this is a randomized controlled clinical trial, evaluating the antibacterial effect of laser diode and zinc oxide nano-particles when used as cavity disinfectants.Two groups, including 15 patients each, will be randomly allocated so that each group receives one of either interventions (15 patients will undergo cavity disinfection by laser diode, while the other 15 patients will undergo cavity disinfection by zinc oxide nano-particles). For each patient, a dentin sample will be taken before and after cavity disinfection. The dentin samples will be microbiologically analyzed to assess the difference in the bacterial count (out come assessment). The results will be statistically analyzed.

This study will be carried out on adult patients attending to the department of conservative dentistry at The Faculty of Oral and Dental Medicine, Cairo University, Egypt. The procedures will be carried out by postgraduate student Sarah Atef Khattab (B.D.S. 2011) from Cairo university without an assistant. The researcher will bear ultimate responsibility for all activities associated with the conduct of a research project including recruitment of patients, explaining and performing the procedures to them.

A : Excavation protocol:

The teeth of the patients that meet inclusion criteria will be anaesthetized, isolated with a rubber dam. Cavity opened using conventional high-speed rotary instruments. The central cariogenic biomass and the superficial parts of the necrotic dentine will be removed with round burs. Caries lesion will be completely removed in the enamel/dentin junction. The excavation procedure will be terminated as soon as the soft and wet dentine was removed and the remaining tissue was leathery but not hard on exploring. A dentinal sample will then be collected from the base of the cavity using sterile spoon excavator as a baseline for bacteriological assessment. Then, after application of either interventions , another dentinal sample will be collected using a sterile excavator and transferred into sterile tubes and transferred to the laboratory for microbiological analysis.

b. Application of the interventions: b.1: laser diode Each cavity will be irradiated in contact mode with continuous wave of radiation. The laser light is transferred through a 600µm flexible fiber optic tip by a special hand piece. The fiber optic will be disinfected for each use by 70% ethyl alcohol and inserted inside the cavity to 1 millimeter with a spiral continuous movement clockwise from the top to the floor and anti-clockwise in the reverse direction. This procedure improves the distribution of the laser light inside the cavity and to avoid excessive heat generation in the internal cavity surface. Irradiation time will be 15 seconds and repeated 5 times with 15 second intervals with contact, according to manufacture instructions. During this study the output power will be adjusted at 1.5 Watt.

b.2: Zinc oxide nano-particles: The zinc oxide nano-powder will be mixed with ethanol and applied in the cavity, where the ethanol evaporates, while the zinc oxide nano-particles infiltrate into the carious floor of the cavity.

N.B: In both methods, a dentin sample will be taken before the application of the disinfectant, then the zinc oxide nano-particles are applied in the cavity and left for 5 minutes, then another sample will be taken. The second specimen will be compared to the first specimen taken before cavity disinfection, regarding the streptococcus mutans count.

c .Assessment of Outcome: Samples of carious dentin will be collected with sterile excavator before and after application of both interventions. The dentin samples will be transferred to sterile container containing a 1millilitre thioglycollate medium used as a carrier, then this sterile container will be kept in an ice box and taken to the microbiology laboratory for processing, within an hour, by another examiner who is blinded to the type of agent applied after partial caries removal. Samples will be vortexed for two minutes, decimally diluted and 0.1 ml will be plated on Mitis Salivarius Bacitracin agar plates, these plates will be incubated anaerobically for 48 hour at 37 degree celcius then aerobically for 24 hour at room temperature. The number of colonies will be expressed as colony-forming units (CFU/ml) compared before and after application of the antibacterial agent.

Inclusion Criteria:

• Normal adults will be recruited in this study, with free medical history
• Have not received antibiotic therapy since 1 month before sampling
• Good oral hygiene
• Co-operative patients approving the trial
• In each patient required one or two deeply carious class v cavities either upper or lower permanent anterior teeth

Exclusion Criteria:

• Subjects with a history of allergy to any of the drugs or chemicals used in the study
• History of pulp pathology of the teeth which indicates root canal treatment is not included in the study
• Pregnancy and Lactating mothers
• Patients with any systematic problems or mental or physical disability
• Heavy smoking
• Xerostomia
• Lack of compliance

the examiner who will carry the microbiological analysis of dentin samples will be blinded to the type of agent applied during treatment. Finally, the treatment results will be assessed blindly by a statistician.

Blinding:

Blinding of the operator is not possible, because the natures of both interventions are obviously different. The operator is blinded until randomization into groups, to avoid biases with regard to the application of antibacterial agent. Also, the examiner who will carry the microbiological analysis of dentin samples will be blinded to the type of agent applied during treatment. Finally, the treatment results will be assessed blindly by a statistician also to avoid the risk of bias of results in favor of one intervention over the other.