Qualitative assessment of stress distribution during insertion of endodontic posts in photoelastic material.

OBJECTIVES: The purpose of this study was to investigate the stress patterns associated with prefabricated endodontic posts during the various stages of insertion according to a number of design characteristics. METHODS: In a photoelastic material with elastic properties comparable to dentin, analyses were performed of the overall stress patterns with polarized light revealing substantial differences in stresses generated by the various posts. The effects of variations in design for certain configurations of the posts were also assessed. RESULTS: One geometric feature was the retentive thread of the post. The stress patterns within the photoelastic material revealed a homogeneous distribution of stress along the entire length of the thread, and more threads induced additional stress. The stress recorded with a vent when the pitch of the thread was 0.8-1.0 mm was classified as minimal-to-mild stress. Another geometric feature considered was the head (coronal extension) of the post. Minimal stress was recorded in the material in contact with the head and the apical end of the post when the contact surface of head was more than 3 mm2. CONCLUSIONS: This study suggests that during insertion of threaded posts the least stress occurs when the head contact surface is sufficient (> or = 3 mm2). A thread with a pitch of 0.8-1.0 mm is most desirable in stress reduction. The number of windings should also be limited (less than six windings) as samples with a substantial number of windings (N = 13 or 30) produce severe stress.

The effect of lactic and acetic acid on the formation of artificial caries lesions.

Fluoride specifically adsorbed to hydroxyapatite reduces the rate of dissolution of hydroxyapatite. Since specific adsorption is a property which anions of all weak acids have in common, it is expected that the anions of lactic and acetic acid will be specifically adsorbed to hydroxyapatite too and in this way will cause a reduction in the rate of dissolution of hydroxyapatite. In order to investigate this, we used the experimental results from Featherstone and Rodgers (1981). Calculations of the rate of increase of the lesion depth in dental enamel per mmol. L-1 of the un-ionized acid revealed that lesion progress was pH-dependent and was lowest at that pH which corresponds with the pK-value of the relevant acid. It is concluded that the anions of lactic and acetic acid are specifically adsorbed to the enamel mineral and in this way reduce the rate of dissolution of the mineral. This suggests that a description of the development of an artificial caries lesion in mathematical terms should include the effect on the rate of dissolution of the mineral of specific adsorption of the relevant acid anions to the mineral.

Improvement of the caries resistance of human dental roots by a two-step conversion of the root mineral into fluoridated hydroxylapatite.

Roots of extracted human molars were treated for both 10 and 30 min with a saturated solution of dicalcium phosphate dihydrate (DCPD) of pH 2.4 and subsequently with a 5.3 mmol.L-1 solution of sodium fluoride of pH 7.0. The objective of these combined treatments was to convert the highly-soluble root mineral into the less-soluble fluoridated hydroxylapatite, with DCPD as an intermediate, to improve caries resistance. The mineral content of the surface layer was not affected in a significant way. Roots treated in this way were subjected to 50 mmol.L-1 acetate buffer solutions of pH 5.5 and with pIOHA-values of 112, 116, and 122. The proportional reduction of the rate of demineralization (Vdem) of similarly treated roots subjected to buffer solutions of pIOHA 112 and 116 appeared to be equal initially. Since fluoridated hydroxylapatite is insoluble under these conditions, these findings are in agreement with expectations. The reduction of V dem amounted to about 45 and 70% for roots treated for 10 and 30 min, respectively. When roots were treated for 10 and 30 min and then exposed to buffer solutions with a pIOHA 122, reductions of V dem of about 30 and 55%, respectively, were found. Conversion of the root mineral into fluoridated hydroxylapatite can eliminate, in principle, the existing difference in caries susceptibility of the root and of the dental enamel.

The demineralization of human dental roots in the presence of fluoride.

Intact roots of human molars, which had not been exposed to the oral environment, were exposed to buffer solutions of pH 5.5, which contained, besides calcium and phosphate, various concentrations of fluoride. Densitometric measurements on contact-microradiograms of transverse sections of these roots produced data which were used to calculate the rate of demineralization (Vdem). Vdem appeared to be reduced with increasing fluoride concentration and with decreasing pIOHA (i.e., the negative logarithm of the ionic product of hydroxyapatite) of the buffer solution. In addition, it was observed that, after some days of exposure, remineralization started to occur within the surface layer at some micron from the outer surface and that the region within which remineralization took place was extended inward with increasing time of exposure. These data suggest that fluoride is effective in the prevention of root surface caries. The mechanism of the interaction of fluoride with the root mineral is discussed.

The mineral solubility of human tooth roots.

Whole roots of molars that had never been exposed to the mouth were exposed to calcium and phosphate-containing buffer solutions with discrete values of pIOHA [i.e. -log(a10Ca2+ X a6PO4(3-) X a2OH-)] at various pH. From densitometric measurements on contact-microradiographs of transverse sections of the roots the rate of demineralization (Vdem) was calculated. Vdem changed non-linearly as a function of pIOHA and became zero at a pIOHA-value of 105.3 +/- 0.4; this is substantially lower than the corresponding value of 118 +/- 1 for enamel. Thus root mineral is more soluble than enamel mineral. The critical pH for root caries appears to be about 6.7, provided that the plaque fluid follows about the same pH-pIOHA- pathway as acidified saliva.

The vulnerability of unexposed human dental roots to demineralization.

Crowns and roots of human molars, the roots from which had not been exposed to the oral environment, were exposed for 0, 3.5, 7, and 14 days to buffer solutions which were undersaturated or supersaturated with respect to hydroxyapatite. Densitometric measurements on contact-microradiograms of transverse sections of the crowns and of the cervical parts of the roots yielded plots of the mineral content as a function of the distance to the outer surface. From these plots, the rate of demineralization was calculated. It was found that the mineral of the roots dissolved even in buffer solutions which were supersaturated with respect to hydroxyapatite. Comparison of the results obtained from the crowns with those from the roots showed that the root hard tissues were more vulnerable to demineralization than was the dental enamel.

Measurement of the permeability of dental enamel and its variation with depth using an electrochemical method.

To investigate the permeability of the dental enamel of erupted and unerupted human pre-molars and its variation with depth, we determined the resistivity of successive 100-micron-thick layers. The electrical resistance of halved tooth crowns at zero Hertz, which according to Scholberg et al. (1982, 1984) is inversely related to the permeability, was measured before and after removal of successive enamel layers from the outer surface toward the dentino-enamel junction (D.E.J.). The resistivity of the successive enamel layers increased from the D.E.J. toward the outer surface in approximately the same way in erupted as well as in unerupted pre-molars, except within the outermost layer of 100 to 200 microns thickness. The resistivity of this layer in erupted pre-molars was considerably higher than that in unerupted pre-molars, which may be due to post-eruptive mineralization.

The effect of cavity preparation on the metabolism of protein in dentin.

Tritiated proline was administered to domestic cats during the development of their permanent dentition. The amount of residual radioactivity was determined autoradiographically at various times after cavity preparation in mature molars and premolars. Two wk after cavity preparation, the amount of radiolabel beneath the axial cavity wall decreased to about 50% of that present in corresponding contralateral cavity-free teeth. The observed decreases are ascribed to an enhancement of the protein metabolism in dentin. At 45 wk, 80-90% of the radiolabel has disappeared. This paper discusses the role of an eventual effect of microleakage on the latter results.

Radiochemical determination of the metabolic activity of collagen in mature dentin.

Tritiated proline was administered to domestic cats during the development of their permanent premolars. The metabolic activity of collagen in the mature premolar dentin was determined by quantitating the amount of tritiated hydroxyproline in the dentin as a function of time. It has been demonstrated that the metabolic activity of the dentinal collagen was extremely low and remained within the experimental error for a period of 45 weeks.