Gary S.P. Cheung

Cyclic fatigue and fracture characteristics of ground and twisted nickel-titanium rotary files.

INTRODUCTION The purposes of this study were to compare the fatigue resistance of traditional, ground nickel-titanium rotary instruments with the Twisted File and to examine the fracture characteristics of the fatigued fragment. METHODS Size #25, 0.06 tapered, TF (SybronEndo), RaCe (FKG Dentaire), Helix (DiaDent), and ProTaper F1 (Dentsply Maillefer) were examined with scanning electron microscope for surface characteristics before subjected to a cyclic (rotational bending) fatigue test. The time until fracture was recorded to calculate the number of revolutions for each instrument. The data were compared for differences by using analysis of variance and post hoc Scheffé test. The fragments were examined with scanning electron microscope both in lateral view and fractographically. RESULTS TF showed a significantly higher resistance to cyclic fatigue than other nickel-titanium files that were manufactured with a grinding process (P < .05). The path of crack propagation appeared to be different for electropolished (TF and RaCe) versus non-electropolished (Helix and ProTaper) instruments. CONCLUSIONS Although all specimens showed similar fractographic appearance, which indicated a similar fracture mechanism, instruments with abundant machining grooves seemed to have a higher risk of fatigue.

Dynamic Torsional Resistance of Nickel-Titanium Rotary Instruments

INTRODUCTION The cyclic fatigue of nickel-titanium (NiTi) rotary instruments has been studied extensively, but there is little information available on torsional fracture. Moreover, a clinical repeated locking effect was not considered in previous studies that evaluated torsional resistance of NiTi instruments. Thus, this study was aimed to compare the repetitive torsional resistance of various NiTi instruments with clinical relevance. MATERIALS AND METHODS Five brands of NiTi rotary instruments were selected: Twisted File (TF; SybronEndo, Orange, CA) and RaCe systems (FKG Dentaire, La Chaux-de-Fonds, Switzerland), both with an equilateral triangular cross-section, and the ProTaper (Dentsply Maillefer, Ballaignes, Switzerland), Helix (DiaDent, Chongju, Korea), and FlexMaster (VDW, Munchen, Germany), which had a convex triangular cross-section. Five millimeters of the tip of each file was embedded in composite resin block, and uniform torsional stresses (300 rpm, 1.0 N.cm) were applied repetitively by an endodontic motor with auto-stop mode until the file succumbed to torsional failure. The number of load applications leading to fracture was recorded. All fracture surfaces were examined under the SEM. Results were analyzed nonparametrically with alpha = 0.05. RESULTS Under the mode of load applications in this study, TF had the lowest and FlexMaster the highest torsional resistance among the groups (p < 0.05). Scanning electron microscopy examination revealed a typical pattern of torsional fracture for TF, RaCe, and ProTaper that was characterized by circular abrasion marks and skewed dimples near the center of rotation. In addition to these marks, Helix and FlexMaster presented a rough, torn-off appearance. CONCLUSION It was concluded that files of same cross-sectional design may exhibit different resistance to fracture probably as a result of the manufacturing process.

Comparison of Forces Generated During Root Canal Shaping and Residual Stresses of Three Nickel–Titanium Rotary Files by Using a Three-Dimensional Finite-element Analysis

The study was aimed to compare the stress distribution during simulated root canal shaping and to estimate the residual stress thereafter for some nickel-titanium rotary instruments. Three brands of instruments (ProFile, ProTaper, and ProTaper Universal; Dentsply Maillefer) were scanned with micro-computed tomography to produce a real-size, 3-dimensional model for each. The stresses on the instrument during simulated shaping of a root canal were analyzed numerically by using a 3-dimensional finite-element package, taking into account the nonlinear mechanical behavior of the nickel-titanium material. From the simulation, the original ProTaper design showed the greatest pull in the apical direction and the highest reaction torque from the root canal wall, whereas ProFile showed the least. In ProTaper, stresses were concentrated at the cutting edge, and the residual stress reached a level close to the critical stress for phase transformation of the material. The residual stress was highest in ProTaper followed by ProTaper Universal and ProFile.

Stress distribution of three NiTi rotary files under bending and torsional conditions using a mathematic analysis

AIM To compare and evaluate the stress distribution of three NiTi instruments of various cross-sectional configurations under bending or torsional condition using a finite-element analysis model. METHODOLOGY Three NiTi files (ProFile, ProTaper and ProTaper Universal) were scanned using Micro-CT to produce a three-dimensional digital model. The behaviour of the instrument under bending or torsional loads was analysed mathematically in software (ABAQUS V6.5-1), taking into consideration the nonlinear mechanical characteristic of NiTi material. RESULTS ProFile showed the greatest flexibility, followed by ProTaper Universal and ProTaper. The highest stress was observed at the surface near the cutting edge and the base of (opposing) flutes during cantilever bending. Concentration of stresses was observed at the bottom of the flutes in ProFile and ProTaper Universal instruments in torsion. The stress was more evenly distributed over the surface of ProTaper initially, which then concentrated at the middle of the convex sides when the amount of angular deflection was increased. CONCLUSION Incorporating a U-shaped groove in the middle of each side of the convex-triangular design lowers the flexural rigidity of the origin ProTaper design. Bending leads to the highest surface stress at or near the cutting edge of the instrument. Stress concentration occurs at the bottom of the flute when the instrument is subjected to torsion.

Three-dimensional reconstruction of microleakage pattern using a sequential grinding technique

Dye penetration tests are very commonly used to detect the absence of a fluid seal at the tooth-restoration interface. Airlocks in the marginal gap, leaching of water-soluble tracers during processing, and the failure of only a few sections to allow interpretation of the full pattern, limit these tests to low reproducibility and precision. The purpose of this present study was to generate high-resolution three-dimensional images of waterfast tracer patterns. Cylindrical class V (3 mm diameter, 2 mm deep) dentine-bonded resin composite restorations in buccal coronal dentine were thermally cycled (1000 x, 8 degrees C, 55 degrees C, 30 s dwell at each temperature) and then silver stained using an initial vacuum (100 mmHg pressure). Each restoration was sequentially abraded from the free surface on wet 180 grit silicon carbide paper, producing up to 30 parallel surfaces at approximately 0.15 mm separation through the restoration down to the pulp. Images of the ground surfaces were captured, and assembled by a computer image analyser program to give a three-dimensional model of the tracer pattern. The maximum depths of tracer penetration below the reference surfaces were 3.00 mm, 2.09 mm, 3.16 mm and > 2.29 mm for the four specimens. Projections of the models were viewed from several directions with sections in various locations to allow investigation of the full tracer pattern. This method allows the creation of high-resolution three-dimensional tracer patterns.

A preliminary investigation into the longevity and causes of failure of single unit extracoronal restorations

Four types of single unit restoration, including porcelain jacket, full and partial veneer, and metal ceramic crowns provided at the Prince Philip Dental Hospital in Hong Kong were investigated for evidence of failure. Of the 4658 units of restoration provided between 1981 and 1989, 100 restorations of each type were selected at random by computer. The relevant patient were identified and invited to return for a review appointment. A total of 132 patients attended and 152 crowns were examined. The average length of service was 34 months at the date of examination. Using strict criteria, 21 (14 per cent) restorations were deemed to have failed. Technical failure was the most prevalent cause of failure, followed by aesthetic complaints and endodontic problems. The failure rates ranged from 2.4 to 7.8 per cent per year for the different crowns in order of: partial veneer less than full veneer less than metal ceramic less than porcelain jacket crowns. Fracture of restoration, which affected metal ceramic and porcelain jacket crowns, was the single most frequent cause of failure observed in this study.

A numerical method for predicting the bending fatigue life of NiTi and stainless steel root canal instruments.

AIM To evaluate the bending fatigue lifetime of nickel-titanium alloy (NiTi) and stainless steel (SS) endodontic files using finite element analysis. METHODOLOGY The strain-life approach was adopted and two theoretical geometry profiles, the triangular (TR) and the square cross-sections, were considered. Both low-cycle fatigue (LCF) lifetime and high-cycle fatigue (HCF) lifetime were evaluated. RESULTS The bending fatigue behaviour was affected by the material property and the cross-sectional configuration of the instrument. Both the cross-section factor and material property had a substantial impact on fatigue lifetime. The NiTi material and TR geometry profiles were associated with better fatigue resistance than that of SS and square cross-sections. CONCLUSIONS Within the limitations of this study, finite element models were established for endodontic files to prejudge their fatigue lifetime, a tool that would be useful for dentist to prevent premature fatigue fracture of endodontic files.

A mathematical model for describing the mechanical behaviour of root canal instruments

AIM The purpose of this study was to establish a general mathematical model for describing the mechanical behaviour of root canal instruments by combining a theoretical analytical approach with a numerical finite-element method. METHOD Mathematical formulas representing the longitudinal (taper, helical angle and pitch) and cross-sectional configurations and area, the bending and torsional inertia, the curvature of the boundary point and the (geometry of) loading condition were derived. Torsional and bending stresses and the resultant deformation were expressed mathematically as a function of these geometric parameters, modulus of elasticity of the material and the applied load. As illustrations, three brands of NiTi endodontic files of different cross-sectional configurations (ProTaper, Hero 642, and Mani NRT) were analysed under pure torsion and pure bending situation by entering the model into a finite-element analysis package (ANSYS). RESULTS Numerical results confirmed that mathematical models were a feasible method to analyse the mechanical properties and predict the stress and deformation for root canal instruments during root canal preparation. CONCLUSIONS Mathematical and numerical model can be a suitable way to examine mechanical behaviours as a criterion of the instrument design and to predict the stress and strain experienced by the endodontic instruments during root canal preparation.