INTRODUCTION

Resin cements have become clinically desirable because of their ability to bond to both the tooth structure and the restoration. They are often designed for specific applications, rather than general use, and are formulated to provide the handling characteristics required for particular applications.¹ Dual-cured resin cements have been considered the material of choice to lute esthetic indirect restorations and fiber posts.^2-4

The setting mechanism of dual-cured resin cements is usually based on a redox reaction of benzoyl peroxide with aromatic tertiary amines (represented by catalyst and base paste, respectively). One or both pastes contain a light-sensitive compound (usually camphorquinone) that is responsible for initiating the light-cured setting mechanism. After the pastes are mixed, and until light is provided, working time is controlled by the ratio of inhibitors to peroxide and aromatic tertiary amines.⁴ Both inhibitors and peroxides are organic chemical compounds that are susceptible to degradation upon storage; therefore, resin cements have a limited shelf life, and the setting mechanism of the cements may be influenced by changes in the chemistry of these components during storage time.⁵ The implications of such changes on the mechanical properties of the resin cements are unknown; however, clinicians handling resin cements with altered working time (WT) and setting time (ST) may experience some clinical difficulties.

Chemical reactions are known to be generally accelerated by heat, and heat causes significant changes in the WT/ST of traditional and resin-based cements.^6,7 Moreover, a slower-setting resin cement is more likely to be affected by adverse reactions with acidic, permeable adhesive systems.^8,9 Consequently, the drawback with such materials and the chemical interactions resulting from polymerization and aging acting on physical properties becomes evident. Therefore, the aim of this study was to evaluate the effects of accelerated aging and temperature variation on the rheologic properties (WT/ST) of several dual-cured resin cements. The null hypothesis was that temperature variation during reaction or after accelerated aging would not affect the WT/ST of the materials.

METHODS AND MATERIALS

Six commercial resin cements were selected for this study (Table 1). One material is representative of an exclusive self-cure reaction, and the others are representative of dual-cure materials. The WT and ST were determined from the chemically initiated reaction of the materials alone, as no light activation was used. WT is defined as the period of time available for material manipulation and handling. ST starts as soon as the redox reaction (mixing the initiator and amine) is initiated and is the time the material takes to reach the limit viscosity, which corresponds to the formation of a solid mass and, consequently, a near complete polymerization.¹⁰

Table 1. Six commercial resin cements selected for the study

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Rheologic Measurements

The WT and ST were determined with the aid of a thermo-controlled oscillating rheometer (Sabri Dental Enterprises Inc, Downers Grove, IL, USA) coupled to a highly sensitive flatbed recorder with an electric pen lift and a chart paper with 10-mm horizontal and vertical markers and a preset chart speed of 1 cm/min (Linseis Inc, Selb, Germany).

The chart recorder and a digital chronometer (Azula Marine, São Paulo, Brazil) were started exactly at the beginning of the mixing of the cements. After mixing per manufacturer's instructions, the material was transferred to the lower stage of the rheometer, which was preset at a temperature of 22°C or 37°C. The upper lever was lowered toward the material and reading continued until oscillation was completely restricted as detected in the chart, which was indicative of hardening of the material (Figure 1). Surface temperature of the oscillating rheometer was regularly monitored before and during the testing for each material with the aid of a laser thermometer (Neiko Tools USA, Homewood, IL, USA) pointed directly to the upper lever/lower stage interface and compared with the oscillating rheometer temperature indication to verify the accuracy of the readings. The WT and ST were determined directly from the chalice-like chart generated from each reading. The inflection points used, from the chart, to determine the WT and ST were 1) start of WT measurement during the begin of mixing per the manufacturer's instruction (vertical chart black line), 2) start of ST measurement (end of vertical chart black line), and 3) end of ST (restricted standardized horizontal chart lines indicative of material hardening; red line) (Figure 1).

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Five mixes were prepared from each material for each of the reading temperatures, both before and after accelerated aging. The WT/ST results were expressed in minutes and seconds (x' y”±standard deviation) and relative percentage contribution of WT to the overall setting period. Repeated measures analysis of variance was used to analyze the mean differences between the effects of temperature and aging on WT and ST for each material and differences examined by Tukey test. All analyses were pre-established at α = 5%.

RESULTS

Both temperature and aging significantly affected WT and ST of the cements (p<0.05), but no significant interactions were observed (p>0.05). In general, the higher temperature always resulted in significant reduction of WT and ST for all materials, regardless of the aging condition (Table 2). Effects of aging varied widely among materials. Calibra, Panavia F 2.0, and Variolink II showed significant increases in both WT and ST after aging. Conversely, C&B and Duolink showed significant reductions in both WT and ST after aging. BisCem showed a significant reduction in WT but a significant increase in ST after aging. Panavia F 2.0 and Variolink II presented the longest WT and ST of all materials, regardless of temperature and aging. The relative percentages of WT over the entire ST of the cements were in the range of 52% to 63% for fresh cements cured at 22°C (Figure 2). Although the overall WT and ST were reduced at 37°C, it appeared that the increased temperature during setting affected WT and ST differently (Figure 2). That was even more evident for the aged groups (Figure 3).

Table 2. Results of working time/setting time according to temperature and aging condition. Mean working time/setting time is expressed in minutes and seconds (x′y″) ± SD. n=5.*

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DISCUSSION

The overall results demonstrate that the WT and ST of all tested cements were affected by temperature and aging. The anticipated null hypothesis should then be rejected. The WT for fresh materials ranged from 2′48″ to 8′54″ when maintained at 22°C. In general, a WT in the range of 1 to 2 minutes would be adequate for most of the luting procedures in clinical practice.¹³ Panavia and Variolink had WTs above 8 minutes, but that should not be a clinical problem as both are dual-cure cements, and procedures can continue after cementation upon light activation of the cement. On the other hand, all other three dual-cured resin cements (Calibra, BisCem, and Duolink) showed WTs that were shorter than those for the self-cure C&B resin cement. This is of clinical importance because clinicians tend to believe dual-cure resin cements are preferable because they cure on demand and offer a longer WT. For all fresh materials, WT was roughly reduced by 50% when allowed to set at 37°C with most cements allowing less than 2 minutes before hardening takes place. This finding is in agreement with a recent study that also demonstrated a significant reduction in WT with an increase in temperature of dual-cured resin cements.¹⁴ Considering that the materials will encounter body temperature when placed in the mouth, this may be of importance in such procedures as post cementations, for example. If the cement is first dispensed into the root canal and the insertion of the post is somehow delayed, body temperature will accelerate the reaction, so clinicians may encounter difficulties in inserting the post if hardening takes place. It is also important to note that clinicians may use the remaining material on the mixing pad to check for setting of the cement used to lute a crown, for example. It is not surprising that the cement on the mixing pad could still appear unset at room temperature while the portion in the mouth has already set because of the body temperature. Accelerated setting reaction of polymers when exposed to heat has also been demonstrated in other studies.^6,7,15 Of greater importance were the findings of WT after aging. It is a common belief that aging degrades the chemistry, thus resulting in slower reaction and longer WT. This, however, proved the opposite for C&B, BisCem, and Duolink, in which WT decreased significantly when materials were aged. Reductions in WT were evident for both temperatures, but dropped below one minute for BisCem and Duolink when setting at 37°C. Such a short WT may translate into clinical difficulties and limitation of procedures. Thus, it would be wise to check the working time of resin cements at chair-side to prevent any unexpected shortened WT during a luting procedure, especially when the resin cement is not used often and is more than halfway to its expiration date. As expected, Calibra, Panavia, and Variolink resulted in increased WT after aging, and the latter two materials reached a WT in the range of 15 to 20 minutes when tested at 22°C, but not exceeding 10 minutes when tested at 37°C. Possible explanations as to why aging had different effects on the cements will be given later in the discussion.

With the exception of Panavia and Variolink, all other cements presented a ST that was within the clinically desirable range (4′00″ < ST < 7′00″). When heated to 37°C, ST dropped for all the materials, but never to a value that would compromise most clinical applications. Except for BisCem, which showed reduced WT and increased ST after aging, all other materials followed the same trend for ST as for WT. For example, C&B and Duolink resulted in reduced ST, and Calibra, Panavia, and Variolink had an increased ST after aging. Of note was the markedly long ST for Panavia and Variolink. Although this long ST of the self-activated reaction is likely to be overlapped by the mandatory light activation of the material in the clinical set, areas where light cannot reach will probably undergo the delayed ST predicted in our findings. This may pose a more serious clinical implication whenever these cements are used in combination with a simplified, acidic adhesive. Adverse reactions and permeability issues between these adhesives and slow-setting, self-cure, or dual-cure resin cements are well known to dramatically compromise the bonding between the cement and the adhesive.^4,9,16-18 Variolink and Panavia have also been previously demonstrated to be highly dependent on light activation to achieve adequate properties.^4,19-28 It has been advised that these cements should only be used when clinicians can guarantee access to light exposure to ensure adequate curing.^4,22 As recently demonstrated, all-ceramic restoration thickness of 3 mm and above adversely affects the polymerization of dual-cured resin cements.²⁹ It was interesting to note that some resin cements presented a reduction in WT and ST after aging, others had the opposite behavior, and BisCem showed the odd reduction of WT in combination with an increase in ST. This different behavior can be explained by the chemistry involved in the formulation of the materials. The self-curing reaction is activated on mixing by the traditional acid-base reaction between benzoyl peroxide in the catalyst paste and aromatic tertiary amines in the base paste. Inhibitors usually control the speed of this reaction, and how well this chemistry functions determines WT and ST. The stability of WT and ST during aging is therefore dependent on the stability of these chemical compounds. It is well known that peroxides and inhibitors are not stable chemical compounds, as they are highly sensitive to heat. As the material ages during storage, particularly when exposed to heat, as in this study, both peroxides and inhibitors may undergo degradation.^30-32 The original concentration of peroxide and the ratio of inhibitors in the formula results in an unbalanced ratio over time because these chemicals degrade at different rates. If inhibitors degrade faster, peroxides will attack amines faster, and this will accelerate the reaction, reducing WT and subsequently ST if peroxides are still in active concentrations to react with all the remaining amines. This is likely what happened to C&B and Duolink. As BisCem presented a reduced WT and extended ST, it is likely that inhibitors degraded first, thus allowing for a faster reaction of peroxides with amines, but later the reaction slowed down to result in increased ST. It is possible that peroxides also degraded to a point that not enough amounts were available to react with amines and the reaction speed was reduced. We cannot rule out, however, that other mechanisms might be involved in this particular finding. BisCem was the only self-adhesive cement in the study, and these types of materials contain acidic monomers in their composition that might also play a role in governing the self-curing reaction. It is also interesting to note that the materials with similar behavior after aging are produced by the same manufacturer (Bisco). It is not known whether manufacturers tend to follow a standard chemistry when developing different products of the same class, but this coincidence warrants such speculation.

Conversely, it is likely that Calibra, Panavia, and Variolink had a faster degradation of the peroxides, which resulted in enhanced action of inhibitors, thus reducing the speed of reaction and increasing both WT and ST. It is probable that the chemistry involved in the determination and subsequent stability of the WT and ST of these resin cements is much more complex than the earlier explanation. This is beyond the scope of this study but warrants further investigation. Aging is an important clinical issue, and little literature is available that relates how it affects the clinical use of the products. Every product has an expiration date printed in the package. What is important to understand is that such shelf-life expectancy is determined by the manufacturer based on accelerated-aging in-house studies.^11,12 This means the actual detailed performance of the product has not necessarily been tested when estimating the shelf life, particularly when new products are launched. Moreover, as organic chemical compounds are expected to be unstable over time, changes in performance are naturally expected from a freshly made product versus an aged one. What is not clear is how such changes affect clinical handling and how clinical handling has to be adjusted to compensate for the changes. This study addressed these important issues to determine how unavoidable factors such as aging of the product and temperature differences may affect the clinical handling characteristics of WT and ST. Because alterations are material related, clinicians are advised to take precautions when using these products. These could include accommodating purchases to permit the use of the freshest material possible and always storing peroxide-containing products in the refrigerator. Cold storage prolongs the stability of the chemistry and thus the stability of performance.

CONCLUSIONS

Both WT and ST were affected by temperature variation and aging condition. Some materials were demonstrated to have extended or shortened WT/ST periods after aging, despite temperature variation. Therefore, clinicians should be aware of such implications in clinical practice.