[Spatial variations of the mechanical properties of dental enamel and dentin]

Cuts with a Vickers diamond were made in five horizontal planes of the upper third of the crown of human lower premolars. At the same time the depth of entry was registered directly during the cutting with adot screen with a distance of about 150 mu between the dots. The depth of the cuts obtained was determined by optical measurement of breadth. Only in the fissure enamel was a slight hardening in the cervical direction observed. With a 5 p loading of the cutter diamond the reversible (elastic) deformation in the dentine is 0,4 +/- 0,1 micrometer greater than in the enamel. The optically determined cutting depth (irreversible deformation) amounted to 1,2 +/- 0,1 micrometer or 2,1 +/- 0,2 micrometer in enamel and dentine.

Effect of cooling techniques on temperature control and cutting rate for high-speed dental drills

This paper describes a series of tests conducted to determine the effectiveness of various cooling techniques used while preparing teeth for restoration. Comparisons are presented for four cooling techniques: air stream, air-water spray, water stream from a separate hand-held syringe, and water applied through a hollow bur. The comparisons were made in two ways, temperature control and influence on cutting rate. It was demonstrated that wet cooling techniques provided better temperature control than cooling with air and also improved the rate of material removal.

Finite element stress analysis of a class I amalgam restoration subjected to setting and thermal expansion

Stress analyses are presented for a second mandibular premolar with a class 1 amalgam restoration. The effects of amalgam setting and thermal expansions are examined. An axisymmetric representation of the structure is used and the analyses are carried out by the finite element method. Lined and unlined restorations are compared with regard to the principal stresses set up in the composite structures and also the patterns of deformation incurred. The effects of various enamel and dentin stiffness combinations are also considered.

Thermal stress in teeth

Observations of crack damage in the tooth structure from in vivo studies and in vitro experimental thermal cycling studies were combined with numerical analysis techniques to identify and isolate the influence of thermal stresses on the creation and propagation of cracks in teeth. The factors considered in this study included: (a) variations in tooth type or geometry (molar, bicuspid, etc.), (b) tooth age, (c) material properties of the tooth, (d) the magnitude of the change in the temperature of the environment surrounding the tooth, and (e) the thermal resistance between the tooth and the medium surrounding the tooth.

Numerical and experimental evaluation of energy inputs, temperature gradients, and thermal stresses during restorative procedures

Wet cutting in enamel should be used during restorative procedures. According to evidence reported here, dry cutting can induce sufficiently high thermal stresses to fracture the enamel. Temperatures resulting from dry cutting in dentin are high enough to cause biologic pulp damage if the cutting is within 1 to 2 mm of the pulp. Cracks induced in the cavity walls by dry cutting may eventually contribute to marginal failure. A hand-held syringe directing a small, high-velocity stream of water into the cutting region is a more effective cooling technique than an air-water spray.

Stresses from loading distal-extension removable partial dentures

A concentrated load of 222 N in the region of the central fossa of the second molar resulted in stresses in the supporting bone which increased gradually from zero in the region of the retromolar pad to about 25 MN/m2 distal to the second molar. Stresses in the alveolar bone supporting the second premolar were about 30 MN/m2 at the cemento-enamel junction, decreased to about 20 MN/m2 distal to the apex of the root, increased to 100 MN/m2 at the apex, and decreased to 3 MN/m2 near the mesial upper third of the root surface. When a distributed load of 222 N was applied to the first and second molar region, the stress distribution was similar to that observed for the concentrated load, and stress magnitudes were about 20% lower. Dramatic decreases in stress, with the maximum stress at the apex being 40 MN/m2, occurred when a 222 N load was distributed in the area of the marginal ridge.

An electrochemical study on the human dental enamel with special reference to isoelectric point

In an attempt to make a dynamic study of the isoelectric point and the chargeability of human dental enamel, an electrochemical approach was utilized. Zeta potential of three kinds of the enamel was measured by a streaming potential method. As a result, these isoelectric points could be decided.

[The effect of active removable orthodontic appliances on the dental enamel]

It could be demonstrated that enamel defects which are caused by maxillo-facial fixing elements of actively removable implements were reproducible in laboratory experiments. With minimal pressure of the wire bow of 50 grams and 250,000 lift, or individual vertical movements on the enamel surface resulted in a micrometer area of scouring.

A saliva substitute for use by xerostomic patients undergoing radiotherapy to the head and neck

A saliva substitute has been developed for use in veteran patients, primarity those undergoing radiotherapy for malignancy of the head and neck. The viscosity of the solution is adjusted to simulate that of whole saliva, and the electrolyte levels are similarly set. Fluoride is included at the level of 5.0 p.p.m. in an effort to enhance the mineralizing capacity of this preparation which is high in calcium and phosphate.

Role of attrition and occlusal contact in the physiology of the rat incisor: XL Kinetics of inner enamel epithelium during severely impeded eruption

Under conditions of high impediment, the rate of eruption of the rat incisor decreases considerably as does the rate of cell production of the inner enamel epithelium (IEE). The latter is acomplished by a decrease in the size of the proliferative compartment and an increase in the generation time. A linear relation exists between IEE cell production and tooth eruption, and cell production can be estimated from eruption measurements.

Finite element stress analysis of the crowns of normal and restored teeth

Stress distributions are presented for a normal and a restored mandibular second premolar under masticatory-type forces. These were obtained using the finite element method of stress analysis applied to two-dimensional models. The effect of the relative stiffness of the materials is examined in each instance.

Sterilisation of teeth for homogenous transplantation

Sterilisation of teeth for homogenous transplantation has been successfully achieved by combined ultrasonic waves and immersion in 6 per cent hydrogen peroxide. This combined treatment was found to have no harmful effect on the tooth integrity as indicated by enamel solubility, hardness, root canal seal and tooth size and weight. It is suggested the method offers the possibility of storing teeth in a "tooth bank".

Wear of dental tissues and materials

Wear may result from physiological or pathological conditions and may be desirable, as in the reduction of an overcontoured restoration, or undesirable as in the production of cervical abrasion cavities. A variety of methods, including clinical testing, the use of wear machines and the measurement of related properties such as hardness or coefficient of friction have been used to investigate wear of tooth tissue and of dental materials. Because these methods may not reveal the nature of the wear process recent work has been directed to the study of surface failure resulting from a single sliding contact. Many clinical studies have been conducted but they are time consuming and difficult to quantify, nor do they allow of evaluation of different parameters contributing to the wear. Laboratory simulation of wear has been shown to be valuable in comparing materials of the same group but between-group comparisons may give anomalous results. The most rewarding studies have been those using a single or small number of passes of a suitable abrading point over the material since these permit determination of the actual process by which wear is produced.

Fracture properties of human enamel and dentin

Fracture in enamel is anisotropic with respect ot the orientation of the enamel rods, with Wf for fracture parallel to the rods being 0.3 X 10(2) J/meter2 but on the order of 2.0 X 10(2) J/meter2 for fracture perpendicular to the rods. Fractographs of enamel showed that the enamel rods behaved as integral units during controlled fracture. A model was proposed to explain the fracture properties of enamel involving the assumption that the hydroxyapatite crystals did not fracture but that their orientation determined the fracture properties. Dentin was also anisotropic, with fracture parallel to the dentinal tubules being the strong direction - Wf was 5.5 X 10(2) J/meter2 for fracture parallel to the tubules, but 2.7 X 10(2) J/meter2 for fracture perpendicular to the tubules. Wf for dentin was of the same magnitude as the high strain rate Wf found for bone (2.0 X 10(2) J/meter2 [ref 3]). The data obtained in this investigation indicated that dentin and enamel are brittle substances and that forces occasionally imposed during mastication or bruxism probably are capable of initiating fracture when the normal anatomy of the tooth has been altered by cavity preparation. As consequence, the design of cavity preparations should include considerations to reduce stress concentrations and thus improve the practical fracture strength of tooth structure.