Journal of Dental Research, Vol 71, 426-433, Copyright © 1992 by International & American Associations for Dental Research Online Journals
Evaluation of smear layers on serial sections of human dentin by means of electrochemical impedance measurements
M. Levinkind, T. J. Vandernoot and J. C. Elliott
London Hospital Medical College, United Kingdom.
The alternating current impedance characteristics of a uni-directional
porous glass membrane and serial sections of dentin from unerupted human
third permanent molars (with and without smear layers) were measured. The
membrane was equilibrated at 20 +/- 1 degrees C in a range of
concentrations of unbuffered KCl, and the tooth sections were equilibrated
in 0.1 mmol/L unbuffered KCl. Each section's impedance was measured for a
50-mV (rms) alternating potential difference applied over 1 Hz to 65 kHz.
By means of complex non-linear least-squares regression analysis, the dc
resistance of each specimen was determined. Scanning electron microscopy
and subsequent image analysis enabled the cross-sectional area of the pores
in the membrane and dentin tubule surface-openings to be measured. The
results showed that the measured cross-sectional area of pores in the
membrane was in good agreement with that calculated from the measured dc
resistance and solution resistivity over the range of electrolyte
concentrations. In dentin, the presence of smear layers on the cut surfaces
increased the measured resistance. The resistance of the smear layer
varied, suggesting that its thickness changed across the surface of the
dentin. This was superimposed on regional variations in resistance of
smear-free dentin which were found to be related to alterations in the
number of tubules present and their cross-sectional area. The resistivities
of the dentin tubule contents and smear layer material were generally lower
than that of the hydrating electrolyte. It is postulated that this could be
due to ions from partial dissolution of the mineral in the dentin.
