Israel Lewinstein

Improving the fit of crowns with die spacers

This investigation studied the dependence of seating crowns on the thickness of layers of spacers applied to dies. Extracted molars were prepared to designated taper angles. Artificial stone dies were prepared in polyether impressions of tooth preparations and covered with one to five layers of new or old spacer material in a predetermined manner. Wax patterns were invested to obtain a uniform and low expansion. Crowns were cast, luted to the teeth, invested in acrylic resin, ground parallel to the axes of the teeth, and inspected microscopically. The average thickness of layers of new and old spacer material was determined. The application of spacers up to the shoulder margins of dies decreased the elevation of the casting above the margin of the tooth preparation until an average minimum elevation above the shoulder of the preparation was obtained. A further increase in the spacer thickness did not affect the elevation, but increased the cement thickness at the axial walls. The average minimum elevation results mainly from individual protrusions on the casting surface. The optimum thickness of the spacer results in the minimum elevation at the margin together with a low cement thickness at the axial walls. Leaving the cervical part of the axial walls near the margin uncovered with spacer negates the effect of a thick spacer on the remaining die surface almost completely and is therefore contraindicated.

Transverse bond strength of repaired acrylic resin strips and temperature rise of dentures relined with VLC reline resin.

This study measured the transverse strength of polymethyl methacrylate heat-cured resin samples repaired with Triad visible light-cured reline resin with and without bonding pretreatments and with autopolymerizing resin, and it measured the temperature rise of Triad resin during relining of complete dentures at various curing cycles. The results indicated that pretreatment with either monomer or Triad bonding agent improved the bond of the Triad visible light-cured reline resin to the heat-cured resin. However, the use of the monomer rather than bonding agent resulted in a stronger bond and obtained values similar to those of samples repaired with autopolymerizing resin. Polymerization of samples repaired with Triad resin in the curing unit for two cycles of 5 minutes with 1 minute between cycles resulted in bubble formation and severe distortion of the heat-cured resin in the samples. Curing of the relined dentures for 10 minutes as recommended by the manufacturer raised the average peak temperature to 120 degrees C. In addition, it was shown that interrupting the light curing cycle attenuated the temperature rise, but it also resulted in a relatively softer reline resin. A continuous light curing of at least 5 minutes with the adjunct temperature rise is required to reach 1-hour hardness of 21.8 Vickers hardness number of the Triad reline resin.

Fit of crown wax patterns after removal from the die.

The effects of loading the wax pattern before investment was determined. Various loads were placed on the pattern after its margin was remodeled. The patterns were replaced on the die with a load of 50 gm before investment. For loads of 0, 250, and 1000 gm, the average elevation of the pattern on the die was found to be greater by 29, 56, and 19 micron, respectively, than before removal from the die. Repetitive loading showed that the plastic deformation in wax was less in the second than in the first cycle for a specific load. This decrease in plastic deformation may explain the improved adaptation of the pattern after a load of 1000 gm.

Effect of manipulative variables on the accuracy of crown wax patterns

An instrument was developed to carve wax patterns in a reproducible manner. The methods of wax application to the die and wax removal during carving were similar to routine laboratory procedures. The wax patterns were invested in stone with the dies on which they were prepared, and sectioned to allow the determination of their adaptation on sectioned samples. Shrinkage of wax patterns on dies was found to create a marginal gap at shoulders and bevels. Further shrinkage observed after removal from the die was attributed to relaxation of elastic stresses in the wax. Carving of bevels improved adaptation to the die for samples with large gaps at the shoulder. This improved adaptation can be attributed to plastic deformation in the wax bevel. Remodeling of pattern margins was found to improve adaptation to the die, especially if carried out after the pattern was removed and replaced on the die. Remodeling the margin should involve heating with a spatula of a narrow band of wax around the margin up to the axial die wall over the entire shoulder width.

Mechanical properties and histology of charge modified bioprosthetic tissue resistant to calcification

Modification of bioprosthetic heart valves tissue by covalently binding protamine sulphate, results in stable covalent links of protamine to the tissue, conferring resistance to calcification. We report here the morphological evaluation and mechanical properties (elastic modulus and ultimate tensile strength) of protamine-bound bioprosthetic tissue that have high anticalcification potential. Protamine-bound bioprosthetic tissue had significantly higher tissue modulus and ultimate tensile strength values than control tissue groups. However, the mechanical properties and tissue architecture were inferior to those of bioprosthetic tissue.