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Electrical Measurements for Use in Caries Clinical Trials
1 Dental School and Dental Health Services Research Unit, Dental School, University of Dundee, Park Place, Dundee, DD1 4HR, Scotland, UK; and Correspondence: * corresponding author, c.longbottom{at}dundee.ac.uk
ABSTRACT This paper reviews the use of electrical measurements of caries, particularly in relation to caries clinical trials. Electrical measurements change as tooth tissue porosity alters in the caries process, but several other variables also have a significant effect on these electrical measurements and hence upon their diagnostic validity. Available electrical-method data, in the context of clinical trials, relate to the use of the Electronic Caries Monitor (ECM), which measures "bulk" resistance. The device is presently limited in scope to occlusal surfaces, and only limited ECM data from clinical trials are available. In the context of clinical trials, more work is needed to determine the potential role of electrical measurements. Such research will need to focus both on an understanding of those electrical parameters which are most valuable in identifying changes and stages in the caries process in individual teeth and also on identifying the extent of the effects of the variables affecting these measurements.
Key Words: caries • electrical measurement • caries clinical trials INTRODUCTION This paper considers electrical methods of caries detection in relation to caries clinical trials. Specifically, it seeks to consider the question: How effective are electrical measurements in caries detection in a caries clinical trial context? OUTLINE OF PAPER To ascertain the effectiveness of a measuring technique, one must first determine and describe how the technique works in the chosen application. This leads to a detailed examination of what the technique actually measures and what factors may or may not influence these measurements. There is more than one way to make electrical measurements, so the different measurement options are reviewed. In terms of the particular application—caries clinical trials (CCTs)—the fundamental question being asked is, "Can a technique be used to measure caries to determine anti-caries effects?" Hence, what is the accuracy of the technique in measuring what it purports to measure—caries? Factors affecting the process of data analysis also need to be considered. Only then can examination and analysis of the available data from the use of electrical measurements in CCTs thus far reported be used to help to determine whether the results can allow conclusions to be drawn about the applicability and effectiveness of the technique in CCTs. The paper will conclude by summarizing the above review and suggesting recommendations regarding electrical measurements in caries clinical trials. HOW DO ELECTRICAL MEASUREMENTS OF DENTAL CARIES WORK? Put in a fairly simple yet reasonably accurate form, as a tooth demineralizes in the caries process, the loss of mineral leads to increased porosity in the tooth structure. This porosity is filled with fluid from the oral environment which contains many ions. Increased ionic content in the pores leads to increased electrical conductivity, or, conversely, increased porosity leads to decreased electrical resistance or impedance. WHAT DO ELECTRICAL MEASUREMENTS ACTUALLY MEASURE? The ECM device, which uses a single fixed-frequency alternating current, attempts to measure the "bulk resistance" of tooth tissue—i.e., it attempts to locate the resistance of teeth to current as close to zero frequency as feasible. This would be the "true" value of the electrical resistance of the tooth, were there no polarizing effects caused by direct-current application. The inherent electrical properties of materials, including tooth tissues, include resistance, capacitance, and inductance. Fortunately, inductance is not an issue with unrestored teeth, so resistance and capacitance are the main parameters of interest. For several decades, Electrical Impedance Spectroscopy (EIS) has been the conventional means of determining the electrical properties of materials. Its principal advantage over single (fixed)-frequency measurements is that, since materials have different electrical responses at different frequencies, EIS can help to determine more accurately the various parameters which demonstrate these differences. However, it is only in the last decade or so that EIS has been applied to teeth. FACTORS AFFECTING ELECTRICAL MEASUREMENTS Whether the technique uses a single frequency or EIS, the factors affecting measurement will be the same. There are at least 6 factors which can have a significant impact on electrical measurements of teeth: the porosity of the tissues, the surface area of the electrical contact, the thickness of the tissues, the extent of hydration of the tissues, and the temperature and the concentration of the ions in the fluid within the tooth. To understand what electrical measurements mean in the context of CCTs, one must examine each of these factors to determine their extent and significance.
Porosity
In relation to porosity, it is well-recognized that teeth undergo a maturation process after eruption; this process has a significant effect on porosity and hence electrical properties. There are variations in the process between and among individuals, as demonstrated recently by Schulte et al.(1999) and by Wang et al.(2001), and there is a period of up to 15–18 months after eruption in which the electrical impedance characteristics of particular teeth can vary. This is of particular relevance to CCTs involving adolescents, in whom several teeth are relatively recently erupted, notably the second permanent molars.
Surface Area
The Thickness of the Tissues
Hydration of the Enamel
Temperature
The Concentrations of Ions in the Dental Tissue Fluids SINGLE- OR MULTIPLE-FREQUENCY MEASUREMENTS The fixed-frequency (23 Hz) measurements obtained by ECM are limited to occlusal sites, and the device attempts to measure the "bulk resistance" of teeth. It is designed to be operated in a "site-specific" mode, but if the airflow is turned off and a contact medium, such as toothpaste, is applied, a "surface-specific" measurement can be carried out. EIS (Electrical Impedance Spectroscopy) scans a range of electrical frequencies and provides information on impedance and capacitance, among other parameters. The development of an Equivalent Circuit for the tissue being measured is a way of describing the electrical behavior of that tissue in terms of resistors, capacitors, and constant-phase elements. Enamel and dentin are relatively "simple" materials, and Equivalent Circuits for teeth have been developed. Caries makes these circuits somewhat more complex, but these are being investigated. The process provides the potential for more detailed information to be obtained about the physical structure of the tooth being measured—hence the determination of its caries status—than the use of a single-frequency measurement. EIS has been applied to approximal sites in vitro, and it has the potential for application to occlusal and free smooth-surface sites in vivo, with the use of suitable contact probes. One prototype EIS device has been developed—the ACIST device—but this is not yet commercially available. The use of a prototype—the pre-ACIST device—was investigated on a subset of subjects in a recent clinical trial, but details are not yet available, since they are still subject to commercial confidentiality. The results will be presented in due course. VALIDITY The validity of electrical caries detection methods was the subject of a critical review by Huysmans (2000) and papers by Ashley et al.(2000) and Wang et al.(2000). Huysmans pointed out that ECM measurements provide quantitative data, and thus it was best to use a correlation with a gold standard of lesion progression. Almost all the published data relate to ECM data; correlations range from 0.47 to 0.82. In terms of traditional caries thresholds used in CCTs, there are sensitivity and specificity data for D1 (enamel) caries and D3 (dentinal) caries for both site-specific and surface-specific ECM methods. For site-specific D1 measurements, the sensitivity (Se) figures from different studies range between 0.70 and 0.92, with specificity (Sp) values of 0.78 to 1.00. The equivalent figures for surface-specific measurements are 0.61 to 0.65 (Se) and 0.73 to 0.86 (Sp). For dentinal caries, the equivalent figures for these ranges are 0.39 to 0.97 (Se) and 0.56 to 0.98 (Sp) for site-specific measurements and 0.68 to 0.78 (Se) and 0.76 to 0.90 (Sp) for surface-specific measurements. HOW DO THESE FIGURES RELATE TO CCTS? The range of sensitivity and specificity values for the ECM is such that the confidence interval for absolute Diagnostic Value (which depends on caries prevalence) may be considerable and, if incorporated into the data analysis, may be too large to permit significant differences in product effects to be shown. Subjects in different CCTs may have relatively different caries prevalence and/or incidence rates at occlusal, approximal, and/or free smooth-surface sites. Thus, the usefulness of ECM measurements—which apply to occlusal sites only—for a CCT may vary according to the relative occlusal to approximal to free smooth-surface caries ratios for prevalence and/or incidence in the subjects in a particular CCT site. Thus, ECM may be useful in determining product differences in CCTs in areas with incidence rates of occlusal caries that are high relative to approximal caries, but be less useful for CCT in an area with a relatively higher approximal to occlusal caries incidence ratio. There is, of course, a less than simple relationship between total (and/or site) caries prevalence and incidence, since this depends on such factors as the age of the subjects at the time of data collection and population levels of caries. FACTORS AFFECTING MEASUREMENTS IN RELATION TO DATA ANALYSIS
Porosity Ten Bosch et al.(2000) confirmed the time-dependent decrease in the porosity of newly erupted teeth but also noted an unexpected seasonal variation in the ECM measurements on recently erupted teeth. The seasonal variations in these measurements were statistically significant, hence should be taken into account in any analysis of data collected over several months in a clinical trial environment.
Surface Area
Thicknesses of Enamel
Hydration
Temperature
Fluid Ionic Concentration LONGITUDINAL CARIES STUDIES The first study to carry out longitudinal electrical measurements of the same teeth was conducted by White et al.(1981) using the now-unavailable Vanguard device. In total, 769 tooth sites, in the posterior teeth of a pediatric clinic sample, were measured at 6, 12, and 18 mos from baseline. The authors concluded that results showed that the instrument was a better predictor of the "clinically evident anatomical lesions" than was the dental explorer. The relevance of these results to today’s CCT environment, with generally lower caries prevalence/incidence and the move away from the use of the explorer, is difficult to assess. Ie et al.(1995) and Fennis-Ie et al.(1998) reported on the use of the ECM in a longitudinal study of the first and second molars of 50 children aged 5–7 and 11–15 yrs. Comparisons were made with clinical-visual and FOTI techniques, and the teeth were monitored on six occasions at six-monthly intervals—a total of 2.5 yrs. Although the differences were reported to be very small, the authors concluded that ECM measurements were superior to FOTI and clinical-visual assessments for the prediction of occlusal caries. CARIES CLINICAL TRIALS The available published data on electrical measurements in CCTs are limited. Ashley (1997) and Ashley et al.(2000) reported on a study in Northwest England, and recently initial results have been presented by Matuliene et al.(2001), Bedinskaite et al.(2001), and Chesters et al.(2001) in relation to a study carried out in Vilnius, Lithuania. As-yet-unpublished data from these and other studies may appear in the public domain in the near future. In relation to root caries, Baysan et al.(2001) reported on a six-month clinical trial carried out in a dental school. In the Ashley study, surface-specific electrical measurements were recorded every 6 mos during the second 18-month period of a three-year clinical trial comparing two products whose relative anti-caries effects were unknown. The second molars in 316 subjects aged 13.9 yrs at the ECM-baseline were measured and caries status for electrical measurements designated according to D1 and D3 cut-off values previously determined by laboratory studies. The findings of this CCT were that, over the 18-month period, ECM measurements demonstrated a significant difference in the mean DFS increment between the test and the control groups at the p < 0.05 level. In contrast, the conventional detection methods used in the trial were unable to discriminate between the products over 3 yrs. In the Vilnius study, which used two products with different effects of known extent, as shown previously by conventional detection methods, site-specific ECM measurements were recorded on all posterior teeth in 2387 subjects (aged 11–14 yrs) at baseline and 12 mos, using pre-determined D1 and D3 threshold points. For analysis, the ECM data were log-transformed. No product differences were found with the ECM, but they were found with a new Clinical Visual system—the Dundee Selectable Threshold Method (DSTM)—on approximal but not occlusal surfaces. In addition, ECM readings were found to be highly significantly tooth-dependent, with higher readings the more posterior the tooth type. ECM readings were highly significantly jaw-dependent, with mandibular teeth generally scoring higher than maxillary teeth, except for first premolars. There was also a strong relationship between the ECM scores and the Clinical Visual Assessment (CVA) scores. With the results of only two studies published for coronal caries available for review, the results are tantalizing: The Ashley study shows a possible but not conclusive ability of ECM to discriminate between products, but it was not conclusive, since the relative anti-caries effects of the two products were not known. This study also shows the inherent scientific invalidity of using an insensitive technique (CV) as the "gold" standard for demonstrating product effects when these are compared with new detection systems. The Vilnius study shows the need for extreme care in data analysis when electrical measurements are used. As noted above, there are many factors which can affect ECM readings. This study also illustrates the possible effect of relative occlusal to approximal caries prevalence and/or incidence ratios in the subjects on the ability of ECM to demonstrate a product effect. The prevalence of occlusal caries in Vilnius was very high. This may have reduced the relative occlusal to approximal caries incidence to a level at which, in the time-scale of the measurement points, the product effects could not be demonstrated at occlusal sites compared with approximal sites. The only reported longitudinal study involving electrical measurements of root caries involved 186 subjects examined at baseline, 3 mos, and 6 mos. All subjects had at least one primary root caries lesion at baseline, and these lesions were assessed clinically and with the ECM. The subjects were randomly assigned to one of two groups, one using a 5000-ppm-fluoride-containing dentifrice, and the other a 1100-ppm-fluoride-containing dentifrice. After 6 mos, 57% of the subjects in the 5000-ppm-F– group and 27% of those in the 1100-ppm-F– group had lesions which had hardened clinically (p = 0.002). Between baseline and 6 mos, the log10 mean ± SD resistance values of lesions for subjects in the 1100-ppm-F– group decreased by 0.004 ± 0.70, whereas in the 5000-ppm-F– group, they increased by 0.56 ± 0.76 (p < 0.0001). The authors noted that the plaque index in the 5000-ppm-F– group was significantly reduced compared with the 1100-ppm-F– group. This latter difference may be a confounding factor in relation to interpreting the results, since plaque removal per se can alter the activity, hence the hardness, of root caries lesions. SUMMARY
RECOMMENDATIONS
In conclusion, electrical measurements are showing early promise as a technique for the clinical detection of caries in the context of caries clinical trials. Careful analysis of current and emerging CCT data will help to determine the extent of the usefulness of the application of the method in this field. FOOTNOTES Presented at the International Consensus Workshop on Caries Clinical Trials, Glasgow, Scotland, January 7–10, 2002 REFERENCES
Journal of Dental Research, Vol. 83, No. suppl 1,
C76-C79 (2004) This article has been cited by other articles:
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