The influence of grain size coating and shaft angulation of different diamond tips on dental cutting.

OBJECTIVES: To evaluate the influence of the grain size coating and shaft angulation of ultrasonic and high-speed diamond burs on the dental cutting effectiveness.
MATERIALS AND METHODS: For the grain size evaluation, cavities were prepared on 40 incisors using high-speed (1092 and 1093F KG Sorensen(®)) and ultrasonic tips (8.2142 and 6.2142-CVDentus(®)). For the shaft angulation evaluation, cavities were prepared on 40 incisors using uniangulated (T1-CVDentus(®)) and biangulated (T1-A CVDentus(®)) ultrasonic tips. The cavities were bisected and examined at ×50 magnification. The width and depth of cavities were measured by Leica QWin software. Kruskal-Wallis non-parametric test was used for analysis.
RESULTS: The grain size did not affect the cutting effectiveness, but the high-speed burs promoted deeper and wider cavities than the ultrasonic tips. The shaft angulation did not affect the cutting effectiveness; both the angulated and biangulated tips had greater cutting efficiency in dentin than in enamel.
CONCLUSIONS: Ultrasonic tips promoted more conservative preparations and seemed promising for cavity preparation.

INTRODUCTION

Since the introduction of the first rotary cutting instruments in dentistry, research has focused on their cutting efficiency, particularly for diamond burs, to improve and develop more effective instruments. In spite of the technological evolution over time, these burs present difficulties. It is hard to automate their production, and they have short lifetimes due to the possibility of losing diamond grains after repeated use and sterilizations.[1-3]

Ultrasound was introduced in dentistry in 1950 and was used for cavity preparations. Although ultrasound instruments reduce noise, vibration and heat, these never became popular for the preparation of cavities, and they were overtaken by the high-speed instruments that demonstrated higher efficiency in cutting tooth tissue.[4]

At the end of the past century, with technological improvements, diamond tips coupled with conventional ultrasound devices emerged. They were marketed under the same brand name as ultrasonic tips, and they have been used for dental cavity preparations. These tips are manufactured using the chemical vapor deposition (CVD) of continuous diamond coatings onto molybdenum shafts, using gases such as methane in an excess hydrogen environment.[5] After physicochemical interactions, a pure diamond film is formed on the surface of the shaft without metallic binder between the crystals, thus providing a cutting surface with thousands of microscopic edges. The CVD technology allows the diamond coating to be strongly adhered to the shaft and to withstand the vibratory oscillations from the ultrasonic cavity preparation.[6]

Ultrasonic diamond tips come in a wide variety of shapes and shaft angulations. The angulations plus the long shafts provide better access and good visibility of the operative field and facilitate the preparation of minimally invasive cavities in areas that are difficult to access without damaging the adjacent teeth.[7] The ultrasonic tips cut more conservatively and have greater longevity than the conventional high-speed diamond tips.[8] Also, during cavity preparation, the water runs over the entire length of the tip shaft, thus providing effective cooling of the tooth with no risk of overheating.

The high-speed air-turbine is one of the most commonly reported triggers of fear and anxiety reactions in children toward dental treatment.[19] Therefore, the use of ultrasound-activated instruments has gained great acceptance among pediatric patients because the operating mode of ultrasound devices reduces the noise, vibration, heat and pressure during cavity preparation and does not cause trauma to the pulp tissue.[10]

Ultrasonic diamond tips could be considered an alternative to the conventional diamond burs used at high speed due to their conservative action[811] and great longevity.[8] Although the conservative feature of the preparations was confirmed by Lima et al.,[12] little information is available about how their cutting efficiency is related to the grain size coating and shaft angulations.

Thus, the aim of this in vitro study was to evaluate the influence of the diamond grain size of the conventional high-speed and ultrasonic tips and shaft angulation of the ultrasonic tips on the dental cutting effectiveness.

MATERIALS AND METHODS

For evaluating the influence of the grain size coating of ultrasonic and high-speed diamond burs on the dental cutting effectiveness, the buccal surfaces of 40 bovine incisor crowns were cut, providing 40 fragments that were assigned to four groups (n = 10), according to the tip used [Table 1]. For evaluating the influence of the shaft angulation of ultrasonic diamond tips on the dental cutting effectiveness, another 40 bovine incisor crowns were cut so that the enamel fragments were obtained from the buccal surfaces, and the dentin fragments were obtained from the proximal surfaces after removal of the overlying enamel layer. The specimens were assigned to four groups (n = 10) according to the dental substrate and shaft angulation [Table 2].

Table 1

Distribution of the groups according to grain size coating of the instruments

Table 2

Distribution of the groups according to dental substrate and shaft angulation of the instruments

The fragments measuring 5 mm in length, 3 mm in depth and 3 mm in width were embedded in self-polymerized acrylic resin (OrtoClass; Clássico Artigos Odontológicos Ltda, São Paulo, SP, Brazil) using 2-cm-diameter plastic rings.

For the grain size evaluation, cavities were prepared by high-speed (1092 and 1093F KG Sorensen®) and ultrasonic tips (8.2142 and 6.2142-CVDentus® – Clorovale Diamantes Ind. Com. Ltda. Epp, São José dos Campos, SP, Brazil). For the shaft angulation evaluation, cavities were prepared using uniangulated (T1-CVDentus®) and biangulated (T1-A CVDentus®) ultrasonic tips.

For standardizing the cavities, an electro-mechanical device, that could be coupled to both a high-speed turbine and an ultrasound appliance and that controlled the speed, time and pressure of the diamond burs against the specimens, was used.[5]

The conventional diamond burs were coupled to a high-speed handpiece turbine (605 C – Kavo SA, Ribeirão Preto, SP, Brazil). For ultrasonic tips, a specific connector coupled to the ultrasound equipment (Profi I AS Ceramic – Dabi Atlante®) was used that allowed the adjustment of the tip. This equipment was set on mode “power II” (50% of the total vibration amplitude of the tip) and “pump I” (minimum irrigation). For both the equipments, the cavities were prepared with 30 sequential movements and a load of 0.012 kgf of the tips against the tooth surface at an approximate speed of 5.3 mm/sec, thus resulting in a 27-sec working time per specimen during preparation.

After preparation, the cavities were cross-sectioned with a cutting machine, and both sides were examined under a stereomicroscope at 50× magnification. The width and depth of the cavity preparations were measured using the Leica QWin image analysis and image processing software (Leica Imaging Systems Ltd., Cambridge, UK) in a blind analysis.

The data were tabulated and analyzed using Kruskal-Wallis non-parametric tests to assess the effect of the tested variables on the width and depth of the cavities. The significance level was set at 5%.

RESULTS

The grain size of the high-speed and ultrasonic diamond tips did not affect the width and depth of the cavities; however, there was statistically significant difference (P > 0.05) among the cavities prepared by high-speed and ultrasonic tips. The high-speed tips produced cavities that were approximately 34% wider and 91% deeper than those produced by ultrasonic tips [Table 3].

Table 3

Width and depth means (±SD) of the cavity (μm) according to grain size coating of the instruments*

The results of the Kruskal–Wallis tests showed no statistically significant differences between the cavities prepared with either angulated or biangulated ultrasonic tips [Table 4]. The shaft angulation of the ultrasonic diamond tips did not affect the width and depth of the cavities; however, both types of tips produced wider and deeper cavities in dentin than in enamel.

Table 4

Width and depth means (±SD) of the cavity (μm) according to dental substrate and shaft angulation of the instruments*

The scanning electron micrographs revealed differences in the sizes and shapes of diamond grains.

DISCUSSION

The characteristics of the shapes and bondings of the diamond grains on the stems of conventional diamond burs[3] and the CVD of the diamond on the molybdenum stem are the factors that are responsible for instrument cutting efficiency.

The cutting efficiency of conventional diamond burs and ultrasonic tips were evaluated by comparing the sizes of the cavities prepared in a standardized protocol. The results showed that the size of the diamond grains did not influence the width or the depth of the cavities when prepared by “regular” conventional high-speed diamond burs with a mean granulation of 126 μm, which are suitable for cutting the tooth structure, and by diamond tips with a mean granulation of 46 μm, which are suitable for polishing. Additionally, no difference was observed in the width and depth of the cavities created by ultrasonic tips suitable for cutting (silver) and finishing (gold), whose granulometry was not reported by the manufacturers. According to Trava-Airoldi,[13] CVD diamond coatings onto molybdenum shafts are formed in layers. Each layer thickness may range from 60 to 80 μm. The diamond is crystalline with grains of around 50 μm. The CVD tip abrasiveness is given by the grain size and by the layers deposited.

The diamond grain size did not affect the cutting efficiency of these tips; however, the width of the cavities prepared by the conventional diamond burs were on average 34% wider than those prepared by ultrasonic tips. Lima et al.[12] reported that the spherical conventional diamond bur created cavities that were 27% wider than that created by the spherical ultrasonic tip. The authors explained that the cut occurs with the impact of the tip against the cavity wall during the oscillatory movement of the active end of the tip. In the established cutting time (27 sec), the ultrasonic tip did not reach its entire lateral diameter, resulting in narrow cavities.

Regarding the depth of the cavities, high-speed diamond burs produced cavities 3.5 times deeper than the ultrasonic tips, which may be related to the type of diamond (grains and continuous film, respectively), the difference between the cutting principle (rotation and oscillation) of the instruments or the penetration resistance offered by dental tissue that could stop the ultrasonic tip vibration. This finding is in accordance with studies which also reported that high-speed conventional diamond burs produced deeper cavities in bovine teeth[8] and human teeth.[12]

Because ultrasonic diamond tips are specially designed for a use that is coupled to ultrasound devices that only have oscillatory movements, they are available as angulated and biangulated tips. Such tips provide better visualization of the operative field and facilitate access to poorly reachable areas in the mouth.

The tip angulation and the ultrasonic action make the active portion of the tip vibrate forward and backward on the same plane as that determined by the shaft angle. According to the manufacturer, the posterior surfaces of the active portion of the CVDentus® tips have greater cutting efficiency. As the lateral surfaces have no impact on the tooth structure, the tip produces a “scaling” movement, thus providing a high-quality finish to the cavity walls. This does not implicate the need for moving the handpiece by traction during cavity preparation because gentle movements of the tip against the tooth surface provide effective cutting of the dental substrate in any direction.[5] For better cutting efficiency, the CVDentus® CVD-coated diamond tips should first be positioned perpendicularly to the tooth surface and then gently pressed using oscillatory or pendulum movements of small amplitude until the desired depth of penetration is reached. The custom-made apparatus used in this study to produce standardized cavities followed the manufacturer's specifications with respect to positioning of the tips and the maintenance of a constant movement during cavity preparation.

We found that the angulations had no influence on the width or depth of the cavities. Lima et al.[5] observed that the widths of cavities prepared with CVD-coated diamond tips were similar to the diameters of the active portions of the spherical or cylindrical tips. These findings demonstrate the high cutting accuracy of these tips.

The cavities prepared in enamel with angulated and biangulated diamond tips were narrower and shallower than the cavities prepared in dentin. This result is probably related to the difference in mineral concentration and modulus of elasticity of enamel and dentin. As the enamel is a more rigid tissue than the dentin, it offers more resistance to tip penetration and could limit the oscillatory movement of the ultrasonic diamond tip. These results are in agreement with those reported by Lima et al.,[5] who observed that the widths and depths of cavities prepared with cylindrical ultrasonic diamond tips were greater in the dentin of the bovine[5] and human teeth.[12]

As the tip angulation had no influence on the cavity size, the presence of angulations might facilitate the access to difficult to reach areas without compromising the cutting accuracy.

Despite concerns about the surface features of the enamel and dentin that will receive the adhesive restorative materials, a few studies have evaluated the new cutting instruments available in the dental market. Ultrasonic tips promoted more conservative cavity preparations and seem promising for the practice of minimally invasive dentistry; however, studies on the characteristics of the cavity prepared by this cutting instrument must still be conducted to assess the changes produced in the cavity walls.

CONCLUSIONS

The grain diamond sizes did not affect the cutting effectiveness of the high-speed and ultrasonic tips. However, the high-speed burs promoted deeper and wider cavities than ultrasonic tips. The cutting efficiency was similar in both the angulated and biangulated ultrasonic tips that produced wider and deeper cavities in dentin than in enamel.