Degradation in the fatigue strength of dentin by diamond bur preparations: Importance of cutting direction.

The objectives of this investigation were to evaluate the degradation in fatigue strength of dentin by diamond bur preparations and to identify the importance of cutting direction. Three groups of coronal dentin specimens were prepared from unrestored third molars, including a flaw free "control," and two groups that received a diamond bur cutting treatment performed parallel or perpendicular to the specimen length. The specimens were subjected to static or cyclic flexural loading to failure and the results were compared with data for carbide bur cutting. Under static loading diamond bur cutting resulted in significantly lower flexure strength (p ≤ 0.05) than the control for both cutting directions (from 154 to ∼124 MPa). However, there was no significant difference in the strength between the control and carbide bur treated specimens. Similarly, the fatigue strength of the diamond bur treated specimens was significantly lower (p ≤ 0.0001) than that of the control for both cutting directions. Cutting in the perpendicular direction resulted in nearly 60% reduction to the endurance limit (from 44 to 19 MPa). Based on the results, diamond bur cutting of cavity preparations causes a reduction in the fatigue strength of dentin, regardless of the cutting direction. To maintain the durability of dentin, cavity preparations introduced using diamond burs must be performed with appropriate cutting direction and followed by a finishing pass.

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

UNLABELLED: 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.

A blended learning approach to teaching pre-clinical complete denture prosthodontics.

PURPOSE: The current study tested the hypothesis that a blended learning curriculum would develop competency in both pre-clinical laboratory and didactic learning with less direct supervision than in a traditional curriculum, and be positively accepted by both the students and the faculty. With the ability to efficiently deliver course material electronically came the ability to blend the traditional delivery of course material with an asynchronous approach involving online lecture presentations and online laboratory demonstrations. METHODS: The class was divided into two groups with bench space available to accommodate self-paced learning. All audio-visual and written support materials for both didactic and laboratory procedures were provided to the students at the beginning of the semester on a DVD. These materials included online access through the Blackboard Learning System™. RESULTS: Students' performance and satisfaction in the course as well as the supporting faculty response to the course were very favourable to the new blended curriculum. CONCLUSIONS: Making use of the technologically advanced environment, the blended curriculum fostered student-focused learning to develop the didactic and laboratory skills necessary for competency in a pre-clinical setting.

Degradation in the fatigue resistance of dentin by bur and abrasive air-jet preparations.

The objective of this investigation was to distinguish whether the instruments commonly used for cutting dentin cause degradation in strength or fatigue behavior. Beams of coronal dentin were obtained from unrestored 3(rd) molars and subjected to either quasi-static or cyclic flexural loading to failure. The surfaces of selected beams were treated with a conventional straight-sided bur or with an abrasive air jet laden with glass particles. Under monotonic loading, there was no difference in the strength or Weibull parameters obtained for the control or treated beams. However, the fatigue strength of dentin receiving bur and air-jet treatments was significantly lower (p ≤ 0.0001) than that of the control. The bur treatment resulted in the largest overall degree of degradation, with nearly 40% reduction in the endurance limit and even more substantial decrease in the fatigue life. The methods currently used for cavity preparations substantially degrade the durability of dentin.

The reduction in fatigue crack growth resistance of dentin with depth.

The fatigue crack growth resistance of dentin was characterized as a function of depth from the dentino-enamel junction. Compact tension (CT) specimens were prepared from the crowns of third molars in the deep, middle, and peripheral dentin. The microstructure was quantified in terms of the average tubule dimensions and density. Fatigue cracks were grown in-plane with the tubules and characterized in terms of the initiation and growth responses. Deep dentin exhibited the lowest resistance to the initiation of fatigue crack growth, as indicated by the stress intensity threshold (ΔK(th) ≈ 0.8 MPa•m(0.5)) and the highest incremental fatigue crack growth rate (over 1000 times that in peripheral dentin). Cracks in deep dentin underwent incremental extension under cyclic stresses that were 40% lower than those required in peripheral dentin. The average fatigue crack growth rates increased significantly with tubule density, indicating the importance of microstructure on the potential for tooth fracture. Molars with deep restorations are more likely to suffer from the cracked-tooth syndrome, because of the lower fatigue crack growth resistance of deep dentin.

Contributions of microstructure and chemical composition to the mechanical properties of dentin.

The influence of microstructural variations and chemical composition to the mechanical properties and apparent flaw sensitivity of dentin were evaluated. Rectangular beams (N = 80) of the deep and superficial coronal dentin were prepared from virgin 3rd molars; twenty beams of each region were nominally flaw free and the remainder possessed a single "surface flaw" via a Vickers indentation. Mechanical properties were estimated in four-point flexure and examined using Weibull statistics. Fourier Transform Infrared Microspectroscopy in Reflectance Mode (FTIR-RM) was used to quantify the relative mineral to collagen ratios. Results showed that the average flexural strength, and strain and energy to fracture of the deep dentin beams were significantly lower (P < 0.005) than for the superficial dentin. While the deep dentin exhibited the highest mineral/collagen ratio and lowest damage tolerance, there was no significant effect of the surface flaws. Weibull analyses suggest that deep dentin possesses a larger distribution of intrinsic flaw sizes that contributes to the location dependence in strength.

Vertical fracture of root filled teeth restored with posts: the effects of patient age and dentine thickness.

AIM: To determine whether patient age contributed to the fracture resistance of teeth subjected to root canal treatment and post placement. METHODOLOGY: Forty-five single-rooted, single-canal human teeth were mounted, instrumented, obturated and prepared for a post. The teeth were divided into young (18 < or = age < or = 35) and old (60 < or = age) groups and subjected to cyclic loading until fracture; those reaching 200,000 cycles without undergoing failure were then subjected to static loading to fracture. Statistical differences between groups were examined using one-way anovas, and correlations were identified using Pearson's r; significance was established at P < or = 0.05. RESULTS: There was no significant difference between the two age groups in terms of the number of cycles to fracture (P > 0.05) or the load to fracture (P > 0.05). However, there was a significant correlation (P < or = 0.05) between the root fracture resistance and individual age, indicating that the susceptibility to root fracture increases significantly with increasing patient age. Also, the dentine thickness of roots that fractured was significantly less than those that did not (P = 0.04). CONCLUSION: Vertical root fracture of teeth receiving root canal treatment with posts is more likely to occur in the teeth of older patients (60+) and particularly in those with low dentine thickness.

Aging and the reduction in fracture toughness of human dentin.

An evaluation of the crack growth resistance of human coronal dentin was performed on tissue obtained from patients between ages 18 and 83. Stable crack extension was achieved over clinically relevant lengths (0< or = a < or =1mm) under Mode I quasi-static loading and perpendicular to the nominal tubule direction. Results distinguished that human dentin exhibits an increase in crack growth resistance with extension (i.e. rising R-curve) and that there is a significant reduction in both the initiation (K(o)) and plateau (K(p)) components of toughness with patient age. In the young dentin (18< or =age< or =35) there was a 25% increase in the crack growth resistance from the onset of extension (K(o)=1.34 MPa m(0.5)) to the maximum or "plateau" toughness (K(p)=1.65 MPa m(0.5)). In comparison, the crack growth resistance of the old dentin (55< or =age) increased with extension by less than 10% from K(o)=1.08 MPa m(0.5) to K(p)=1.17 MPa m(0.5). In young dentin toughening was achieved by a combination of inelastic deformation of the mineralized collagen matrix and microcracking of the peritubular cuffs. These mechanisms facilitated further toughening via the development of unbroken ligaments of tissue and posterior crack-bridging. Microstructural changes with aging decreased the capacity for near-tip inelastic deformation and microcracking of the tubules, which in turn suppressed the formation of unbroken ligaments and the degree of extrinsic toughening.

Dehydration and the dynamic dimensional changes within dentin and enamel.

OBJECTIVES: The objectives of this study were to quantify the dimensional changes in dentin and enamel during dehydration, and to determine if there are differences between the responses of these tissues from young and old patients. METHODS: Microscopic digital image correlation (DIC) was used to evaluate deformation of dentin and enamel as a function of water loss resulting from free convection in air. Dimensional changes within both tissues were quantified for two patient age groups (i.e. young 18< or =age< or =30 and old 50< or =age) and in two orthogonal directions (i.e. parallel and perpendicular to the prevailing structural feature (dentin tubules or enamel prisms)). The deformation histories were used to estimate effective dehydration coefficients that can be used in quantifying the strains induced by dehydration. RESULTS: Both dentin and enamel underwent contraction with water loss, regardless of the patient age. There was no significant difference between responses of the two age groups or the two orthogonal directions. Over 1h of free convection, the average water loss in dentin was 6% and resulted in approximately 0.5% shrinkage. In the same time period the average water loss in the enamel was approximately 1% and resulted in 0.03% shrinkage. The estimated effective dehydration coefficients were -810microm/m/(% weight loss) and -50microm/m/(% weight loss) for dentin and enamel, respectively. SIGNIFICANCE: The degree of deformation shrinkage resulting from dehydration is over a factor of magnitude larger in dentin than enamel.

On the brittleness of enamel and selected dental materials.

UNLABELLED: Although brittle material behavior is often considered undesirable, a quantitative measure of "brittleness" is currently not used in assessing the clinical merits of dental materials. OBJECTIVE: To quantify and compare the brittleness of human enamel and common dental restorative materials used for crown replacement. METHODS: Specimens of human enamel were prepared from the third molars of "young" (18< or =age< or =25) and "old" (50< or =age) patients. The hardness, elastic modulus and apparent fracture toughness were characterized as a function of distance from the DEJ using indentation approaches. These properties were then used in estimating the brittleness according to a model that accounts for the competing dissipative processes of deformation and fracture. The brittleness of selected porcelain, ceramic and micaceous glass ceramic (MGC) dental materials was estimated and compared with that of the enamel. RESULTS: The average brittleness of the young and old enamel increased with distance from the DEJ. For the old enamel the average brittleness increased from approximately 300 microm(-1) at the DEJ to nearly 900 microm(-1) at the occlusal surface. While there was no significant difference between the two age groups at the DEJ, the brittleness of the old enamel was significantly greater (and up to four times higher) than that of the young enamel near the occlusal surface. The brittleness numbers for the restorative materials were up to 90% lower than that of young occlusal enamel. SIGNIFICANCE: The brittleness index could serve as a useful scale in the design of materials used for crown replacement, as well as a quantitative tool for characterizing degradation in the mechanical behavior of enamel.

Transition behavior in fatigue of human dentin: structure and anisotropy.

The influence of tubule orientation on the transition from fatigue to fatigue crack growth in human dentin was examined. Compact tension (CT) and rectangular beam specimens were prepared from the coronal dentin of molars with three unique tubule orientations (i.e., 0 degrees , 45 degrees and 90 degrees). The CT specimens (N=25) were used to characterize fatigue crack initiation and steady-state cyclic extension, whereas the rectangular beams (N=132) were subjected to 4-pt flexure and used in quantifying the stress-life fatigue response. The transition behavior was analyzed using both the Kitagawa-Takahashi and El Haddad approaches. Results showed that both the fatigue crack growth and stress-life responses were dependent on the tubule orientation. The average Paris Law exponent for crack growth perpendicular (90 degrees) to the tubules (m=13.3+/-1.1) was significantly greater (p<0.05) than that for crack growth oblique (45 degrees) to the tubules (m=11.5+/-1.87). Similarly, the fatigue strength of dentin with 90 degrees tubule orientation was significantly lower (p<0.05) than that for the other two orientations, regardless of the range of cyclic stress. The apparent endurance strengths of specimens with 0 degrees (44MPa) and 45 degrees (53MPa) orientations were nearly twice that of the 90 degrees (24MPa) orientation. Based on these results, human dentin exhibits the largest degree of anisotropy within the stress-life regime and the transition from fatigue to fatigue crack growth occurs under the lowest cyclic stress range when the tubules are aligned with the cyclic normal stress (90 degrees orientation).

A clinical evaluation of 10 percent vs. 15 percent carbamide peroxide tooth-whitening agents.

BACKGROUND: Agents with carbamide peroxide, or CP, in various concentrations are widely prescribed for at-home tooth whitening. It is not clear, however, if the more concentrated gels will whitening teeth to a greater extent, as no controlled clinical trials have been reported. The authors conducted a double-blind study of human subjects to evaluate whether a 15 percent CP tooth-whitening system was more effective than a 10 percent CP system, and to determine if tooth sensitivity increased with use of the higher concentration. METHODS: The authors recruited 57 subjects with maxillary anterior teeth of shade A3 or darker (as gauged against a value-oriented shade guide). The subjects were 18 to 65 years of age and in good general and dental health. After matching the subjects by sex and age, the authors randomly assigned them to either a control group, which used a 10 percent CP whitening agent, or an experimental group, which used a 15 percent CP agent. RESULTS: The results indicated that there was no significant difference in shade change between the groups after one week of treatment (t = 1.455, P = .05), but there was a significant difference at the end of the treatment period (t = 2.303, P < .05), as well as two weeks after treatment concluded (t = 2.248, P < .05). There was no significant difference in sensitivity (t = 1.399, P > .05). CONCLUSIONS: There was a significant difference in color change between the 10 percent CP and 15 percent CP groups at the end of the study period. There was no significant difference in level of tooth sensitivity between the two groups, and the incidence was equal; there was, however, a significant difference in variability of tooth sensitivity between the two groups. CLINICAL IMPLICATIONS: If performed under the careful guidance of a dentist, at-home whitening is an effective treatment, regardless of whether 10 percent CP or 15 percent CP is used. There may be added color change and varying sensitivity with the use of 15 percent CP.

The effect of tooth preparation on microleakage behavior.

Many factors contribute to the microleakage of a restoration. One of the more important is the method of cavity preparation. This study compared the microleakage behavior of composite restorations placed in cavities prepared by different techniques. It also compared and correlated the microleakage data produced by an electrochemical vs a staining technique. Class V cavities were prepared in 48 premolars by four techniques: (1) tungsten carbide bur in a high-speed handpiece followed by acid etching; (2) air abrasion (27 microns Al2O3) followed by acid etching; (3) air abrasion (50 microns Al2O3) and (4) air abrasion (27 microns Al2O3), with n = 12 in each group. All teeth were restored with Prime and Bond 2.1 and Tetric Flow, then thermocycled between 5 degrees and 55 degrees C for 5000 cycles with a one minute dwell at each temperature. After thermocycling, a PVC-covered Cu wire was inserted apically into the pulp chamber of each tooth and sealed into position. Leakage was continuously followed by a conductimetric method for 75 days. The teeth then were immersed in 50% AgNO3 for two hours, rinsed in distilled water for 60 seconds, then placed in a rapid photographic developer solution for two hours, followed by rinsing and sectioning for microscopic examination. Electrochemical data were examined by ANOVA and Newman-Keuls multiple comparison tests, while Kruskal-Wallis and Rank Sum Difference tests were used on the staining evaluations. Spearman's rho test was used to correlate the two test techniques. Electrochemical data for cavities prepared with a bur or air abrasion followed by acid etching prior to restoration showed significantly less (p < or = 0.05) microleakage (mean leakage currents of 1.89 & 1.57 microA, respectively) than teeth prepared with air abrasion alone (mean leakage currents of 3.60 & 3.40 microA, respectively). Rank sum AgNO3 staining data (196 & 242 vs 371 & 368) supported these findings. The correlation between the electrochemical and staining data was significant (p < or = 0.05) for all four groups of test specimens.

Gingivitis in children with malnutrition.

The purpose of the current study was to compare the prevalence and severity of gingivitis and plaque among 291 well- and malnourished children between the ages of 4 to 7 years. Using the National Center for Health Statistics criteria, the children were identified as normal (well nourished), having a height-of-age > 95% of standard. Selected primary teeth of each child were assessed using the Plaque Index (PlI) and the Modified Gingival Index (MGI). The results demonstrated a 100% prevalence of plaque and gingivitis. Also, there was no significant difference in the PlI and MGI in the well-nourished and malnourished groups, nor between males and females. However, there was a trend in the well-nourished group to less plaque and gingivitis when broken down into different stature-by-age percentiles.

Indentation damage and mechanical properties of human enamel and dentin.

Understanding the mechanical properties of human teeth is important to clinical tooth preparation and to the development of "tooth-like" restorative materials. Previous studies have focused on the macroscopic fracture behavior of enamel and dentin. In the present study, we performed indentation studies to understand the microfracture and deformation and the microcrack-microstructure interactions of teeth. It was hypothesized that crack propagation would be influenced by enamel rods and the dentino-enamel junction (DEJ), and the mechanical properties would be influenced by enamel rod orientation and tooth-to-tooth variation. Twenty-eight human third molars were used for the measurement of hardness, fracture toughness, elastic modulus, and energy absorbed during indentation. We examined the effect of enamel rod orientation by propagating cracks in the occlusal surface, and in the axial section in directions parallel and perpendicular to the occlusal surface. The results showed that the cracks in the enamel axial section were significantly longer in the direction perpendicular to the occlusal surface than parallel. The cracks propagating toward the DEJ were always arrested and unable to penetrate dentin. The fracture toughness of enamel was not single-valued but varied by a factor of three as a function of enamel rod orientation. The elastic modulus of enamel showed a significant difference between the occlusal surface and the axial section. It is concluded that the cracks strongly interact with the DEJ and the enamel rods, and that the mechanical properties of teeth are functions of microstructural orientations; hence, single values of properties (e.g., a single toughness value or a single modulus value) should not be used without information on microstructural orientation.

Clinical evaluation of a new 10% carbamide peroxide tooth-whitening agent.

NUPRO Gold Tooth Whitening System was evaluated for efficacy according to the proposed ADA guidelines for acceptance. Sixty participants with discolored anterior teeth participated in a 14-day, double-blind, clinical trial. The participants were matched for age, gender, and oral health status and were given either a placebo gel without the active agent or the NUPRO Gold active gel, which they wore in a custom-fabricated mouth guard for home use. The shade of each participant's maxillary anterior teeth was evaluated using a value-oriented Vita Lumin Vacuum Shade Guide before the study. The same shade guide was used to determine shade changes. Time of use of the agent and potential side effects, such as tooth and gingival hypersensitivity and tissue irritation, were assessed at all recall examinations and were recorded by participants in daily diaries. The average shade change for the placebo users was less than one shade. The average shade change for the NUPRO Gold users was 6.96 shades. Tooth hypersensitivity varied from none to severe. Tissue irritation was minimal. The results of these evaluations indicate that NUPRO Gold is effective as a tooth-whitening system, when administered properly under the supervision of a dentist, with commonly reported side effects of transient tooth sensitivity and minimal gingival sensitivity. Little or no change in tissue health was noted. This study was supported by Dentsply Preventive Care (York, Pennsylvania).

Carbon dioxide laser for de-epithelialization of periodontal flaps.

Regeneration of mineralized and soft connective tissue components of the attachment apparatus is the main goal in the treatment of periodontal diseases. Often, apical migration of epithelium (long junctional epithelium) effectively prevents the formation of bone and connective tissue attachment after periodontal surgery. The purpose of the present study was to compare conventional periodontal surgery combined with carbon dioxide laser and conventional periodontal surgery alone with respect to epithelial elimination and degree of necrosis of mucoperiosteal flaps. After signing a consent form, five patients with at least two comparable bilateral periodontal defects needing pocket elimination surgery participated in this study. The investigators randomly divided each side into test and control sites. Each patient received oral hygiene instruction and initial therapy prior to surgery. At surgery, the test site received a sulcular incision and carbon dioxide laser de-epithelialization of the outer and inner aspects of the flap. The control group received reverse bevel incision only. The surgeon performed open flap debridement on all teeth. At the time of surgery, the surgeon did a biopsy of each site and submitted specimens for histologic evaluation. A matched pairs t-test was used to analyze the data. The results show significant differences between the carbon dioxide laser and reverse bevel incision with respect to sulcular (P < or = 0.025) and gingival (external) (P < or = 0.01) flap surface epithelial elimination and tissue necrosis (P < or = 0.005). These results should be replicated with a larger number of subjects. The carbon dioxide laser eliminated sulcular and gingival (external) epithelium without disturbing underlying connective tissue. This finding supports the concept that the carbon dioxide wavelength has little or no effect on tissues beyond the target. However, neither laser nor blade eliminated all the epithelium. Researchers observed chronic inflammation in the control and test sites, with a predominance of plasma cells. Lining the sulcular and gingival (external) lased areas, investigators found coagulation necrosis covered by fibrin and coagulated blood. The laser appears to effectively remove epithelium at the time of surgery; however, future long-term, well-controlled quantitative histologic studies are needed to evaluate the effect of repeated carbon dioxide laser de-epithelialization of the gingival (external) surface of mucoperiosteal flaps at intervals during the healing period.

Ten-year clinical and microscopic evaluation of resin-bonded restorations.

A clinical recall was conducted for 103 patients who were functioning with resin-bonded restorations for approximately 10 years. The purpose of this study was to evaluate marginal adaptation of the cast metal retainers. Following clinical evaluations, replicas of incisal and occlusal margins were examined in the scanning electron microscope to determine marginal separation and microscopic patterns of the resin composite interface between metal and tooth. Anterior abutments exhibited better clinical adaptation and fewer voids and debonds than did posterior abutments. Larger microscopic marginal separations were associated with those retainers that had clinically detectable voids or defects.