Kai Chiu Chan

Build-up and Repair of Light-cured Composites: Bond Strength

Interfacial bond strengths of light-activated composites were measured as a function of age using a transverse strength test. Bond strength between layers decreased with the age of the initial layer and reflected the setting curves of the composites. The highly-filled composites exhibited the greatest bond strengths. Uncut surfaces provided a better substrate for bonding than did ground surfaces. Use of a bonding agent on both uncut and ground surfaces improved bond strengths. Mean repair strengths of light-activated composites were similar to those of selfcuring composites. Composites with ground surfaces aged for one wk had mean repair strengths 27% of the cohesive strength without bonding agent and 48% with bonding agent.

The ability of foods to stain two composite resins

In this bench study, coffee and soy sauce stained composite resin restorations to a significantly greater degree than did tea or cola beverage. Generally, the greatest degree of staining with all samples occurred during the first week of the study time. The stain penetration was superficial and was estimated to be 5 mu or less.

The strength of multilayer and repaired composite resin

Composite resin surfaces that formed against a plastic matrix or that polymerized while exposed to air were excellent substrates for the adhesion of new resin. Samples formed by the addition of composite resin to a cut surface had a tensile strength of one half of the cohesive strength of the resin. Coating the cut surface with a thin layer of unfilled resin enhanced bonding of the second composite resin layer. The use of a thin layer of unfilled catalyst resin as a bonding agent caused the most rapid development of strength and the greatest strength in samples tested after 7 days.

Curing Light-activated Composite Cement through Porcelain

The purpose of the study was to determine the exposure times needed to harden a light-activated composite cement through various thicknesses of porcelain. A negative exponential model was developed with which the relative hardness of a composite could be predicted from the product of the exposure time and the square root of the light intensity at 470 nm. Hardness data obtained by varying the exposure time and porcelain thickness were found to fit this model. A relationship was derived to predict the required exposure time, t, of a composite under porcelain with a transmission coefficient, Tc, and a thickness, 1 : t = toT c -½, where to is the exposure time of the composite alone.

The effect of bonding agents on the interfacial bond strength of repaired composites

Abstract The interfacial bond strengths of repaired samples of 2 macrofilled and 2 microfilled composites were measured using a transverse strength test. Specimens matured for 7 days in 37°C water were ground with 600 grit silicon carbide paper and coated with either an enamel, dentin, porcelain, or acrylic veneer bonding agent or a solvent, chloroform or ethyl acetate, before the fresh composite was added. In general, the repair strengths were less than the cohesive strengths of the unrepaired composites. The dentin bonding agent, Scotchbond, gave the highest repair strengths. The repaired microfilled composites had higher interfacial strengths, as a percentage of cohesive strength, than the macrofilled materials.

Effect of microwave oven drying on surface hardness of dental gypsum products

D entists often find it necessary to work with gypsum casts soon after separating them from impressions. Since wet casts usually have inadequate surface hardness, dentists normally must wait 24 to 48 hours before the casts are strong enough for manipulation. To save time, the microwave oven-drying technique has been suggested.’ However, the effect of microwave oven drying on gypsum products has not been thoroughly examined. The purpose of this study was to investigate and compare the surface hardness of eight dental gypsum products dried by microwave oven (1485 W, Maytag CME, Maytag Corp., Newton, Iowa) or ambient air.

Mercury Vapor Emission from Dental Amalgam

The patterns of mercury vapor emission from carved, burnished, and polished surfaces made from four commonly used dental amalgam alloys were determined with the use of a selenium sulfide detection method. Mercury vapor was emitted more intensely from the marginal region; carved and burnished surfaces emitted the same amount. There was a difference in the amounts emitted from the different alloys tested.

Curing light-activated composite resins through dentin

L ight-activated composite resins have become widely used because of the advantage of control of the working time. However, these composite resins are limited in that the entire material must be exposed to sufficient light to cause thorough polymerization. In many situations, tooth structure blocks direct access of the light to all portions of the restoration, as in some Class III restorations and others with undercut retentive areas. Similarly, bonding metal appliances such as orthodontic brackets, acid-etch fixed partial denture retainers, and splints with light-activated composite resins requires exposing the composite resin indirectly through tooth structure. Enamel is quite translucent and has been shown not to affect the depth of cure of composite resins when the enamel is 0.5 to 1 mm thick.’ Dentin is more opaque than enamel and, therefore, might be expected to be more limiting to the cure of composite resins. The purpose of this study was to determine the ability of light to penetrate thicknesses of dentin and to cure lightactivated composite resin.

Auxiliary retention for complete crowns provided by cement keys

1. Cement keys were an effective means of increasing the retention of complete cast gold crowns to preparations in extracted teeth. 2. Single grooves placed in either the dentin or in the crown alone were not effective in increasing retention in 30-degree crowns. In 7-degree crowns, a single groove in the crown gave better retention than a single groove in the dentin. 3. Crowns with a 7-degree taper had greater retentive strengths than 30-degree crowns treated identically. The retentive strength of 7-degree crowns with cement keys approached the tensile strength of zinc phosphate cement.

Microleakage and enamel finish

Cavity preparations that are finished with hand instruments allow significantly less marginal leakage than those which are completed only with rotary instruments. This does not mean that hand-instruments walls are perfectly smooth; it only means that these walls allow less marginal leakage. Cavity preparations restored with composite resin showed more marginal leakage than those restored with amalgam. Also, the degree of marginal leakage was greater in cavity preparations restored with composite resin than in those restored with amalgam. Cavity preparations are more prone to marginal leakage at sharply defined cavosurface acute angles than at the smooth or straight surfaces of the cavity. This pattern of marginal leakage was true for both amalgam restorations and composite restorations, but the degree of marginal leakage was greater when composite resin was used as a restorative material. Since less marginal leakage was found in the finished cavity preparations, it is obvious that all cavity preparations must be finished with hand instruments. This may not completely eliminate the problem of marginal leakage, but it will reduce the potential secondary caries both in degree and frequency. Also, since sharply defined acute angles did exhibit more marginal leakage, this part of the cavity preparation needs special attention. With amalgam and composite restorations at least, these angles should not be sharply defined but should be slightly round, so that better condensation of restorative materials can be obtained. This may reduce the degree of marginal leakage.