James L. Sandrik

Composition and Mechanical Properties of Gutta-Percha Endodontic Points

Gutta-percha endodontic filling points were found to contain approximately 20% gutta-percha (matrix), 66% zinc oxide (filler), 11% heavy metal sulfates (radiopacifier), and 3% waxes and/or resins (plasticizer). The mechanical properties were indicative of a partially crystalline viscoelastic polymerfc material. They were found to obey Hookes law and displayed a prominent upper and lower yield point when stressed beyond the proportional limit. The essential differences in mechanical properties of individual brands were found to be a function of the gutta-percha and zinc oxide concentration.

Composition and physical properties of gutta-percha endodontic filling materials

Gutta-percha has been the predominant root canal filling material for over a century. It has been used in various filling techniques and its effect on biologic tissues has been studied; however, knowledge of its chemical and mechanical properties has grown little since its introduction into dentistry. In this investigation, the mechanical properties of five brands of gutta-percha endodontic filling materials were determined, and their chemical compositions were compared. Significant correlations were found to exist between some of these parameters.

Peroxide bleaches: Effect on tensile strength of composite resins

Research concerning bleaching agents on the mechanical properties of composite resin restorative materials has produced conflicting reports. Representative brands of composite resin restorative materials were studied, including microfil, hybrid, and posterior types. The bleaching agents selected were 30% hydrogen peroxide and two commercial brands of 10% carbamide peroxide. Ten specimens of each material, prepared according to American National Standards Institute/American Dental Association specification No. 27 for diametral tensile testing, were stored in bleaching agents for 1 hour and 168 hours at 37 degrees C. An equal number of specimens stored in deionized water with identical conditions were designated controls. Qualitative examination of bleached specimens revealed a marked change in color, especially the microfilled composite resins. There was no significant difference in tensile strength between controls and exposed samples at either time except for the diminished strength of microfilled composite resins in 30% hydrogen peroxide for one week (p < 0.01).

Variations in bracket placement in the preadjusted orthodontic appliance

This study was conducted to determine the accuracy of bracket placement with the direct bonded technique. Ten orthodontic faculty members bonded a preadjusted orthodontic appliance on models of five cases of malocclusion in a simulated clinical situation (mannequin). A total of 50 sets of models served as the population of the study. Photographs of the models were measured to determine vertical and angular discrepancies in position between adjacent bracket pairs from a constructed reference line. Variations are evaluated with respect to the classification of malocclusion, specific tooth type, and intra/inter operator differences. A mean of 0.34 mm for the vertical discrepancies and a mean of 5.54 degrees for the angular discrepancies are found in placement of the orthodontic brackets.

The influence of preloading on stress relaxation of orthodontic elastic polymers.

The main objective of the investigation was the determination of whether prestretching would decrease the rapid force loss of the elastic polymers. The results indicated that Alastik CK chains showed a significant decrease in force loss whereas the C2 chains were unaffected. Since overextending the chain to compensate for force loss during clinical placement could result in capillary bed ischemia, increasing the force loss of the CK chains must be done before application. To accomplish this the chains should be prestretched by the manufacturer or operator. An improvement in the material component or manufacturing process would provide a more lasting solution to a material for which orthodontists have found increasing use.

Accuracy and dimensional stability of four interocclusal recording materials

Four materials, zinc oxide-eugenol paste, eugenol-free-zinc oxide paste, a silicone, and polyether jaw relations registration material were studied in a controlled laboratory environment to determine their accuracy and stability in intervals over a 1-week period. The results are as follows: 1. The eugenol-free zinc-oxide paste was the only material which exhibited no statistically significant difference between the die scribes and those of the sample. 2. The setting reaction of the eugenol-free-zinc oxide paste was a saponification reaction, resulting in the production of an insoluble soap. 3. With polyether, silicone, and zinc oxide-eugenol paste, there was a statistical difference between the die and the respective samples at the immediate reading and throughout the experiment. Polyether showed the least difference, and zinc oxide-eugenol paste the greatest.

Bond strength of three orthodontic adhesives

A method was devised to study the bond strength of direct-bonding orthodontic adhesives. A shear load was applied parallel to the tooth-adhesive-bracket interface by means of an Instron Universal Testing Machine. The bond strength was found to increase for some products after 27 days. However, no significant differences could be detected 1 day after placement. The interface was studied to determine the mode of failure. In all cases bond failure occurred as mixed adhesive-cohesive phenomena.

Tensile and bond strength of putty-wash elastomeric impression materials

n dimensional instability of elastomeric impression materials has been the subject of numerous reports.‘” The instability of these materials is a function of a number of variables, including evaporation of byproducts formed during polymerization, ability to bond to a custom tray, thickness of material within the tray, rigidity of the tray, polymerization shrinkage, and viscosity of the elastomer (light body versus heavy body). When these materials are bonded to an acrylic resin custom tray, their dimensional stability has been shown to be increased significantly.4 Usually, low-viscosity elastomers (light body, syringe, or wash material) are polymerized against high-viscosity elastomers (heavy body or tray materials), which are bonded with adhesive to acrylic resin custom trays. Cherim-AmkieS showed that lowviscosity elastomers shrink less when polymerized against high-viscosity elastomers bonded to a polymethyl methacrylate substrate-a custom tray. Davis et a1.6 found the clinical effectiveness of tray adhesives to be related to their drying time and elastomer type. Polyether elastomers were found to exhibit the greatest bond strength to trays, and polysulfide elastomers adhered better than silicone elastomers. To simulate custom trays, high-viscosity elastomers (putty materials) are used in stock trays, usually without an adhesive. An internal coating of lowviscosity material provides detail. Putty materials typically exhibit good dimensional stability because of a low polymer-to-filler ratio. Their dimensional stability is related to high polymer concentration, particularly in regard to by-product formation and polymerization shrinkage. Therefore, these materials shrink less than lower viscosity materials. The dimensional stability of the putty-wash combination materials would be expected to be related to the factors described as well as to the quality of the bond

Rotation of rectangular wire in rectangular molar tubes

The degree of angular rotation of rectangular orthodontic wires in rectangular molar tubes was measured using a rotatable mechanical stage on a Unitron metallograph and compared to theoretical data. It was found that square 0.016 inch wire did not bind in 0.018 by 0.022 inch or larger mandrel- or inconel-formed tubes, whereas rotations of 11 to 23 degrees were noted in cast tubes of this dimension. Although 0.016 by 0.022 inch wire did bind in these tubes, the degree of rotation was far greater than expected on the basis of theoretical calculations or clinical expectations. The results indicated that it was unrealistic to expect fine control of a few degrees of torque to occur as expected for delicate clinical control.

Tensile strength of elastomeric impression materials, adhesive and cohesive bonding

Numerous factors are known to effect the accuracy of elastomeric impression materials. Factors often overlooked are the quality of the bond between low-viscosity and high-viscosity elastomers and the presumed cohesive bond between materials during an impression reline procedure. This study examined the bond strength of condensation polymerization polysiloxanes, addition polymerization polysiloxanes, polyethers, and a light-cured polyether urethane dimethacrylate. Of the tested materials, light-cured polyether urethane dimethacrylate exhibited the greatest bond strength. The reline technique, light-body cured to previously cured heavy-body putty, yielded results similar to the simultaneous-cure technique. In terms of bond strength there is no advantage of one technique over the other. The corrective reline technique yielded acceptable results in bond strength. In general, the tested materials exhibited greater tensile strength than bond strength.