Effect of canal preparation and residual root filling material on root impedance.

AIM: To investigate the effect of root canal preparation and residual root filling material on the impedance characteristics of extracted human roots. METHODOLOGY: Thirty extracted, human single-rooted teeth were mounted in a custom-made apparatus that allowed strict temperature control. Impedance measurements of the roots were made with a file acting as the internal electrode, using a frequency response analyser. The measurements were made under three canal conditions: (i) before chemo-mechanical preparation; (ii) after chemo-mechanical preparation; (iii) after root filling removal to re-establish patency (following placement of root filling). The measurements were taken at 0, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 mm coronal to the apical terminus and also at 0.5 and 1 mm past the apical terminus. Impedance values were viewed using Nyquist plots and comparisons made within each tooth, between measurement points along the length of the canal, as well as under the different canal conditions. Equivalent circuits were modelled for different test conditions. RESULTS: The impedance decreased from the coronal to the apical levels in all canal conditions in a characteristic way, with an exaggerated drop at the apical terminus. Impedance decreased after chemo-mechanical preparation, but gave higher values compared with before or after instrumentation, once canal filling had been removed. Equivalent circuits remained consistent at the tested positions within the canal, regardless of canal condition, but the circuit component values changed with the impedance. CONCLUSIONS: Impedance was influenced by corono-apical position, chemo-mechanical preparation and residual root canal filling material.

The development and validation of an occlusal site-specific plaque index to evaluate the effects of cleaning by tooth brushes and chewing gum.

These studies sought to develop and validate an occlusal site-specific plaque index to be used to measure plaque removal by brushing or chewing gum. The index divides the occlusal surfaces into imaginary zones from which scores are apportioned on a 0-4 basis dependent on the perceived % plaque coverage of each zone. Examiner calibration was conducted over 2 studies assessing inter-examiner reproducibility and intra-examiner repeatibility, respectively. Study 1 involved 2 examiners who recorded scores from the same 3 groups of subjects who had suspended tooth cleaning for 4 days. Analyses for inter-examiner reproducibility showed no significant mean differences between examiners or no significant differences between variances of the 2 examiners scores. Study 2 involved the same 2 examiners individually scoring 3 groups of subjects 2 x (approximately 60 min apart) for occlusal plaque. Analysis for intra-examiner repeatability showed no significant mean differences between the 2 scorings of each examiner. Furthermore, there were no significant differences between the variances of each examiner's scores except for 1 examiner in the repeatability exercise for the 1st group of subjects. Study 3 involved groups of subjects at 2 separate clinical sites (Bristol, England and Berne, Switzerland) being scored for occlusal plaque before and after toothbrushing with water or after no toothbrushing. Data from individual examiners and examiners combined revealed a significant reduction in occlusal plaque with brushing compared to no brushing. Study 4 was the same as study 3 but occlusal plaque was scored before and after chewing gum or not chewing gum. The Bristol examiner recorded a significant reduction in plaque by chewing gum compared to not chewing gum but the Berne examiner did not. The latter may have resulted from a considerable disparity in the number of evaluable occlusal surfaces between the two study sites. The index could be employed as part of the overall assessment or oral hygiene or used in clinical trials to study mechanical and chemical plaque control agents.

Scanning microradiographic studies of rates of in vitro demineralization in human and bovine dental enamel.

The aim was to measure frequently and with precision the local integrated mineral loss through small areas of the natural surface of human and bovine enamel during in vitro demineralization using an X-ray photon-counting system (scanning microradiography). The method used was an adaptation of photographic longitudinal microradiography in which the attenuation of X-rays through the enamel is measured in the direction of acid attack, i.e., normal to the enamel surface. The mass of mineral (assumed to be hydroxyapatite) per unit exposed area was measured over 15 microm dia. circles at a series of positions as a function of time in blocks of human and bovine enamel immersed in 0.1 mol/l acetic acid buffered to pH 4.0 with NaOH. There was an initial period (approx. 45 h for human, approx. 75 h for bovine enamel) during which the mineral loss with time was sigmoidal, followed by a nearly linear loss for the remainder of the experiment, in some cases up to 500 h. The initial sigmoidal period may be due to properties of surface enamel or be associated with the development of a surface layer overlying subsurface demineralization. The essentially constant rate of mineral loss after the surface layer has formed confirms earlier observations and is consistent with a rate-limiting process occurring at the dissolving enamel surfaces of the advancing front, and not by transport of ions within the lesion. Small perturbations from a linear loss were seen, which were approximately periodic for human enamel. The slope of the linear period was rather constant within one human or bovine block, but variable between blocks without a clear distinction between human and bovine enamel.

Evaluation of smear layers on serial sections of human dentin by means of electrochemical impedance measurements.

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.

Electrochemical impedance characterization of human and bovine enamel.

The alternating current impedance characteristics of human and bovine enamel slices were measured in vitro. An electrochemical cell containing two platinum gauze electrodes and 0.01 mol/L KCl buffered to pH 7.4 at 20 degrees C was used. The electrodes were attached to a potentiostat that was connected to a microcomputer-controlled frequency-response analyzer. Measurements were made at discrete frequencies between 1 Hz and 65 kHz by application of a sinusoidal potential of 50 mV (rms) across the assembled cell. The impedance was calculated from the input potential and the resulting measured flow of current. For quantitative evaluation of these measurements, an equivalent circuit was postulated. It contained five passive electronic components and accurately modeled the different specimens. Values for individual electrical components in the equivalent circuit were estimated by complex non-linear least-squares regression analysis. This study demonstrated that it is possible to measure and quantitatively distinguish between the impedance characteristics of: permanent human, deciduous human, and bovine enamel, as well as the enamel from a variety of sites from human teeth, some of which had been extracted prior to being fully erupted. Proposals are made as to which physical properties in the specimens are modeled by individual components in the equivalent circuit.