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 »  Home  »  Endodontic Articles 3  »  An in vitro comparison of the bactericidal efficacy of lethal photosensitization or sodium hyphochlorite irrigation on Streptococcus intermedius biofilms in root canals
An in vitro comparison of the bactericidal efficacy of lethal photosensitization or sodium hyphochlorite irrigation on Streptococcus intermedius biofilms in root canals

G. J. Seal, Y.-L. Ng, D. Spratt, M. Bhatti & K. Gulabivala
Departments of Conservative Dentistry and Microbiology, Eastman Dental Institute for Oral Health Care Sciences, University College London, London, UK.

Microorganisms play a crucial role in the development of pulpal and periapical disease (Kakehashi et al . 1965). Infected root canals have a complex microbial flora consisting of cocci, rods, spirochaetes, filaments and fungi (Nair 1987) that are distributed along the root canal (Thilo et al . 1986) and may exist as loose collections in the moist canal lumen or as dense aggregates (biofilms) adhering to the dentine wall (Nair 1987). They may also penetrate the dentine to variable depths (Shovelton 1964, Ando & Hoshino 1990), up to 300 m (Horiba et al . 1990) or more. Eliminating such a polymicrobial infection from the root canal system to allow healing of the associated periapical lesion is the ultimate goal of root canal treatment. Contemporary treatment procedures to eliminate the infection, include mechanical enlargement of the main canal, irrigation with an antibacterial agent, interappointment dressing of the canal with an antibacterial medicament and finally, obturation of the resulting dead space. A range of different techniques result in similar success rates (Smith et al . 1993) but 20–30% may fail and if retreatments are considered, the failure rate is even higher (Sjögren et al . 1990). This may be attributed to many factors, but a major hindrance is the complex anatomy of the root canal space (Haapasalo & Ørstavik 1987, Wada et al . 1998). Further disadvantages of conventional treatment procedures include their skill-dependent nature, long treatment time, possible weakening of teeth due to widening of the root canal and use of medicaments such as sodium hypochlorite (Sim et al . 2001) and calcium hydroxide (Grigoratos et al . 2001).
New approaches to eliminate the infection from root canal systems include the non-instrumented technique (Lussi et al . 1995) and use of laser technology (Kimura et al . 2000). A key problem in achieving total kills of bacteria in root canals is that the antimicrobial agent may not have access to the bacteria because of anatomical barriers. A common property of these new techniques is that anatomical complexities do not pose the same barriers. Laser light shone on the crown surface could potentially be redirected in multiple directions by virtue of its transmission through enamel prisms and dentinal tubules, which effectively act as fibre optic channels (Odor et al . 1996). High power lasers such as CO 2 (Zakariasen et al . 1986, Moshonov et al . 1995, Le Goff et al . 1999), Nd:YAG (Rooney et al . 1994, Hardee et al . 1994, Fegan & Steiman 1995, Gutknecht et al . 1996, Ramsköld et al . 1997, Berkiten et al . 2000), Nd:YAP (Blum et al . 1997), diode (Moritz et al . 1997) and Er:YAG (Mehl et al . 1999) are commonly used but not all deliver the optic fibres into the canal. The antibacterial effects of these lasers are a function of dose-dependent heat generation. The amount of heat delivered can vary but has the potential to char dentine, ankylose roots, melt cementum, cause root resorption and periradicular necrosis (Bahcall et al . 1992, Hardee et al . 1994, Ramsköld et al . 1997). These disadvantages may however, be overcome by sensitizing the microorganisms with a photosensitive agent capable of triggering the release of bactericidal radicals when stimulated by light of an appropriate wavelength. This technique called ‘lethal photosensitization’, has been laboratory tested on planktonic microorganisms associated with periodontal disease (Dobson & Wilson 1992, Wilson et al . 1992, 1993, Sarkar & Wilson 1993), root canal infection (Poh et al . 2000) and carious lesions (Burns et al . 1993, 1995) with encouraging results.
The concept of the root canal infection as a biofilm has not been given much consideration thus far. A biofilm is defined as an aggregation of bacteria associated with a surface, embedded in an extra-cellular matrix of polysaccharide. Biofilm bacteria differ greatly in phenotype when compared with their planktonic counterparts and are far less susceptible to antimicrobial killing (Millward & Wilson 1989, Nichols 1991, Wilson 1994). Laboratory models of single-species biofilms have shown that large numbers of viable bacteria remain following 15 min contact time with sodium hypochlorite or povidone iodine (Spratt et al . 2001). To date, studies on the lethal photosensitization of bacteria that cause root canal infection have been limited to laboratory studies of bacterial suspensions or biofilms grown on nitrocellulose discs (Dobson & Wilson 1992, Poh et al . 2000). The lethal photosensitization of biofilms has shown promise (Dobson & Wilson 1992), but has not been applied to biofilms covering a root canal surface in situ . It is postulated that the formation and diffusion of the reactive free radicals responsible for the antimicrobial effect may be able to penetrate the conventionally unreachable areas within the root canal system and achieve the necessary bacterial killing.
The aim of this study was to determine the effect of lethal photosensitization of S. intermedius biofilms in root canals using Toluidine Blue O (TBO) photosensitizer and a 35 mW helium–neon (He–Ne) laser and compare the antibacterial effect of lethal photosensitization with 3% NaOCl irrigation. S intermedius was chosen for the test biofilm because (i) it has the ability to adhere to dentine and is a probable player in primary colonization (Tarsi et al . 1998); (ii) it has been frequently isolated from root canal infections (Vigil et al . 1997, Sunde et al . 2000); and (iii) it has also been isolated from root canals following debridement procedures (Sundqvist et al . 1998), indicating its ability to resist treatment and survive as a single colonizer.