CO22 laser and the diagnosis of occlusal caries: in vitro study

J. Dent. 1993;

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21: 234-239

CO2 laser and the diagnosis of occlusal caries: in vitro study C. Longbottom* and N. B. Pitts*t *Department of Dental Health and jDental

Health Services Research Unit, Dental School, Dundee, UK

ABSTRACT The diagnosis of small lesions in pit and fissure sites is becoming increasingly problematical. This study was designed to evaluate, in vitro, the potential use of a carbon dioxide (CO,) laser technique as an aid to the diagnosis of incipient pit and fissure caries. Vaporization of the organic material in the ‘early’ carious lesion should lead to its carbonization and thus make it more conspicuous. Pilot studies were carried out to identify lasing parameters which produced no visible effect on sound enamel but which caused charring (carbonization)ofwhite spot fissure lesions. Fifty extracted human molars and premolars were air-polished on the occlusal surfaces and independently scored clinically for caries. both before and after lasing. The teeth were subsequently sectioned and examined histologically. Of the 37 sites histologically scored as sound or exhibiting precavitation lesions, eight were correctly scored as sound both prelasing and postlasing. Of the 29 precavitation lesions detected histologically, five were detected clinically prelasing and 11 were detected postlasing. This 21% difference in the sensitivity of caries diagnosis between the prelasing and postlasing examinations was statistically significant (at the 95% level). There were no false-positive caries diagnoses. Further research, in particular the refining of lasing parameters employed. is indicated. KEY WORDS: J. Dent. 1993; February 1993)

Caries, diagnosis; 21:

234-239

Laser (Received

7 December

1990;

reviewed

15 January

1991;

accepted

20

Correspondence should be addressed to: Dr C. Longbottom, Preventive and Children’s Section, Department of Dental Health, Dental School, Park Place, Dundee DDI 4HR. UK.

INTRODUCTION Caries in pit and fissure sites represents the greatest proportion of lesions, especially in children (Brunelle and Carlos, 1982; Bohannen, 1983: Ripa, 1985). However, the process of caries diagnosis in these sites is a notoriously difficult task (Kidd and Joyston-Bechal, 1987) which appears to have become more difficult with the widespread use of fluoride dentifrices (Sawle and Andlaw. 1988; Creanor et al., 1990). The validity of caries diagnosis in pits and fissures by use of a mirror and probe was questioned by Parfitt (1954) and Miller and Hobson (1956). and currently visual diagnosis of clean dry teeth without the use of a probe is the recommended procedure (Kidd, 1984: Pitts. 1991). despite its low sensitivity (Lussi. 1991). A number of aids to caries diagnosis in pits and fissures have been and are being investigated (Rock, 1987: Levenkind. 1988). these include fibreoptic transillumination (Friedman and Marcus. 1970; Menzel and des Bordes. 1974: Dooland and Smales. 1982), bitewing @ 1993 Butterworth-heinemann 0300-s7 12/93/040234-06

Ltd.

radiography, in its conventional form (Creanor et al.. 1990; Kidd et al., 1991) and with the aid of digital enhancement (Russell and Pitts, 1991; Wenzeletal.. 1991). the measurement of electrical resistance (Rock and Kidd. 1988; Pieper et al.. 1990), fluorescence under visible-laser light (Bjelkhagen et al., 1982; Sundstrom et al., 1985) and endoscopy, with and without filtered fluorescence (Pitts and Longbottom, 1987; Longbottom and Pitts, 1990). The ideal method would produce a sensitivity of 1.00 with a specificity of 1.00. i.e.. identifying all carious and sound sites correctly on every occasion. Any method which increases sensitivity above that produced by visual diagnosis alone but does not decrease specificity (below 1.00) would be of potential value in the clinical diagnosis of pit and fissure caries.

Lasers Benedetto and Antonson (1988) suggested that the CO? laser, which emits infrared radiation, might be used to aid caries diagnosis in pits and fissures. The term laser is an

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acronym for ‘Light Amplification by Stimulated Emission of Radiation’. A laser beam is an intense beam of monochromatic electromagnetic radiation. The potential applications of lasers in dentistry have been studied for over 20 years (Featherstone and Nelson, 1987). The basic physics of lasers has been detailed by a number of workers (Fisher, 1983; Birngruber, 1989; Hillenkamp, 1989). and was summarized recently by Midda and Renton-Harper (1991). The effects of lasers on calcified tissue have been extensively studied and possible applications include enamel ‘etching’, caries prevention, caries removal, as well as endodontic and periodontal procedures (Willenborg, 1989, Zakariasen et al., 1991). The CO, laser produces radiation which coincides closely with some absorption bands of apatite (Nelson and Featherstone, 1982; Nelson and Williamson, 1982). The nature of the interaction of CO, laser radiation with calcified dental tissue has been summarized by Jeffrey et al. (1990) and the effects have been reported by a number of workers. Stern et al. (1972) using a pulsed CO, laser, demonstrated surface changes in enamel indicating fusion of the apatite crystals, together with microscopic fissure formation. Lobene et al. (1968) and Kantola et al. (1973) demonstrated that CO, laser irradiation produced phase changes in hydroxyapatite and recrystallization and growth of crystal size. SEM studies by Brune (1980) and von Lenz et al. (1982) confirmed these ultrastructural and crystallographic effects. These changes indicate that a surface temperature of 1400°C is reached at the irradiated tooth site. It is of note that a number of workers have shown that laser-treated enamel demonstrates a reduction in enamel solubility (Stern et al., 1972; Boehm et al., 1977; Kuroda and Nakahara, 1981; Nelson et al., 1986). The reasons for this effect are. as yet, unclear, though they are probably related to larger crystal size and loss of prismatic structure (Ferreira et al., 1989; Oho and Morioka, 1990). Nelson et al. (1986) confirmed previous findings that surface enamel was altered by lasing-a localized. surface melt with a glaze-like appearance, together with significant macroroughening and a reduction of up to 50% in acid solubility. Subsequently, Nelson et al. (1987) confirmed these findings and estimated that the zone of enamel denatured by lasing was lo-20 urn below the surface, but varied with the laser wavelength, the deposited peak power and the number of pulses. Pate1 et al. (1988) using a continuous wave (CW) CO, laser, demonstrated four distinct surface changes in lased enamel: (i) ablation; (ii) melting; (iii) resoliditication, and (iv) cracking and flaking. A review of the literature (Jeffrey et al., 1990) suggested that using a CO, laser of 10.6 urn, energy densities of l-100 J cm-2 delivered in a period of ltY-1e4s will produce rapid melting and recrystallization of enamel. Increasing the energy density and laser interaction time will produce a ‘drilling’ or ‘cutting’ effect (Nelson et al.. 1986). The effects of CO, laser radiation on dentine have also been studied (Lobene et al.. 1968; Kantola et al., 1973; Lhuisset, 1979; Bonin et al., 1985; Nelson er al., 1986; Launay et al., 1987). In summary, using a CW CO, laser,at

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energy densities less than those required to produce enamel changes, carbonization of the organic material in dentine will occur, together with a melted and recrystallized surface zone of inorganic material which resembles enamel rather than dentine. Increasing the laser power will eventually produce hard tissue destruction/removal but the extent of this varies, depending on the power level and interaction time used (Jeffrey et al., 1990).

Caries diagnosis Radiation of wavelength 10.6 urn, the most commonly used for CO, lasers, is strongly absorbed by water to the extent that it can be vaporized instantaneously (Carruth and McKenzie, 1986). The fact that most biological tissues contain large quantities of water makes them vulnerable to destruction by irradiation from a laser beam of such a wavelength-the water in the tissue is vaporized leaving a residue of carbon-based material (Absten and Joffe, 1989). The earliest stage of a carious lesion comprises a small subsurface zone of demineralization which is more porous than the surrounding enamel (Silverstone and Mjor, 1988). The philosophy underlying this application of a laser as a diagnostic aid assumes that this zone contains more organic material (derived from the mouth) than does sound enamel. Stack (1954) demonstrated a threefold increase in the organic content of the ‘early’ enamel carious lesion compared with adjacent sound enamel. Photovaporization, by a CO, laser, of this organic material in the incipient carious lesion will leave a carbonized residue which will appear black (Benedetto and Antonson, 1988). At low power levels and short, interaction times the inorganic substances of sound enamel, with a minimal watercontent. will be much less affected by the CO, laser beam (Featherstone and Nelson. 1987). Benedetto and Antonson used this technique on 50 recently extracted molars and premolars in order to visualize enamel fissure caries. These workers found differences between the number of teeth diagnosed as carious by this method compared with diagnosis using a sharp probe, but failed to report validation of the diagnoses by reference to histological examinations. Caries diagnosis by probing has been demonstrated to be inaccurate (Pa&t, 1954; Miller and Hobson, 1956) and histological appearance is generally recognized to be the appropriate ‘gold standard’ with which to evaluate such diagnoses (Pittsl991). In addition, Benedetto and Antonson did not preclean the occlusal surfaces to remove (any) extraneous organic material prior to lasing. This apparent oversight may have influenced their findings. The aim of the present study was to evaluate,in vitro and with histological validation, the potential use of a CO, laser technique as a possible aid to the diagnosis of incipient caries in occlusal pits and fissures.

MATERIALS

AND METHODS

Using a prototype Continuous-Wave CO, laser (Laser Ecosse. Dundee and Edinburgh Instruments, Edinburgh,

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Table 1. Diagnostic criteria used by the independent examiner for the examination of the 50 test teeth to assess caries status

Score 0 1 2

3 4 5

Visual criteria No lesion-sound enamel White spot lesion in enamel Brown spot lesion (with or without a white spot lesion) including black lesions, but excluding areas assessed as ‘stained’ but non-carious Shadow in dentine (with or without a white spot lesion), i.e. subsurface opacity or darkness Cavity which appears to be 0.5 mm in diameter or less Cavity which appears to be > 0.5 mm in diameter

The examiner was asked to annotate any unusual appearance on the surface of any teeth. No probe was used for the diagnosis of caries.

UK), a pilot study was carried out to determine lasing parameters which would produce no visible effect on sound enamel but which would cause charring of natural white spot lesions in human extracted teeth. Appropriate parameters were found to be: beam diameter = 1 mm; power = 3 W; pulse time = 0.5 s; pulse interval = 0.12 s; power density = 381 W cm-2. Fifty extracted posterior teeth (48 permanent and two deciduous teeth), which had been stored in formal-saline (to prevent bacterial growth) and water, were air-polished on their occlusal surfaces using a Prophy-Jet (Dentsply, Weybridge, UK) instrument for 1 min (using Prophy-Jet powder No. 1, at an approximate pressure of 60 p.s.i. (414 kPa)) in order to remove all extraneous organic material (Garcia-Godoy and Medlock, 1988; Strand and Raadal. 1988). The teeth were rinsed with water and stored in thymolized water. They were then air-dried and photographed in colour. to produce X 4 magnification prints of the occlusal surface of each tooth. Pencil tracings of the occlusal surfaces of each tooth were made from the prints and copies of these tracings produced. The teeth were then examined individually by an independent examiner not involved with the previous methods in this study (A.N.), using the specified examination conditions and visual diagnostic criteria given in Table Z (after WHO, (World Health Organisation, 1987)). The examiner marked on a copy of the tracing of each tooth any areas designated as carious according to the specified criteria. A randomly selected 20% of the teeth were re-examined 1 week later to test for intraexaminer reliability. The teeth were positioned at the focal distance of the laser (300 mm), by, first, placement approximately at the focal distance and, second, by visual adjustment, until the helium-neon guide-beam was in focus. Each tooth was thus at the focal point of the CO, laser beam. The teeth were then dabbed dry with paper tissues and the entire occlusal surface of each respective tooth was exposed in turn to a CO, laser beam utilizing the previously stated parameters and using a scanning action. The laser was aimed using X 16 magnification binoculars and the builtin helium-neon laser aiming beam of the prototype

Table II. Caries scores used for assessment sections Score 0 1 2

3 4

of histological

Histological criteria No caries apparent Carious lesion(s) in outer half of enamel only Carious lesion(s) into inner half of enamel but not into dentine (up to and including the amelodentinal junction) Carious lesion(s) through enamel and into outer half of dentine Carious lesion(s) through enamel and into the inner half of dentine

apparatus. The scanning action involved movement of the guide-beam across the occlusal surface by the distance of the beam diameter between each pulse, the path being traced back and forth across the surface until the whole surface had been scanned. The teeth were positioned so that the plane of the occlusal surface was perpendicular, laterally and vertically, to the direction of the laser beam. Oneoperator(C.L.)carriedoutirradiationofallofthe teeth. All the teeth were then re-photographed under the same conditions as previously and re-examined. For this reexamination, the caries scores were marked onto a new set of the original tracings without reference to the first results. As previously, a random selection of 10 teeth were re-examined to test for intraexaminer reliability. The teeth were then serially sectioned, using a Microslice (Metals Research, Cambridge, UK), to produce longitudinal ground sections of 400 pm thickness. The plane of sectioning was chosen to provide optimal examination of the fissure patterns of each tooth. Generally this was buccolingually for premolars and mesiodistally for molars. Sections were mounted in Canada Balsam and examined under a microscope, at X 63 magnification using polarized light. The presence and extent of any caries in enamel and/or dentine was noted for each tooth and scored according to the criteria given in Table ZZ. Examples of photographs of prelased and postlased teeth are given in Figs Z-4.

RESULTS The caries scores for each of the three different examinations for the 50 teeth were tabulated so that the prelasing and postlasing scores could be validated by reference to the histological scores. Thirty-three sites were diagnosed clinically as having cavitated lesions both prelasing and postlasing, and were excluded from further analyses since they required no aid for diagnosis. This left a total of 37 sites for statistical analysis. A comparison of the prelasing visual diagnosis for these 37 non-cavitated sites with the reference histological carious/sound diagnoses is set out in Table ZZZ.The sensitivity of the prelasing diagnoses was 17%; the specificity was 100%. Acomparison ofthe postlasing diagnoses for these same 37 non-cavitated sites and the reference histological diagnosis is given in Table IV. The postlasing sensitivity

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Fig. 1. A premolar tooth prior to laser application. Note the white spot lesion in the base of the fissure (arrow).

fig. 2. The same tooth as in Fig. 1 after application of the CO, laser beam. Note the distinct dark areas in the base of the fissure (arrows).

fig. 3. A molar tooth prior to laser application. Note the intact white surface at the centre of the mesial fossa (arrow).

Fig. 4. The same tooth as in Fig. 3 after application of the CO, laser beam. Note the darkened cavitation area at the centre of the mesial fossa (arrow).

was 38%; the specificity was again 100%. The 11 sites correctly diagnosed as carious in the postlasing examination included the five sites diagnosed as carious prior to lasing. The postlasing scores resulted in an increase in the sensitivity to 38% in comparison to the prelasing diagnoses of 17% (Table ZZZ,Iv). This was statistically significant when tested for differences in proportions with a paired case (Table v). For these 37 sample sites there were no false-positive caries diagnoses in either the prelasing or postlasing examinations. Kappa values for the reproducibility studies were: prelasing - I.00 and postlasing 0.80.

DISCUSSION The application of a laser beam to tooth tissue results in a complex interaction involving many variables. This present study utilized one particular set of lasing parameters and a specific lasing technique in order to evaluate the potential usefulness of a CO, laser as an aid to caries diagnosis in pits and fissures. The results suggest that,

whilst the particular diagnostic technique utilized is not totally accurate, it does indeed facilitate the diagnosis of incipient occlusal carious lesions and produces a higher sensitivity than when visual diagnosis alone is employed. The variation of any of the lasing parameters-beam diameter, power, pulse time, pulse interval and duration of lasing-might result in greater accuracy in the diagnosis of carious pit and fissure sites. In view of the difficulty of caries diagnosis in pit and fissures there is an urgent need for the development of any technique which facilitates caries diagnosis in these sites. Furtherin vitro research into the precise lasing technique and parameters which will produce the optimal levels of sensitivity and specificity in caries diagnosis is therefore indicated. If a more sensitivie technique were to be developed in vitro. a number of factors would need to be considered before any recommendations for in viva studies could be carried out. First of all, the safety aspects (for both patient and operator) would need to be thoroughly investigated.

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Tab/e 111. Comparison of the prelasing visual and the histological carious/sound diagnoses for the 37 noncavitated sites

Histological

diagnosis

Prelasing visual diagnosis Carious Sound Total

Tab/e IV. Comparison of the postlasing visual and the histological carious/sound diagnoses for the 37 noncavitated sites

Histological

diagnosis

Postlasing visual diagnosis Total Carious Sound

Carious Sound

5 0

24 8

29 8

Carious Sound

11 0

18 8

29 8

Total

5

32

37

Total

11

26

37

Sensitivity = 17%; specificity = 100%.

Sensitivity = 38%; specificity = 100%.

Tab/e K Prelasing and postlasing diagnosis for 29 lesions diagnosed carious histologically

incipient occlusal lesions when compared with visual diagnosis alone. Research to attempt to further refine the lasing technique would appear to be indicated.

Prelasing visual diagnosis Carious Sound

Postlasing visual diagnosis Carious Sound 5 6

0 18

Difference in sensitivity = 0.2069. t = 2.75, P < 0.05, % confidence interval 0.059-0.354.

Whilst CO, laser surgery of soft tissue has been studied and is an accepted technique, the problems associated with heating of pulpal tissue and reflection of the laser beams from teeth and/or filling materials require to be clarified (Jeffrey et al.. 1990). Secondly, the effects of CO, laser application on carious lesions themselves need to be clarified. It could also be of value to clarify the theoretical ability of lasing to sterilize carious lesions (bacteria present ought to be carbonized). The effects of lasing on subsequent fissure sealant application or attempts at lesion remineralization also need to be investigated, since carbonization of the enamel organic matrix will probably have occurred. Recently, Zhang et al. (1992) have demonstrated that a combined CO, laser/fluoride treatment of occlusal surfaces of molars can inhibit further lesion progression of artiticially induced lesions. Thirdly, the effect of increasing the visibility of noncavitated lesions on patients’ opinions of aesthetics and on dentists’ decisions to restore teeth needs to be assessed. It would be unfortunate if small precavitation lesions which were not at a stage which required operative intervention were made more conspicuous as an aid to risk classification and planning of preventive care but were then treated operatively. The preventive concept dictates that these lesions should receive active preventive care in the first instance. The correlation between true lesion size and postlasing visual scores also needs to be investigated. Further research involving larger numbers of lesions is therefore indicated. It is concluded that a technique involving the application of a CO, laser beam to the occlusal surface of human posterior teeth in order to facilitate caries diagnosis at these sites has been tested in vitro and been found to produce an increase in the sensitivity of diagnosis for

Acknowledgements The authors are grateful to Ferranti Industrial Electronics (Laser Ecosse) and Dr I. D. Duncan and his staff at the Department of Obstetrics and Gynaecology, Dundee University for access to the CO, laser. Especial thanks go to Dr A. Neilson of the Department of Dental Surgery, University of Dundee, for acting as the independent examiner for the caries assessments, to Mrs E. Murison for assistance with the photography, and to MS J. Davies and Dr Z. Nugent for help with the statistical analysis. Professor Pitts acknowledges financial support from the Chief Scientist Office of the Scottish Office Home and Health Department. The views expressed above are those of the authors and not necessarily those of the Scottish Office.

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