Abstract: Objective This study aimed to evaluate the effects of treatment with different powers of Nd: YAG laser irradiationon root surfaces and Streptococcus mutans (S. mutans) adhesion. Methods Extracted teeth because of severe periodontaldisease were divided into the following four groups: control group, laser group 1, laser group 2, and laser group 3. Afterscaling and root planning, laser group 1, laser group 2, and laser group 3 were separately treated with Nd: YAG laser irradiation(4/6/8 W, 60 s); however, the control group did not receive the treatment. Scanning electron microscopy (SEM) was used todetermine the morphology. S. mutans were cultured with root slices from each group. Colony forming unit per mL (CFU·mL-1)was used to count and compare the amounts of bacteria adhesion among groups. SEM was used to observe the difference ofbacteria adhesion to root surfaces between control group (scaling) and laser group 2 (6 W, 60 s), thereby indicating the differentbacteria adhesions because of different treatments. Results Morphology alterations indicated that root surfaces in controlgroup contain obvious smear layer, debris, and biofilm; whereas the root surfaces in laser group contain more cracks withless smear layer and debris. The bacteria counting indicated that S. mutans adhesion to laser group was weaker than that ofcontrol group (P<0.05). No statistical significance among the laser groups (P>0.05) was observed. Morphology alterationsalso verified that S. mutans adhesion to laser group 2 (6 W, 60 s) was weaker than that of control group (scaling). ConclusionThis study demonstrated that Nd: YAG laser irradiation treatment after scaling can reduce smear layer, debris, and biofilmon the root surfaces as compared with conventional scaling. The laser treatment reduces the adhesion of S. mutans as well.However, Nd: YAG laser irradiation can cause cracks on the root surfaces. In this experiment, the optimum laser power of 6 W thoroughly remove the smear layer and debris, as wellas relatively improve the control of thermal damagee.

Key words: Nd: YAG laser, Streptococcus mutans, root surfaces