Effect of repetitive up-and-down movements on torque/force generation, surface defects and shaping ability of nickel-titanium rotary instruments: an ex vivo study.

BACKGROUND: The screw-in effect is a tendency of a nickel-titanium (NiTi) rotary endodontic file to be pulled into the canal, which can result in a sudden increase in stress leading to instrument fracture, and over-instrumentation beyond the apex. To reduce screw-in force, repeated up-and-down movements are recommended to distribute flexural stress during instrumentation, especially in curved and constricted canals. However, there is no consensus on the optimal number of repetitions. Therefore, this study aimed to examine how repeated up-and-down movements at the working length affect torque/force generation, surface defects, and canal shaping ability of JIZAI and TruNatomy instruments. METHODS: An original automated root canal instrumentation device was used to prepare canals and to record torque/force changes. The mesial roots of human mandibular molars with approximately 30˚ of canal curvature were selected through geometric matching using micro-computed tomography. The samples were divided into three groups according to the number of up-and-down movements at the working length (1, 3, and 6 times; n = 24 each) and subdivided according to the instruments: JIZAI (#13/0.04 taper, #25/0.04 taper, and #35/0.04 taper) or TruNatomy (#17/0.02 taper, #26/0.04 taper, and #36/0.03 tape) (n = 12 each). The design, surface defects, phase transformation temperatures, nickel-titanium ratios, torque, force, shaping ability, and surface deformation were evaluated. Data were analyzed with the Kruskal-Wallis and Dunn's tests (α = 0.05). RESULTS: The instruments had different designs and phase transformation temperatures. The 3 and 6 up-and-down movements resulted in a smaller upward force compared to 1 movement (p < 0.05). TruNatomy generated significantly less maximum torque, force, and surface wear than JIZAI (p < 0.05). However, TruNatomy exhibited a larger canal deviation (p < 0.05). No statistical differences in shaping ability were detected between different up-and-down movements. CONCLUSIONS: Under laboratory conditions with JIZAI and TruNatomy, a single up-and-down movement at the working length increased the screw-in force of subsequent instruments in severely curved canals in the single-length instrumentation technique. A single up-and-down movement generated more surface defects on the file when using JIZAI. TruNatomy resulted in less stress generation during instrumentation, while JIZAI better maintained the curvature of root canals.

Effect of preset torque setting on torque/force generation, shaping ability and surface changes of nickel titanium rotary instrument in different root canal curvature locations: An ex vivo study.

The aim of this study was to evaluate how preset torque settings influence the torque, vertical force, and root canal-centering ability of ProGlider and ProTaper NEXT nickel-titanium rotary instruments in canals with different curvature locations. Based on micro-computed tomography, mesial roots of human mandibular molars (25°-40° curvature) were allocated to the apical curvature (apical 1-5 mm) or the middle curvature (apical 5-9 mm) groups, and mandibular incisors (curvature <5°) to the straight canal group. Each group was subjected to automated instrumentation and torque/force measurement with the preset torque of 1, 2.5, or 5 N•cm. Canal-centering ratios were determined with micro-computed tomography. Instrument fracture occurred only in the 2.5 and 5 N•cm groups in curved canals. The preset torque setting and curvature location did not influence canal shaping ability.

Evaluation of Root Canal Anatomy of Maxillary Premolars Using Swept-Source Optical Coherence Tomography in Comparison with Dental Operating Microscope and Cone Beam Computed Tomography.

OBJECTIVE: This study aimed to evaluate the ability of swept-source optical coherence tomography (OCT) to detect internal anatomy of maxillary premolars in comparison with dental operating microscope (DOM) and cone beam computed tomography (CBCT). BACKGROUND DATA: The ability of OCT to observe the pulp horn during cavity preparation and assess the remaining dentin thickness (RDT) has been demonstrated, whereas validation of OCT in comparison with other imaging techniques seems required. METHODS: Ten extracted human maxillary premolars were sectioned perpendicular to the tooth axis from the occlusal surface at approximately 2 mm increments. OCT and DOM were performed after each cut, and microfocus X-ray computed tomography (micro-CT; reference standard) and CBCT were conducted before sectioning and after the first and second cuts. Three examiners evaluated all images for presence of the pulp horn/pulp chamber, isthmus, lateral canals, and the number of root canals. RDT was determined from OCT, micro-CT, and CBCT images. Correlations were analyzed with Pearson's correlation coefficient. RESULTS: OCT had a sensitivity and specificity of 0.90 and 0.80 in detecting the pulp horn/pulp chamber and 0.84 and 0.71 in detecting the isthmus, respectively. The three techniques showed strong correlations in detecting the number of root canals compared with micro-CT. OCT and DOM did not detect lateral canals. For RDT values, strong correlations were observed between micro-CT and CBCT, micro-CT and OCT, and CBCT and OCT (p < 0.01 for all). CONCLUSIONS: Under the present experimental condition, OCT accurately measured RDT and detected internal tooth anatomy such as the pulp horn, isthmus, and root canals.

Application of optical coherence tomography to identify pulp exposure during access cavity preparation using an Er:YAG laser.

OBJECTIVE: The study aimed to evaluate the ability of optical coherence tomography (OCT) to guide and identify pulp exposure using an erbium: yttrium-aluminum-garnet (Er:YAG) laser. BACKGROUND DATA: The Er:YAG laser has been proven to be effective in ablating dental hard tissue and offers advantages, as there is none of the vibration and noise you get with conventional methods, but it has limitations in relation to the tactile feedback that would aid in identification of entry into the pulp chamber. Based on depth-resolved optical reflectivity, OCT technology has been developed to provide high-resolution, cross-sectional images of the internal structure of biological tissues. MATERIALS AND METHODS: The pulp chambers of 20 human mandibular incisors were examined, and the average thickness of hard tissue covering the pulp chamber was assessed using micro-computed tomography (micro-CT) images. An Er:YAG laser was used to gradually penetrate the hard tissue over the pulp chamber under microscopic guidance. The preparation was constantly imaged using a swept-source OCT at 10 sec intervals until a pulp chamber exposure was identified using the technology. The pulp exposure was re-examined under the microscope and compared with micro-CT images for verification. RESULTS: The pulp exposures of 20 incisors were all verified microscopically and with micro-CT images. The thickness of hard tissue penetrated by the laser ranged from 0.44 to 1.69 mm. CONCLUSIONS: Swept-source OCT is a useful tool for identifying pulp exposure during access opening with the Er: YAG laser.