Effects of temporary cementation on permanent retention to composite resin cores
Philip L. Millstein, DMD, MS, and Dan Nathanson, DMD, MSD Boston University Goldman School of Graduate Dentistry, Boston, Mass. This in vitro study compared the effects on retention of base metal cylindrical retainers placed on composite resin cores when pretreated with eugenol and noneugenol temporary cements. Sixty composite cores and base metal cylindrical retainers were tested. The cores were pretreated with eugenol and noneugenol temporary cements before eventual cementation with resin and zinc phosphate cements. Cemented core retention was measured by application of a compressive force to the cores in an Instron machine. Differences were found between the two permanent cements. Pretreatment with eugenol cement reduced retainer retention with resin cement, but had no effect with zinc phosphate cement. Pretreatment with noneugenol cement did not reduce retainer retention. (J PROSTHET DENT 1992;67:856-9.)
omposite resin is commonly used in restorative dentistry as a core buildup material. It is relatively inexpensive, easy to manipullate, quick-setting, and reaches final hardness in minutes. The properties of this material allow a clinician to complete a core buildup and make final impressions in one visit. Provisional and permanent crowns and fixed partial dentures are commonly cemented with eugenol-containing temporary cements. Eugenol is known to be incompatible with resin polymers. It has been shown that eugenol-containing bases and liners partially inhibit the polymerization of freshly mixed composite resin restorations and that there is a softening of the resin surface adjacent to the eugenol liner.lm3 The effect of eugenol on cured composite resin was investigated by the authors, who showed that eugenolcontaining temporary cements could not be completely removed from cured composite resin surfaces and that, on removal, the surface of the resin had been altered.4 The effect of incomplete removal of a temporary cement from a cured composite resin on retention of a permanently cemented restoration has not been investigated. This in vitro study was designed to measure the retention of retainers cemented with zinc phosphate and composite resin cement after pretreatment of composite resin cores with eugenol and noneugenol temporary cements. MATERIAL
Sixty experimental cylindrical composite cores measuring 6 x 6 mm were made by mixing and syringing (C-R Syringe, Centrix, Inc., Stratford, Conn.) composite core maPresentedat the 69th General Sessionof the International Association for Dental Researchin Montreal, Que. aAssociateClinical Professor,Department of Biomaterials. bProfessorand Chairman, Department of Biomaterials. 10/l/35590
terial (Core Paste, DenMat, Santa Maria, Calif.) into aluminum molds coated with Teflon (DuPont Co., Wilmington, Del.) (Fig. 1). On hardening, the composite cores were separated from the molds and stored in 100 % humidity at 37” C for 48 hours. Sharp edges were rounded with an abrasive cutting instrument, and the cores were cleaned in an ultrasonic bath with distilled water. Imperfections that appeared as ridges and grooves on the outer surface of the hardened composite were not removed. Sixty base metal (Rexillium III, Rx Jeneric, Wallingford, Conn.) hollow cylinders with an outside diameter of 12 mm, a 6 mm height, and a centered hole 6.1 mm in diameter were used as experimental retainers in this study. The composite cores fit into them with a uniform cement space of 0.05 mm. The cylinders were prepared from Rexillium rods that were sectioned and machined to specification. The internal surface of each cylindrical retainer was sandblasted with a No. 240 grit fine aluminum oxide abrasive. Before cementation, the composite cores were divided into three experimental groups for the following pretreatments. Group A. No pretreatment was provided (control group). Group B. The external walls of the composite cores were covered with a eugenol-containing temporary cement (Temp-Bond, Kerr Co., Romulus, Mich.). Group C. The external walls of the composite cores were covered with a noneugenol-containing temporary cement (Freegenol, G-C International, Scottsdale, Ariz.). After application of temporary cement, the composite cores were stored at 37O C and 100% humidity for 7 days; then the temporary cements were mechanically removed. The cores were cemented into the retainers as follows: 30 cores were cemented with a resin cement (Crown Cementation Paste, DenMat Corp., Santa Maria, Calif.) and the remaining 30 cores were cemented with zinc phosphate cement (S.S. White, Mission/White, Tinton Falls, N.J.). With the resin cement, equal amounts of base and catalyst
1. Schematic view of core fabrication.
EErpressive Core Cement Metal Casting
2, Schematic view of core retainer separation,
paste were mixed according to manufacturer’s instructions and applied to the internal surfaces of the retainers and the external surface of the cores. Each core was positioned in the retainer and pressed against a flat glass mixing slab. Complete seating was ensured by the double open-ended retainers. Cores were cemented individually with mixing plus insertion times not exceeding 60 seconds per retainer. For the zinc phosphate cement group, premeasured amounts of cement liquid and powder were mixed on a cooled slab at room temperature (25O C). Mixed cement was applied to the intern.al surfaces of the retainer and external surface of the core with a sable brush. Cores were seated in the retainer with finger pressure against a glass slab. Again, mixing plus insertion times did not exceed 60 seconds per retainer. When set, excesscement was removed and copal varnish was applied to the open sides of the retainers cemented with zinc phosphate cement. After cementation all samples were stored in 100 % humidity at 37O C for 7 days. Retention levels of cores cemented to the retainers were measured in an Instron universal testing machine (In&on Corp., Canton, Mass.). Retainers were placed in a machined metal base to make room for core ex-
trusion. The cores were forced out of the retainers with a hardened steel compression rod at a crosshead speed of 0.02 cm/min (Fig. 2). Peak separation loads were recorded.
RESULTS Mean separation loads for the three groups are shown in the histogram (Fig. 3). The mean separation loads (pounds) for the resin cements were: group A, 953 + 261; group B, 119 4 54; and group C, 836 + 261. Mean separation loads for the zinc phosphate cements were: group A, 349 f 35; group B, 400 2 44; and group C, 357 f 52. A two-way ANOVA showed significant differences (JJ< 0.001) among the groups. Further differences were identified with a Newman-Keuls test. The resin cores pretreated with eugenol-containing temporary cement (group B) and cemented with resin cement clearly had lower separation loads than the other groups. Neither the resin-cemented nor the zinc phosphate-cemented retainers showed statistically different retention from the controls when cores were pretreated with a noneugenol temporary cement (group C). Pretreatment with a eugenol-containing temporary cement did not have a sig-
A B -RESIN-
GROUPS Fig. 3. Histogram showing separation forces of resin cores cemented with resin and zinc phosphate cements. A, No pretreatment, B, pretreatment with eugenol-containing temporary cement and C, pretreatment with noneugenol temporary cement.
nificant effect on the bond strength of zinc phosphate cement. With the exception of groups pretreated with eugenol cements, retainers cemented on resin cores with resin cement were significantly more retentive than retainers cemented with zinc phosphate cement. DISCUSSION Limiting the variables in this study was important in demonstrating that the application of eugenol-containing cement to cured composite resin cores before final cementation with a resin cement significantly reduced retention of the retainers. Apparently a eugenol-containing residue remains on the core after temporary cement is applied and cannot be effectively removed by simple mechanical means. On the contrary, pretreatment of a resin core with eugenolcontaining cement did not reduce crown retention when the core was cemented with zinc phosphate cement. Apparently eugenol-containing cement does not affect the properties of zinc phosphate cement. Even though the outer surface of the core may be altered as shown in a previous study, the overall bond strength is not reduced. However, long-term clinical effects of eugenol-containing cements on composite cores are unknown. The effect of the residual eugenol on resin cores cemented with a resin cement is profound and reduces retention dramatically. Eugenol is known to interfere with 858
the polymerization of resins, and it is possible that its presence on the core surface inhibits polymerization of self-curing resin cement. Clinicians use eugenol-containing temporary cements because the eugenol acts as an obtundent, and there is sufficient retention for the temporary cementation of crowns and bridges. Recent tests indicate that noneugenol-containing temporary cements (as compared with eugenol-containing cements) are equally retentive over time.5 The therapeutic effects of a eugenol cement would not generally be applicable to teeth with resin core buildups because these teeth may be devitalized or, if vital, covered mostly with composite resin. Using a noneugenol-containing temporary cement for teeth with resin cores is a viable alternative. In this study the noneugenol temporary cement tested had no adverse effect on retention of resin cores, even when a resin cement was used for final cementation. Indeed, the combination of resin core and resin cement produced retention values that were two to three times greater than retention produced by zinc phosphate cement. It is possible that a chemical bond occurred between the resin cement and the resin core, enhancing retention. Clinical
Resin cements can be used to increase core crown retention, provided the core has not been pretreated with a eu-
genol-containing temporary cement. When resin cores are coupled with a resin cement for final cementation, there is a potential for unprecedented retention values. The interim use of a eugenol-containing temporary cement is definitely contraindicated, and probably causes great reduction in retention. This observation is peculiar to resin cement only, in that there was no effect on the final outcome with zinc phosphate cement. With the availability of more final cements and several new resin cements, additional research is necessary to establish material compatibility. CONCLUSIONS
REFERENCES 1. Phillips 2.
3. 4. 5.
In vitro comparison of the retention of base metal cylinders placed on composite resin cores showed that resin cements produced retention values that were two to three times greater than retention produced with zinc phosphate cement. Retention was greatly reduced when sam-
ples were pretreated with eugenol-containing temporary cement.
of the strength
RW. Skinner’s science of dental materials. 8th ed. Philadelphia: WB Saunders, 1982498. Grajower R, Hirschfeld Z, Zalkind M. Compatibility of a composite resin with pulp insulating materials. A scanning electron microscope study. J PROSTHET DENT 1974;32:70-7. Grajower R, Hirschfeld Z, Zalkind M. Observations on cavity liners for composite resin restorations. J PROSTHET DENT 1976;36:265-73. Millstein PL, and Nathanson D. Effect of eugenol and eugenol cements on cured composite resin. J PROSTHET DENT 1983;50:211-15. Millstein PL, Nathanson D, Hazan E, Pierce A. Effect of aging on temporary cement retention in vitro. J PROSTHET DENT 1991;65:768-71.
DR. PHIUP L. MILLSTEIN 609 ALBANY ST., RM J601 BOSTON, MA 02118
Steven 0. Hondrum, DDS, MS’ The U.S. Army Institute of Dental Research,Washington, D.C. New ceramic materials for restorative dentistry have been developed and introduced in recent years. This article reviews advantages and disadvantages of dental ceramics, concentrating on strength properties. Included are factors affecting the strength of dental ceramic materials and the most common mechanisms for increasing the strength of dental ceramics. The properties of presently available materials such as dispersion-strengthened ceramics, cast ceramics, and foilreinforced materials are discussed. Current research efforts to improve the fracture resistance of ceramic restorative materials are reviewed. A description of methods to evaluate the strength of ceramics is included, as is a caution concerning the interpretation of strength data reported in the literature. (J PROSTHET DENT 1992;67:869-66.)
ew ceramic materials for restorative dentistry have been developed and introduced in recent years. Besides esthetics, the most important property of these new materials is strength. The purpose of this paper is to review the advantages and disadvantages of dental ceramics, concentrating on strength properties. Included are mechanisms for increasing the strength of dental ceramics, the strength of products presently available to the dentist,
The views expressedherein are those of the author and do not necessarilyreflect the views of the United States Army or the Department of Defense. %hief, Dental Materials. 10/l/36072
and present and future research efforts to improve the strength of restorative ceramic materials. BACKGROUND Dental ceramics are used as restorative materials to provide esthetic realism. Dental ceramics allow regular and diffuse t.ransmission, as well as diffuse and specular reflectance of light, and therefore have the potential to reproduce the depth of translucency, depth of color, and texture of natural teeth.‘, 2 In addition, dental ceramics are resistant to degradation in the oral cavity, are biologically compatible, and have a coefficient of thermal expansion that is similar to that of tooth structure. The major disadvantage of dental ceramics is their susceptibility to fracture during placement, mastication, and 859