| laser therapy Effects of 10,600 nm carbon dioxide lasers on preventing caries A literature review Kenneth Luk1, 2, Ollie Yiru Yu2, May Lei Mei2, Norbert Gutknecht3, Chun Hung Chu2 & Irene Shuping Zhao1, 2 1 School of Dentistry, Shenzhen University Health Science Center, Shenzhen, China; 2 Faculty of Dentistry, The University of Hong Kong, Pok Fu Lam, Hong Kong, China; 3 Department of Operative Dentistry, RWTH Aachen University, Aachen, Germany Introduction Although dentistry has benefitted from technological advancements in recent years, dental caries remains a major oral health problem in most industrialised and non-industrialised countries affecting more than half of schoolchildren and a large majority of adults.1 Dental caries is a localised chemical dissolution of dental hard tissues caused by the action of acidic by-products of the metabolic processes of cariogenic biofilm.2 Conventional non-invasive strategies for caries prevention include oral health education, reduction of sugar consumption, use of fluoride, and application of pit and fissure sealants. Flu- oride principally works by hindering the process of de- mineralisation of enamel and dentine and promoting re- mineralisation of tooth surfaces with early signs of mineral loss. Pit and fissure sealants generate a physical barrier to prevent the access of dental plaque and its acid prod- ucts from damaging the enamel surface. However, den- tal sealants are only effective on the pit and fissures and not the smooth surfaces of teeth. New strategies, such as casein phosphopeptide amorphous calcium phos- phate3 and micro- or nanohydroxyapatite compounds, have been proposed to control the balance between de- mineralisation and remineralisation. It is essential to de- velop new caries preventive methods to control the dis- ease. A novel, non-invasive approach is the use of laser irradiation on enamel or dentine in preventing caries de- velopment. A number of studies reported the potential of laser irradiation on tooth roots or enamel in inhibiting formation of caries lesions.4–9 as the active laser medium.10 Subsequent CO2 lasers produced various emission laser lines with wavelengths ranging from 9,000 to 11,000 nm. The most common laser lines of CO2 lasers are centred at 9,300; 9,600; 10,300; and 10,600 nm, respectively. CO2 laser wave- lengths have a higher absorption coefficient to hydroxy- apatite than water. A conventional CO2 laser-emitting light at 10,600 nm is well-absorbed by minerals, while 9,600 nm has the best absorption coefficient to hydroxyapatite followed by 9,300 nm.1 However, the 10,600 nm CO2 laser has been commonly used in medicine and den- tistry, and most of the commercially available CO2 lasers operate only at this wavelength. The effect of CO2 lasers in caries prevention has been studied since the 1960s,1 when CO2 lasers (10,600 nm) were discovered to signifi- cantly inhibit enamel caries progression.7 Significantly less demineralisation was also found in CO2 laser-treated ( = 10,600 nm) dentine than non-lased dentine.9 Further- more, promising effects of combined CO2 laser irradiation and fluoride treatment in preventing enamel and dentine caries were reported.11–13 However, the mechanism of caries prevention of CO2 lasers remains to be elucidated. A literature search using the databases PubMed, Scopus and Web of Science revealed no comprehensive review to evaluate studies investigating the effects of actions of CO2 lasers in caries prevention. Therefore, the objective of this paper was to systematically review the evidence regarding the effects of CO2 lasers ( = 10,600 nm) on preventing dental caries. Methods Several types of laser were studied for caries preven- tion. The wavelengths of neodymium-doped yttrium alu- minium garnet (Nd:YAG) lasers ( = 1,064 nm) and argon lasers ( = 488–514 nm) make them difficult to be ab- sorbed by enamel. On the other hand, carbon dioxide (CO2) lasers ( = 9,000–11,000 nm) are highly absorbed by dental hard tissues and thus have good potential for use in caries prevention.7, 9 The first CO2 laser adopted a mixture of nitrogen, helium, and CO2, and CO2 acted Search strategy Two investigators (KL and ISZ) performed a systematic search of articles archived in three databases, PubMed, Scopus and Web of Science. The following keywords were used to identify relevant articles: [(CO2 laser) OR (carbon dioxide laser)] AND [(dental caries) OR (tooth remineralisation)]. There was no publication year limit, and the last search was made on 31 January 2019. Stud- ies published in English through 2018 and archived in the 26 prevention 2 2020