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J Am Dent Assoc, Vol 137, No 5, 638-644.
© 2006 American Dental Association
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RESEARCH

In vitro caries formation in primary tooth enamel

Role of argon laser irradiation and remineralizing solution treatment



Gary H. Westerman, DDS, MS, M. John Hicks, DDS, MS, PhD, MD, Catherine M. Flaitz, DDS, MS and G. Lynn Powell, DDS


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Background. The authors evaluated the effects of argon laser (AL) irradiation and remineralizing solution (RS) treatment alone and in combination on carieslike lesion formation in primary tooth enamel in an in vitro study.

Materials and Methods. The authors divided 10 caries free primary tooth enamel surfaces into four segments and assigned them to one of four treatment groups: no treatment control, AL irradiation alone at 13.5 joules per square centimeters (0.270 watts, 5-millimeter beam, 10 seconds), RS treatment alone for two minutes and AL irradiation before RS treatment. The authors created in vitro caries using a modified ten Cate solution. They evaluated longitudinal sections (three per tooth segment, 30 per treatment group) for mean lesion depth.

Results. After lesion formation, mean lesion depths (± standard deviation) were 179 ± 16 micrometers for the no treatment controls, 137 ± 19 µm for AL irradiation alone, 87 ± 9 µm for RS treatment alone and 68 ± 12 µm for AL irradiation before RS treatment. All treatment groups had mean lesion depths that were significantly less than those for the matched no-treatment control group (analysis of variance [ANOVA], Duncan multiple range [DMR] test, P < .05). AL irradiation before RS treatment significantly reduced lesion depth compared with AL irradiation alone or RS treatment alone (ANOVA, DMR test, P < .05).

Conclusions. The maximum reduction in lesion depth in primary tooth enamel was achieved when the RS—which contained calcium, phosphate and fluoride in a carbopol base—was combined with AL irradiation.

Clinical Implications. It would appear that to improve clinical caries resistance to enamel dissolution, AL irradiation before RS treatment could be used.

Key Words: Primary teeth; enamel; caries; argon laser; calcifying solution; remineralization; demineralization; artificial caries

Despite the decline in dental caries prevalence in the primary dentition, as documented in the First and Third National Health and Nutrition Examination Surveys,1–3 dental caries remains the most common chronic childhood disease.4 This transmittable bacterial disease affects more children than any other disorder and is particularly prevalent in families with low socioeconomic status5–9 and in immunocompromised children.10,11 Early childhood caries appears to be on the rise and is considered by some to be an epidemic.12,13 The American Academy of Pediatric Dentistry Policy Statement on the Dental Home advocates early oral health examinations and early dental treatment for preventive measures and restorative needs.14 This policy encourages establishing a "dental home" early in life and recommends that the first dental visit should occur before 12 months of age.

Prevention of the complex multi-factorial disease dental caries requires a risk assessment for future caries development and the institution of appropriate preventive modalities and oral hygiene education.13,15–19 Preventive modalities include use of systemic and topical fluoride, application of fluoride varnish, reduction of dietary cariogenic refined carbohydrates, use of improved plaque removal and oral hygiene techniques, placement of pit and fissure sealants, use of fluoride-releasing preventive and restorative materials, and the prescribing of antimicrobials such as chlorhexidine gluconate. A relatively simple and noninvasive caries preventive regimen is treating primary and permanent tooth enamel with low-fluence argon laser (AL) irradiation, either alone or in combination with topical fluoride treatment, resulting in reduced enamel solubility and dissolution rates.20–31

A recent in vitro study32 evaluated the influence of a commercially available remineralizing solution (RS) on caries formation and its progression in permanent tooth enamel. The broad-spectrum RS—which contained calcium, phosphate and fluoride ions in a carbopol base—enhanced enamel resistance to initial caries formation and caries progression. The ability of the enamel to resist in vitro caries formation was improved significantly over that of matched enamel surfaces treated with an acidulated phosphate fluoride (APF) gel.

The effect of the RS on primary tooth enamel is not known. Therefore, we conducted this in vitro study to evaluate the effects of AL irradiation alone and RS treatment alone and in combination on caries-like lesion formations in primary tooth enamel using polarized light microscopic techniques.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
We selected 10 extracted or exfoliated primary teeth with macroscopically caries-free buccal and lingual enamel surfaces, as determined with a stereo-zoom dissecting microscope (original magnification x 16, for this in vitro study. After we performed soft-tissue débridement and fluoride-free prophylaxis, we applied an acid-resistant varnish to the teeth, leaving two buccal and two lingual windows of sound enamel exposed. We divided the teeth into four segments (distobuccal, mesiobuccal, distolingual and mesiolingual) to provide four segments with sound enamel windows from each tooth and assigned each segment to one of the four treatment groups. This allowed each tooth to serve as a matched internal control for each of the treatments. The four treatment groups were no-treatment control, AL irradiation alone, RS treatment alone and AL irradiation before RS treatment.

We carried out AL irradiation using an AL unit (ARAGO, LaserMed, West Jordan, Utah) with low-fluence irradiation at 13.5 joules per square centimeter (0.270 watts, 5-millimeter beam, 10 seconds). We applied the RS (Remin+, Raintree Essix, Metairie, La.) to the enamel surface for two minutes per the manufacturer’s recommendation, followed by copious air-water spray rinsing. The RS contained calcium, phosphate and sodium fluoride in a carbopol base (trisodium phosphate, calcium phosphate, calcium chloride, dibasic sodium phosphate, sodium fluoride, carbopol).

We rinsed the tooth specimens in distilled/deionized water and then exposed them to synthetic saliva (20 millimolar sodium carbonate, 3 mmol/L phosphate, 1 mmol/L calcium, pH 7.0) for 24 hours. We created in vitro caries-like enamel lesions using a modified ten Cate solution (2.2 mM calcium, 2.2 mM phosphate, 5.0 mM fluoride, pH 3.9). After exposing the specimens to the artificial caries solution for seven days, we prepared three longitudinal sections from each tooth segment, resulting in 30 caries-risk sites per treatment group. We imbibed the longitudinal sections with water and examined them with polarized light microscopy in a blinded fashion. We captured and evaluated images of the lesions using a computer-interfaced image software program (UTHSCSA ImageTool, Version 3.0 Final, University of Texas Health Science Center at San Antonio, "http://ddsdx.uthscsa.edu/dig/download.html") for mean lesion depth determination. We made comparisons among the four treatment groups (no treatment control, AL irradiation alone, RS treatment alone, AL irradiation before RS treatment) using analysis of variance (ANOVA) and Duncan multiple range test analysis for paired samples (alpha level of ≥ P < .05).


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The tableGo shows mean lesion depths for the four treatment groups. When compared with the matched no-treatment control group (179 ± 16 micrometers), mean lesion depths were significantly less for AL-irradiation-alone group (137 ± 19 µm), RS-treatment-alone group (87 ± 9 µm) and the AL-irradiation-before-RS-treatment group (68 ± 12 µm). Mean lesion depths decreased by 23 percent for the AL-irradiation-alone group compared with the matched no-treatment control group (P < .05). A comparison of mean lesion depths between the no-treatment control and the RS-treatment-alone groups revealed a 51 percent reduction in mean lesion depth (P < .05). The mean lesion depth for the AL-irradiation-beforeRS-treatment group was 62 percent less than that for the no-treatment control group (P < .05). When compared with the AL-irradiation-alone group, the RS-treatment-alone group had an additional 36 percent reduction in mean lesion depth (P < .05). AL irradiation before RS treatment significantly reduced mean lesion depth compared with either AL irradiation alone (50 percent reduction, P < .05) or RS treatment alone (22 percent reduction, P < .05).


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  		TABLE Lesion depth by treatment group.

 
The histopathologic appearances of the representative carieslike enamel lesions from each group (Figures 1Go–4GoGoGo) may be correlated readily with the mean lesion depth findings among the four treatment groups. The representative lesions from each of the three treatment groups (Figures 2Go–4GoGo) demonstrated dramatic decreases in the body of the lesion depths compared with the representative lesion from the no-treatment control group (Figure 1Go). Not only were lesion depths affected, but also we noticed certain qualitative differences with the lesions. The body of the lesion in the AL-irradiation-before-RS-treatment group (Figure 4Go) showed a qualitatively decreased degree of positive birefringence (> 5 percent pore volume) and a negatively birefringent surface zone (5 percent pore volume). This indicates a lessened degree of demineralization. In contrast, the no-treatment control lesion (Figure 1Go) showed a relatively high degree of positive birefringence qualitatively, with loss of the typical striae of Retzius and prism markings within the body of the lesion.


Figure 1
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  		Figure 1. No-treatment control representative lesion (polarized light microscopy, water imbibition, original magnification x 200).

 

Figure 2
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  		Figure 2. Argon-laser-irradiation-alone representative lesion (polarized light microscopy, water imbibitions, original magnification x 200).

 

Figure 3
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  		Figure 3. Remineralizing-solution-treatment alone representative lesion (polarized light microscopy, water imbibitions, original magnification x 200).

 

Figure 4
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  		Figure 4. Argon-laser-irradiation-before-remineralizing-solution-treatment representative lesion (polarized light microscopy, water imbibitions, original magnification x 200).

 
While the surface enamels overlying the caries-like lesions for all four groups were intact, we noted a certain degree of irregularity in the surface enamel in the no-treatment control (Figure 1Go) and AL-irradiation-alone (Figure 2Go) groups. The lesions in these groups had positively birefringent surface layers (> 5 percent pore volume) that merged with their underlying positively birefringent bodies. In contrast, with the lesion in the RS-treatment-alone group (Figure 3Go), the surface zone was composed of interspersed areas of pseudoisotropy (5 percent pore volume) and negative birefringence (< 5 percent pore volume), which are indicative of a lessened mineral loss, compared with the lesions in the no-treatment control (Figure 1Go) and AL-irradiation-alone (Figure 2Go) groups. With the lesion in the AL-irradiation-before-RS-treatment group (Figure 4Go), the surface zone was uniformly negatively birefringent (< 5 percent pore volume) and composed a considerable proportion of the entire lesion depth (at least 50 percent).


   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Although the majority of clinicians associate laser use in clinical practice with soft tissue disease management, the U.S. Food and Drug Administration has approved lasers for several hard-tissue applications. These include, but are not limited to, polymerization of visible light–cured preventive and restorative materials, tooth-whitening procedures and cavity preparations for restoration placement.15,16,33,34 In the past 13 years, researchers have shown that lasers improve enamel’s resistance to dissolution, enhance micro-hardness, and lessen in vitro and in vivo caries formation and progression.20–31 In particular, the AL used at low-fluence irradiation levels significantly improves the ability of sound enamel, dentin and root surfaces to ward off a caries like challenge. In vitro caries models that create lesions closely resembling enamel caries have consistently shown a 30 to 40 percent reduction in mean lesion depth after caries initiation.20–26,31 Investigators have noted similar reductions when the lesions have undergone in vitro caries progression.23,24,31 This reduction in lesion progression was afforded by only a single 10-second exposure of the sound enamel to AL irradiation before lesion formation. The effect of the AL on sound enamel would appear to endure for a considerable time, as demonstrated by the effect of AL irradiation on in vitro lesion progression.

The mechanisms for the caries resistance20,21,31,35 imparted by low-fluence AL irradiation may be related to the creation of a microsieve network within tooth mineral substance. This network within the tooth mineral substance may promote rapid redeposition of calcium and phosphate mineral phases mobilized during a cariogenic challenge in an acidic environment. Also, researchers have suggested that the crystalline structure of tooth mineral substance may be altered and become more resistant to demineralization by removal of organic material and carbonate and reduction in internal crystalline strain. Caries resistance may be enhanced further by the increased fluoride uptake that occurs with laser treatment. The presence of increased fluoride within tooth mineral substance may result in the formation of less acid-soluble fluoridated hydroxyapatite (FHAP) from more soluble tooth mineral substance phases, such as octacalcium phosphate (OCP), dicalcium phosphate dihydrate (DCPD) and tricalcium phosphate (TCP).17–19,32–40 Researchers also have shown that swelling of the organic matrix of tooth mineral substance occurs, and this may make the pore structure of tooth mineral substance less accessible to organic acids produced by dental plaque.13,15,16,20,21,31,34,35

The ability of laser irradiation to affect enamel solubility is reflected by the change in the critical pH at which enamel dissolution occurs41–43; the critical pH for sound enamel is pH 5.5. After laser exposure, the critical pH is reduced to pH 4.8. This means that a fivefold increase in organic acid would be necessary to start demineralization of lased enamel. Using the results from our study, it is evident that AL irradiation of primary tooth enamel alone provided a significant level of caries resistance compared with untreated primary tooth enamel from the matched no-treatment controls (P < .05).

Clinical pilot studies25,27–29 have confirmed the in vitro caries-resistant effect of AL irradiation.20–24,26,30,31 These investigations have taken advantage of the well-established method for creating accelerated natural caries of placing orthodontic bands with open plaque-retentive slots on teeth scheduled for extraction before orthodontic therapy. Well-defined natural caries develop within the enamel adjacent to these plaque-retentive slots during a four- to six-week period. After tooth extraction, this methodology allows for laboratory assessment of the effects of experimental treatment within the confines of the oral environment. These in vivo studies, using the accelerated natural caries model, have found that sound enamel exposed to low-fluence irradiation AL for 10 seconds resulted in a 29 to 44 percent reduction in lesion depth, compared with matched untreated sound enamel.25,27–29 When low-fluence AL irradiation was combined with topical fluoride treatment, there was a 62 percent reduction in lesion depth in this accelerated natural caries model. The caries-protective effect of AL irradiation found in these clinical studies is similar to that reported in laboratory investigations using artificial caries media to produce in vitro enamel lesions.20–24,26,30,31 It appears that in vitro techniques successfully mimic the accelerated in vivo natural caries model and provide a certain degree of validity to laboratory in vitro caries methods for evaluating caries-preventive agents.

Recently, an RS (Remin+) became commercially available for use in clinical practice. It contains a proprietary formulation of calcium, phosphate and sodium fluoride in a carbopol base (trisodium phosphate, calcium phosphate, calcium chloride, dibasic sodium phosphate, sodium fluoride and carbopol) and is marketed primarily to orthodontists for the prevention of white-spot lesions adjacent to orthodontic brackets and appliances. Promising results, with respect to enhancing the caries resistance of sound permanent tooth enamel, have emerged from an in vitro caries study comparing the RS with traditional topical APF treatment.32 After caries formation, the RS-treatment-alone group had a 69 percent reduction in lesion depth compared with the matched no-treatment control group and a 46 percent reduction in lesion depth compared with the APF treatment group. The study yielded similar findings when the in vitro carieslike enamel lesions underwent lesion progression. Lesion depth was reduced by more than 50 percent with RS treatment compared with matched no-treatment controls and by slightly less than 40 percent when compared with APF treatment.

In our study of primary tooth enamel, the RS-treatment-alone group had a slightly greater than 50 percent reduction in lesion depth compared with the matched no-treatment control group, and a 36 percent reduction when compared with the AL-irradiation-alone group. The RS provides a source of calcium, phosphate and fluoride in remineralizing hypomineralized or clinically undetectable demineralized sound enamel. The mineral contained in the RS may result in precipitation of fluoride-rich calcium and phosphate mineral phases that could form stable, less soluble mineral phases within the superficial enamel or result in fluoride-rich mineral deposits on the enamel surface that become mobilized during cariogenic challenges.13,15,16,19,36–40 These fluoride-rich mineral phases may affect the dissolution rate of enamel.

It is well-known that only a small concentration of fluoride is required for remineralization to be favored over demineralization. Dental plaque and salivary fluoride concentrations in the range of 0.03 to 0.08 parts per million are required for remineralization to be favored over demineralization, even in acidic conditions.13,15–19 Fluoride also may act as a catalyst in the conversion of more soluble, less stable mineral phases (DCPD, TCP, OCP) to less soluble, more stable mineral phases (hydroxyapatite [HAP], FHAP, fluorapatite).19,36–40 It also is possible that calcium fluoride may be formed from the mineral and fluoride components in the RS. Calcium fluoride on the tooth surface and within the superficial layers of the enamel acts as a reservoir for fluoride release to the adjacent enamel, dental plaque or saliva.19,36–40 During an acidic cariogenic attack, fluoride is released from the calcium fluoride and has been shown to inhibit HAP dissolution and enhance FHAP formation.19,36–40 Interestingly, investigators have found that calcium fluoride may be hydrolyzed to partially FHAP in the presence of acid phosphate or phosphate ions.19,36–40

We noted a synergistic effect on in vitro caries formation in primary enamel when AL irradiation took place before RS treatment in our study. This combined treatment of sound primary tooth enamel surfaces resulted in the greatest reduction in lesion depth. With the AL irradiation before RS treatment, carieslike enamel lesion depths were reduced by 62 percent when compared with matched no-treatment controls, by 50 percent when compared with AL irradiation alone, and by 22 percent when compared with RS treatment alone (P < .05). The mechanisms13,15,16,19–21,31,34–43 for increasing the resistance of tooth mineral substance to cariogenic challenges previously described for lasers and RS no doubt play a role in the lesion depth reductions with primary tooth enamel found in our study. A similar synergistic effect has been reported between AL irradiation and topical APF application both in vitro laboratory studies20–24,26,30,31 and in vivo clinical pilot studies.25,27–29 As we noted earlier, AL irradiation of permanent and primary tooth enamel leads to a 30 to 40 percent reduction in lesion depth. When AL irradiation occurs either before or after topical APF application, a 50 percent to slightly greater than 60 percent reduction in lesion depth occurs. It would appear that dentists should combine AL irradiation with a remineralizing agent for optimal caries resistance.


   CONCLUSIONS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
AL irradiation alone or RS treatment alone provided significant caries reduction in primary tooth enamel when compared with matched no treatment controls (P < .05). The susceptibility of primary tooth enamel to a continuous cariogenic challenge decreased significantly with RS treatment alone compared with AL irradiation alone (P < .05). We achieved the maximum reduction in lesion depth in primary tooth enamel when we combined the RS, which contained calcium, phosphate and fluoride in a carbopol base, with AL irradiation (P < .05).


   FOOTNOTES
 

Dr. Westerman is a professor and the chair, Department of Community and Preventive Dentistry, Creighton University Medical Center, School of Dentistry, 2500 California Plaza, Omaha, Neb. 68178, e-mail "ghwester{at}creighton.edu". Address reprint requests to Dr. Westerman.


Dr. Hicks is a professor, Department of Pathology, Baylor College of Medicine and Texas Children’s Hospital, Houston, and an adjunct professor, Department of Pediatric Dentistry, Dental Branch, University of Texas Health Science Center at Houston.


Dr. Flaitz is the dean, Dental Branch, University of Texas Health Science Center at Houston, and a professor, Department of Diagnostic Sciences and Department of Pediatric Dentistry, Dental Branch, University of Texas Health Science Center at Houston.


Dr. Powell is the assistant dean, Dental Education, School of Medicine, University of Utah, Salt Lake City, and a professor, Department of Operative Dentistry, School of Medicine, University of Utah.


This article is based on Dr. Westerman’s oral presentation at the American Academy of Pediatric Dentistry 58th Annual Session, Orlando, May 28, 2005.


   REFERENCES
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 

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