arrangement used for quasi-static evaluation ( Fig. 1 (b)) with a stress ratio ( R = min load/max load) of 0.1 and frequency of 5 Hz. These conditions are consistent with previous studies . Each beam was subjected to cyclic loading until failure at a cyclic stress that resulted in failures in between 100 cycles and 1200 kcycles. For specimens that withstood 1200 kcycles without failure, the experiment was discontinued as that is near the apparent endurance limit identified in previous studies . The fatigue life distribution of the specimens that underwent fatigue failure in each group was modeled using a Basquin-type model according to

<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='σ=A(N)B’>σ=A(N)Bσ=A(N)B
σ = A ( N ) B
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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Degradation in the fatigue strength of dentin by cutting, etching and adhesive bonding
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