To investigate the influence of curing mode and temperature on the shrinkage kinetics of self-adhesive resin cements in comparison to a conventional multi-step resin cement.
The shrinkage of self-adhesive resin cements Maxcem Elite (MX), Speedcem (SPC), Smartcem2 (SMC), iCem (IC) and RelyX Unicem (RX) and Nexus Third Generation (NX3) as a multi-step resin cement was measured continuously for 1 h using the bonded disk method. All materials were tested with dual-curing (dc) and self-curing (sc) mode. All measurements ( n = 5 per group) were conducted at room temperature (23 °C) as well as at body temperature (37 °C). Shrinkage time constants were obtained from a simple exponential growth model. Data were statistically analyzed by ANOVA and the p -values were adjusted for multiplicity according to Hothorn et al. (2008) using the R-package “multcomp”.
Shrinkages ranged between 1.84 (RX sc23) and 7.09 (IC sc37). The curing-mode changing from sc to dc had the dominant effect for several materials, especially RX, both on final shrinkage and time constant for setting. Temperature increase had an effect on setting and shrinkage for all materials except RX. Final shrinkage for SPC, SMC and NX3 was statistically equivalent ( p > 0.05).
The 3-fold variation in final shrinkage for these materials is significant for clinical material selection. Light curing can lead to a 10-fold increase in the rate of setting. A self-adhesive universal resin cement (RX) had the lowest shrinkage in the groups examined.
Until recently only a small number of cement types for cementation of inlays, crowns or fixed partial dentures has been available, such as zinc phosphate cement or glass ionomer cement. Zinc phosphate cement has been regarded to be the gold-standard for cementation of metal crowns, porcelain-fused-to-metal crowns, cast posts and bridges due to its good clinical performance and the long-lasting clinical experience. It shows far more success with clinical evidence than any other luting cement although its mechanical and biological properties seem to be poor . High solubility, lower compressive strength, lack of adhesion and a low pH which may lead to pulp irritation have to be mentioned in this context .
A resin composite was firstly introduced as a crown and bridge cement in the early 1970s . Resin cements with their excellent esthetic shade-matching potential became popular due to the higher esthetical demands of patients for dental restorations . Other advantages of resin cements comprise better fracture resistance and higher bond strength compared to phosphate cement, which only adheres due to micromechanical interaction with the tooth substrate .
For the cementation of all-ceramic restorations multi step systems were used, requiring etching, priming, bonding and the application of a composite cement. This complicated procedure resulted in high technique sensitivity. As further development one or two bottle systems for dentin bonding were introduced by the manufacturers. Most recently the number of application steps was further reduced by the development of self-adhesive resin cements. The benefit of these materials lies in the ability to bond dentin without any type of pre-treatment . Due to the fact that the number of steps is affecting the risk of failure of a restoration and leads to a time expansion followed by an increasing risk of contamination with salvia or blood, one-step systems seem to be more favorable . Furthermore these self-etching composite cements promise a cementation procedure as simple as with conventional cements like zinc phosphate cements and at the same time a bond strength comparable to classical composite cements requiring a three-step dentin bonding procedure .
A number of studies have evaluated the bond strength of self-adhesive resin cements compared to conventional multistep luting agents. The results showed favorable bond strength behavior on dentin, while lower bond strengths were found on enamel surfaces compared to those provided by multistep luting agents . Therefore the self-adhesive cements became popular among dentists and have been increasingly used during the past few years. For clinical success reliable adhesion of dental materials to tooth substrates has to be guaranteed. Possible consequences of bad adhesion are microleakage, secondary caries, pulp reaction, plaque accumulation and periodontal disease . One of the main reasons of adhesion failure is polymerization shrinkage .
The aim of this study was to investigate the shrinkage strain as an important factor of polymerization shrinkage kinetics of five self-adhesive resin cements in comparison to one multi-step adhesive cement according to different curing modes and temperatures. The objectives included characterization of the kinetics by a simple time-constant.
The hypotheses to be tested were that
the magnitude of the final shrinkage increases with (A) temperature and (B) changing cure mode from sc to dc
the time constant of shrinkage decreases with (A) temperature and (B) changing from sc to dc
The null-hypotheses to be tested were:
changing curing mode from self-curing to dual-curing does not influence the magnitude of the final polymerization shrinkage strain
environmental temperatures (room temperature/23 °C versus body temperature/37 °C) do not affect the final strain
there is no difference in the final shrinkage strain between recently introduced self-adhesive resin cements and a conventional multi-step adhesive cement
Materials and methods
Five self-adhesive resin cements and one adhesive resin cement were selected for investigation. Material code, manufacturer and batch number and general composition details are given in Table 1 .
|Code||Material||Manufacturer||Lot number||Mixing-mode||Recommended cementation mode||Mean particle size (μm)||Filler loading (wt%)||Filler loading (vol%)||Fillers||Monomers||Monomers (wt%)|
|MX||Maxcem Elite||Kerr Corp., USA||3235810||Automix-syringe||Self-adhesive||3,6||69||46||Ytterbiumflourid, mineral fillers||MA HEMA||19–40|
|SPC||SpeedCEM||Ivoclar Vivadent, Netherlands||M31940||Automix-syringe||Self-adhesive||5||75 (base)||40||Barium glass ytterbium triflourid, copolymer, silicon dioxide||DMA, acidic monomers||20–30|
|SMC||SmartCEM 2||Dentsply, USA||0904261||Automix-syringe||Self-adhesive||3,6||69||46||Fluoride glass||UDMA, EBPADMA, DMA, TMA,||31|
|IC||iCEM||Heraeus Kulzer, Germany||305360||Automix-syringe||Self-adhesive||N/A||N/A||41||N/A||UDMA, DMAs, TMAs||50–75|
|RX||RelyX Unicem||3 M Espe, Germany||369233||Capsulated||Self-adhesive||d  = 90% of the fillers) being <12.5||70||50||Alkaline (basic) fillers, silanated fillers||MA, acidic monomers||25|
|NX3||Nexus third generation||Kerr Corp., USA||3256099||Automix-Syringe||Adhesive: OptiBond Solo Plus (total-etch) + gel-etchant or OptiBond All-In-One (self-etch)||0,6||67,5||47||Mineral fillers||MA||20–40|
All materials were tested at room temperature (23 °C) and body temperature (37 °C) at two curing modes: 23 °C/dual-cured (dc); 23 °C/self-cured (sc); 37 °C/dc; 37 °C/sc. Five specimens of each group were tested. All resin cements were applied and light cured by an Optilux 500 light curing unit (Kerr Dental Products, USA; light intensity ∼500 W/m 2 ) according to the manufacturer’s instructions. All measurements were performed continuously for 60 min except RelyX Unicem sc, which was recorded for 80 min due to its initial expansion. For specimen production the order of the materials was randomized.
Bonded disc method
The bonded disc method was used to measure the polymerization shrinkage strain kinetics. This method was developed by Watts and Cash , based on a method originated by Wilson . A rigid glass slide was lightly sandblasted and carried a brass ring of 15 mm inner diameter and a height of 1 mm. Inside of the brass ring a standardized soft-wax ring ( h = 1 mm; d = 8 mm) was fixed to retain the unset resin cement. A flexible glass cover slip of 0.1 mm thickness and a size of 22 mm 2 was slightly pressed on the brass ring by using another glass slide. Hence the unset material was brought in firm contact with the cover slip as well. The unset cement specimens of circular disk geometry and a mass with a range of 0.096–0.118 g were shaped with dimensions of 8 mm in diameter and 1 mm in thickness. These measures accorded to a C-factor (bonded to unbonded surface) of 0.6 periodically .
The glass plate was mounted to a horizontal aluminum table using two stainless steel clamps. The Optilux 500 light curing unit was positioned in its prevented space below the table using an adjustable clamp. The heart of the apparatus was the Linear Variable Displacement Transducer (LVDT) (GTX 2500, RDP Electronics, Wolverhampton, UK) which was clamped to a horizontal table of an aluminum stand. The sensitivity of the LVDT was better than 0.1 μm. The sensitive tip of the LVDT was vertically positioned in the center of the disk-geometry specimen on the cover slip. The deflection of the cover slip corresponding to the curing of the material was then recorded. The LVDT was connected to a signal conditioning unit (Type E309, RDP Electronics, Wolverhampton, UK) which monitored the displacement in units of voltage recorded by a computer data recorder and Picolog Software (Pico Technology Ltd., St. Neots, Cambridgshire, UK; Version 5.08.6.).
The displacement of the cover slip was equal to the amount of linear contraction: Δ L = L 0 − L where L 0 stood for the original specimen thickness and L was the final thickness. The calibration coefficient of displacement/voltage was used to calculate the deflection.
The LVDT was calibrated using a modified and calibrated digital micrometer (Mitutoyo, Tokio, Japan) with a display precision of 1 μm. The voltage/displacement calibration factor was calculated by linear regression ( r ≈ 0.9999), resulting in a factor of 0.0218 μm/mV. To calculate the shrinkage strain (%) of the 1 mm high specimen the given displacement/voltage calibration factor was divided over 10.
Determination of time-constants for shrinkage kinetics
Shrinkage (%) was re-expressed as normalized shrinkage ( S N ) by dividing each measured shrinkage value point by the maximum shrinkage observed (at 60 min). S N data points varied then between 0 and 1. The shrinkage curve was treated as an exponential decay. The mathematical equation for a simple exponential approach was expressed in terms of a time constant τ :
S N = 1 − e − t / τ