Electrical Conductance


Brief Description

The method of using electrical conductance to measure carious lesions in the teeth was first adopted in the field of dentistry in 1959. This diagnostic method is a novel application of the concept of using electricity to measure the bulk resistance of materials to carious teeth. Similar applications would have probably been in use in the fields of engineering and medicine before it was introduced to dentistry as a useful diagnostic tool. Nevertheless, it provides clinicians with quantifiable data which allows for an objective assessment of the extent of caries development, something which could not previously be done by mere visual inspection.


Working Principle

The tooth consists of 4 basic structures: the enamel, the dentine, the cementum as well as the pulp. Each of these structures has a different chemical composition, hence each of the structures exhibit different electrical signatures when electricity is passed through them. The propagation of electric currents through the tooth is made possible through the movement of ions within the enamel and dentine pores when a potential difference is applied via two electrodes. Mature enamel is less conductive than immature enamel and dentine is more conductive than enamel. Since most of the structures have a greater conductivity than enamel owing to the abundance of electrolytes, the conductance of the tooth in general is mainly influenced by the enamel. In the case of demineralization of the enamel layer, the reduced mineral content of the enamel naturally translates to a decrease in the bulk resistance of the tooth and an increase in electrical conductivity. In other words, an increase in the porosity of the tooth leads to a decrease in the electrical resistance or impedance. Water and other soluble electrolytes from the saliva may enter the carious pores and increase electrical conductance of the entire tooth. (Pretty, 2006)


Electrical Conductance - Cariology
Fig.6 A demonstration of an ECM profile obtained from a primary root caries lesion in vitro demonstrating the sites accessed
(Pretty, 2006)


Instrumentation

There are various instruments used to measure the electrical conductance of the tooth. These instruments have evolved along the years and some of them are no longer in use or replaced by newer and more efficient models of measuring instruments. Affixed below is a table which shows the various instruments used to measure electrical conductance of a tooth and their various manufacturers.

The modified AC Ohmmeter
Caries Meter L (G-C International Corp., Leuven, Belgium)
Vanguard Electronic Caries Detector (Massachusetts Manufacturing Corp., Cambridge, Mass., USA)
Electronic Caries Monitor(ECM) I, II, III and IV (LODE Diagnostic, Groningen, The Netherlands)
Modified Electrochemical Impedance Spectroscopy (EIS)
Electrical Impedance Tomography (EIT)
















ECM

Fig. 5 – The ECM device (Version 4) and its clinical application. (a) The ECM machine, (b) the ECM handpiece, (c) site specific
measurement technique, (d) surface specific measurement technique.
(Pretty, 2006)

The modified AC Ohmmeter as well as the Caries Meter L has been made obsolete and is not in use today. The focus shall thus be placed on the ECM as well as the modified electrochemical impedance spectroscopy (EIS).

The ECM makes use of a single fixed frequency alternating current to measure the bulk resistance of the tooth. When the electrical properties of a point site on a tooth is to be determined, the ECM probe is placed with its tip on the site, which is very often a fissure and measurements are hence taken over a 5-seconds duration. During this time compressed air is emitted from the tip of the probe to obtain a drying profile, which is essentially a collection of data which can be used to characterize the target lesion. An airflow is applied to dry the tooth surface around the probe so as to prevent current from "leaking" through a superficial layer of moisture to the gingiva, thus affecting the accuracy of the data obtained. A contra-electrode is held in the patient’s hand to complete the electrical circuit. In certain cases, the clinician may decide to coat the surface measured with a conducting medium to facilitate flow of current and to standardize the procedure to minimize variables so as to produce accurate results. In order to analyse the experimental results, a thorough understanding of impedance is required.


It is crucial to highlight the concept of impedance. Impedance is the measure of the degree to which an electric circuit resists electric current flow when a voltage is impressed across 2 electrodes. In direct current circuits, impedance corresponds to resistance. In alternating current circuits, impedance is a function of resistance, inductance and capacitance. (O.Fejerskov & E.Kidd, 2008, p.97)

As mentioned in this report, a carious tooth would confer a lower resistance to electricity. This device makes use of this physical property to locate the resistance of the tooth to electrical currents as close to zero frequency as possible. This would be the “true” resistance of the tooth, in the absence of the polarizing effects caused by the application of a direct current. Inductance, capacitance and resistance are the three electrical properties occurring naturally within tooth tissues. Of the three, the latter two are the main parameters of interest since inductance is not of major concern in tooth that has no restorations done on it.



Electrical Impedance Spectroscopy (EIS)

As opposed to the single frequency ECM, the modified electrochemical impedance spectroscopy (EIS)adopts multiple frequencies to detect early carious lesions at different parts of the tooth structure. Its main advantage over the ECM is that, as different dental structures have different molecular composition and hence respond to electric currents of differing frequencies, it aids in determining the various parameters which demonstrates these differences. The EIS scans a range of electrical frequencies and provides information on capacitance and impedance. The usefulness of this technique lies in the fact that hard dental tissues like the dentine and enamel can be characterized by its unique electrical signature to provide a comprehensive view on the overall structural stability and anatomy of the tissues. This provides clinicians and researchers with quantifiable data which can be used as a basis of comparison between normal dental tissues and caries of varying severity. The dentist can then better decide if invasive treatment is required in the event that remineralisation is not able to reverse and remedy the situation. The ECM could be used to predict the probability that a sealant or a sealant restoration would be required within 18-24 months after eruption. (O.Fejerskov & E.Kidd, 2008, p.98).

The true clinical significance of this technique is that it allows for detection of very minor increases in the porosity of tissue in early carious lesions which may not manifest on the enamel surface and impossible to detect with visual inspection. A derivation of the EIS, the Alternating Current Impedance Spectroscopy Technique (ACIST), uses alternating current to prevent polarizing the electrolyte solution and allows for the characterization of hard dental tissues with pores of ionic dimensions. In carious dental tissues, the pores are larger than in normal healthy hard dental tissues. The size of the pores could be affected by caries and is monitored closely to reflect the stage of caries development.

The concept of using the electrical impedance of teeth to monitor caries progression was adopted in research to detect caries lesion at approximal sites. The in vitro results obtained were promising but no follow-up research findings have been reported since. Hence, it is difficult to ascertain the effectiveness of the EIS given the limited information that were available at the time of finalising this writeup. (O.Fejerskov & E.Kidd, 2008, p.98).


Effectiveness

ECM-ROC areas under the curve

ROC-area
Diagnostic Threshold
Tooth type
Surface or site specific measurements
Study
0.82
D1
Premolars
Site specific
52
0.80
D1
Molars
Site specific
53
0.84
D3
Premolars
Site specific
52
0.82
D3
Molars
Site specific
54
0.80
D1
Premolars
Surface specific
19
0.67
D1
Premolars
Surface specific
19
0.94
D3
Premolars
Surface specific
19
0.79
D3
Molars
Surface specific
55
(Pretty, 2006)

These figures reflect a good to excellent range of AUC with the exception of surface specific premolars when assessing at the D1 level (lesions restricted to the enamel).


Site specific measurements
Surface specific measurements
Sensitivity 74.8 ± 11.9 63.0 ± 2.8
Specificity 87.6 ± 10.0 79.5 ± 9.2


The lower efficacy in surface specific measurements has led researchers to focus their efforts on site specific measurements instead.


Site specific measurements
Surface specific measurements
Intra class correlation coefficient
0.76
0.93
(Pretty, 2006)

Reproducability of data readings is rated as good to excellent for both measurement techniques. It must be noted, however, that these figures are obtained under a strictly controlled in vitro setup. More studies have to be conducted before the device is used to measure lesions longitudinally. The limits of agreement can be up to
±580kΩ and this could be a signifcant source of error. Huysmans et al. (2005) concluded that there was a consistent, systematic, non-random measurement variation due to insufficient and unpredictable probe contact.

A longitudinal study was conducted on children using the ECM. From the results, it was shown that ECM values was significantly higher for carious sites than non-carious sites, and values obtained from sites with carious dentine were also critically higher than non-carious sites or sites with caries restricted to the enamel. The ECM could thus be potentially used to detect fissure caries in newly erupted molar teeth and at the same time be used to predict the probability that a sealant restoration would be needed 18-24 months after eruption. In another study (Lussi et al., 1995), an in vivo application of the ECM on human third molars without any preexisting restorations done on them or any caries has shown that the ECM is effective in detecting in vivo occlusal caries under clinically intact fissures. The entire procedure involves using the ECM on the molar teeth in vivo, with the teeth subsequently extracted, histologically prepared, serially sectioned perpendicularly to the occlusal surface and examined for the presence of caries. It was shown however that there was a significantly high value of 0.23 for false positive ratings, which might lead to sound teeth being restored needlessly. (O.Fejerskov & E.Kidd, 2008, p.98)


Factors influencing electrical measurements


Porosity Porosity can mean either the pore depth, the pore volume or the three-dimensional configuration of the pore, all three of which can affect ionic migrations within dental hard tissues. Maturation of teeth also causes changes in the porosity of the dental tissues, as demonstrated by Schulte et al.(1999) and by Wang et al.(2001).
Surface Area Surface area of electrode contact with the tooth yields very different electrical measurements. Measurements taken can either be site specific (one point of contact) or surface specific, with each having a different measurement.
Thickness of Tissues The range of enamel fissure thickness will affect electrical measurements, and it can be inferred that different tooth types and different sites of measurement yield different electrical measurements as well.
Hydration of the enamel Hydration of the enamel affects concentration of water and soluble electrolytes. Care must be taken not to dehydrate the teeth prior to air flow treatment and application of contact medium, and if possible, to keep the extent of dehydration constant for all teeth measured.
Temperature Huysmans et al.(2000) recently demonstrated that the temperature of the teeth affects the electrical values obtained, though the fact that this effect was linear simplifies in vitro to in vivo extrapolations of absolute values of parameters.
The concentration of ions in the dental tissue fluids There exist fluctuations and variations of ion concentration within dental hard tissues in vivo due to diet or food intake. Hence, the extent to which this factor affects measurements taken in vivo is difficult to elicit.

(Longbottom et al., 2004)


Advantages and Disadvantages of the Electrical Conductance diagnostic method

Advantages Disadvantages
Quantification of data helps to minimize human errors Low specificity; low chances of a negative test amongst patients with without the disease.
Allows for reproducibility and recording of data for future follow ups or to track progress of caries Lower efficiency in surface specific measurements
Allows for the detection of non-cavitated lesions Background resonance may lead to cardiac disorders. Minimum frequency do not fall below 100Hz so as not to mimic the frequency of cardiac discharge. (Kaczmarek et al., 2007, p. 14)
Minimally intrusive technique Alternating current administered may infringe on the patient’s pain threshold. Intense stimulation of the nerves and muscles may cause muscle fatigue and pain. Since frequency of current affects threshold values of pain, in vivo conditions call for high frequency alternating current to be used for patient’s safety and comfort. This may compromise on the accuracy of the measurements obtained since frequency of current should be close to zero ideally to obtain an accurate value for electrical impedence. Hence, this technique is more suitable for in vitro measurements. (Kaczmarek et al., 2007, p. 14)

Diagnostic method is time consuming, hence it is not popular amongst dental practices and is mostly used in research. Even then, insufficient and unpredictable probe contact with the site on the tooth makes tracking of the carious process inefficient. (O. Fejerskov & E.Kidd, 2008, p.99)




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References

1) Fejerskov, O., & Kidd, E. (2008), Dental Caries: The Disease and Its Clinical Management 2nd edition. Wiley-Blackwell.

2) Longbottom, C., & Huysmans, M. C. (2004). Electrical measurements for use in caries clinical trials. J Dent Res, 83 Spec No C, C76-79.

3) Murdoch-Kinch, C. A. (1999). Oral medicine: advances in diagnostic procedures. J Calif Dent Assoc, 27(10), 773-780, 782-774.

4) Naba’a, L. Advanced methods of quantification of occlusal caries. Retrieved from http://www.the-o-zone.cc/HTMLOzoneF/pdf/AdDiag01.pdf.

5) Pretty, I. A. (2006). Caries detection and diagnosis: novel technologies. J Dent, 34(10), 727-739.

6) Wozniak,J. (2007). The Reproducibility of Tooth Impedance Spectroscopy Measurements: an in vitro Study. Dent. Med. Probl. 2007, 44, 1, 11–17.






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