Enamel subsurface damage due to tooth preparation with diamonds

Heat generated during dental treatments affecting intrapulpal temperature: a review

INTRODUCTION

Heat is generated and transferred to the dentine-pulp complex during various dental procedures, such as from friction during cavity preparations, exothermic reactions during the polymerisation of restorative materials and when polishing restorations. For in vitro studies, detrimental effects are possible when intra-pulpal temperature increases by more than 5.5°C (that is, the intra-pulpal temperature exceeds 42.4°C). This excessive heat transfer results in inflammation and necrosis of the pulp. Despite numerous studies stating the importance of heat transfer and control during dental procedures, there are limited studies that have quantified the significance. Past studies incorporated an experimental setup where a thermocouple is placed inside the pulp of an extracted human tooth and connected to an electronic digital thermometer.

METHODS

This review identified the opportunity for future research and develop both the understanding of various influencing factors on heat generation and the different sensor systems to measure the intrapulpal temperature.

CONCLUSION

Various steps of dental restorative procedures have the potential to generate considerable amounts of heat which can permanently damage the pulp, leading to pulp necrosis, discoloration of the tooth and eventually tooth loss. Thus, measures should be undertaken to limit pulp irritation and injury during procedures. This review highlighted the gap for future research and a need for an experimental setup which can simulate pulp blood flow, temperature, intraoral temperature and intraoral humidity to accurately simulate the intraoral conditions and record temperature changes during various dental procedures.

Novel Fluoride- and Chloride-containing Bioactive Glasses for Use in Air Abrasion

OBJECTIVES

To investigate the degradation, fluorapatite formation, biological safety and cutting efficiency on dentine of the mixed fluoride- and chloride-containing bioactive glasses (BGs).

METHODS

Two series of mixed fluoride- and chloride-containing glasses (GPFCl and GPF2.3Cl series) were synthesized using a melt-quench method. Glass transition temperature (T_g) and the bioactivity in term of glass degradation and fluorapatite formation were evaluated in Tris buffer solution. The cutting efficiency of the powdered BGs (GPF2.3Cl series) on dentine via air abrasion was investigated using white light profilometry and scanning electron microscope. The cytotoxicity of GPF2.3Cl series on human periodontal ligament stem cells (hPLSCs) and oral fibroblasts (OFB) were examined by MTT.

RESULTS

These BGs are highly degradable and able to form fluorapatite within 3h of immersion. The formation of CaF₂ was also found in the high fluoride-containing BGs. The faster glass degradation was evidenced in the BGs with higher chloride. A significant reduction of T_g from 790°C to 463°C was seen with increasing in calcium halide content. Air abrasion on dentine using the low and intermediate chloride-containing glasses demonstrates clear depressions, while no depression was found using the high chloride-containing glass. Moreover, the studied BGs showed no cytotoxicity to hPLSCs and OFB.

CONCLUSIONS

The glasses with mixed fluoride and chloride integrate the benefits from the presence of both, showing rapid glass degradation, fast fluorapatite formation, excellent biocompatibility and controllable hardness to provide a selective cutting efficiency on dentine.

CLINICAL SIGNIFICANCE

The developed BGs air abrasive with tunable hardness by varying chloride content can selectively cut different dental tissues. In clinic, a relatively hard BG is of great interest for caries preparation, while a soft glass is attractive for tooth cleaning.

Cutting Efficiency of Welded Diamond and Vacuum Diffusion Technology Burs and Conventional Electroplated Burs on the Surface Changes of the Teeth - An In vitro Study

Context:

Welded diamond and vacuum diffusion technology (WDVDT) burs in comparison to electroplated burs claim to approach the solution of dental hard tissues by increased cutting efficiency, decreasing the overheating of oral tissues and thus reducing the microcracks on the prepared tooth surface.

Aims:

This study aimed to evaluate the cutting efficiency of two different rotary diamond burs used for tooth preparation with their repeated usage on the surface changes of the prepared tooth.

Settings and Design:

This in vitro comparative study evaluated the cutting efficiency and surface changes of the teeth prepared with conventional electroplated burs and WDVDT burs.

Subjects and Methods:

Thirty freshly extracted, healthy, noncarious human premolars were divided into Group A and Group B with 15 each, and were further subdivided into three subgroups depending on the different usage intervals as first, fifth, and tenth (A1–A3 and B1–B3). The surface of each prepared specimen was evaluated quantitatively using a surface profilometer, and qualitative analysis was done using a scanning electron microscope.

Statistical Analysis Used:

Two-way ANOVA and Turkey's multiple post hoc tests were used.

Results:

The mean surface roughness of Groups A1, A2, and A3 was 1.50 ± 0.40, 2.39 ± 0.39, and 2.65 ± 0.65 Ra, respectively. The mean surface roughness of Groups B1, B2, and B3 was 0.76 ± 0.23, 0.92 ± 0.10, and 1.24 ± 0.07 Ra, respectively. The mean surface roughness of the prepared tooth surface was significantly higher in Group A and its subgroups when compared to that of Group B and its subgroups.

Conclusions:

The study results showed that surface roughness was considerably lesser and also had less wear and increased cutting efficiency of tooth preparations using burs made with WDVDT compared to the preparations using conventional electroplated burs.

Three-dimensional non-destructive visualization of teeth enamel microcracks using X-ray micro-computed tomography

Although the topic of tooth fractures has been extensively analyzed in the dental literature, there is still insufficient information about the potential effect of enamel microcracks (EMCs) on the underlying tooth structures. For a precise examination of the extent of the damage to the tooth structure in the area of EMCs, it is necessary to carry out their volumetric [(three-dimensional (3D)] evaluation. The aim of this study was to validate an X-ray micro-computed tomography ([Formula: see text]CT) as a technique suitable for 3D non-destructive visualization and qualitative analysis of teeth EMCs of different severity. Extracted human maxillary premolars were examined using a [Formula: see text]CT instrument ZEISS Xradia 520 Versa. In order to separate crack, dentin, and enamel volumes a Deep Learning (DL) algorithm, part of the Dragonfly's segmentation toolkit, was utilized. For segmentation needs we implemented Dragonfly's pre-built UNet neural network. The scanning technique which was used made it possible to recognize and detect not only EMCs that are visible on the outer surface but also those that are buried deep inside the tooth. The 3D visualization, combined with DL assisted segmentation, enabled the evaluation of the dynamics of an EMC and precise examination of its position with respect to the dentin-enamel junction.

Machinability of enamel under grinding process using diamond dental burrs

Enamel grinding is a critical dental surgery process. However, tooth damage during the process remains a significant problem. Grinding forces, burr wear, and surface quality were characterised in relation to grinding speed, enamel orientation, grinding depth, and burr grit grain size. Results indicated that enamel rod orientation, grinding depth, and grinding speed critically affected enamel grinding. Occlusal surface grinding resulted in significantly higher normal forces, surface roughness, and marginally greater tangential forces than axial surface grinding. Damage to enamel machined surfaces indicated the significant impact of diamond grit size and rod orientation. Burr wear was primarily diamond grit peeling off and breakage. Surface roughness of axial and occlusal sections was largely influenced by grinding speed and diamond grit size. Improving the surface quality of machined enamel surfaces could be realised using fine burrs, reducing the grinding speed and grinding depth, and adjusting the feed direction vertical to the rod orientation. Enamel surface quality and roughness could be improved by reducing brittle failure and circular runout during the grinding process, respectively.

Impact of Fluorosis on the Tensile Bond Strength of Metal Brackets and the Prevalence of Enamel Microcracks

The objective of this in vitro study was to determine the effects of dental fluorosis on the tensile bond strength of metal brackets bonded to human teeth and to evaluate the changes in the tooth enamel surface after debonding. The study sample consisted of 68 recently extracted human upper premolars, which were divided into 2 groups: the fluorosis group (34 fluorosed teeth) and the control group (34 healthy teeth). Identical premolar metal brackets were bonded on the buccal surfaces of the teeth. Both groups were submitted to a tension test using a universal testing machine. The number and length of the enamel microcracks in the buccal surface of each tooth were determined before bracket bonding and after debonding using a stereomicroscope. The percentage adhesive remnant index (PARI) was calculated after debonding. The results showed that the tensile bond strength (TBS) of brackets bonded to fluorosed teeth was 21.08% lower than that of brackets bonded to healthy teeth (p < 0.0001). The length of the enamel microcracks on fluorosed teeth increased by 47.4% after debonding, whereas the control group showed an increase of only 12.6%. The PARI showed lower values for fluorosed teeth in comparison to the control group (p = 0.047). In conclusion, dental fluorosis has a negative impact on tensile bond strength and the length of microcracks formed after bracket debonding.

The effect of ultrafast fiber laser application on the bond strength of resin cement to titanium

The purpose of this study was to investigate the effect of ultrafast fiber laser treatment on the bond strength between titanium and resin cement. A total of 60 pure titanium discs (15 mm × 2 mm) were divided into six test groups (n = 10) according to the surface treatment used: group (1) control, machining; group (2) grinding with a diamond bur; group (3) ultrafast fiber laser application; group (4) resorbable blast media (RBM) application; group (5) electro-erosion with copper; and group (6) sandblasting. After surface treatments, resin cements were applied to the treated titanium surfaces. Shear bond strength testing of the samples was performed with a universal testing machine after storing in distilled water at 37 °C for 24 h. One-way ANOVA and Tukey's HSD post hoc test were used to analyse the data (P < 0.05). The highest bond strength values were observed in the laser application group, while the lowest values were observed in the grinding group. Sandblasting and laser application resulted in significantly higher bond strengths than control treatment (P < 0.05). Ultrafast fiber laser treatment and sandblasting may improve the bond strength between resin cement and titanium.

Fracture Forces of Dentin after Surface Treatment with High Speed Drill Compared to Er:YAG and Er,Cr:YSGG Laser Irradiation

Dental tooth restorative procedures may weaken the structural integrity of the tooth, with the possibility of leading to fracture. In this study we present findings of coronal dentin strength after different techniques of surface modification. The fracture strength of dentin beams after superficial material removal with a fine diamond bur high speed drill hand piece, Er:YAG (2.94 μm, 8 J/cm(2)), and Er,Cr:YSGG (2.78 μm, 7.8 J/cm(2)) laser irradiation slightly above the ablation threshold was measured by a four-point bending apparatus. Untreated dentin beams served as a control. A total of 58 dentin beams were manufactured from sterilized human extracted molars using the coronal part of the available dentin. Mean values of fracture strength were calculated as 82.0 ± 27.3 MPa for the control group (n = 10), 104.5 ± 26.3 MPa for high speed drill treatment (n = 10), 96.1 ± 28.1 MPa for Er,Cr:YSGG laser irradiation (n = 20), and 89.1 ± 36.3 MPa for Er:YAG laser irradiation (n = 18). Independent Student's t-tests showed no significant difference between each two groups (p > 0.05). Within the parameter settings and the limits of the experimental setup used in this study, both lasers systems as well as the high speed drill do not significantly weaken coronal dentin after surface treatment.

Restoration of Severely Compromised Teeth With Modern Operative Techniques

This case report illustrates how to restore severely compromised teeth with direct composite restorations. The size of the restorations presented is often considered by dentists as being a contraindication for direct composites. Hence, the technique is explained step by step, addressing the crucial points.

The prognostic value of visually assessing enamel microcracks: Do debonding and adhesive removal contribute to their increase?

OBJECTIVE

To find a correlation between the severity of enamel microcracks (EMCs) and their increase during debonding and residual adhesive removal (RAR).

MATERIALS AND METHODS

Following their examination with scanning electron microscopy (SEM), 90 extracted human premolars were divided into three groups of 30: group 1, teeth having pronounced EMCs (visible with the naked eye under normal room illumination); group 2, teeth showing weak EMCs (not apparent under normal room illumination but visible by SEM); and group 3, a control group. EMCs have been classified into weak and pronounced, based on their visibility. Metal brackets (MB) and ceramic brackets (CB), 15 of each type, were bonded to all the teeth from groups 1 and 2. Debonding was performed with pliers, followed by RAR. The location, length, and width of the longest EMCs were measured using SEM before and after debonding.

RESULTS

The mean overall width (Woverall) was higher for pronounced EMCs before and after debonding CB (P < .05), and after the removal of MB. Pronounced EMCs showed greater length values using both types of brackets. After debonding, the increase in Woverall of pronounced EMCs was 0.57 µm with MB (P < .05) and 0.30 µm with CB; for weak EMCs, - 0.32 µm with MB and 0.30 µm with CB.

CONCLUSIONS

Although the teeth having pronounced EMCs showed higher width and length values, this did not predispose to greater EMCs increase after debonding MB and CB followed by RAR.

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

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.

On the Mechanics of Fatigue and Fracture in Teeth

Tooth fracture is a major concern in the field of restorative dentistry. However, knowledge of the causes for tooth fracture has developed from contributions that are largely based within the field of mechanics. The present manuscript presents a technical review of advances in understanding the fracture of teeth and the fatigue and fracture behavior of their hard tissues (i.e., dentin and enamel). The importance of evaluating the fracture resistance of these materials, and the role of applied mechanics in developing this knowledge will be reviewed. In addition, the complex microstructures of tooth tissues, their roles in resisting tooth fracture, and the importance of hydration and aging on the fracture resistance of tooth tissues will be discussed. Studies in this area are essential for increasing the success of current treatments in dentistry, as well as in facilitating the development of novel bio-inspired restorative materials for the future.

Hidden contributions of the enamel rods on the fracture resistance of human teeth

The enamel of human teeth is generally regarded as a brittle material with low fracture toughness. Consequently, the contributions of this tissue in resisting tooth fracture and the importance of its complex microstructure have been largely overlooked. In this study an experimental evaluation of the crack growth resistance of human enamel was conducted to characterize the role of rod (i.e. prism) orientation and degree of decussation on the fracture behavior of this tissue. Incremental crack growth was achieved in-plane, with the rods in directions longitudinal or transverse to their axes. Results showed that the fracture resistance of enamel is both inhomogeneous and spatially anisotropic. Cracks extending transverse to the rods in the outer enamel undergo a lower rise in toughness with extension, and achieve significantly lower fracture resistance than in the longitudinal direction. Though cracks initiating at the surface of teeth may begin extension towards the dentin-enamel junction, they are deflected by the decussated rods and continue growth about the tooth's periphery, transverse to the rods in the outer enamel. This process facilitates dissipation of fracture energy and averts cracks from extending towards the dentin and vital pulp.

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.

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.

Topography and presence of a smear layer in deciduous molars prepared with high-speed cutting and ultrasonic abrasion: an in-vitro study

OBJECTIVE

The aim of this in-vitro study was to compare the effect of high-speed cutting (HS) with ultrasonic abrasion (US) concerning the internal topography and the presence of a smear layer in a cavity preparation performed in healthy deciduous molars.

MATERIAL AND METHODS

Seven first deciduous molars were used. Two occlusal cavity preparation were done, one in the medial fossula and another in the distal fossula, which were chosen randomly. One preparation was carried out with a diamond point adapted to the HS system (GI), while a chemical vapor deposition (CVD) point adapted to a US device (GII) was used for the other preparation. Subsequently, all samples (n = 14) were cleaved to observe the inside and then prepared for evaluation using scanning electron photomicroscopy. The internal topography of the prepared cavities was descriptively analyzed. In order to assess the presence of a smear layer, scores were tabulated using the 2000 GMC program and analyzed using the Mann-Whitney test.

RESULTS

Concerning the internal topography, the presence of striae was verified in both groups. In the GI group they were finer, found in a greater number, and with narrower spaces between them. In the GII group, the striae were undulating, similar to the effect of wheels on sand, and with wider spaces between them. As regards the presence of a smear layer, there was no statistically significant difference between the groups (P > 0.05).

CONCLUSIONS

In view of the methodology employed, it may be concluded that cavity preparation with a CVD point in a US abrasion system led to the formation of fewer striae and both devices promoted the marked presence of a smear layer, obstructing dentinal tubuli.

The Quantitative Effect of Diamond Grit Size on the Subsurface Damage Induced in Dental Adjustment of Porcelain Surfaces

Diamond burs with different grit sizes are often applied to adjust ceramic prostheses in restorative dentistry. However, the quantitative influence of diamond grit size on subsurface damage in adjusting ceramic prostheses is unknown. The aim of this study was to investigate and visualize the quantitative effect of diamond bur grit size on subsurface damage in dental adjusting of a feldspar prosthetic porcelain. Diamond burs with coarse (106—125 μm), medium (53—60 μm), and fine (10—20 μm) grit sizes were selected. Dental adjusting-induced subsurface damage was quantitatively investigated with the aid of finite element analysis (FEA) and scanning electron microscopy (SEM). Significant differences in subsurface damage depth were found among the coarse, medium, and fine diamond burs (ANOVA, p < 0.05). Coarse diamond burs induced approximately 6—8 times deeper subsurface damage than fine burs. Diamond grit size is confirmed to be a controlling factor in determining the degree of subsurface damage. Subsurface damage depths also significantly increased with removal rate (ANOVA, p < 0.05). The correlation of the SEM-measured subsurface damage depths and the diamond grit sizes supports the FEA predictions. From a practical standpoint, dental porcelains should be adjusted using smaller diamond grit sizes with lower removal rates to minimize subsurface damage.

Calculation of contraction stresses in dental composites by analysis of crack propagation in the matrix surrounding a cavity

OBJECTIVES

Polymerization contraction of dental composite produces a stress field in the bonded surrounding substrate that may be capable of propagating cracks from pre-existing flaws. The objectives of this study were to assess the extent of crack propagation from flaws in the surrounding ceramic substrate caused by composite contraction stresses, and to propose a method to calculate the contraction stress in the ceramic using indentation fracture.

METHODS

Initial cracks were introduced with a Vickers indenter near a cylindrical hole drilled into a glass-ceramic simulating enamel. Lengths of the radial indentation cracks were measured. Three composites having different contraction stresses were cured within the hole using one- or two-step light-activation methods and the crack lengths were measured. The contraction stress in the ceramic was calculated from the crack length and the fracture toughness of the glass-ceramic. Interfacial gaps between the composite and the ceramic were expressed as the ratio of the gap length to the hole perimeter, as well as the maximum gap width.

RESULTS

All groups revealed crack propagation and the formation of contraction gaps. The calculated contraction stresses ranged from 4.2 MPa to 7.0 MPa. There was no correlation between the stress values and the contraction gaps.

SIGNIFICANCE

This method for calculating the stresses produced by composites is a relatively simple technique requiring a conventional hardness tester. The method can investigate two clinical phenomena that may occur during the placement of composite restorations, i.e. simulated enamel cracking near the margins and the formation of contraction gaps.

A comparison of fatigue crack growth in human enamel and hydroxyapatite

Cracks and craze lines are often observed in the enamel of human teeth, but they rarely cause tooth fracture. The present study evaluates fatigue crack growth in human enamel, and compares that to the fatigue response of sintered hydroxyapatite (HAp) with similar crystallinity, chemistry and density. Miniature inset compact tension (CT) specimens were prepared that embodied a small piece of enamel (N=8) or HAp (N=6). The specimens were subjected to mode I cyclic loads and the steady state crack growth responses were modeled using the Paris Law. Results showed that the fatigue crack growth exponent (m) for enamel (m=7.7+/-1.0) was similar to that for HAp (m=7.9+/-1.4), whereas the crack growth coefficient (C) for enamel (C=8.7 E-04 (mm/cycle)x(MPa m(0.5))(-m)) was significantly lower (p<0.0001) than that for HAp (C=2.0 E+00 (mm/cycle)x(MPa m(0.5))(-m)). Micrographs of the fracture surfaces showed that crack growth in the enamel occurred primarily along the prism boundaries. In regions of decussation, the microstructure promoted microcracking, crack bridging, crack deflection and crack bifurcation. Working in concert, these mechanisms increased the crack growth resistance and resulted in a sensitivity to crack growth (m) similar to bone and lower than that of human dentin. These mechanisms of toughening were not observed in the crack growth response of the sintered HAp. While enamel is the most highly mineralized tissue of the human body, the microstructural arrangement of the prisms promotes exceptional resistance to crack growth.

Vibration characteristics of dental high-speed turbines and speed-increasing handpieces

OBJECTIVES

Vibrations of dental handpieces may contribute to symptoms of hand-arm vibration syndrome in dental personnel and iatrogenic enamel cracking in teeth. However, methods for measuring dental handpiece vibrations have previously been limited and information about vibration characteristics is sparse. This preliminary study aimed to use a novel approach to assess the vibrations of unloaded high-speed handpieces in vitro.

METHODS

Maximum vibration displacement amplitudes of five air turbines and two speed-increasing handpieces were recorded whilst they were operated with and without a rotary cutting instrument (RCI) using a scanning laser vibrometer (SLV).

RESULTS

RCI rotation speeds, calculated from frequency peaks, were consistent with expected values. ANOVA statistical analysis indicated significant differences in vibrations between handpiece models (p<0.01), although post hoc tests revealed that differences between most individual models were not significant (p>0.11). Operating handpieces with a RCI resulted in greater vibrations than with no RCI (p<0.01). Points on the head of the handpiece showed greater vibration displacement amplitudes than points along the body (p<0.01).

CONCLUSIONS

Although no single measurement exceeded 4 microm for the handpieces in the current test setup (implying that these vibrations may be unlikely to cause adverse effects), this study has formed the basis for future work which will include handpiece vibration measurements whilst cutting under clinically representative loads.

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).

Engineering long term clinical success of advanced ceramic prostheses

Biocompatability and, in some applications, esthetics make all-ceramic prostheses compelling choices but despite significant improvements in materials properties and toughening mechanisms, these still have significant failure rates. Factors that contribute to the degradation in strength and survival include material selection and prosthesis design which set the upper limit for performance. However, fabrication operations introduce damage that can be exacerbated by environmental conditions and clinical function. Using all-ceramic dental crowns as an example, experimentally derived models provide insight into the relationships between materials properties and initial critical loads to failure. Analysis of fabrication operations suggests strategies to minimize damage. Environmental conditions can create viscoplastic flow of supporting components which can contribute additional stress within the prosthesis. Fatigue is a particularly challenging problem, not only providing the energy to propagate existing damage but, when combined with the wet environment, can create new damage modes. While much is known, the influence of these new damage modes has not been completely elucidated. The role of complex prosthesis geometry and its interaction with other factors on damage initiation and propagation has yet to be well characterized.

Performance evaluation of a dental handpiece in simulation of clinical finishing using a novel 2DOF in vitro apparatus

This paper reports on the performance evaluation of a dental handpiece in simulation of clinical finishing using a novel two-degrees-of-freedom (2DOF) in vitro apparatus. The instrumented apparatus consisted of a two-dimensional computer-controlled coordinate worktable carrying a dental handpiece, a piezoelectric force dynamometer, and a high-speed data acquisition and signal conditioning system for simulating the clinical operations and monitoring the dental finishing processes. The performance of the dental handpiece was experimentally evaluated with respect to rotational speed, torque, and specific finishing energy under the applied clinical finishing conditions. The results show that the rotational speeds of the dental handpiece decreased by increasing either the depth of cut or the feed rate at a constant clinically applied air pressure and water flowrate. They also decreased when increasing both the tangential and normal finishing forces. The specific finishing energy decreased with an increase in either depth of cut or feed rate, while the finishing torque increased as either the depth of cut or the feed rate was increased. Implications of these results were to provide guidance for proper applications of dental handpieces in clinical practice.

Surface integrity and removal mechanism in simulated dental finishing of a feldspathic porcelain

Dental abrasive finishing of a fine-grained feldspathic porcelain was performed on a computer-assisted apparatus for simulation of a 2-degrees-of-freedom restorative operation with a dental handpiece and a coarse diamond bur of grit size of 106-125 mum. Finishing forces, surface roughness, and morphology were investigated as functions of finishing conditions. The tangential and normal forces were measured using a piezoelectric dynamometer and a data processing system. The results indicated that these forces increased with either the depth of cut or with the feed rate, in the ranges of 0.12-0.31 N and 0.45-1.09 N, respectively. However, an increase in either depth of cut or feed rate affected neither the surface roughness measured using a stylus profilometer nor the morphology observed under a scanning electron microscope. The finished porcelain surfaces were found to consist of the microfracture and chipping areas, ductile removal areas, smeared areas, and debris. Irregular fracture and chipping resulted from the extension of lateral/median cracks; ductile micromachining was attributed to the plastic deformation accompanied by distributed microcracks. It was determined that a combination of the microfracture and ductile micromachining was the primary mechanism for material removal.

Influence of Er:YAG Laser on Cavity Preparation and Surface Treatment in Microleakage of Composite Resin Restorations

OBJECTIVE

The purpose of this study was to assess microleakage on class V cavities prepared by Er:YAG laser with varying surface treatment.

BACKGROUND DATA

There has been little reported research on microleakage at cavities prepared and treated with Er:YAG laser.

METHODS

Fifty preparations using Er:YAG laser or a highspeed handpiece were developed. The surface treatment was performed as follows: Er:YAG laser (group 1); 35% phosphoric acid (group 2); laser + acid (group 3); finishing with low-speed + laser + acid (group 4); conventional preparation developed with a high-speed handpiece + acid (group 5--control). The samples were restored with Single Bond/Z250, thermocycled, isolated, and immersed in a 50% AgNO(3) solution. The restorations were sectioned, and the microleakage was measured in milimeters using a specific computer software. Data were analyzed employing ANOVAand Tukey test.

RESULTS

Statistical analysis showed that group 5 (G5) obtained the lowest results of microleakage (22.05%), and it was statistically similar (p > 0.05) to G1 (37.7%) and different (p < 0.05) from the other groups (G2 = 50.4%; G3 = 43%; G4 = 44.2%). The occlusal margins (22.62%) demonstrated less microleakage (p < 0.05) than the cervical margins (56.32%).

CONCLUSION

The use of Er:YAG laser for cavity preparation and surface treatment negatively influenced the marginal sealing of composite resin restorations.

Cutting characteristics of dental diamond burs made with CVD technology

The aim of this study was to determine the cutting ability of chemical vapor deposition (CVD) diamond burs coupled to an ultrasonic dental unit handpiece for minimally invasive cavity preparation. One standard cavity was prepared on the mesial and distal surfaces of 40 extracted human third molars either with cylindrical or with spherical CVD burs. The cutting ability was compared regarding type of substrate (enamel and dentin) and direction of handpiece motion. The morphological characteristics, width and depth of the cavities were analyzed and measured using scanning electron micrographs. Statistical analysis using the Kruskal-Wallis test (p < 0.05) revealed that the width and depth of the cavities were significantly greater when they were prepared on dentin. Wider cavities were prepared when the cylindrical CVD bur was used, and deeper cavities resulted from preparation with the spherical CVD bur. The direction of handpiece motion did not influence the size of the cavities, and the CVD burs produced precise and conservative cutting.

Stress ratio contributes to fatigue crack growth in dentin

An experimental study of fatigue crack growth in dentin was conducted, and the influence of stress ratio (R) on the crack growth rate and mechanisms of cyclic extension were examined. Double Cantilever Beam (DCB) fatigue specimens were sectioned from bovine molars and then subjected to high cycle fatigue loading (10(5) < N < 10(6)) under hydrated conditions. The evaluation consisted of Mode I loads with stress ratios that ranged from -0.5 to 0.5. The fatigue crack growth rates were measured and used to estimate the crack growth exponent (m) and coefficient (C) according to the Paris Law model. The fatigue crack growth rates for steady-state extension (Region II) ranged from 1E-7 to 1E-4 mm/cycle. It was found that the rate of cyclic extension increased significantly with increasing R, and that the highest average crack growth rate occurred at a stress ratio of 0.5. However, the crack growth exponent decreased with increasing R from an average of 4.6 (R = 0.10) to 2.7 (R = 0.50). The stress intensity threshold for crack growth decreased with increasing R as well. Results from this study suggest that an increase in the cyclic stress ratio facilitates fatigue crack growth in dentin and increases the rate of cyclic extension, both of which are critical concerns in minimizing tooth fractures and maintaining lifelong oral health.

The effects of tubule orientation on fatigue crack growth in dentin

The fracture of restored teeth is a significant obstacle to lifelong oral health. Recent studies suggest that fatigue cracks originate at flaws introduced during cavity preparation and that fatigue crack growth is a principle cause of restored tooth fractures. In this study, the rate of fatigue crack growth in bovine dentin was estimated under mode I cyclic loading. Double cantilever beam (DCB) specimens were obtained from bovine molars and subjected to high cycle fatigue loading (10(5) < N < 10(6)). The fatigue crack growth rates were measured and used to estimate the crack growth exponent and coefficient according to the Paris Law. The average fatigue crack growth exponent was 4.7 +/- 0.6 for crack growth parallel to the dentin tubules, which was significantly larger than 4.3 +/- 0.5 for crack growth perpendicular to the tubules (t-test, CI > 80%). Although the crack growth rates varied considerably, there was no significant dependence on tubule orientation or tubule density. However, specific features of the fracture surfaces and tendencies for crack curving away from the tubules suggested preferential fatigue crack growth perpendicular to the dentin tubules. Results from this study are being used to guide an experimental investigation of fatigue crack growth in human dentin.

Air abrasion: an emerging standard of care in conservative operative dentistry

Air abrasion, in conjunction with microdentistry, has opened up a whole new method of treatment for patients that preserves far more tooth structure than was ever previously possible, with greater patient comfort and superior aesthetics. The resulting preparations and surfaces are also far better prepared to receive and retain bonded restorations compared with traditional methods of preparation. Air abrasion eliminates the needless destruction of sound tooth structure associated with traditional restorative techniques, while leaving the tooth ideally conditioned for the beneficial caries-inhibiting and strengthening properties of bonded restorations.

Strong and macroporous calcium phosphate cement: Effects of porosity and fiber reinforcement on mechanical properties

Because of its excellent osteoconductivity and bone-replacement capability, self-setting calcium phosphate cement (CPC) has been used in a number of clinical procedures. For more rapid resorption and concomitant osseointegration, methods were desired to build macropores into CPC; however, this decreased its mechanical properties. The aims of this study, therefore, were to use fibers to strengthen macroporous CPC and to investigate the effects of the pore volume fraction on its mechanical properties. Water-soluble mannitol crystals were incorporated into CPC paste; the set CPC was then immersed in water to dissolve mannitol, producing macropores. Mannitol/(mannitol + CPC powder) mass fractions of 0, 10, 20, 30, and 40% were used. An aramid fiber volume fraction of 6% was incorporated into the CPC-mannitol specimens, which were set in 3 mm x 4 mm x 25 mm molds and then fractured in three-point flexure to measure the strength, work of fracture, and modulus. The dissolution of mannitol created well-formed macropores, with CPC at 40% mannitol having a total porosity of a 70.8% volume fraction. Increasing the mannitol content significantly decreased the properties of CPC without fibers (analysis of variance; p < 0.001). The strength (mean +/- standard deviation; n = 6) of CPC at 0% mannitol was 15.0 +/- 1.8 MPa; at 40% mannitol, it decreased to 1.4 +/- 0.4 MPa. Fiber reinforcement improved the properties, with the strength increasing threefold at 0% mannitol, sevenfold at 30% mannitol, and nearly fourfold at 40% mannitol. The work of fracture increased by 2 orders of magnitude, but the modulus was not changed as a result of fiber reinforcement. A scanning electron microscopy examination of specimens indicated crack deflection and bridging by fibers, matrix multiple cracking, and frictional pullout of fibers as the reinforcement mechanisms. Macroporous CPCs were substantially strengthened and toughened via fiber reinforcement. This may help extend the use of CPCs with macropores for bony ingrowth to the repair of larger defects in stress-bearing locations.

The influence of three Erbium:YAG laser energies on the in vitro microleakage of Class V compomer resin restorations

OBJECTIVES

To investigate the influence of three Erbium:YAG pulse energies (200 mJ, 240 mJ, and 300 mJ with a 100 mJ dentine finish) on the microleakage of Class V compomer restorations (Compoglass).

DESIGN

In vitro study.

SAMPLE AND METHODS

Sixty-one extracted human premolar teeth were randomly allocated to three groups (according to pulse energy). Each tooth hosted one test cavity prepared with one of the three Er:YAG pulse energies using a pulse repetition rate of 5 Hz and a pulse duration of 250 microseconds, and one control cavity prepared with a conventional diamond bur in a high-speed hand piece. Both cavities were placed at the cervical margin of the tooth and were restored and finished according to the manufacturer's instructions. Each tooth was then stored in 0.12% thymol solution at 37 degrees C for three months, before being thermocycled through water baths between 5 degrees C and 55 degrees C for 240 cycles. Microleakage was assessed using a methylene blue dye penetration technique and was quantified using a score 0 (none) to 4 (to and into the axial wall). The data was analysed using the Kruskal-Wallis and Mann-Whitney U-tests.

RESULTS

Leakage was seen in all groups at both the dentine and enamel margins. There were no statistically significant differences in leakage (P < 0.05) at either the enamel or dentine margins, except for the 240 mJ lased enamel margins. This group performed significantly better than the enamel margins prepared with 200 mJ (P = 0.03) and the 300/100 mJ (P = 0.01) laser energies as well as the conventionally prepared enamel margins (P < 0.001).

CONCLUSION

Cavity preparation with this Erbium:YAG laser did not influence the microleakage of Compoglass restorations adversely. Different pulse energies were required for optimum cavity sealing at the enamel and dentine margins and for different materials.

Cutting efficiency of three diamond bur grit sizes

BACKGROUND

Tooth preparation requires safe, efficient and rapid cutting, and diamond burs routinely are used for extracoronal preparation and gross tooth reduction. Coarser-grit diamond burs often are used for gross tooth reduction, with tooth surface finishes being sacrificed for the presumed greater cutting rates, or CRs, of the coarser diamond burs. The authors compared the CRs of medium-, coarse- and super-coarse-grit diamond burs.

METHODS

The authors used a self-contained dental treatment system with digitally controlled handpiece speed, torque and water flow rate to cut a machinable glass ceramic cutting substrate with medium-, coarse- and super-coarse-grit diamond burs from the same manufacturer under a load of 147.5 grams (0.9 kilonewton at the bur tip) and a coolant flow rate of 22 milliliters per minute. They made three cuts through 13-millimeter bars of the cutting substrate with six diamond burs of each grit size. They determined CRs as the transection time per millimeter and analyzed CR data by one-way analysis of variance and post hoc Scheffé tests.

RESULTS

The authors found no statistically significant difference in CR (P > .05) between the three diamond bur grit sizes for the first (13 mm) cuts. When they compared the three cuts (39 mm total cut length), they found no difference (P > .05) between CRs for coarse- and super-coarse-grit diamond burs, but they did find that the super-coarse-grit diamond burs cut faster than the medium-grit diamond burs (P < .01).

CONCLUSION

Differences in CR for the three diamond bur grit sizes are due to the greater decrease in CR for the medium-grit diamond burs (50 percent) compared with the CRs of the coarse- and super-coarse-grit diamond burs (35 percent and 25 percent, respectively) over the total cutting period.

CLINICAL IMPLICATIONS

Coarser-grit diamond burs may be useful for extensive gross tooth preparations, but dental professionals should be aware of the associated effects of the coarser grit on surface finish, heat generation and enamel damage.

High and low torque handpieces: cutting dynamics, enamel cracking and tooth temperature

OBJECTIVE

The aim of these experiments was to compare the cutting dynamics of high-speed high-torque (speed-increasing) and high-speed low-torque (air-turbine) handpieces and evaluate the effect of handpiece torque and bur type on sub-surface enamel cracking. Temperature changes were also recorded in teeth during cavity preparation with high and low torque handpieces with diamond and tungsten carbide (TC) burs. The null hypothesis of this study was that high torque handpieces cause more damage to tooth structure during cutting and lead to a rise in temperature within the pulp-chamber.

MATERIALS AND METHODS

Images of the dynamic interactions between burs and enamel were recorded at video rate using a confocal microscope. Central incisors were mounted on a specially made servomotor driven stage for cutting with a type 57 TC bur. The two handpiece types were used with simultaneous recording of cutting load and rate. Sub-surface enamel cracking caused by the use of diamond and TC burs with high and low torque was also examined. Lower third molars were sectioned horizontally to remove the cusp tips and then the two remaining crowns cemented together with cyanoacrylate adhesive, by their flat surfaces. Axial surfaces of the crowns were then prepared with the burs and handpieces. The teeth were then separated and the original sectioned surface examined for any cracks using a confocal microscope. Heat generation was measured using thermocouples placed into the pulp chambers of extracted premolars, with diamond and TC burs/high-low torque handpiece variables, when cutting occlusal and cervical cavities.

RESULTS

When lightly loaded the two handpiece types performed similarly. However, marked differences in cutting mechanisms were noted when increased forces were applied to the handpieces with, generally, an increase in cutting rate. The air turbine could not cope with steady heavy loads, tending to stall. 'Rippling' was seen in the interface as this stall developed, coinciding with the bur 'clearing' itself. No differences were noted between different handpieces and burs, in terms of sub-surface enamel cracking. Similarly, no differences were recorded for temperature rise during cavity preparation.

CONCLUSIONS

Differences in cutting mechanisms were seen between handpieces with high and low torque, especially when the loads and cutting rates were increased. The speed increasing handpiece was better able to cope with increased loading. Nevertheless, there was no evidence of increased tooth cracking or heating with this type handpiece, indicating that these do not have any deleterious effects on the tooth.

Surface roughness and bond strength of enamel to composite

OBJECTIVES

The aim of the study was to assess the influence of pretreating enamel by different rotary instruments on shear-bond strength of composite to enamel after the acid etch technique.

METHODS

Ninety human enamel specimens with transversely and longitudinally sectioned prisms were ground flat, and half of the enamel area of each specimen was prepared with one of the eight different rotary instruments: a 120 microns diamond bur, three finishing diamonds (30, 15 and 8 microns), two tungsten carbide finishing instruments (8- and 30-fluted) and two stones (Dura green and Dura white). The other half of the enamel area was left untreated. Enamel roughness was measured by laser-stylus profilometry. Two cylinders filled with the hybrid composite Pertac were adhesively fixed on each enamel surface, one on the prepared side and the other on the untreated side. After seven days of water storage, shear-bond strength was measured with a Universal testing machine.

RESULTS

Rz values of the treated enamel surfaces were between 4.14 and 17.75 microns. The shear-bond strength data varied between 16.8 and 29.4 MPa and were similar for transversely and longitudinally orientated prisms. The different roughness data had no significant influence on shear-bond strength of composite to enamel. Additional SEM investigation showed that the surface geometry caused by the rotary instruments largely persisted after acid etching.

SIGNIFICANCE

With regard to shear-bond strength, there was no advantage in using a special bur for finishing enamel surfaces prior to the acid etch technique. Burs causing extreme roughness should not be used for this purpose.

Indentation creep behavior of a direct-filling silver alternative to amalgam

Amalgam creep has been identified as a key parameter associated with marginal breakdown and corrosion. The aim of this study was to evaluate the time-dependent deformation (creep) of a novel silver filling material as an alternative to amalgam. We made the silver specimens by pressing a precipitated powder at room temperature to a density that can be achieved in clinical hand consolidation. The surface of the silver was either polished or burnished. To examine local contact creep and the effect of surface finishing, we used an indentation creep method in which a Vickers indenter was loaded on the specimen surface at a load of 10 N with dwell times of 5 sec to 6x10(4) sec. We used a bonded-interface technique to examine subsurface creep mechanisms. The flexural strength (mean+/-SD; n = 10) was 86+/-20 MPa for amalgam, 180+/-21 MPa for polished silver, and 209+/-19 MPa for burnished silver-values which are significantly different from each other (family confidence coefficient = 0.95; Tukey's multiple-comparison test). Indentation creep manifested as hardness number decreasing with increased dwell time. With dwell time increasing from 5 sec to 6x10(4) sec, the hardness number of amalgam was reduced by approximately 80%; that of the polished silver and the burnished silver was reduced by only 40%. Subsurface creep in amalgam consisted of the shape change of the alloy particles from spherical to elongated shapes, and the separation of matrix grains from each other, possibly due to grain-boundary sliding. Creep of the polished silver occurred by densification reducing porosity and increasing hardness; that of the burnished silver occurred by the displacement of the burnished layer. These results suggest that, due to creep-induced subsurface work-hardening and densification, the consolidated silver exhibits a higher resistance to indentation creep than does amalgam. The hardness number of silver approaches that of amalgam after prolonged indentation loading.

In vitro milling accuracy and fit of 'natural inlays'

OBJECTIVE

This study aimed to investigate the milling behaviour of natural inlays (NI) dental restorations constructed from sound extracted teeth. This was done by comparing the milling accuracy and fit of NI to those of industrial porcelain inlays (PI), milled in the same way.

METHODS

A calibration pro-inlay was used to mill three NI and three PI. These were cemented in six acrylic cavities reproduced from the calibration mould, using composite luting cement. No etching and bonding were done. After storage for 24 h in water at room temperature the specimens were sliced buccolingually in a standardised way. For each specimen, two sections were photographed under a light-microscope, resulting in enlarged pictures of the sections. An acetate matrix with the measurement points was placed over each picture in a standardised way. The interfaces between inlays and moulds were measured at 13 fixed points per section using computerised image analysis software. The mean vertical- and floor-interfaces were calculated for each picture, and the overall means were found for each group. Confidence intervals were used for comparison of the differences. The profiles of the milled materials were examined using scanning electron microscopy.

RESULTS

There were no differences between NI and PI in the mean interfaces (NI, 102 +/- 8 micrometers; PI, 107 +/- 8 micrometers). Electron microscopy revealed no apparent differences in the profiles of the milled surfaces.

CONCLUSION

These findings indicate that the milling accuracy and the fit of natural inlays and milled porcelain inlays are comparable.