Retentive strength, luting cements
Sergio Gorodovsky, BDS, MS,a and Omar Zidan, BDS, HDD, MS, PhDb Intercontinental University, Mexico City, Mexico, and University of Minnesota, Minneapolis, Minn. This study evaluated the retention of complete crowns by using five diRerent methods of cementation. Complete crowns were prepared with standardized dimensions on extracted human molars. Metal crowns were cast with a high noble gold ceramic alloy and were cemented with zinc phosphate cement, glass ionomer cement, composite resin cement, composite resin cement with a dentinal bonding agent, and adhesive resin cement. The retention was measured by subjecting the specimens to tensile load until fracture occurred. The disintegration was measured according to American Dental Association Specification No. 8, and the condition of the cements at the margins of crowns was analyzed by use of a scanning electron microscope. Kruskal-Wallis one-way analysis of variance revealed statistically significant differences between the mean retentive strehgths. The retention of the zinc phosphate and the glass ionomer groups was significantly different from that of the adhesive resin group. The retention of the adhesive resin cement was 65% greater than the retention of the composite resin and the composite resimdentinal bonding agent group, but the Mann-Whitney Wilcoxon rank sum test did not depict this difference as significant. The mean i- SD of the disintegration for the zinc phosphate, the glass ionomer cement, and the composite resin cement was 0.025 + 0.013, 0.023 + 0.011, and 0.017 * 0.001, respectively. The scanning electron microscope analysis of the margins revealed that the composite resin cement was almost intact, the zinc phosphate was subjected to limited disintegration, and the glass ionomer displayed the worst marginal integrity. (J PROSTHET DENT
hree main groups of dental materials are available for cementation of casting-namely the zinc phosphate cement, the glass ionomer cement, and the resin cement. Zinc phosphate is the oldest but most popular dental cement, and current formulations remain similar to those introduced 90 years ago.l Glass ionomer was introduced in 1969 by Wilson and Kent,2 and after continual improvements, Ketac-cem material was distributed as the first commercial water-hardening luting agent in 1979. Resin cements have been available since the 1940s but their use has been limited despite the potential for this group of materials.3, 4 The resin cement Comspan was developed in 1980 by the L.D. Caulk Co., Milford., Del., in conjunction with the etched-metal technique at the University of Maryland and represents an improved generation over early formulations. The new Comspan material displayed dramatic improvement in mechanical properties, film thickness, and handling characteristic. s, 6 In the mid 1980s Superbond crown and bridge system (Sun Medical Co., Kyoto, Japan), a newer class of resin cement with adhesive properties, was formulated from the research of Nakabayashi.7 aAssistant Professor, Department bAssociate Professor, Department
of Operative Dentistry. of Restorative Sciences.
This study measured the retention of crowns cemented or bonded with five methods using a zinc phosphate cement, a glass ionomer cement, a resin cement, a resin with a dental bonding agent, and an adhesive resin cement. The disintegrations of zinc phosphate, a glass ionomer, and a composite resin cement were measured, and the condition of these cements at the margins of artificial crowns was analyzed microscopically.
MATERIALS AND Retention test
The retention of. complete crowns cemented with five methods was measured by a tensile strength test using a zinc phosphate cement, a glass ionomer cement, a resin cement, a resin cement with a dentinal bonding agent, and an adhesive resin cement. The materials investigated were: the zinc phosphate cement-Fleck’s (Mizzy, Inc., Clifton Forge, Va.); the glassionomer cement-Ketac-Cem (ESPEPremier, Norristown, Pa.); the composite resin cementComspan (Caulk Dentsply, Milford, Del.); the adhesive resin cement-Superbond C & B; and the dentinal bonding agent-Prisma Bond-(Caulk Dentsply). Extracted intact human molars were cleaned and then refrigerated in demineralized water until used. Horizontal notches were prepared on the roots for retention, and the teeth were then aligned in a chlorinated polyvinyl chloride
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Retentive Notches CP VC ring -
Fig. 1. Tooth preparation nated polyvinyl chloride.
in this study. CPVC, Chlori-
ring with their axes perpendicu1a.r to the horizontal plane. The rings were filled with self-curing acrylic resin (Trucure, Teledyne Dental, Elk Grove Village, Ill.). Complete crowns were prepared with the standardized dimensions using a lathe (Southbend Lathe, South Bend, Ind.). The resinous cylinders were fastened to a three-jaw chuck and a stylized complete veneer crown preparation was prepared with a solid carbide single-end four fluted cutting tool (Cleveland Twist Drill, Cleveland, Ohio). The three-jaw chuck and the cutting tool were revolved at 340 and 18,000 rpm, respectively, and demineralized water was used as a coolant. The axial walls were prepared with a 4-degree taper on each wall, and the occlusal surface was flat and perpendicular to the long axis of the tooth, leaving only part of the occlusal anatomy for a seating key of the future metal crown, while a shoulder was the finishing line. A uniform surface finish was realized by lightly touching the rotating occlusal and axial walls of the preparation with a fine Carborundum disk. A bevel approximately 1 mm wide was prepared with a flame-shaped la-blade finishing bur at the junction of the occlusal and axial walls with a highspeed drill and water spray coolant. All preparations were measured in three dimensions to an accuracy of 0.5 mm with a manostat device (Tyoe 5921, KWB, Switzerland), and preparations that deviated more than 0.5 mm from the desired configuration were rejected. The samples were then rinsed and refrigerated in demineralized water (Fig. I). Individual impression trays were fabricated for each prepared tooth. An impression was made with a polyvinylsiloxane material (Mirror 3, Kerr-Sybron, Romulus, Mich.) using the single mix technique, and was poured after 1 hour with Die-Keen, a dental gypsum material (Columbus Dental, St. Louis, MO.). Three layers of Tru-Fit (Gee, Taub 270
2. Specimenassembledand mounted on MTS testing machine. Fig.
Products and Fusion Co., JerseyCity, N.J.) wereapplied as die relief while avoiding the margin. Blue inlay castingwax wasusedto form the pattern and wax wasreadapted to the margin with a heated instrument while the excesswasremoved with a PKT No. 4 under a stereomicroscopeat 10 power. A round wax sprue was shapedas a loop and was attached to the occlusalsurfaceof the wax pattern to provide a connection for the tensile testing machine, and the wax patterns were then numbered. The wax pattern was invested in a phosphate-bonded investment (Ceramigold, Whip-Mix, Louisville, Ky.) following the manufacturer’s instructions. The metal crownswere cast using a high noble gold ceramic alloy, Lodestar (Williams Dental Co.,Amherst, N.Y.) with a metal content asfollows: 51.5% gold, 38.5% pailadium, 8.5% indium, , or larger than, precedes the mean and standard deviation values of this group, denoting that the actual value is slightly greater than the reported value. The means ranged from a minimum of 3.08 MPa with the zinc phosphate cement to a maximum of >6.40 MPa with 271
Fig. 3. SEM of margin of crown cemented with composite resin cement. Superior portion of micrograph is crown and inferior portion is tooth structure. Note that composite resin cement is almost intact at marginal area.
the adhesive resin. The minimal standard error (>0.58) was recorded by the adhesive resin, while the greatest range (5.54) was for the composite resin cement. Nonparametric methods were considered more suitable for the statistical analysis because of the nature of the data. A Kruskal-Wallis one-way analysis of variance revealed statistically significant differences (p > 0.005) between the groups, so Mann-Whitney Wilcoxon rank sum test analyses were performed to compare within-groups. Because of multiple inferences (10 pairwise), no differences between groups was designated statistically significant unless it exhibited a p value no greater than 0.005. The retentive strengths of the zinc phosphate and the glass ionomer cements were significantly different from those of the adhesive resin group. The retention of the adhesive resin cement was 65% greater than that of the composite resin cement and the composite resin cement/DBA groups, but the statistical test did not depict this difference as significant (p = 0.0295, p = 0.0080, respectively).
Disintegration The mean I SD of the disintegration for the zinc phosphate, the glass ionomer, and the composite resin cements was 0.025 t 0.013, 0.023 i 0.011, and 0.017 _t 0.001, re-
Fig. 4. SEM of margin of crown cemented phosphate cement with minimal disintegration.
spectively, and the percentage of integration were 0.914 +_ 0.527, 1.130 i 0.554, and 0.815 + 0.039, respectively. Although this study followed American Dental Association Specification No. 8, the limited number of specimens was not well suited for statistical analysis.
of the marginal
Representative scanning electron photographs depicting the disintegration of the cement at the margins are presented in Figs. 3 to 5. The composite resin cement was almost intact at the marginal area (Fig. 3). The zinc phosphate cement exhibited limited disintegration during the testing, with sparse scattered porosities visible at the cement line (Fig. 4). The glass ionomer cement possessed the worst marginal integrity (Fig. 5), with the disintegration of the cement from almost the entire interface between the tooth and the artificial crown.
DISCUSSION The retentive strength of five methods of crown cementation was evaluated in vitro. Ten metal crowns were fabricated for each group and were cemented on the prepared teeth. At the completion of the study, the number of specimens within the groups varied from the original design because the teeth were unexpectedly dislodged from the acrylic resin.
The retentive strength between the various methods revealed a significant difference, and this disparity was attributed to the high retentive strengths of adhesive resin Superbond C & B. The artificial crowns were not separated from the preparation in 46 % of the composite resin/DBA group or in 90 % of the adhesive resin group. This indicated that for these two groups the retentive strengths reported in the Results section and analyzed were below the actual values. Zinc phosphate is a nonadhesive cement with limited mechanical properties, but it is reliable. The primary retention of a cast restoration cemented with zinc phosphate is influenced by the configuration of the tooth preparation, namely the taper, the length, and the surface area.g-13 The luting ability of the cement is considered a secondary role in retention, which is achieved mainly from mechanical interlocking. I4 Glass ionomer cement attains retentive strength both through mechanical interlocking and physiochemical bonding.15 This study could not verify the claims for the superiority of the glass ionomer compared with the zinc phosphate with respect to retentive strength. Both materials measured the same retentive strength, and only one specimen in each group showed unexpected results. The adhesion properties of the glass ionomer cementI did not improve the retention of the castings, so it could be inferred from these findings that the retentive parameters described for the zinc phosphate were applicable also for the glass ionomer; that is, retention should be primarily the function of the geometric configuration of the preparation. The same would apply for the composite resin cement, because the slight increase in the retentive strength could be considered a secondary function and could be attributed to stronger mechanical properties. The use of a dentinal bonding agent (DBA) with the composite resin cement for the luting of a crown modified the nature of the result because 46 % of the specimens could be classified as unexpected failures. Although the mean retentive value for this group was not statistically greater than that of the zinc phosphate, glass ionomer, or composite resin cement, and not significantly lower than the adhesive resin, the real retentive value commonly exceeded the value for the statistical analysis because of the unexpected failures. The DBA had a pronounced effect on the result, but the actual importance of this effect could not be established from this study and requires further investigation. The newer generations of DBAs that induce stronger bonding strengths than the agent used in this study should be included in future research. The adhesive resin cement recorded the highest retentive strength of all the groups, and this was attributed to the intense bond of 15.5 MPa effected by Superbond C & B to dentin.17 The retention induced by both the geometric configuration of the preparation and the strong bonding properties of the adhesive resin resulted in 90 % unexpected results. This also proposed concerns about the ease
5. SEM of margin of a crown luted with glassionomer cement. Cementhasalmost totally disintegrated from interface between tooth preparation and artificial crown. Fig.
of removing castingscementedwith adhesiveresin on ideal preparations. Conversely, an adhesiveresin cement would improve the retention of castings on tooth preparations compromisedby excessivetaper or by lack of adequate length. Future research could be directed toward establishing a correlation betweenthe taper and the length of the tooth. The method to evaluate the disintegration of three luting cementswasthe American Dental Association Specification No. 8 for zinc phosphate cement.8 Although this specification wasintended for the quality control of commercial products, it has often been used to evaluate new materials and comparedifferent dental cements.The reasonis that other techniqueshave not been adopted by any international dental organizations. This specification has obvious limitations: the test is too brief, water asa test medium differs from oral fluids, and there is no element of abrasion.An additional limitation wasthe paucity of specimens,making any statistical inference basedon thesedata of limited value. The SEM analysisrevealed that the marginsof the composite resin and the zinc phosphate cementswere almost intact, while the glassionomer cement was substantially dissolvedfrom the margins. If water causedan inordinate degree of disintegration of the cement in this study, an 273
acidic media would accelerate the dissolution of glass ionomer cement at an alarming rate. SUMMARY
The retention of castings cemented using zinc phosphate, glass ionomer, composite resin cement, composite resin cement with a DBA, and adhesive resin cement were evaluated. Complete crown preparations with standardized dimensions and an $-degree total taper were prepared on extracted intact human teeth and were restored with a cast gold ceramic alloy crown. The castings were cemented and the retention was tested under tension. The disintegration of cement was measured according to the American Dental Association Specification No. 8 for the zinc phosphate cement, and the margins of crowns were examined using the SEM after storage for 6 to 8 weeks. If the retentive test caused a separation of the metal crown from the tooth preparation, the results were considered “expected.” If the tooth was dislodged from the embedding medial, the results were termed “unexpected.” The means + SD in MPa (number expected - number unexpected) of the retentive strength for the different groups were: zinc phosphate, 3.08 i- 0.9 (9 - 1); glass ionomer, 3.12 -t 1.2 (9 - 1); resin cement, 4.21 j, 1.8 (9 - 2); resin cement/DBA, 4.01 F 1.8 (7 - 6); and the adhesive resin cement, >6.40 +- 1.8 (1 - 9). The statistically significant difference between the groups was attributed to the elevated retentive strength of the adhesive resin cement. The zinc phosphate and the glass ionomer cements were statistically significantly different from the adhesive resin group. The purported superiority of the glass ionomer compared with the zinc phosphate with respect to retention was not verified in this study. 1. The use of a dentinal bonding agent with the resin cement resulted in 46 % of unexpected failures; this could be the reason why the statistically significant difference remained undetected for this group. The adhesive resin cement resulted in 90% unexpected failures. 2. The mean + SD of the disintegration for the zinc phosphate, glass ionomer, and the composite resin cement was: 0.025 F 0.013, 0.023 F 0.011, and 0.017 + 0.001, respectively, but the method of measuringthe disintegration of cement had obvious limitations.
3. The SEM photomicrographsof the marginsrevealed that the zinc phosphate and the compositeresin cements were almostintact, while the glassionomer cement nearly disappearedfrom the margins. REFERENCES
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