Effects of the Nd:YAP laser on coronal restorative materials: implications for endodontic retreatment.

With new wavelengths that allow light transmission by optical fibers, the laser is now often used in endodontics either during treatment or retreatment. The aims of this study were to (i) specify the effects of laser irradiation on restorative materials in terms of topographic effects and (ii) describe different protocols for the first steps of retreatment. The laser used in this study was an optical fiber Nd:YAP (Lokki dt, Vienne, France) with a wavelength of 1.34 microns. Samples of the following restorative materials were prepared: amalgam, composite, permanent and temporary cements, and prosthodontic alloy. The handpiece of the Nd:YAP laser was fixed perpendicular to the surface of the different preparations. All trials were performed with the fiber tip either in contact with or at a distance of 2 mm from the material. The surface effects in all cases were (i) the creation of craters in the center of the lased areas, (ii) a border of fusion material at the edge of the craters, and (iii) cracks or fractures at a distance from the target areas. For the cements, amalgam and composite, the effects included a projection of material from the center to the edge of the lased area and/or the apparent photovolatization of light particles. For all materials the laser induced deeper absorption in the areas of direct contact than when it was held at a distance. This suggests that the fiber should be in contact with the restorative material for lasing in the straight part of the canal when the objective is either to pass through the material or alongside it between material and tooth. If the practitioner cannot determine the direction of the curve of the canal, lasing should be performed at a distance to weaken the material and thus permit more efficient use of an ultrasonic device. Lasing should in all cases be performed under close X-ray monitoring. Provided that sufficient caution is used, the laser may be helpful in removing restorative materials during retreatment.

An evaluation of the bactericidal effect of the Nd:YAP laser.

The aim of this in vitro study was to compare the efficacy of a classical irrigant with that of a laser in disinfecting a contaminated root canal. Thirty canals of extracted single-root teeth were prepared with files to size #20. The teeth were sterilized with Germispad (Spad, France) for 30 min and then inoculated with Streptococcus mitis ATCC 33399. By randomization, the teeth were divided into six groups of five teeth each. In the first group, teeth were neither inoculated nor prepared. This was the sterility control group (1). In the second group (2) teeth were inoculated without any preparation: as positive controls. The third group was inoculated and then hand-instrumented with files to size #30 with 5.25% NaOCl as irrigant. This was the hand instrumentation group. The other groups were prepared with hand instrumentation with files to size #30, using sterile water as an irrigant, and the canal was then lased with different frequencies as follows: group 4, frequency of 5 Hz and power of 260 mJ; group 5, frequency of 10 Hz and power of 310 mJ; and group 6, frequency of 30 Hz and power of 300 mJ. After experimentation, the residual colonies were counted. The results indicated that (i) the treatment with NaOCl and manual instrumentation effectively inhibited the growth of Streptococcus mitis ATCC 33399; and (ii) the antibacterial effect of the Nd:YAP laser depended on the frequency. Only a frequency of 30 Hz of the Nd:YAP laser inhibited the growth of Streptococcus mitis ATCC 333999.

Study of the Nd:YAP laser. Effect on canal cleanliness.

The aim of this study was to evaluate the canal cleanliness achieved by five different preparation techniques, including use of the laser. By randomization, the palatal roots of 50 maxillary molars were assigned to one of five groups. During all preparations, a sodium hypochlorite irrigant was used. The following techniques were studied: (A) manual instrumentation (serial preparation), (B) laser preparation (Nd:YAP laser), (C) manual preparation with laser as adjunct, (D) manual preparation with a subsonic device as adjunct (MM 3000 with shapers), and (E) manual instrumentation with a subsonic device and laser as adjuncts (MM 3000 with shapers, Nd:YAP laser). The canal wall surfaces were examined under a scanning electron microscope at all levels with a new method using grid incrustation on the microscope screen. Techniques A and C differed from each other only by the size of the debris particles, which were smaller for the C preparation. For laser preparation (B) there was little increase in canal diameter increase, and a substantial amount of debris was present. The differences between techniques A, C, and D were not significant. The use of the subsonic device and laser together as adjuncts (E) showed the cleanest preparation with very little debris, opened tubules, and very small particle size. This result suggests that the laser has a potential in ensuring optimal canal cleanliness.

An in vivo recording of variations in oral temperature during meals: a pilot study.

Thermocycling is often used in the in vitro evaluation of marginal leakage of dental restorations. The in vivo temperatures occurring during meals were recorded at different surfaces of restored teeth, to compare the variations in temperature with laboratory thermocycling. Temperatures were recorded with thermocouples at three different sites: (1) on the facial surface of a silver amalgam restoration, (2) at the base of a silver amalgam restoration and (3) within the root canal. The maximal temperature differences between upper and lower extremes were 29.6 degrees C at the base of a coronal restoration, 27.1 degrees C at the facial surface, and 11.8 degrees C within the root canal. A practical regimen for these experiments is suggested in the light of these ranges because many thermocycling regimens in in vitro studies appear extreme or unrealistic.

Stereochemical structure characterization of dental gutta-percha.

Natural gutta-percha is well known for its mechanical properties which derive from its particular stereochemical 1-4 trans structure. The gutta-percha contained in the various commercially available points is made of natural gutta-percha which has undergone a heating process to allow inclusion of the other elements found in its composition, especially a nonspecified coloring agent. The purpose of this study was to confirm that the stereochemical 1-4 trans structure contained in commercial points is identical to that of natural gutta-percha and that it has not been altered by the heating process inherent to its fabrication. Furthermore, we wanted to identify the coloring agent also included in its composition. Samples of commercial gutta-percha points were studied using nuclear magnetic resonance spectral analysis and compared with that of natural gutta-percha. Our results show that both natural and commercial gutta-percha mainly have a 1-4 trans stereochemical structure and that the coloring agent is erythrosin.

Thermomechanical analysis of dental gutta-percha.

Samples of three common commercial gutta-percha endodontic filling points (Hygienic, Mynol, and Maillefer Pink) and a sample of natural gutta-percha were submitted to thermomechanical analysis under pressures ranging from 0.01 to 0.2 N. Samples of Mynol and Maillefer filling points were thermally treated and then submitted to thermomechanical analysis in parallel with differential scanning calorimetric analysis. The results show that thermomechanical analysis gives results distinct from those obtained by classic dilatometry and that it is a technique well suited to the study of the thermoplastic properties of gutta-percha. An analysis of the results shows that the amount of the inorganic component used in a commercially available endodontic point has a strong influence on its thermomechanical properties. Thermodynamical properties and the "thermal history" of the gutta-percha are also important. Both temperature and force should be controlled in order to assess the thermomechanical properties of endodontic filling points, while the latter have not yet been codified in clinical procedures.

Photocalorimetry of light-cured dental composites.

The reactivity of six light-curable dental composite materials has been examined by a novel technique, photocalorimetry, which permits in situ analysis of photoreactions in opaque cross-linking systems. According to photocalorimetry, the rates of reaction of dental composites decrease in the following order: Uvio Fil, Scintilux 2, Visio Dispers, Visio Fil, Silux, and Prisma Fil. By comparing the heats of reaction of the composites and of the resins, one may assume that at least two types of reactive resins are used in these dental composites: one with a heat of reaction of about 35 cal/g, and others with considerably less reactivity, ranging from 14 cal/g to 25 cal/g. Photocalorimetry allows for a rapid assessment of the rate of polymerization (photospeed) of light-curing dental composites and is ideally suited for the study of kinetics and for optimization of formulations.