The physical properties of a visible light-cured temporary fixed partial denture material

Z. Khan, D.D.S.,* R. Razavi, B.S.,** and J. A. von Fraunhofer, M.Sc., Ph.D.*** University of Louisville, School of Dentistry, Louisville, Ky.

T he fit and function of a permanent cast restoration is often determined by the temporary coverage provided while the cast restoration is being made. The temporary crown should isolate the prepared tooth or teeth from the oral environment, stabilize the occluding and adjacent teeth, and provide compatible function, form, and esthe- tics.

A satisfactory temporary restoration can be made from autopolymerizing acrylic resin. However, the placement of unpolymerized acrylic resins on dentin and the gingivae may lead to thermal irritation from the exothermic polymerization reaction to the resin or chem- ical irritation from free or residual monomer.‘,’

To combine reduced tissue toxicity and thermal irrita- tion of the alternative resin systems with the ease of processing acrylic resins, a new visible light-cured (VLC) crown and fixed partial denture system that contains no methyl methacrylate has been introduced. This provisional fixed partial denture resin (Triad, Dentsply, York, Pa.) is ready to use from the package and remains workable until placed and cured by irradi- ation with visible light.

Fig. 1. Rectangular specimen used in testing proce- dures.

The physical properties and dimensional stability of a new VLC denture base resin system have recently been reported3 and the staining characteristics and selected physical properties of the system have been considered.4 This study compares the mechanical properties and surface characteristics of the Triad VLC fixed partial denture material with those of a conventional methyl methacrylate fixed partial denture material.

determined by the standard flexural test (ADA Specifi- cation No. 125). Specimens prepared as described were stored in distilled water for 48 hours at 37” + 1°C and then subjected to a three-point bend test using a univer- sal testing machine (United Calibration Corp., Gar- den Grove, Calif.) at a crosshead speed of 20 mm/min. All tests were performed in quadruplicate. The trans- verse strengths were calculated using the standard re- lation:

s, = 3LP 2WT’

MATERIAL AND METHODS

Rectangular specimens of dimensions 75 X 25 X 3 mm were prepared from Triad VLC material and Jet (Lang Dental Mfg. Co., Chicago, Ill.) acrylic resin for staining, water sorption, and surface hardness testing (Fig. 1). Both materials were invested in gypsum molds. The Triad VLC specimens were cured as directed by the manufacturer and the Jet acrylic resin specimens were allowed to autopolymerize under 25 psi.

where L = distance between supports, P = load at fail- ure, W = specimen width, and T = specimen thickness. Mean values and their standard deviations were calcu- lated for the strength data from the four specimens of each material.

The transverse strengths, S,, of the two materials were

*Associate Professor and Director of Postgraduate Prosthodontics. **Junior dental student. ***Professor of Biomaterials Science.

Staining tests were performed in solutions prepared from 800 ml of distilled water at 100” C to which one large tea bag (7.6 g) was added. The tea-bag was allowed to steep undisturbed in the water for 5 minutes, and after its removal, 0.5 g sodium azide was added to the solution as an antifungal and antibacterial agent. Four specimens of each material were placed in separate tea solutions that were incubated at 37 -+ 1” C and 50 + 1” C. Immersion periods of up to 1 month were used. The degree of staining was determined by comparing the postimmersion specimens with controls, which were immersed in distilled water at the same temperatures for the same time period.

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KHAN, RAZAVI, AND VON FRAUNHOFER

Table I. Properties of Triad VLC material and Jet acrylic resin

Triad VLC Jet acrylic resin

Fig. 2. Cylindrical specimens used in wear/abrasion studies.

The surface hardnesses of both the Triad and Jet acrylic resin specimens were determined by means of a Buehler semiautomatic microhardness tester using a 20 gm load. Microhardness tests were performed following 0, 1, 2, 4, 6, 8, 14, and 20 days of immersion in the tea solution at 37 ? lo C. A minimum of six indentations were made on each of the four specimens of the Triad VLC material and Jet acrylic resin. Mean values and their standard deviations were calculated for the micro- hardness values obtained from all four test specimens at each test period.

Specimens for abrasion testing were made by investing the Triad VLC material and Jet acrylic resin into 2 cm long and 0.5 cm diameter cylindrical tubes and cured as described (Fig. 2). These specimens were then imbedded in dental artificial stone blocks to provide a base that could be mounted onto the arm of the abrader machine (R. E. L. Reciprocating Single Arm abrader, Hampton Hill, Middlesex, U.K.). A total of eight blocks were used for abrasion testing, four of each material. The height of the cylinders was measured from the base of the stone block with a 0.02 mm dial caliper (Mitutoyo, Tokyo, Japan). Each block was then mounted in the arm of the abrader and subjected to 500 abrasions with 100 gm overweight across water-lubricated 600-grit silicon car- bide abrasive paper. After remeasurement of heights, the blocks were subjected to 500 additional abrasions and the loss of height was measured again.

The wear of the cylinders, expressed as percent loss in height, was determined with the formula:

Wear=Ho-HXlOO% - - Ho 1

where Ho is the original height and H the height after abrasion of the cylinder.

Water sorption 13.30 f 0.75 13.66 + 1.50 &/mm*)

Transverse strength 7.95 f 2.20 7.07 + 0.48 (kg/mm*)

Abrasion loss (%) 1.30 3.88 Microhardness (kg/mmr)

Initial 16.90 + 0.91 11.72 ? 1.07 20-day 17.90 f 0.52 12.50 + 0.42

Staining No difference

RESULTS

Jet acrylic resin and Triad VLC material exhibited similar staining characteristics and comparable values for transverse strength and water sorption. Over a 20-day immersion period, the respective water sorptions were 13.66 and 13.30 kg/mm2. The transverse strengths were 7.07 kg/mm2 for Jet acrylic resin and 7.95 kg/mm2 for Triad VLC resin; these values were unchanged over a 3-week immersion period. The initial microhardness of Triad VLC resin (16.9 kg/mm’) was greater than that of Jet acrylic resin (11.72 kg/n&). This difference was reflected in the greater abrasion of Jet acrylic resin (3.88%) compared with that for the Triad VLC material (1.3%). These data are summarized in Table I and are in reasonable agreement with the findings of other workers.3

CONCLUSIONS

Triad VLC material was found to have similar mechanical properties but wear and abrasion resistance superior to an acrylic resin temporary crown and fixed partial denture material. Because Triad VLC material does not contain methyl methacrylate, it could be advan- tageous for patients sensitive to poly(methy1 methacry- late), and its superior abrasion resistance suggests its usefulness for patients requiring extended use of provi- sional coverage of prepared teeth.

We plan to undertake evaluation studies by experi- enced dentists to determine whether the Triad VLC system offers significant advantages for chairside opera- tions and to assess whether the in vitro findings are reflected in clinical practice.

REFERENCES 1. Phillips RW. Skinner’s science of dental materials. 8th ed.

Philadelphia: WB Saunders Co, 1982. 2. Shillingburg HT, Hobo S, Whitsett LD. Fundamentals of fixed

prosthodontics. 2nd ed. Chicago: Quintessence Publishing Co, 1981.

3. Ogle RE, Sorensen SE, Lewis EA. A new visible light-cured resin system applied to removable prosthodontics. J PROSTHET DENT 1986;56:497-506.

4. Khan Z, van Fraunhofer JA, Razavi R. The staining character-

544 NOVEMBER 1988 VOLUME 60 NUMBER 5

istics, transverse strength, and microhardness of a visible-light cured denture base material. J PROSTHET DENT 1987;57:384-

6. American Dental Association. ADA Specification No. 15 for acrylic resin teeth. J Am Dent Assoc 1964;56:53.

Reprint requests to: DR. ZAFRULLA KHAN

UNIVERSITYOF LOUISVILLE

SCHWLOF DENTISTRY

LOUISVILLE, KY 40292

The efficacy of liquid and gel acid etchants

H. Baharav, D.M.D.,* H. S. Cardash, B.D.S., L.D.S., R.C.S.(Eng.),*+ R. Pilo, D.M.D.,* and M. Helft, D.M.D.*** Tel Aviv University, The Maurice and Gabriela Goldschleger School of Dental Medicine, Tel Aviv, Israel

T he acid-etch technique is based on establishing a mechanical bond between the composite bonding agent and enamel. Silverstone et al.’ described three types of etching patterns following exposure of enamel to etching agents. Type 1 showed preferential removal of prism core material leaving the periphery intact. In type 2, the periphery of the prisms dissolved leaving the prism core relatively unaffected. In type 3, the etching pattern could not be related to prism morphology because all three patterns were observed in adjacent areas of the same tooth. However, the type 3 etching pattern does show a pattern compatible with crystal morphology.2 The mechanical agitation of an acid-etching agent on the enamel surface results in a more debris-free surface.2 Dissolution of some of the apatite crystals and a reduc- tion in size of others increases the potential spaces between the crystals for the retention of the bonding agent.

The acid-etching agent, consisting of 30% to 60% phosphoric acid, is supplied by the commercial manufac- turers in a liquid and a gel form. This in vitro study compares the efficacy of liquid and gel type acid agents in preparing the enamel surface for penetration of the bonding agent.

METHOD AND MATERIAL

Five freshly extracted noncarious human lower third molars were cleaned with a prophylaxis brush and a water slurry of flour of pumice, washed, and dried with compressed air. The buccal surface of each tooth was divided into four quadrants.

The liquid etching agent, 35% phosphoric acid (Esti- cid, Kulzer, Friedrichsdorf, W. Germany) was applied

Fig. 1. SEM photographs of enamel surface after appli- cation of liquid etching agent. A, Magnification X1000; B, magnification ~5000.

*Instructor, Department of Oral Rehabilitation. **Senior Clinical Lecturer, Department of Oral Rehabilitation. ***Chairman, Department of Oral Rehabilitation.

to the first quadrant and allowed to remain for 1 minute. The tooth was washed with tap water from a syringe and dried with an air syringe.

The liquid etching agent was actively rubbed onto the

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