Laser-Induced Fluorescence for Subgingival Calculus Detection: Scientific Rational and Clinical Application in Periodontology

Introduction

P eriodontitis is an inflammatory disease of infectious etiology. ^1,2 The periodontal pathogens present in the bacterial biofilm ³ initiate and maintain the inflammatory host response. The latter is responsible for the periodontal breakdown, attachment loss, and the formation of pockets. The main treatment strategy for periodontitis consists of professional scaling and root planing (RP), accompanied by the implementation of rigorous oral hygiene. ⁴ Therefore, controlling the supra- and subgingival biofilm is essential for periodontal healing. All plaque-retaining factors, such as iatrogenic restorations, caries, supragingival calculus, should be eliminated prior to the root debridement (RD). Calculus is formed when the bacterial biofilms calcify. ^5,6 The roughness of the calculus makes it an ideal surface for bacterial adhesion and proliferation. Subgingival calculus (SGC) removal is the main purpose of root instrumentation, ⁷ which requires mechanical or photomechanical (laser) instrumentation. RD reestablishes the composition of the subgingival biofilm, which is compatible with periodontal healing. Therefore, periodontal pockets (PP) could heal (reestablishing healthy sulci), mainly through the formation of a long junctional epithelium. Alternatively, the complete elimination of SGC was not found to be realistic. Residual SGC varied from 12 to 30% after the nonsurgical RP. A surgical access flap was made to obtain a direct visual detection of SGC, especially when treating the deep pockets. However, microscopic calculus could not be seen. Therefore, the negative effects of surgery (e.g., patient discomfort, increased gingival recession) should not be ignored. So far, the endpoint of RD was based on a clinical assessment by the practitioner, guided by the tactile sensation using a dental probe. Research has been conducted to develop a reliable tool for detecting SGC. New calculus detection techniques have been suggested in the literature, and subsequently placed in the market. These techniques rely on ultrasound or laser-induced fluorescence (LIF). The latter was used to develop one of the SGC detection tools in the market (Kavo® Diagnodent®, Germany), which could be coupled to an erbium:yttrium-aluminium-garnet (Er:YAG) laser device (Kavo Laser Key 3®, Germany). The latter was used to perform the LIF-controlled laser debridement (LD). The present review focuses on the scientific rationale behind LIF in SGC detection and discusses the clinical relevance of using this device.

LIF of Dental Calculus

Fluorescence is a luminescence in which the absorption of a photon triggers the emission of another photon with a longer wavelength. Several reports have described the autofluorescence of hard dental tissues, which vary depending on the pathological changes. ^{8, 9} Carious lesions have greater fluorescence intensity (FI), followed by monochromatic light excitation. The successful use of a red light (638 nm, 655 nm) to induce fluorescence has been used to differentiate the sound and the carious tissue. The abovementioned phenomenon facilitated the development of a dental caries detection tool, which was commercialized in 1998 (Diagnodent). It was suggested that this difference in FI was caused by the demineralization and alteration of the crystalline structure of the hard dental tissues. ¹⁰ Therefore, Hibst et al. explored the origin of fluorescence in the dental hard tissues. ¹¹ The dominant components of enamel and dentine are calcium phosphates, organized in hydroxyapatite crystals and are essentially organic components. In another study, Hibst et al. compared induced fluorescence signals (FS) in enamel and various calcium phosphates to find out whether inorganic components contributed to the signal. ¹² It was observed that calcium phosphates were not responsible for the baseline fluorescence of sound teeth. The carious process started with a demineralization phenomenon, which resulted in the dissolution of calcium. The impact of this demineralization was analyzed in the induced fluorescence. It was observed that demineralization increased the FS. Alternatively, red light excitation of the incubated bacterial strains revealed the fluorescing metabolites. This fluorescence was caused by the presence of bacterial fluorophores in the carious lesions. These molecules belonged to the porphyrins group. Protoporphyrin IX was found to be largely responsible for the induced fluorescence observed in the carious lesions. Porphyrins are metabolites that occur as an intermediate bioproduct in the synthesis of heme. Several types of oral periodontopathic bacteria, such as Porphyromonas gingivalis or Prevotella intermedia, synthesize the porphyrins and the other fluorophores. Calculus, especially in the subgingival area, contains large amounts of porphyrins because of the involvement of P. gingivalis and P. intermedia in the periodontal diseases. In fact, these periodontopathogens are frequently present in the subgingival pocket's biofilm. Therefore, using bacterial autofluorescence to detect SGC has been suggested for the periodontal diagnosis as well as as therapy. The fluorescence spectroscopy of the dental calculus was studied by Buchalla et al. The emission spectra was investigated from supra- and SGC at a wide range of excitation wavelengths in the ultraviolet and visible range (from 360 to 740 nm). ¹³ The emission spectra from calculus differed significantly from the clean root spectra. However, supra- and SGC had similar emission signals. Emission peaks at excitations of 570 and 740 nm were observed from calculus, but not from the clean root surfaces.

Kurihara et al. found that the subgingival calculus had 700 and 720 nm fluorescence peaks when excited at 635 and 655 nm, ¹⁴ respectively. Under these conditions, the fluorescence measurements of blood and periodontopathogenic bacteria showed no fluorescence peaks. It was concluded that the blood clot and the bacterial colonies masked the fluorescence peaks and prevented calculus detection. Therefore, a total elimination of the bacterial and blood deposits on the root surface was recommended before attempting to detect the calculus autofluorescence. This could be achieved by a thorough irrigation of any PP.

Diagnodent is a caries detection tool that relies on the fluorescence of demineralized carious lesions, as was mentioned previously. It has been suggested that Diagnodent could also be used as a clinical diagnostic tool for detecting the subgingival calculus. For the purpose of excitation, the device used a diode laser beam with a wavelength (InGaAsP, 655 nm) in the appropriate range to induce the calculus fluorescence peaks. A laser light was beamed through a central quartz fiber on to the root surface. Around this fiber, the additional secondary fibers collected the FS from the calculus and the hard dental tissues. The FS was transmitted to an optical detection unit after filtering the ambient and the reflected light. A detector (photodiode) measured the FS, and expressed it in relative units [U] (0–99 range). The relative unit was displayed on a digital screen, which informed the practitioner of the presence of calculus or a carious lesion.

Recently, this device has been coupled with an Er:YAG laser (Kavo, Laser key 3) to detect calculus by means of laser-fluorescence. The Er:YAG laser was found to be effective in the debridement of PP and based on experimental and clinical data the Er:YAG laser appears to possess characteristics most suitable for the nonsurgical treatment of chronic periodontitis ¹⁵ (for review see Schwarz et al. ¹⁵ ). In the abovementioned hybrid laser device, the Er:YAG beam is only triggered when SGC is detected on the root surface. Therefore, the FS of SGC is a feedback signal for the selective elimination of calculus.

Basic Studies

Krause et al. explored the efficacy of the Diagnodent system in detecting calculus. ¹⁶ The FS were analyzed on the extracted teeth with a periodontal involvement. A histometric analysis was conducted for the same areas tested with the Diagnodent. Toluidine blue staining was used to visualize the subgingival calculus in the histological sections. A correlation was found between the FI detected by the device and the presence of SGC. The addition of blood or saline solution to the root did not disturb the FS. Folwaczny et al. evaluated 30 extracted teeth for the FI of the root surfaces. ¹⁷ The samples were divided into three groups depending upon the media in which the FI was measured: air, electrolytic salt solution (ES) and bovine blood (BB). In air, the Diagnodent system detected a FI of 0.4 (±0.51) for cementum and 54.1 (±29.09) for calculus. The signal was weakened in the presence of BB or the ES. The difference in FI between cementum and calculus was nevertheless significant in each group, which made it possible to distinctly detect the calculus FS. In the present study, the threshold value of FI for determining calculus was 5 [U], which made it possible to detect SGC with 100% specificity and sensitivity. Recently, the same group explored the effect in vitro of the threshold value on the residual calculus. ¹⁸ The residual calculus decreased with a decrease in the threshold level. The mean cementum thickness was 80 μm after LD and 90 μm on the untreated control roots. The loss of cementum after the laser irradiation was considered to be clinically acceptable. It was concluded that a threshold of <5 could be used in clinical practice to reduce the amount of the residual calculus.

Another study evaluated the amount of residual calculus after the mechanical RP of 40 extracted teeth. ¹⁹ The treatment endpoint was determined using a probe (control specimens) or the Diagnodent device (FI <5 [U]). On molars, the latter permits the reduction of the amount of residual calculus after RP, in comparison to the subjective probe appreciation. The detection superiority of Diagnodent was absent on single-rooted teeth.

Schwarz et al. compared the nonsurgical fluorescence controlled LD with ultrasonic debridement (UD). They used an experimental periodontitis model in beagles. ²⁰ Both the treatments reduced the periodontal inflammation and allowed for new cementum to be formed, thereby making the connective attachment possible. Periodontal regeneration was more pronounced in the group receiving the laser treatment. LD was less time-consuming than UD.

In addition, when compared to UD, fluorescence-controlled Er:YAG LD performed in vitro appeared to have the same efficacy in removing the root calculus. Furthermore, it permitted selective removal of calculus without any noticeable modification of the root surface (i.e., dentinal exposure). ²¹ The present findings are in accordance with an in vivo/histologic study (i.e., periodontally compromised teeth, considered for extraction), where the control therapy was scaling and RP. ²²

Schwarz et al. compared the residual subgingival calculus (RSC) on the root surfaces after laser (Kavo Key Laser 3), ultrasonic (Vector® system) or manual debridement. ²³ The treated teeth were periodontally compromised and programmed for extraction. Debridement was performed in vivo before the extraction. In the laser group, the LIF detection tool was used for all the specimens, and the absence of FI was considered to be the endpoint of the treatment. There were no significant differences between the laser and the ultrasonic groups in terms of the RSC values when the appropriate settings (140 mJ, 10 Hz) were used. Both the treatments were more effective in eliminating SGC than was the manual RP. Alternatively, the RSC in the laser-treated groups were proportional to the pocket depth, which suggested a lesser effective calculus detection in the deep pockets. Regarding the length of the procedure, LD was found to be less time consuming than the ultrasonic RP. LD resulted in a homogeneous root surface with rare alterations. Optimal cementum conservation was observed and the mean depth of the root changes was significantly lower than that found after UD.

Clinical Studies

To the best of our knowledge, no clinical research has been performed to determine the advantages of LIF over the manual detection tools, when performing the same therapy. In addition, few clinical studies have evaluated the treatment outcomes after LD by using an LIF-controlled Er:YAG device (Kavo Key Laser 3). These studies were not designed to evaluate the efficacy of the calculus detection device.

In the split-mouth study of Sculean et al. it was observed that LIF-controlled LD (160 mJ, 10 Hz) led to clinical improvements at 3 and 6 months, similar to UD. ²⁴ In another study, Tomasi et al. compared LD (160 mJ, 10 Hz) using LIF calculus detection, to UD. ²⁵ For the first treatment modality, the endpoint was the absence of calculus detection, whereas for the second it was based on subjective judgement by the practitioner. The mean debridement duration per pocket was 3.6 min (±1.3) for the LD group and 4.0 (±1.1) for the UD group. Both the procedures were effective in reducing the pocket depth and increasing the clinical attachment levels, at 4 months post-treatment. Based on the patients' perceptions, it was observed that the laser treatment generated less discomfort than did UD.

Recently, a study evaluated the adjunctive effect of LIF-controlled LD to scaling and RP. ²⁶ The addition of LD (160 mJ, 10 Hz) to the mechanical therapy did not improve the clinical outcome at 1 and 2 months. However, LIF-controlled LD permitted a reduction of the Il-1β and TNF-α amounts in the gingival crevicular fluid. Furthermore, it also permitted a reduction of the microbial recolonization rate.

Conclusions

LIF of the subgingival calculus is a proven phenomenon. All the in vitro studies have demonstrated the differential FI between the calculus and cementum, regardless of the environment (e.g., blood or water). However, the FS may be perturbed by the bacterial biofilms, cellular debris, and initial deep pockets. Prior rinsing of PP could be useful for preventing a weakened FS that can undermine the reliability of the system. Grafting the detection system onto an Er:YAG laser device represented a progress in the LD of PP. The data available confirmed the efficacy of this hybrid system for selectively ablating SGC without undermining the cementum layer. However, further research is required to determine the optimal detection threshold that would make it possible to increase the detection sensitivity with clinically acceptable root surface damage. In addition, the detection device seems to reduce the time needed for LD. In a clinical study, where the Er:YAG laser device was used in a continuous mode, the mean working time for the laser treatment in single-rooted teeth was 5 min, whereas it was 4 min for the ultrasonic treatment. It was observed that in all the other studies, where LD was found to be less time consuming, the LIF detection was responsible for the saving of time.

Calculus detection could be a valuable aid for the less-experienced practitioners who wish to perform laser pocket debridement, as it makes it possible to determine the endpoint of the treatment in a simple manner.

Future clinical studies should make it possible to confirm the data available on the advantages of LIF detection of subgingival calculus in the clinical periodontal practice. There is also a need to determine the relevance of using the standard threshold indicated by the manufacturer.