Braz Dent J (1998) 9(1): 25-31 ISSN 0103-6440
| Introduction | Material and Methods | Results | Discussion | Conclusion | References |
This study evaluated the influence of three cleaning agents on the bond strength of complete crowns cemented with zinc phosphate and polycarboxylate cements. For the polycarboxylate cement group, three cleaning agents were used: Tergentol; Tergentol + polyacrylic acid; Tergentol + citric acid. Zinc phosphate cement was used as a control group with Tergentol. There were no statistically significant differences of mean retentive force for either cement regardless of the test conditions. The marginal fit after cementation was also evaluated and the results of zinc polycarboxylate were lower than zinc phosphate cement and were statistically significant.
Key Words: cements, cleaning agents.
During tooth preparation, there is deposition of a smear layer
on the worn surfaces basically comprised of an enamel crystal mixture,
dentin debris, small particles from the rotatory instruments, oil and components
of saliva which can interfere with the cemented bond strength of cast restorations,
thus favoring the infiltration and leakage of marginal sealing.
A number of studies have been developed with the aim of finding cleaning procedures for dental preparation which provided better conditions for true chemical bonding between the cements with adhesive features and the enamel and dentin. This would provide better retention in the marginal sealing of the cast restorations.
The purpose of this study was to evaluate the influence cavity cleansers would have on chemical adhesion with zinc polycarboxylate cement and consequently, on the retentive ability of this material when utilized as a luting agent for complete crowns. An additional goal was to measure the film thickness of the cements utilized in this study.
Forty recently extracted human maxillary molars were embedded in a plastic cylinder with acrylic resin and stored in 100% humidity at room temperature. The teeth were prepared to receive a complete metal crown with the *chanfered cervical margin placed in the cement-enamel junction (Figure 1).
With the aim of simulating clinical procedures, the teeth were molded with an elastomeric impression material (Unilastic, Kerr Manufacturing Co., Michigan, USA) and from the stone dies obtained, patterns were waxed with a loop attached on the occlusal surface to provide a connection to the tensile testing machine (Figure 2). The patterns were invested with a phosphate-bonded investment (Termocast, Polidental Indústria Comércio Ltda., São Paulo, SP, Brazil).
The restorations were cast with a Ni-Cr alloy (Durabond MS, Dental Gaúcho,
São Paulo, SP, Brazil) according to manufacturer’s directions, cleaned
and adjusted on the dies. The crowns were then adapted on the respective
teeth and final evaluation was made with an exploring probe.
Based on the fact that a high frictional retention of the uncemented crown has a negative influence on marginal fit and on retentive strength of complete crowns, an attempt was made to standardize the frictional retention of the crowns around 0.500 Kgf (Pandolfi et al., 1985). For this, retentive force measurement was carried out in a Universal testing machine (Kratos, Dinamômetros Kratos Ltda., São Paulo, SP, Brazil) at a crosshead speed of 0.5 mm/min, and in those cases where the retention was above 0.500 Kgf, internal localized adjustments were made to reduce the frictional retention. The interference spots on the internal casting surfaces were detected by a paint-on indicator and the adjustments carried out also improved the fit of cast restorations to the prepared teeth.
Two types of cements and three cleaning agents were used according to
the combinations given in Table 1.
In group I (control), Type 1 zinc phosphate cement (S.S. White Ltda., Rio de Janeiro, RJ, Brazil) was used only with Tergentol (Laboratório Sintético Ltda., São Paulo, SP, Brazil) as recommended by the Prosthodontics Department of the Dental School at Bauru, São Paulo, Brazil. In the other groups, polycarboxylate cement (Durelon, ESPE, GMPH, Germany) was used with the following cleaning agents: a) Tergentol - 28% sodium sulfate lauryldietyleneglycol ether (detergent - pH 6.4) with cotton for 50 seconds; b) 32% polyacrylic acid - with cotton for 10 seconds; c) 50% citric acid - with cotton for 10 seconds.
Before cementation, each tooth was subjected to the previously described cleaning and the crowns were cleaned with Cavidry (Dental Fillings Ltda., Rio de Janeiro, RJ, Brazil).
Zinc phosphate cement was used in a proportion of 0.2 g/0.125 ml (Pandolfi et al., 1985) and 0.14 g/0.125 ml (Eames et al., 1977) for the polycarboxylate cement. After mixed, the cement was painted onto the inner walls of the crown from the cervical to the beginning of the occlusal surface (Pandolfi et al., 1985).
The casting was initially placed on the tooth with finger pressure and loaded with 9 kg in a stastic press (Figure 3) and after the final setting, the cemented castings were stored in water at 37oC for 24 hours.
Measuring seating of castings
Marginal fit was evaluated with a Mitutotyo Microscope with 0.005 mm accuracy and to facilitate of readings, marks were made on the four surfaces of the teeth and crowns. Three measurements were carried out on each surface before and after cementation and the difference between the initial and final adaptation measurements indicated the loss of fit due to cementation.
Testing crown retention
Twenty-four hours after cementation, the tensile forces required to dislodge the castings were determined by a Universal testing machine with a cross-head speed of 0.5 mm/min (Figure 4).
Table 1 presents the retention of complete
crowns cemented with zinc phosphate and polycarboxylate cements using three
different cleaning agents. One-way analysis of variance revealed no significant
differences between cements and cleaning agents.
Table 2 shows the marginal fit discrepancy of restorations cemented with both cements using different cleaning agents. Analysis of variance indicated statistical differences among the four groups. Multiple comparison using the Tukey-Kramer test revealed significant differences among groups, I x II, I x III and I x IV (P<0.05).
The importance of achieving a cement bond to the dental structure is
not only related to the possibility of increasing restoration retention,
but also to provide a better marginal sealing, decreasing cement degradation
in this area, and as a consequence, the establishment of carious processes.
The formation of a effective bond depends upon the physical nature of these surfaces and an obstacle to an effective bond is the presence of a smear layer (Gwinnett, 1984) (a layer aggregated in dentin). This layer is formed by a 0.5-?m thick organic pellicle and which consists of coagulated proteins formed from the denaturation of the collagen due to heat formed when using rotatory instruments. Non-organic particles originating from dentin and enamel are present in this pellicle, varying between 0.5 and 15 µm and blood, saliva and microorganisms are also present (Eick et al., 1970; Diamond and Carrel, 1984). This layer which acts as protection against bacterial penetration into the dentinal tubules (Diamond and Carrel, 1984) has begun to call the attention of a number of investigators who observed the possibility of removing this layer, thus opening the dentinal tubules with the consequent increase in the roughness of the prepared surface and, therefore, a possible increase in bond strength (Gwinnett, 1984). Thus, many studies have been carried out to evaluate the action of various substances on the dentinal surface, more specifically on the smear layer.
There has been great interest in acid substances due to their effective action in cleaning the prepared dentinal surface. In this investigation, 50% citric and 32% polyacrylic acids were used. According to Going (1972), citric acid is indicated for removing dentinary debris, blood and denatured collagen on exposed dentinal surfaces. However, Aboush and Jenkins (1987) advocate polyacrylic acid because it induces less alteration of dentinal topography and has low toxicity.
The bond strengths obtained in this study with Durelon cement and with the two cleansing agents - polyacrylic acid (group III) and citric acid (group IV) were 54.14 and 52.82 Kgf, respectively, which were lower than that obtained with group II (Tergentol + Durelon) which was 62.81 Kgf. Thus, one can observe that the use of acids not only promoted similar retention values but also lower values (around 16%) than the group which used only Tergentol as a cleansing agent and, consequently, also lower than group I (control group). These results were higher than those of other investigations of complete crowns cemented with polycarboxylate or zinc phosphate cements tested in a similar way (Richter et al., 1970; Dahl and Oilo, 1986).
Research related to the action of these substances shows that citric acid can completely remove the smear layer, widening the opening of the dentinal tubules. While polyacrylic acid also removes this layer, its action is lower in intensity, that is, a smaller alteration of the dentin topography occurs (Aboush and Jenkins, 1987; Meryon et al., 1987). This cannot be confirmed in this study because of the similarity between the results obtained with groups III and IV.
According to Powis et al. (1982), the removal of this layer does not favor the bonding process, as the widening of the dentinal tubule openings causes points of stress on the cement/dentin interface and as this layer is rich in calcium and phosphorus decreasing these elements causes an alteration in bond strength. This statement is in agreement with the results obtained in this investigation because, as previously mentioned, the use of polycarboxylate cement with Tergentol as a cleaning agent (group II) shows slightly superior values to group III and similar to zinc phosphate cement (group I). Thus, polycarboxylate cement can be used for the cementation of cast metal restorations, especially in vital teeth.
Durelon showed no significant differences in marginal fit in the three tested conditions and it was superior to the zinc phosphate cement with statistically significant differences. Comparing these results to other studies, one can observe that for the zinc phosphate cement the results in general were similar to those of Eames et al. (1978) (33 µm) and for the Durelon cement, the values were lower than those obtained by Richter et al. (1979) (112 µm) and Eames et al. (1978) (20 µm).
Thus, we can state that, regardless of the cleaning agent used, both cements had similar bond strengths and, in clinical terms, would not require the use of acid substances as dentin cleaning agents to increase bond strength, thus preserving the smear layer, which is important for the protection of the pulp crown.
1) For the polycarboxylate cement, the results of bond strength of group
II were superior to groups III and IV, with no statistically significant
2) Both cements tested under the same conditions (groups I and II) showed similar bond strength results.
3) For the polycarboxylate cement, the marginal fit of group III was lower than groups IV and II, with no statistically significant differences.
4) The crowns cemented with polycarboxylate cement showed lower marginal discrepancies than zinc phosphate cement, with statistically significant differences.
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Correspondence: Dr. Liuz Fernando Pegoraro, Departamento de Prosthodontics, Faculdade de Odontologia de Bauru, USP, Bauru, SP, Brasil.
Accepted March 17, 1998
Electronic publication: October, 1998