The aims of this investigation were to investigate dual-cured luting-agents as to whether their bond strength, dimensional change and flexural modulus influence no interfacial-gap incidence parameters of composite inlay restorations during the early stages. The correlations of interest were between: (a) their shear bond strength to dentin, (b) their dimensional change on setting, (c) their flexural modulus, and (d) no interfacial-gap incidence with indirect restorations.
Seven dual-cured luting-agents, one self-adhesive resin cement, six resin cements and one resin-modified glass-ionomer for luting, were investigated with specimen sub-groups ( n = 10) for each property measured. The principal series of experiments were conducted in dentin cavities with interfacial polishing either immediately (3 min) after setting or after 1-day water-storage. After the finishing procedure, each tooth was sectioned in a buccolingual direction through the center of the restoration, and the presence or absence of interfacial-gaps was measured (and then summed for each cavity) at 14 points (each 0.5 mm apart) along the composite inlay restoration interface ( n = 10 per group; total points measured = 140), and was expressed by percentage of measured total points. The shear bond strengths to dentin, setting shrinkage-strain and flexural modulus were measured. To estimate the dimensional change of luting-agents, the maximum marginal gap-width and the opposing-width that occurred with luting-gents in a Teflon mold were measured. Moduli were measured in 3-point bending.
For all composite inlay restorations, polished immediately after setting, an incidence of summed no-gaps of 69–88% was observed. For specimens polished after 1 day, a significantly ( p < 0.05) decreased number of 91–96% summed no-gaps occurred. After 1-day storage, shear bond strengths to dentin and flexural modulus increased highly significantly ( p < 0.001) for many materials, whereas dimensional changes in the Teflon mold were non-significantly different ( p > 0.05). There was a highly significant correlation between no interfacial-gap incidence and shear bond strength ( r = 0.702, p = 0.002, n = 16). As the dimensional change (shrinkage) within Teflon molds increased, the no interfacial-gap incidence of dentin/inlay interfaces with ‘no-gaps’ decreased ( r = −0.574, p = 0.02, n = 16). Flexural moduli significantly correlated with no interfacial-gap incidence in the composite inlay restorations ( r = 0.695, p = 0.003, n = 16).
For three classes of luting-agents during the early stage of setting (<1 day), the shear bond strength to dentin, the dimensional change measured by marginal gaps in Teflon mold and flexural moduli correlated with no interfacial-gap incidence in the composite inlay restoration.
In clinical practice, the popularity of tooth-colored posterior restorations has increased due to demand for esthetic restoratives and also a growing concern about the biocompatibility of amalgam. Resin composite and resin-modified glass-ionomer cement (RMGIC) showed better performance than luting-agents for indirect esthetic restorations . Luting-agents for composite inlay restorations are produced in dual-polymerized formulations, which are indicated for restorations with material opacity sufficient to inhibit light energy from transmission to the cement. Although light irradiance reaching the cement may often initiate the surface polymerization process, a self-cure chemical agent and some time is needed to ensure a maximal cure . RMGIC materials are unlike light-cured resin composites or conventional glass-ionomer cements. These systems embody dual-setting processes consisting of photo-polymerization and an acid–base reaction. The final set material has glass particles sheathed in a matrix consisting of two networks, one derived from the resin, the other from a glass-ionomer type reaction .
The resin cement and RMGIC for luting-agents are claimed to improve marginal adaptation by enhancing the bond strength and hygroscopic expansion during the day after light-activation . Restorations which use ceramic or resin composite should be firmly bonded and sealed to the underlying tooth substrate with adhesive luting-agents . Gap-formation might be used directly as a parameter for restorative material bond-ability . However, the mean luting-agent film thickness with composite inlay restorations was previously reported as highly variable, ranging between 50 and 100 μm . Therefore, in the interests of eliminating uncontrolled variables in a scientific study, it is preferable to measure the gap-formation for luting-agents with them placed directly in tooth cavities of constant size, without the complicating presence of restorative-material inlays, and thus variable luting-agent dimensions.
Shrinkage stresses in composite restorations or RMGICs for luting generated during setting are still one of the major problems in indirect restorative dentistry. Excessive shrinkage stresses being placed on the tooth cavities, due to wall-to-wall contraction, may lead to marginal discrepancies, postoperative hypersensitivity and secondary caries . Our method for measuring the relative setting shrinkage-strain, for comparison with marginal gaps in tooth cavities, was described previously . This method is based upon determining marginal gap-widths in non-bonding Teflon cavities. These showed a significant correlation with marginal gap-widths in tooth cavities . The effect of water-storage on gap-widths in both Teflon and tooth cavities was also studied . However, other studies suggested that as shear bond strength increased, the marginal gap in the tooth cavity decreased, that is, the higher the bond strength shown the smaller the marginal gap-width . With resin-composite restorations, a stiffer composite places higher stress on the adherence than does one of lesser stiffness. The shrinkage stress is reduced by flow of the resin composite from the non-bonded surface . However, the flow of unset luting-agents is very important for inlay placement with the lute between restorative material and tooth substrate.
This investigation was, therefore, carried out with multiple types of dual-cured luting-agents to evaluate early stage behavior (both immediate and after 1-day storage). Performance was to be assessed with regard to: (a) their early no interfacial-gap incidence around butt-joints in composite inlay restoration, along all dentin margins; (b) their early shear bond strengths to dentin; (c) their immediate free setting shrinkage-strain and hygroscopic expansion after 1-day storage, determined by the marginal gap-width in a non-bonding Teflon mold; and (d) their flexural modulus of elasticity. The hypothesis to be tested was that, among the set of materials studied, trends in one or more of properties (b), (c) and (d) would correlate with trends in property (a).
Materials and methods
The sources, compositional details and classification of the eight luting-agents used in this study, together with their pretreatment agents, are summarized in Tables 1 and 2 . All procedures were performed in accordance with the manufacturers’ instructions. Capsules of RelyX Unicem and Fuji Plus were triturated using a high-speed mixer (Silamat, Vivadent, Schaan, Liechtenstein) for 15 s or 10 s, respectively. For light-activation, a curing unit (New Light VL-II, GC, Tokyo, Japan; optic diameter: 8 mm) was used. The light irradiance was checked immediately before each application to the materials, using a radiometer (Demetron/Kerr, Danbury, CT, USA). During the experiment the light irradiance was maintained at 450 mW/cm 2 . Human premolars, extracted for orthodontic reasons, were used. After extraction, the teeth were immediately stored in cold, distilled water at about 4 °C for 1–2 months before use. Ten specimens were made for each material, storage period and property were investigated. Ten specimen composite inlay restorations were produced using each luting-agent and one composite inlay material (Z 250, 3M ESPE, St. Paul, MN, USA; Batch No. 5LYJ; Shade: A3; filler: 80 wt%, 60 vol.%) and each bonded area to luting-agents of composite inlay restorations was pretreated by a silanized primer (RelyX Ceramic Primer, 3 M ESPE, St. Paul, MN, USA; Batch No. 6XF; 1% gamma-methacryloxypropyl trimethoxy silane, 70–80% ethanol, and 20–30% water). All procedures, except for cavity preparation and mechanical testing, were performed in a thermo-hygrostatic room kept at 23 ± 0.5 °C and 50 ± 2% relative humidity. The results were analyzed statistically using the Mann–Whitney U -Test, Tukey Test (parametric), and Tukey Test (non-parametric or t -Test).
|Self-adhesive resin cement|
|RelyX Unicem Aplicap||3M ESPE, Seefeld, Germany||Filler content 72 wt% (aluminosilicate, silanized filler)|
|Methacrylates, initiators, acidic methacrylates|
|Adhesive resin cement|
|Calibra||Dentsply/Caulk, Milford, DE, USA||Filler content 67–68 wt% (silica fume)|
|Bis-GMA, TEGDMA, titanium dioxide, catalyst|
|NEXUS 2||Kerr, Orange, CA, USA||Filler content 70 wt% (fumed silica and barium aluminosilicate)|
|Bis-GMA, TEGDMA, EBPADMA, HEMA, UDMA, catalyst|
|Panavia F||Kuraray Medical, Kurashiki, Japan||Filler content 78 wt%|
|Paste A: MDP, comonomer, filler, NaF, BPO|
|Paste B: comonomer, filler, NaF, amine, initiator|
|LINK MAX||GC, Tokyo, Japan||Filler content 68 wt% (fluoroalumonisilicate glass, SiO)|
|UDMA, HEMA, dimethacrylate, catalyst|
|Bistite II||Tokuyama Dental Tokyo, Japan||Filler content 77 wt% (silica–zirconia)|
|MAC-10, EBPADMA, monomer, initiator|
|Chemiace II||Sun Medical, Moriyama, Japan||P: complexed filler, SiO 2 , ZrO 2 , amine|
|L: 4-META, HEMA, dimethacrylate, BPO,|
|Resin-modified glass-ionomer cement|
|Fuji Plus||GC, Tokyo, Japan||P: fluoro-alumino-silicate|
|L: copolymer of acrylic and maleic acid, HEMA, water, initiator|
|Materials||Manufacturer||Composition and surface pretreatment|
|Self-adhesive resin cement|
|RelyX Unicem Aplicap||3M ESPE, Seefeld, Germany||Non|
|Aadhesive resin cement|
|Calibra||Dentsply/Caulk, Milford, DE, USA||Tooth conditioner gel: phosphoric acid (3.4%), water|
|Prime & Bond NT: acetone, dipentaerythritol pentaacrylate phosphate, UDMA, polymerizable dimethacylate resins|
|Prime & Bond Self-Cure Activator: acetone, ethyl alcohol sodium p-toluenesulfinate|
|Tooth Conditioner Gel (15 s) → wash & dry → Prime & Bond NT → Prime & Bond Self-Cure Activator (20 s)|
|NEXUS 2||Kerr, Orange, CA, USA||Kerr Gel Etchant: 37.5% phosphoric acid, water|
|Optibond Solo Plus: Bis-GMA, HEMA, GDM, filler CQ, ethanol|
|Kerr Gel Etchant (15 s) → wash & dry → Optibond Solo Plus (15 s) → air (3 s) → light cure (20 s)|
|Panavia F||Kuraray Medical Kurashiki, Japan||ED Primer II Liquid A: HEMA, MDP, 5-NMSA, accelerator, water|
|ED Primer II Liquid B: 5-NMSA, accelerator, water|
|ED Primer II A + B (30 s) → gently dry|
|LINK MAX||GC, Tokyo, Japan||Self-etching primer EP-A: 4-MET, HEMA, dimethacrylate, distilled water, ethanol, catalyst|
|Self-etching primer EP-B: ethanol, catalyst|
|Self-etching primer EP- A + B B (30 s) → gently dry|
|Bistite II||Tokuyama Dental, Tokyo, Japan||Primer-1: phosphoric acid monomer, acetone, alcohol, water, catalyst,|
|Primer-2: HEMA, acetone, catalyst|
|Primer-1 (30 s) → dry → Primer-2 (20 s) → dry|
|Chemiace II||Sun Medical||Treating agent (green): citric acid (10%), ferric chloride (3%), water|
|Treating agent (10 s) → rinse & dry|
|Resin-modified glass-ionomer cement|
|Fuji Plus||GC, Tokyo, Japan||Fuji Plus Conditioner: citric acid (10%), ferric chloride (2%) water|
|Fuji Plus Conditioner (20 s) → wash & dry|
No interfacial-gap incidence in composite inlay restorations.
It is preferable to measure the no interfacial-gap incidence for luting-agents in the same tooth structure of constant size. Therefore we have measured in dentin cavity for the case of luting-agents testing . A flat surface of dentin was obtained by grinding the tooth with wet silicon carbide paper (#220). Then, a cylindrical cavity was prepared with a tungsten carbide bur (200,000 rpm) and a fissure bur (8000 rpm) under wet conditions to a depth of approximately 1.5 mm with a diameter of 3.5 mm. One cavity was prepared in each tooth on the coronal region and medial surface. A total of 140 cavities were prepared in 140 teeth for this study (7 materials × 2 polishing or inspecting times × 10 repeats = 140). Incidence of dentin/cement interfaces with no gaps for 10 specimens was expressed as a percentage of measured total points ( Fig. 1 ). The prepared cavity surface was pretreated with the conditioner/primer according to each manufacturer’s instruction as described in Table 2 . The appropriate luting-agent ( Table 1 ) was applied in the treated cavity the using a syringe tip (Centrix C-R Syringe System, Centrix, Connecticut, USA). Each composite inlay was inserted, then covered with a plastic strip and exposed to a visible light source for 20 s. The Fuji Plus restored specimens were stored in an incubator at 37 °C and 100% relative humidity for 5 min after mixing.
The surface was polished immediately after light-activation or setting or after storage in distilled water at 37 °C for 1 day. The excess luting-agent was removed with a tungsten carbide bur and wet grinding with silicon carbide paper (#1000), followed by polishing with linen with an aqueous slurry of 0.3 μm aluminum oxide (Alfa Micropolish, Buehler Ltd., Chicago, USA) and rinsing with distilled water immediately after polishing. Each tooth was sectioned in a buccolingual direction through the center of the restoration with a low-speed diamond saw (Isomet, Buehler Ltd., Lake Bluff, IL). The presence or absence of marginal gaps was measured with a traveling microscope (1000×, Measurescope, MM-11, Nikon, Tokyo, Japan) at 14 points (each 0.5 mm apart) along the cavity restoration interface ( n = 10; total points measured = 140) and the absence-of-gap data were summed for each cavity.
Shear bond strengths to dentin
Bond strengths to flat dentin surfaces were determined both immediately after light-activation (or setting) and after 1-day distilled water-storage at 37 °C. The specimens ( n = 10/group) were obtained from human premolars embedded in slow-setting epoxy resin (Epofix Resin, Struers, Copenhagen, Denmark) and flat dentin surfaces were obtained by grinding with wet silicon carbide paper (#1000), then pretreated with the conditioner/primer according to the manufacturer’s instructions or with a silanized primer, as described above. Each luting-agent was placed into Teflon molds (3.6 mm diameter and 2.0 mm height) set on the dentinal surface, and hardened as described above. The specimens thus obtained were mounted on a testing machine (5565, Instron, Canton, MA, USA), and shear stress was applied at a cross-head speed of 0.5 mm/min. After the shear measurements, all the failed specimens were analyzed utilizing a light microscope (4×) (SMZ-10, Nikon, Tokyo, Japan) to determine the nature of their fractures .
Marginal gaps in Teflon molds
Since Teflon does not react with luting-agents, it was used as a mold to measure the degree of setting shrinkage-strain (immediately after setting) or measure the degree of hygroscopic expansion for 1 day of the luting-agents. Each prepared Teflon mold ( n = 10/group), with a depth of 1.5 mm and a diameter of 3.5 mm, was placed on a silicone oil-coated glass plate, and filled with luting-agents using a syringe tip, then covered with a plastic strip until set. After setting, the degree of the setting shrinkage was determined as previously described , again at a time of 3 min from start of light-activation or the degree of the hygroscopic expansion was determined as previously described . The sum of the maximum gap-width and the opposing gap-width (if any) was expressed as a percentage to the measured diameter in Teflon mold.
Flexural modulus of elasticity
Teflon molds (25 mm × 2 mm × 2 mm) were used to prepare flexural specimens ( n = 10/group). Bistite II, Chemiace II, Compolute and XenoCem were cured in three overlapping sections, each cured for 30 s and Fuji Plus was hardened as described above. Flexural moduli were measured, both immediately after setting and after 1-day storage, using the 3-point bending method with a 20-mm span and a load speed of 0.5 mm/min (5565, Instron, Canton, MA, USA) as outlined in ISO 9917-2 (1996) and were calculated (Software Series IX, Instron, Canton, MA, USA).