Abstract
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The presence of cariogenic biofilm is closely related to the development of dental caries. Cariogenic bacteria produce acids that promote and increase lesion development by perpetuating the acidic environment represented by net mineral loss and carious lesion formation. 5 The suppression of these disease-related micro-organisms is an important requirement for the prevention and therapy of dental caries. 6 Streptococcus mutans remains deserving of detailed study, as an exemplar of an acidogenic, aciduric plaque bacterium, and because it shows a strong association with caries; however, the set of oral pathogens that form complex biofilms has so far been considered to be particularly difficult to suppress. 7 For a sufficient elimination of bacteria organized in biofilms, high doses of antimicrobial drugs are needed that might lead to unfavorable side effects.
For cavitated teeth in advanced stages, the traditional management has involved removal of all soft demineralized dentine before a filling is placed. 8 The rationale for this has been to prevent further caries development caused by remaining bacteria in the demineralized inner layer. The difficulty with this approach is the imminent risk of pulp exposure during the caries excavation procedures, with damage to the nerve of the tooth. 9 In addition, removal of all of the caries lesions has some disadvantages, including toothache and possibly weakening of the tooth structure. 10 New scientific developments in cariology and diagnostic systems have shifted dentistry's approach toward minimal intervention. Contemporary restorative dentistry has considered removing the infected layer of dentine, leaving the affected layer to preserve the integrity of the tooth. However, the total disinfection of the remaining dental tissue cannot be guaranteed. Eradication of bacterial pathogens related to dental caries with a noninvasive method is an important issue for oral care.
Antimicrobial photodynamic therapy, also known as photodynamic antimicrobial chemotherapy (PACT), has entered the dentistry field as a well-suited avenue for the control of bacteria in oral plaque biofilms where there is relatively easy access for application of the photosensitizing (PS) agent and light sources to areas requiring treatment. 11 Therefore, use of PACT as modality for the treatment of localized microbial infections following similar principles to those of photodynamic therapy (PDT) has gained greater attention with its applicability to different fields of dentistry such as periodontics, endodontics, and oral medicine. 12 Large classes of micro-organisms, such as gram-positive and gram-negative bacteria, fungi, and viruses, including a large number of oral micro-organisms, have been effectively inactivated by PACT with a wide range of PS, both natural and synthetic, available with differing physicochemical makeup and light-absorption properties. 13,14
Considering this, some investigators have also advocated the clinical application of PACT therapy as a possible adjunctive tool for a conservative approach for caries management in restorative dentistry, and to fight against cariogenic bacteria organized in biofilms that present increased resistance to conventional antibacterial treatment. 15 Because of its specific mechanism of action, PACT offers the particular advantage of immediate bacterial reduction; a condition only reached by other approaches after the treatment stages of two visits with a long time interval between them (∼3–6 months). 10
PACT for caries-related bacteria has been characterized as a process in which micro-organisms are treated with a PS drug and then irradiated with low-intensity visible lights of the appropriate wavelength. Usually, red or blue light sources, such as noncoherent light-emitting diodes (LED) and low-intensity lasers, are used, because these wavelengths are compatible with the most used PS available in dentistry. These light sources are easily available and present other clinical applications in restorative dentistry. Blue LED is often used in restorative dentistry for photocuring of resin-based dental materials. 16
A number of study groups have investigated the susceptibility of cariogenic species to photoinactivation. With this approach, gram-positive species such as S. mutans, Streptococcus sobrinus, various lactobacilli, and Actinomyces viscosus do appear to have their numbers greater reduced by more than tenfold by PACT when growing in liquid (planktonic) cultures and biofilms. 17,18
The initial experience with PACT in carious tissue suggested positive response and prolonged control expressed by Williams et al. in a collagen matrix that resembled demineralized dentine 19 and by Wilson et al. in extracted carious teeth. 20 Our group experience with PACT reported the bacterial killing response of this approach using different in vitro and in situ models that expose dentin samples to the oral environment. The bacterial reduction using similar protocol [red LED; 94 J/cm2; toluidine blue O (TBO) at 100 μg mL−1] are slightly minimized in carious substrate produced by these models reaching significant reduction (>3 log10). 21 –23
In view of promising pre-clinical results, in vivo studies have been suggested to support the bacterial reduction response obtained by PACT; however, relatively few in-depth studies have been published on this subject. Longo et al. 24 reported that PACT promoted a mean reduction of 82% of total bacteria in the treated cavities using aluminum-chloride-phthalocyanine associated with red laser. Guglielmi et al. 25 showed a range of log reduction from 0.91 to 1.38 using methylene blue dye and a red low-power laser. A similar outcome was reported by a clinical trial underway that evaluated microbial outcome promoted by PACT in dental caries. 26
Although the data from clinical trials cannot yet provide substantial evidence to support the effectiveness promoted by PACT, these studies identify challenges being faced to improve the performance of this therapy for caries management and oral biofilm inhibition.
To promote the desired therapeutic effect, knowledge of both light and PS distribution through target-tooth substrate is essential for dental caries treatment. 27 The depth of diffusion of the PS and the light certifying that they reach the full extent of carious tissue may influence the outcome. Moreover, the assignment of the appropriate light intensity (energy delivered) and PS dose (concentration, solubility, and polarity) to be used in each specific treatment would contribute to the development of an optimized practical protocol to be adopted for PACT in restorative dentistry. Notwithstanding these concerns, the information obtained can make possible investigative overcoming strategies and end-points for further research trials.
Despite all of the abovementioned considerations, PACT is returning to its origins in microbiology and is offering an alternative treatment for destroying oral micro-organisms. PACT in contemporary restorative dentistry possesses a future related to its potential for one-step high-level disinfection of the tooth cavity, and has led to a plethora of publications and patent applications. The recent advances in pre-clinical research and the development of reliable in vivo data (animal, in situ models) and clinical trials under investigation continue to provide cautious optimism that an effective clinical decision support to PACT for dental caries will eventually emerge. We expect to see more enhanced application of PACT to improve dental caries management in the years to come.