Electrical conductance measurements are being used experimentally to d
iagnose caries. Current equipment, e.g. the electronic caries monitor
(ECM), uses a probe to scan occlusal fissures. For full-mouth examinat
ion this method is rather time-consuming. A method with which only one
measurement is needed for an entire (occlusal) surface would be prefe
rable. However, the enlargement of the area being measured will influe
nce the conductance. It was the purpose of this study to investigate t
he relationship between the electrical conductance of human teeth and
the enamel electrode area, and to compare the range of results of surf
ace measurements with those of the scanning method. Twenty-five sound
extracted teeth were selected for the study. The reference electrode o
f the ECM was connected to the roots. The buccal surface was blotted d
ry, and a coloured dentifrice was syringed in increments onto the surf
ace. After each increment the surface was photographed together with a
metric reference, a conductance measurement was performed by holding
the ECM probe tip in the dentifrice, and the ECM reading was recorded.
For each tooth between 5 and 10 increments were applied and measured.
The photographs were digitised and the electrode areas were calculate
d. A least squares curve fitting procedure yielded a linear relationsh
ip between conductance and electrode area (0.88 less than or equal to
R(2) less than or equal to 1.0, mean R(2) = 0.97). For most teeth the
threshold for dentinal caries as used for scanning ECM measurements (E
CM reading = 6.00) was reached only when the electrode area exceeded 1
2 mm(2). For 6 teeth this conductance was already reached between 5 an
d 12 mm(2) Electrical conductance measurements of the occlusal surface
s of 18 teeth, with electrode areas as projected two-dimensionally of
between 5 and 10 mm(2), showed only 3 measurements above the threshold
. None of those 3 came from the group of 6 teeth mentioned before. It
was concluded that the relationship between electrical conductance of
teeth and electrode area is linear. It was also concluded that when el
ectrical conductance measurements of occlusal surfaces are to be made,
the threshold for dentinal caries will probably have to be shifted to
a higher conductance level to limit false-positive results.