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.

A comparison of two nickel-titanium instrumentation techniques in teeth using microcomputed tomography

AIM

The aim of the study was to compare the shaping of root canals by two nickel-titanium instrumentation techniques using microcomputed tomography (MCT).

METHODOLOGY

Ten mandibular first molar teeth (30 canals) that had intact crowns and fully formed roots were scanned using MCT. Fifteen canals were instrumented using NiTiFlex hand files (Maillefer) using balanced force. The remainder were instrumented using prototype ProFile 0.04 Taper instruments (Dentsply) in a crown-down manner to an apical size ISO 25. The teeth were scanned again following instrumentation. The two instrumentation techniques were compared in a total of 27 canals. The area of dentine removed at predetermined levels (2.0, 3.0, 4.5, 6.0, 7.5 mm) from the apex was measured. Transportation and centring were recorded. Images constructed at these levels were compared with video images of equivalent physical sections created after the second scan. The volume of dentine removed in the apical 7.5 mm of the root canals of each tooth was calculated and the different techniques compared. Rendered three-dimensional images were used to assess the preparations qualitatively. The time taken for preparation was recorded.

RESULTS

There was no significant difference between hand instrumentation with NitiFlex files and machine instrumentation with prototype ProFile 0.04 Taper instruments for any of the variables tested.

CONCLUSIONS

Both techniques produced well centred and tapered preparations.

Effects of irrigation on debris and smear layer on canal walls prepared by two rotary techniques: a scanning electron microscopic study

Debris and smear layer were evaluated in canals prepared with either Lightspeed (LS) or ProFile (PF) rotary instruments. Irrigants used were tap water (group A) or alternating 5.25% NaOCl and 17% EDTA (group B). Apical stops were prepared to size 52.5 in the LS and to size 6 (approximately ISO size 36) in the PF techniques. The roots were split longitudinally and examined at apical, middle and coronal levels for debris and the smear layer using a 5-step scale. Using only water, mean debris scores were similar for LS and PF. In contrast, with EDTA/NaOCl, LS- and PF-prepared canals had similar debris scores at the apical and coronal levels, but there was a significant difference at the middle level. Mean smear layer scores were similar in LS- and PF-shaped canals when water was the sole irrigant. In contrast, with NaOCl and EDTA, mean smear layer scores were significantly different at the apical and middle levels, but not at the coronal level. Neither technique was superior in removing debris, but larger canal preparations obtained in this study with LS instruments enabled a more effective removal of the smear layer in the EDTA-NaOCl group.

Dental burs--what bur for which application? A survey of dental schools

PURPOSE

The purpose of this study was to survey North American dental schools regarding recommendations for rotary instrumentation for fixed prosthodontic and operative procedures at the predoctoral and postgraduate level.

MATERIALS AND METHODS

A 25-question survey was sent to 64 North American dental schools, of which 15 questions specifically related to rotary instrument recommendations. One questionnaire was addressed to the individual at each school having administrative responsibility for teaching tooth preparation techniques. A high response rate was assured by follow-up telephone calls and faxes. The distributions of bur recommendations for both predoctoral and advanced prosthodontic programs were analyzed by chi-squared tests at an a priori alpha = 0.05.

RESULTS

Completed surveys were received from 58 of 64 dental schools, a response rate > 90%. Medium grit burs predominate in predoctoral education for gross tooth reduction for fixed prosthodontics, whereas coarse grit burs predominate at the graduate level (p < .05). The use of the diamond bur alone predominates for axial wall refinement, whereas the use of carbide burs or carbide burs in combination with diamond burs prevails for marginal refinement (p < .05). In predoctoral operative dentistry, recommendations for cavity outline form were similar at all dental schools (p > .05) and were principally tungsten carbide (WC) burs. Carbide burs are the instrument of choice for internal walls, but the WC bur alone or in combination with diamond burs is preferred for refining composite margins (p < .05).

CONCLUSIONS

There is a broad consensus within North American dental schools on rotary instrumentation used by dental students. There is a greater use of coarser grit burs for gross tooth reduction in fixed prosthodontics at the postgraduate than predoctoral level.

Shaping and cleaning the root canal system: a scanning electron microscopic evaluation of a new instrumentation and irrigation technique

The purpose of the present scanning electron microscopic study was to investigate the efficacy of a combination of EDTA, NaOCl, and surface-active irrigating solutions during and after root canal preparation with ProFile nickel-titanium rotary instruments. Thirty canals were divided into three groups, instrumented and irrigated as follows: 5% NaOCl and a final flush with 17% EDTA were used for group A; group B specimens were irrigated using 17% EDTA, followed 15 s later by 1% TRITON X-100 (tensioactive agent) and then by 5% NaOCl; and group C specimens were irrigated with the same combination, but once shaping procedures were completed the irrigating sequence was repeated three times. After scanning electron microscopic evaluation, group C specimens exhibited the most effective debridement of the root canals. Results showed that tensioactive agent contributed to enhanced debridement. Cleaning was significantly improved once shaping procedures were completed.

Effect of rotational speed on nickel-titanium file distortion

Separation of instruments while performing root canals is something that has plagued all practitioners. The purpose of this study was to evaluate the effect of rotational speed on nickel-titanium file distortion and separation. Forty extracted human molars were instrumented with Profile (Dentsply Tulsa Dental Products, Tulsa, OK) ISO .04 rotary nickel-titanium instruments at 166.67 or 333.33 rpm. The instruments were evaluated after instrumentation for separation and/or distortion. The results of this study were statistically significant and indicate that file distortion and/or separation is four times as likely to occur at 333.33 rpm vs. 166.67 rpm.

Effects of simulated clinical conditions on nickel-titanium rotary files

When rotary nickel-titanium files are used in highly curved canals, there is a significant risk of separation. This study exposed files to saline or 5.25% NaOCl, while freely rotating in axial motion with a range of approximately equal to 6 mm at 150 rpm for 30 s in a 1.5 mm inner diameter, thin-walled metal tube of 15 mm length with a 90 degrees curve and a 5 mm radius of curvature. Files of 0.04 taper; 25 mm length; and in ISO sizes 25, 30, and 35 were each used three times. Groups were sterilized in a Chemiclave at 132 degrees C for 30 min after each use or tested without sterilization. Controls were not subjected to simulated use. Torsional moment (N-cm) and angular deflection (degrees) at failure was determined on a Torsiometer/Memocouple. Data (n = 5) were analyzed by analysis of variance. Tukey-Kramer intervals at the 0.05 significance level were determined. Exposure to cyclic fatigue, irrigants, and sterilization did not consistently affect torsional moment or angular deflection, compared with controls. Torsional moment significantly increased with an increase in size, regardless of condition (unsterilized vs. sterilized) or solution (saline vs. NaOCl).

Rotary Ni-Ti profile systems for preparing curved canals in resin blocks: influence of operator on instrument breakage

AIM

The aim of this study was to determine the incidence of fracture of ProFile 0.4 and 0.6 taper Series 29 nickel-titanium instruments with respect to operator experience.

METHODOLOGY

A total of 125 simulated root canals in resin blocks with the same geometrical shape in terms of angle and radius of curvature and coronal and apical orifice diameter were used. Five operators prepared all the specimens using an identical step-down instrument sequence, each one preparing 25 canals. The operators included two endodontists and three general practitioners. Statistical data concerning the incidence of instrument failure was compiled using Statlab and Fisher's partial least square difference analysis of variance.

RESULTS

A total of 21 (16.8%) instruments fractured, all had 0.04 tapers. Nine size 25 instruments failed, 9 size 20 instruments failed and 3 size 15. During the study, the Binary Tree analysis of instrument failure revealed two operator populations belonging to two different study periods. The first period, which represented the first 13 root canal preparations, was called the 'learning period', and the second period, which represented the next 12 sample preparations, was called the 'application period'. A greater number of instruments failed during the first period than during the second. In the 'learning period', both groups of operators learned the same way. In the 'application period', two groups could be distinguished; the first group consisted of a general practitioner who produced worse results, and the second group consisted of the other four operators.

CONCLUSIONS

The results indicate the necessity of mastering this rotary canal preparation technique, and the importance of improving competence through learning and experience.

Dynamic and cyclic fatigue of engine-driven rotary nickel-titanium endodontic instruments

The absence of adequate testing standards for engine-driven nickel-titanium (NiTi) instruments necessitates further study of these instruments in all areas. This study examined three groups of engine-driven rotary NiTi endodontic instruments (Profile, Hero, and Quantec) and assessed the times for dynamic fracture in relation to the radius of curvature to which the instruments were subjected during preparation, with the instrument diameter determined by size and taper and the mode by which the fracture occurred. Ten instruments were randomly selected representing each size and taper for each group and for each radius of curvature: 600 in total. The instruments were rotated at 350 rpm and introduced into a tempered steel curve that simulated a canal. Two radii of curvature of canals were used: 5 and 10 mm. Time at fracture was noted for all files, and the fracture faces of each file were analyzed with scanning electron microscopy. Radius of curvature was found to be the most significant factor in determining the fatigue resistance of the files. As radius of curvature decreased, fracture time decreased. Taper of files was found to be significant in determining fracture time. As diameter increased, fracture time decreased. In all cases, fracture was found to be of a ductile nature, thus implicating cyclic fatigue as a major cause of failure and necessitating further analyses and setting of standards in this area.

A comparison of instrumentation using Naviflex and Profile nickel-titanium engine-driven rotary instruments

This study was designed to compare the changes in canal configuration resulting from instrumentation by either Profile or Naviflex instruments. Forty mesial canals in extracted human molar teeth were embedded and sectioned at two root levels. Reassembled teeth were instrumented with a modified crown-down technique as described in the Profile training video for Profile files and in a similar manner for Naviflex instruments. Superimposed pre- and postinstrumented cross-sectional root images were projected, traced, and scanned into a computer for analysis. Canal movement, in relation to the furca, and canal area change were recorded. The results showed no significant difference in canal center movement or canal area change between the Profile or Naviflex groups. The degree of canal curvature had no effect on canal center movement or canal area change.

Clinical problems associated with unusual cases of talon cusp

CASE REPORT

Talon cusp is an uncommon dental anomaly manifested as an accessory cusp-like structure on the crown of anterior teeth. This report describes two unusual cases of talon cusp. Case 1 showed bilateral anomalous cusps on the palatal aspects of maxillary supernumerary teeth, causing premature contact and tooth impaction. In case 2, a double-fused talon cusp was projected from the palatal surface of a large geminated central incisor. A talon cusp is not an innocuous defect, as it may provide a substantial diagnostic, treatment planning and procedural challenge. Early diagnosis and management are important to avoid complications.

The removal of the smear layer using the Quantec system. A study using the scanning electron microscope

AIM

The aim of this study was to determine the ability of the Quantec Series 2000 rotary nickel-titanium endodontic system to remove dentinal debris and smear layer produced during canal preparation.

METHODOLOGY

A first group (control) of 12 curved root canals was prepared using conventional manual instruments and the step-back technique. A second group of 12 curved root canals was instrumented using the complete Quantec sequence according to the manufacturer's instructions. In both groups, irrigation was performed using a 3% NaOCl solution. The canal walls were observed under a scanning electron microscope and the coronal, middle and apical thirds of each canal photographed at a magnification of 500. The views were divided into 10 subareas by overlaying a grid, and the absence or presence of a smear layer was rated and scored on three appearances using the scale described by Ciucchi et al. (1989).

RESULTS

The scores were higher (i.e. less debris was present) in the middle third (P < 0.0001) and in the apical third (P < 0.0001) of canals prepared with the Quantec system when compared with those prepared with K-files. Nevertheless, in canals prepared with Quantec instruments, the scores were significantly higher in the coronal third compared with the apical third (P < 0.005).

CONCLUSIONS

The Quantec rotary system produced cleaner canal walls than conventional manual instrumentation, particularly in the middle and apical thirds. This finding may imply that stresses applied to the cutting regions of Quantec instruments by accumulation and compression of the smear layer are minimized.

[Oscillating procedures in the preparation technic (I)]

An optimal preparation technique should meet the following requirements: a cavity that does not extend beyond the carious lesion; maximum preservation of sound tooth substance, preparation and margin design adapted to the requirements of the restorative material and its adhesion properties, prevention of adjacent tooth damage, and low technique sensitivity. Oscillating instruments with one diamond-coated and one polished surface have been developed to overcome the limitations of conventional preparation designs dictated by rotary burs. Previously untreated carious lesions were prepared using a modified air-scaler handpiece (Sonicsys) with various working tips allowing interproximal cavity preparation with minimal extension and a low risk of damaging the neighboring tooth. Another oscillating system is based on the EVA-System. It uses files for the preparation of interproximal bevels and for the finishing of margins of interproximal box, crown and veneer preparations. These oscillating instruments overcome the major limitations of rotary burs for tooth preparation and facilitate conventional cavity preparation. In addition, they allow the use of cavity designs that could not be achieved with conventional instruments.

[Preparation with air abrasion]

Air abrasive technology has re-emerged in dentistry. The history, characteristics and clinical application of this approach are reviewed, including advantages and limitations for the removal of enamel, dentin and decay. Air-abrasive technology may be especially suited for use in bonded restorations as well as for repair of all types of restorations. The factors affecting adequate bond strength are discussed.

Cyclic fatigue of Profile rotary instruments after simulated clinical use

AIM

The purpose of this study was to evaluate cyclic fatigue of Profile Ni-Ti rotary instruments (PRIs) after dry heat sterilization and up to 10 times simulated clinical use.

METHODOLOGY

Instruments of size 40-15 were used in a crown-down technique. Three groups were included in this study. In groups 1 and 2, each set of instruments was used in five and 10 canals, respectively. Group 3 was the control group. NaOCl at a concentration of 2.5% was used as an irrigant. Each set of instruments was sterilized before each use.

RESULTS

The PRI size 40 demonstrated the lowest incidence of rotations to breakage. One-way analysis of variance and Turkey's HSD test showed statistically significant differences among different file sizes within each group.

CONCLUSIONS

The results showed that dry heat sterilization and simulated clinical use in the presence of NaOCl did not lead to a decrease in the number of rotations to breakage of the files.

Location of contact areas on rotary Profile instruments in relationship to the forces developed during mechanical preparation on extracted teeth

AIM

The aim of this study was to locate the areas of direct instrument contact with dentine in the root canal system during rotary preparation and to analyse the relationship between these areas and the vertical forces and torque developed during the preparation.

METHODOLOGY

Canal preparations were performed by endodontists either with the step-back (SB) or the crown-down (CD) technique. In order to locate the areas of contact, the instruments were coated with two layers of gold by electro-deposition. They were photographed before and after use, and a coding system, based on mm from the instrument tip, was devised to designate areas of gold removal or instrument wear due to friction. To standarise the conditions of instrument manipulation, the teeth were fixed in the Endographe holder, and this device was used to measure vertical forces and torque.

RESULTS

The results showed that the first series of instruments used for the CD technique (taper 0.06) left 2 +/- 1 mm of the tip with the gold intact, indicating that these instruments and this step of the CD technique are the safest part of the preparation. For all other instruments (taper 0.04 series), the areas of gold removal involved the 3 mm around the tip and this finding was independent of the order of instrument use and preparation technique (SB or CD). The differences between the two techniques were significant in terms of the mean area of decolouration and the mean force and torque values. For the SB and CD techniques, the contact areas were, respectively, 10 +/- 3 and 7 +/- 2 mm. The forces and torque values were correspondingly higher for SB vs. CD; the mean values were, respectively, 19 and 21 N for vertical forces and 16 10(5) and 13 10(5) Nm for torque.

CONCLUSIONS

The recorded torque values and the location on the instruments of the areas of contact with dentine during this development of torque i.e. at or near the tip, indicate that great caution should be used with the rotary technique, particularly with the taper 0.04 instruments, regardless of preparation technique.

A scanning electron microscopic study of the effect of Gluma CPS bonding system on dentinal smear layers produced by different bur types and rotational speeds and on the resin-dentin interface

OBJECTIVE

The purpose of this study was to investigate the effect of conditioners on smear layers produced by different bur types and rotational speeds and to study the interaction of subsequently applied primer and sealer with these conditioned surfaces.

METHOD AND MATERIALS

Smear layers were produced on human teeth immediately after extraction by burs rotating at approximately 6,000 rpm without water spray and 400,000 rpm with water spray. Gluma CPS etchant was applied for 15, 30, and 60 seconds, and a 20% phosphoric acid liquid (control) was applied for 30 seconds to smear layers. The specimens were prepared by critical point drying for scanning electron microscopic imaging within 24 hours. Additional specimens were prepared and treated with Gluma CPS primer and sealer, according to the manufacturer's instructions.

RESULTS

There were only small variations in the smear layer thicknesses with different bur types or speeds of rotation. Gluma CPS conditioner, applied for the recommended times, did not completely remove the smear layer. An altered smear layer, composed of a reaction product, remained on the dentinal surface. A zone of demineralization did occur, however, beneath the partially removed smear layer in both the 15- and 30-second specimens but was only partly filled by primer and sealer. Smear layers were completely removed by the 20% phosphoric acid liquid to expose a delicate collagen network.

CONCLUSION

An understanding of the appropriate treatment for smear layers is crucial to the development of improved dentin bonding systems.

Bacterial reduction with nickel-titanium rotary instrumentation

The purpose of this study was to compare intracanal bacterial reduction on teeth instrumented with 0.04 tapered nickel-titanium (NiTi) rotary instrumentation to bacterial reduction when using a stainless-steel K-file step-back technique using sterile saline irrigation. Forty-eight patients with apical periodontitis were randomly assigned treatment type. The canals were sampled before, during, and after instrumentation. The samples were incubated anaerobically for 7 days at 37 degrees C, colony-forming unit numbers calculated, and a log transformation performed to normalize the counts. Teeth exhibiting apical periodontitis were uniformly infected, whereas vital control teeth were not. A similar and uniform reduction occurred with progressive filing, regardless of technique (p < 0.0001). There was no detectable difference in colony-forming unit count after NiTi rotary or stainless-steel hand instrumentation (p = 0.42). Neither technique could predictably render canals free of bacteria. The results of this study indicate NiTi rotary and stainless-steel hand K-file step-back instrumentation techniques were not significantly different in their ability to reduce intracanal bacteria.

A useful evacuation aid for intraoral air-abrasive devices

Air-abrasive technology offers several advantages over conventional handpieces. Although modern high-speed evacuation systems result in limited powder accumulation, the microabrasive debris continues to be a major concern. This article highlights the need for rubber dam isolation and introduces a makeshift evacuation device fabricated from a plastic soft drink container.