A surgical technique for administering laser irradiation to rat incisor dentin

With the advent of lasers in the field of dentistry, various methods have been used to expose dentin to laser irradiation. This was done indirectly through the alveolar bone, as well as directly on dentin wafers and on extracted human teeth. However, dentin is a vital tissue and the only way to show the ramifications of laser irradiation is by directly exposing the dentin to this energy source under in vivo conditions. In the present study we describe a highly reproducible method for accomplishing this. Male Sprague Dawley rats were anesthetized and an incision was made at the junction of the attached palatal mucosa and the unattached buccal mucosa parallel to the maxillary bones on both sides. This procedure exposed the underlying bone where a 2 mm square window, at a point 3 mm caudal and parallel to the gingival attachment, was cut out to the depth of the periodontal ligament. The site was flushed with a 0.9% sterile sodium chloride solution and gently dried with gauze. The beam of the laser was focused in the middle of the 2 mm square window and a pre-determined energy level and pulse duration (2 W 50 ms or 10 W 50 ms) were selected. The focal length guide was held at right angles to the dentin surface and in contact with the maxilla at all times. The energy densities of either 52.6 or 263.2 J/cm(2) were delivered to the 'root' dentin by a single pulse. The survival rate was 92%. The animals that died did so while under anesthesia and not after exposure to laser irradiation. The animals that survived tolerated the surgery and the laser irradition very well, regaining (and eventually surpassing) their initial weight 3 days after the procedures were performed. The thickness of the dentin that was exposed to laser irradiation was not completely penetrated by the energy levels used. At both energy levels some dentinal tubules appeared blocked and evidence of melted dentin was apparent. However, at the higher energy level two distinct zones were observed in the funnel-shaped lesion. In the inner zone distinct dentinal tubules were not readily visible, while in the outer zone the tubules were opened and completely free of debris. This technique is highly reproducible and yields consistent results with little or no stress to the animals. Although the energy levels used did not penetrate the entire thickness of the dentin, the depth of penetration of the laser beam appears to be related to the energy level used and the thickness of the dentin layer present.