Effects of four Ni-Ti preparation techniques on root canal geometry assessed by micro computed tomography

The effect of cleaning procedures on fracture properties and corrosion of NiTi files

AIM

To evaluate the effect of repeated cleaning procedures on fracture properties and corrosion of nickel-titanium (NiTi) files.

METHODOLOGY

New NiTi instruments were subjected to 2, 5 and 10 cleaning cycles with the use of either diluted bleach (1% NaOCl) or Milton's solution (1% NaOCl plus 19% NaCl) as disinfectant. Each cleaning cycle consisted of scrubbing, rinsing and immersing in NaOCl for 10 min followed by 5 min of ultrasonication. Files were then tested for torsional failure and flexural fatigue, and observed for evidence of corrosion using scanning electron microscope (SEM). Four brands of NiTi files were immersed in either Milton's solution or diluted bleach overnight and evaluated for corrosion.

RESULTS

Up to 10 cleaning cycles did not significantly reduce the torque at fracture or number of revolutions to flexural fatigue (P > 0.05, two-way anova), although decreasing values were noted with increasing number of cleaning cycles using Milton's solution. No corrosion was detected on the surface of these files. Files immersed in 1% NaOCl overnight displayed a variety of corrosion patterns. The extent of corrosion was variable amongst different brands of files and amongst files in each brand. Overall, Milton's solution was much more corrosive than diluted bleach. Corrosion of file handles was often extreme.

CONCLUSIONS

Files can be cleaned up to 10 times without affecting fracture susceptibility or corrosion, but should not be immersed in NaOCl overnight. Milton's solution is much more corrosive than bleach with the same NaOCl concentration.

Root-canal preparation with FlexMaster: canal shapes analysed by micro-computed tomography

AIM

To evaluate the relative performance of FlexMaster nickel-titanium instruments shaping maxillary molar root canals in vitro.

METHODOLOGY

Extracted human maxillary molars were scanned, before and after root-canal shaping, with FlexMaster, employing micro-computed tomography (microCT) at a resolution of 36 microm. Canals were three-dimensionally reconstructed and evaluated for volume, surface area, 'thickness' (diameter), canal transportation and prepared surface. Based on median canal volume, the canals were divided into 'wide' and 'constricted' groups. Comparisons were made between mesiobuccal (mb), distobuccal (db) and palatal (p), as well as 'wide' and 'constricted' canals, using anova and Scheffé post-hoc tests.

RESULTS

Volume and surface area increased significantly and similarly in mb, db and p canals, and no gross preparation errors were found. Mean root-canal diameters, 5 mm coronal to the apex, increased from 0.45 to 0.65 mm, from 0.41 to 0.56 mm and from 0.79 to 0.85 mm for mb, db and p canals, respectively. Apical canal transportation ranged from 0.01 to 0.29 mm and was independent of canal type; 'wide' canals had a significantly higher (P < 0.05) proportion of unprepared surfaces than 'constricted' canals had.

CONCLUSIONS

FlexMaster instruments shaped curved and narrow root canals in maxillary molars to sizes 40 and 45 without significant shaping errors. Canal anatomy had an insignificant impact on preparation, indicating that FlexMaster instruments were able to shape 'constricted' canals as well as 'wide' ones.

Preparation of curved root canals with rotary FlexMaster instruments compared to Lightspeed instruments and NiTi hand files

AIM

To compare the recently introduced rotary FlexMaster instruments with Lightspeed instruments and NiTi hand files in preparing curved root canals.

METHODOLOGY

Root canals of extracted molars were shaped with rotary FlexMaster instruments ('FM': n = 45), Lightspeed instruments ('LS': n = 46), and NiTi hand files (n = 45) using the balanced-force technique. The apical preparation size was 40. Root canal instrumentation was carried out in a phantom head under clinical conditions. A re-assembly technique allowed a comparison of the canal outline before and after preparation. Root sections 2.5 mm short of working length were used to calculate the percentage of prepared canal outline (= PPO) and the amount of root canal transportation. Loss of working length, fracture rate and time spent on complete preparation were also recorded.

RESULTS

The highest PPO values were found in the LS group (mean = 63% [95% CI: 55%; 70%]). Although not statistically significant, lower PPO values were detected for FM instruments (mean = 55% [95% CI: 49%; 62%]) and for hand files (mean = 53% [95% CI: 47%; 59%]). The incidence of root canal transportation exceeding 0.1 mm was significantly lower in the LS group than in the FM group. Loss of working length of 0.5 mm occurred in five cases (LS group: 4x; FM group: 1x). Two LS instruments fractured. Half the time was needed for root canal preparation with FM instruments than with hand files.

CONCLUSIONS

Rotary FM instruments are suitable for preparing curved root canals. They provided results similar to LS instruments with minimal risk of instrument fracture but increased risk of root canal transportation.

Progressive versus constant tapered shaft design using NiTi rotary instruments

AIM

To evaluate the influence of a progressive versus constant tapered shaft design on canal preparation by NiTi rotary techniques.

METHODOLOGY

A XMCT-scanner and custom-made software were used to nondestructively analyse the mesial canals of 10 extracted mandibular molars in 3D with a spatial resolution of 12.5 microm. Specimens (n = 10 per group) were scanned before and after preparation using ProTaper (progressive tapered) or K3 (constant tapered) files. Numerical values for volume, curvature, dentine removal and centring ratio were obtained in addition to a visual inspection for canal aberrations. Data were analysed by (multiway factorial) anova, Wilcoxon tests and t-tests.

RESULTS

The volume of total dentine removal (mean +/- SD) was 1.21 +/- 0.66 mm(3) (ProTaper) and 1.06 +/- 0.23 mm(3) (K3) (P > 0.05), and the amount of dentine removal at all separate horizontal regions examined was comparable for both groups. The mean linear dentine removal (transportation) was in the range of 8-212 microm (ProTaper) and 4-187 microm (K3). The resultant centring ratio varied from 0.01 to 0.24 (ProTaper) and from 0.01 to 0.17 (K3), whilst different straightening patterns were observed. A centre displacement towards the furcation coronally was most pronounced for the ProTaper group whereas a centre displacement towards the outer side of the curvature more apically was only observed for the K3 group. No severe canal aberrations were found.

CONCLUSIONS

The progressive tapered shaft design of the ProTaper instrument was less influenced by the mid-root curvature than the constant tapered design of the K3 instrument thereby providing a good centred apical preparation. However, ProTaper instruments tended to transport towards the furcation in the coronal region.

ProTaper rotary root canal preparation: effects of canal anatomy on final shape analysed by micro CT

AIM

To evaluate the relative performance of ProTaper nickel-titanium (Ni-Ti) instruments shaping root canals of varying preoperative canal geometry.

METHODOLOGY

Extracted human maxillary molars were scanned, before and after shaping with ProTaper, employing micro computed tomography (muCT) at a resolution of 36 mum. Canals were three-dimensionally reconstructed and evaluated for volume, surface area, 'thickness' (diameter), canal transportation and prepared surface. Based on median canal volume, canals were divided into 'wide' and 'constricted' groups. Comparisons were made between mesiobuccal (mb), distobuccal (db) and palatal (p), as well as 'wide' and 'constricted' canals, using repeated-measures anova and Scheffé posthoc tests.

RESULTS

Volume and surface area increased significantly and similarly in mb, db and p canals, and gross preparation errors were found infrequently. Root canal diameters, 5-mm coronal to the apex, increased from 0.38 to 0.65 mm, 0.42 to 0.66 mm and 0.57 to 0.79 mm for mb, db and p canals, respectively. Apical canal transportation ranged from 0.02 to 0.40 mm and was independent of canal type; 'wide' canals had a significantly higher (P < 0.05) proportion of unprepared surfaces than 'constricted' canals.

CONCLUSIONS

Canals in maxillary molars were prepared in vitro using ProTaper instruments without major procedural errors. These instruments may be more effective in shaping narrow canals than wider, immature ones.

ProTaper rotary root canal preparation: assessment of torque and force in relation to canal anatomy

AIM

To investigate physical parameters for ProTaper nickel-titanium (NiTi) rotary instruments whilst preparing curved canals in maxillary molars in vitro.

METHODOLOGY

A novel torque-testing platform was used to prepare root canals in 15 extracted human maxillary molars with ProTaper rotary instruments. Peak torque and force was registered along with numbers of rotations required to shape the canals. Canals were divided into 'wide' and 'constricted' groups depending on canal volumes assessed by micro computed tomography. Mean scores for each instrument type were calculated and statistically compared using anova and Scheffé posthoc tests.

RESULTS

Mean torque varied between 0.8 +/- 0.5 and 2.2 +/- 1.4 N cm whilst mean force ranged from 4.6 +/- 2.6 to 6.2 +/- 2.7 N. Mean numbers of rotations totalled up to 21. All three variables registered were significantly correlated to preoperative canal volumes (P < 0.001) and differed significantly between 'wide' and 'constricted' canals (P < 0.001).

CONCLUSIONS

Whilst high forces were used in some cases, no ProTaper instrument fractured when a patent glide path was present. There were significant positive correlations between canal geometry and physical parameters during shaping.

Smooth flexible versus active tapered shaft design using NiTi rotary instruments

AIM

The aim of this study was to evaluate the influence of a smooth flexible versus active tapered shaft design on canal preparation by NiTi rotary techniques.

METHODOLOGY

A XMCT-scanner (SkyScan 1072) and developed software (Bergmans et al. 2001) were used to nondestructively analyze the mesial canals of 10 extracted mandibular molars in 3D with a spatial resolution of 30 microm. Specimens (n = 10 per group) were scanned before (PRE) and after (POST) preparation using Lightspeed (smooth flexible) or GT-rotary (active tapered) files. Numerical values for volumes, dentine removal (net) transportation and centring ability were obtained in addition to a visual inspection on canal aberrations. Data were analyzed by Shapiro Wilk test, multiway factorial anova, Tukey-Kramer test, Wilcoxon test and t-test.

RESULTS

Results indicated that the active tapered shaft removed significantly more dentine in the middle to apical portion of the root compared to the smooth flexible design. Both groups demonstrated some straightening, but no significant differences were found with respect to instrument types. However, absolute values for net transportation and centering ratio were small and no canal aberrations could be found.

CONCLUSIONS

The smooth flexible shaft design did not improve the morphological characteristics of canal preparation by NiTi rotary instruments when compared with the active tapered design. Therefore, system selection should be based upon other criteria.

Effect of sterilization on the cutting efficiency of PVD-coated nickel-titanium endodontic instruments

AIM

To investigate alterations in cutting efficiency when conventional and titanium nitride (TiN) coated nickel-titanium (NiTi) K-files were exposed to repeated sterilization using an autoclave.

METHODOLOGY

A total of 96 NiTi K-files (size 35) were randomly divided into two groups (A and B) of 48 instruments each. Whilst the instruments of group B were exposed to physical vapour deposition (PVD) creating a coating of a TiN layer, the files of group A were not coated. The instruments of groups A and B were randomly divided into four subgroups of 12 instruments each. A.1/B.1: Instruments were exposed to five cycles of sterilization. A.2/B.2: Instruments were exposed to 10 cycles of sterilization. A.3/B.3: Instruments were immersed in NaOCl for 30 min, rinsed in water, and exposed to five cycles of sterilization. A.C/B.C: Instruments were not sterilized (controls). The cutting efficiency of all files was determined by means of a computer-driven testing device. Special plastic samples with cylindrical canals were used and the maximum penetration depth of the files into the lumen was assessed.

RESULTS

The TiN-coated instruments of groups B.1, B.2 and B.3 showed no significant difference in comparison with the penetration depths of the controls (P> 0.05). The uncoated files of groups A.1, A.2 and A.3 displayed significantly lower maximum penetration depths (P< 0.05) when compared to the control files.

CONCLUSIONS

Repeated sterilization under autoclave or exposure to sodium hypochlorite (NaOCl) prior to sterilization did not alter the cutting efficiency of PVD-coated NiTi K-files.

Efficiency of hand and rotary instruments in shaping oval root canals

This study was designed to determine the efficiency of hand and rotary instruments in shaping oval root canals. Seventy-five oval canals were equally divided into three groups. The apical third was prepared with rotary LightSpeed instruments either to size 52.5 (mandibular incisors) or to size 57.5 (distal root of mandibular molars). The middle third with an oval cross-section was shaped with Hedström hand files using circumferential technique, with 6% taper rotary Hero files in a circumferential filing movement or with rotary LightSpeed instruments in a step-back technique. Instrumentation was performed under clinical conditions in a phantom head. The teeth were sectioned at two levels in the middle third of the root. An assembly technique allowed comparing the canal outline before and after instrumentation. The photographed root sections were superimposed and traced under a stereomicroscope. The ratio of prepared to unprepared canal outline was calculated for each section. The lowest values were observed in the LightSpeed group (mean: 0.42; 95% confidence interval (CI): 0.37; 0.47). Significantly higher values were recorded in the Hero group (mean: 0.58; 95% CI: 0.53; 0.64) and in the Hedström group (mean: 0.56; 95% CI: 0.49; 0.62). No instrumentation technique was capable of completely preparing dentin walls of oval root canals. Circumferential filing of the middle third of oval root canals with either 6% taper Hero files or conventional Hedström hand files gave comparable results.

Dynamic torque and apical forces of ProFile.04 rotary instruments during preparation of curved canals

AIM

To analyse torque and force generated whilst shaping curved canals using rotary instruments.

METHODOLOGY

A specially designed computer-controlled testing platform was used to record events during the shaping of straight and curved canals in plastic blocks and in extracted human teeth using ProFile.04 instruments. Size 40 apical stops were prepared using crown-down, apical preparation and step-back procedures. Maximum torque, apically directed force and the numbers of revolutions were recorded at a resolution of 100 samples s-1. Load causing separation as required by the ISO 3630-1 test and cyclic fatigue was also recorded. Mean maximum scores were calculated and statistically tested using one- and two-way analyses of variance.

RESULTS

Highest and lowest torque scores were recorded, respectively, in straight canals in plastic blocks at 25 Nmm and in natural canals at 14 Nmm. Significant differences were recorded for canal type and preparation phase (P < 0.0001). Loads causing separation varied from 3.7 to 32.3 Nmm. Apically directed forces ranged from 1 to 7.5 N. Again, there were significant differences depending on canal type and preparation phase (P < 0.0001). The number of revolutions during preparation ranged from 18 to 41. Size 15, 30 and 45 ProFile.04 instruments separated after 581, 430 and 402 revolutions, respectively, in a standard cyclic fatigue test.

CONCLUSIONS

The new torque-testing platform details physical parameters during preparation of curved canals. To improve predictability, instrumentation sequences must be tested for excessively high torsional moments or forces. This study indicated that up to 10 curved canals could be safely prepared with a sequence of ProFile. 04 rotary instruments without separation due to cyclic fatigue. Efforts should continue to correlate root canal anatomy with torque and force generated during rotary root canal preparation.

Engine-driven preparation of curved root canals: measuring cyclic fatigue and other physical parameters

An increasing number of engine-driven rotary systems are marketed to shape root canals. Although these systems may improve the quality of canal preparations, the risk for instrument fracture is also increased. Unfortunately, the stresses generated in rotary instruments when shaping curved root canals have not been adequately studied. Consequently, the aim of an ongoing project was to develop a measurement platform that could more accurately detail physical parameters generated in a simulated clinical situation. Such a platform was constructed by fitting a torque-measuring device between the rotating endodontic instrument and the motor driving it. Apically directed force and instrument insertion depth were also recorded. Additional devices were constructed to assess cyclic fatigue and static fracture loads. The current pilot study evaluated GT rotary instruments during the shaping of curved canals in plastic blocks as well as "ISO 3630-1 torque to fracture" and number of rotations required for fatigue fracture. Results indicated that torques in excess of 40 Nmm were generated by rotary GT-Files, a significantly higher figure than static fracture loads (less than 13 Nmm for the size 20. 12 GT-File). Furthermore, the number of rotations needed to shape simulated canals with a 5 mm radius of curvature in plastic blocks was 10 times lower than the number of rotations needed to fracture instruments in a "cyclic fatigue test". Apical forces were always greater than 1 N, and in some specimens, scores of 8 N or more were recorded. Further studies are required using extracted natural teeth, with their wide anatomical variation, in order to reduce the incidence of fracture of rotary instruments. In this way, the clinical potential of engine-driven rotary instruments to safely prepare curved canals can be fully appreciated.