The aim of the study was to evaluate the contraction stress, microhardness and polymerization kinetics of three self-adhesive cements vs. conventional dual-cure resin cement.
Cements tested were: RelyX Unicem (3M ESPE, St. Paul, MN, USA), MaxCem Elite (Kerr, Orange, CA, USA), Clearfil SA Cement (Kuraray, Tokyo, Japan) and Duolink (Bisco Inc., Schaumburg, IL, USA). Cements were irradiated with a LED-curing unit (bluephase, IvoclarVivadent) for 20 or 40 s and the contraction forces (N) generated during polymerization were continuously recorded for 6 h with a universal testing machine. Polymerization kinetics were monitored using micro-Raman spectroscopy and degree of conversion was calculated. Vickers microhardness was also recorded. All measurements were performed at 10 min and 6 h. Data were statistically analyzed by three-way ANOVA with repeated measures and Tukey’s post hoc test ( α = 0.05).
Irrespective of exposure time, stress analysis ranked in the following order: Clearfil SA Cement < MaxCem < RelyX Unicem ≤ Duolink ( p < 0.05). Stress was correlated with microhardness values ( p < 0.05). Kinetic curves showed that maximum degree of conversion was attained more quickly than maximum stress after light activation.
The conventional resin-based cement showed higher stress values than the self-adhesive cements. The results were material-dependent and probably correlated to the composition of each material.
Simplified self-adhesive resin cements have been marketed in the last decade for luting inlays, onlays, crowns, and posts . In comparison with multi-step systems, self-adhesive cements are claimed to be less technique sensitive, as they do not need any pretreatment of the tooth surface with a separate adhesive or etchant . For this reason they gained rapidly in popularity, satisfying the clinicians’ demands for the simplification of luting procedures.
Self-adhesive cements are dual-curing hybrid materials that resemble compomers with the addition of acid-functionalized monomers to demineralize the tooth substrate . Due to their resinous nature, the polymerization of these materials is characterized by an inevitable volumetric contraction . Moreover, when used to lute posts, inlays and crowns, resin cements are applied in thin layers, characterized by an extremely high level of external constraint, i.e. C-factor . Bouillaguet et al. posited that the endodontic C-factor is higher than 200, whereas coronal restoration values range between 1 and 5. A high C-factor may generate sufficient stress to cause debonding of the luting material, thereby decreasing retention and increasing microleakage .
Nevertheless, information on stress generated by these materials is still lacking. Moreover, even though many commercial self-adhesive cements are currently available, most studies have focused attention on the properties of RelyX Unicem, the first self-adhesive material introduced to the market , and detailed information on other products is still limited.
The aim of this study was to compare curing characteristics of three contemporary resin-cements in comparison with a dual-cure resin cement. The main objectives were to measure the degree of conversion, hardness and contraction stress of the resin cements after light activation.
The hypotheses tested were: (a) self-adhesive cements exhibit similar final stress values as the conventional dual-cure resin based cement; (b) contraction stress values are correlated with microhardness; (c) self-adhesive cements exhibit similar polymerization kinetics, as measured by the acquisition of degree of conversion, as the conventional dual-cure resin based cement; and (d) the kinetics of stress development is directly related to the polymerization kinetics.
Materials and methods
The self-adhesive cements tested were: RelyX Unicem (3M ESPE, St. Paul, MN, USA), MaxCem Elite (Kerr, Orange, CA, USA) and Clearfil SA (Kuraray, Tokyo, Japan). The conventional dual-cure resin-based cement Duolink (Bisco Inc., Schaumburg, IL, USA) was used as a control ( Table 1 ).
|Cement||Delivery system||Shade||% filler weight/volume||Composition||Recommended curing time (s)|
|RelyX Unicem||Capsules (Aplicap: 0.01 ml)||A2 Universal||70/50||Powder: glass fillers, silica, calcium hydroxide, self-curing initiators Liquid: methacrylated phosphoric esters, dimethacrylates, acetate, stabilizers, self-curing initiators, light curing initiators||20|
|MaxCem Elite||Paste/paste dual syringe; direct dispensing through a mixing tip||Clear||69/46||Glycerol phosphate dimethacrylate (GPDM), co-monomers (mono-, di- and tri-functional methacrylate monomers, water, acetone, ethanol, barium, glass, fumed silica, sodium hexafluorosilicate||20|
|Clearfil SA Cement||Paste/paste dual syringe; direct dispensing through a mixing tip||White||66/45||Bis phenol A diglycidylmethacrylate (Bis-GMA), triethylene glycol dimethacrylate (TEGDMA), 10-methacryloyloxydecyl dihydrogenphosphate (MDP), hydrophobicaromaticdimethacrylate, hydrophobicaliphaticdimethacrylate, silanated barium glass filler, silanated colloidal silica, surface treated sodium fluoride, dl-camphorquinone, benzoylperoxide, initiator, accelerators, pigments||20|
|Duo-link||Paste/paste dual syringe; direct dispensing through a mixing tip||Traslucent||66/42||Base: bis-GMA, triethyleneglycoldimethacrylate urethane dimethacrylate, glass filler
Catalyst: bis-GMA, triethyleneglycoldimethacrylate, glass filler
The experimental setup consisted of two stainless steel cylinders as bonding substrates with a diameter of 2 mm and height of 25 mm . The two cylinders were fixed to the upper and lower clamps of a universal testing machine (Sun 500, Galdabini, Cardano al Campo, VA, Italy). The lateral surface of the stainless steel cylinder was threaded to improve the retention of the testing machine clamps. Before each test, the attachments were ground by 180 grit sandpaper and air-abraded before the application of the self-adhesive cements using a silica-containing abrasive (Cojet, 3M ESPE, St. Paul, MN, USA). When Duolink was tested, a layer of hydrophobic unfilled resin (Scotchbond Multipurpose Plus, 3M ESPE) was applied on the sandblasted surface and polymerized for 20 s with a LED curing unit (bluephase, IvoclarVivadent, Shaan, Lichtenstein) before cement application to ensure appropriate bonding to the stress analyzer. The irradiance of the curing unit was 1200 mW/cm 2 as measured using a commercial dentalradiometer (100 Optilux radiometer; SDS Kerr, Danbury, CT, USA).
A Mylar film was placed around the lower rod and filled up with the cement, then the upper cylinder was lowered and inserted into the upper hole of the mold, and the distance between the two cylinders was set to 2 mm ( 2 mm, 2 mm in height; C-factor = 0.5). An extensometer (model 2630-101, Instron, Norwood, MA, USA) was attached to the cylinders to provide an electronic feedback loop in the system to keep the specimen height constant during the test. Any approximation between the fixation points of the extensometer caused by resin cement shrinkage was immediately compensated for by controlled movement of the crosshead in the opposite direction (within 0.1 μm).
A defined quantity of cement paste (14 mg) for each tested material was placed in the mold in bulk and polymerized both for 20 s or for 40 s ( Table 1 ).
The contraction force (N) generated during polymerization to keep specimen height constant in opposition to the force exerted by composite shrinkage was continuously recorded for 6 h after photo-initiation. Preliminary investigations revealed that no stress occurred after that time (i.e. a plateau of the curve was achieved). Each experiment was conducted at room temperature (23–24 °C) and repeated 5 times for each material ( N = 5). Sample size was determined to reach 80% power for the analysis.
Contraction stress (MPa) was calculated at 10 min and 6 h as the force value (N) per area unit (force value/bonded surface area).
Microhardness (MH) measurements were performed with a Leica VMHT microhardness tester (Leica Microsystems S.p.A., Milano, Italy) equipped with a Vickers indenter. Cement specimens ( 5 mm, 2 mm in height, N = 5 for each cement at each curing time) were covered with a Mylar film and polymerized for 20 s or 40 s as above. Specimens were then polished with 1000 grit SiC paper to remove the resin-rich layer formed against the matrix. Microhardness was measured at 10 min and 6 h on the exposed surface at three randomized points using a Vickers indenter at 50 gf of load and 15 s dwell time and the mean hardness value for each specimen was calculated.
Polymerization kinetics and degree of conversion
A modular research spectrograph (Renishaw InVia; Renishaw plc, Gloucestershire, UK) connected to an optical microscope (Leica DM/LM optical microscope; Leica, Wetzlar, Germany) was used to investigate the degree of conversion (DC) of the self-adhesive cements. A near-infrared diode laser operating at 785 nm was used to induce the Raman scattering effect. The spectral coverage ranged from 100 to 3450 cm −1 with an average spectral resolution of 5 cm −1 . Instrument calibration was determined before data acquisition by comparison with the spectrum of single crystal silicon. Specimens were placed under the optical microscope on a computer-controlled X-Y-Z stage, focusing the laser beam with a 20× lens (NA 0.4; Leica N Plan objective, Wetzlar, Germany). The exposure time for each scan was 20 s with a spectral region of 400–1900 cm −1 , including the fingerprint region of the methacrylate-based polymers.
Specimens were prepared as for microhardness testing and polymerized for 20 s or 40 s as above. Five spectra were continuously acquired for 6 h on the exposed surface for each specimen as described before. The acquired data were then analyzed with a curve fitting application (Fityk, GNU General Public License) to analyze the polymerization kinetics and a spectrographic analysis software (Grams/AI 7.02; Thermo Galactic Industries Corp., Salem, NH, USA) to calculate the degree of conversion. Spectral acquisition was initiated immediately upon specimen deposition to obtain the Raman spectra of the material in the uncured state. The reaction peak was set at 1640 cm −1 (C C), while the reference peak was set at 1610 cm −1 (phenyl C C). The degree of conversion (DC) was calculated at 10 min and 6 h using the ratio between the reactive ( A rnx ) and the reference ( A ref ) peak intensities as follows:
DC % conversion = 1 − A rnx ( p ) / A ref ( p ) A rnx ( u ) / A ref ( u ) × 100 %