This study aimed to compare and contrast two resin–ceramic bond strength tests, the tensile bond strength and the four-point bending tests. The effects of hydrofluoric acid (HF) etching time and storage condition on bond strength were also studied.
Ceramic beams (N = 480) with the dimensions of 2.00 × 2.00 × 12.45 mm 3 were sectioned from lithium disilicate ceramic ingots (IPS e.max CAD), then polished and fired for final crystallization. The joint surfaces were etched with HF gel (IPS Ceramic etching gel) for 20 s, 40 s, or 60 s of each group (n = 160). Then, a silane coupling agent (Vitasil ® ) was applied in a single application on the HF etched surfaces, left for 60 s before air-drying. Two beams were bonded together with resin composite cement (Variolink II ® ) in a tailored-mold (2.00 × 2.00 × 25.00 mm 3 ) to control cement thickness to 0.10 mm and then light cured on both sides. The bonded specimens were further divided into two groups (n = 40): (1) tested one day after luting (dry); and (2) tested after storage in 37° C distilled water for 4 weeks. Two mechanical tests were used (n = 20): the tensile bond strength and four-point bending tests. Bond strength results were subjected to two-way AoV, and Weibull statistics with α = 0.05. Fracture surfaces were examined visually and verified using light microscopy.
The four-point bending test showed a higher consistency than the tensile bond strength test using Weibull statistics ( p < 0.01). The effect of HF etching time on the flexural strength was significant, with longer HF etching times decreasing the flexural strength ( p < 0.01). Storage also had a significant effect on the flexural strength ( p < 0.01). However, HF etching time did not have a significant effect on the tensile bond strengths ( p > 0.05) and the influence of the storage time was marginally significant ( p < 0.05). More than 75% of specimens failed adhesively in the four-point bend test while a mixture of adhesive, cohesive and mixed failures was observed in the tensile bond test.
The four-point bending test might be a better approach to evaluate bond strengths. Increased HF etching time and a longer storage period resulted in a decrease in the flexural bond strength. However, both HF etching time and storage time had no significant effect on the tensile bond strengths.
It is unquestionable that ceramic has become widely employed in dental restorations as its superior mechanical properties and better appearance have improved substantially . However, there remains a major constraint for clinical application of ceramic materials, they are vulnerable when subjected to tensile stresses , i.e . ceramics are prone to fracture that is a failure. Despite most restoration failure originating from ceramic fracture, the bonding reliability of the ceramic is a possible explanation for restoration failure and should not be overlooked.
Other than the performance of ceramic or resin cement, adhesion to a ceramic material is the one of the key factors to evaluate the bond durability (adhesion strength) , i.e., long term success in clinical application for an adhesion system of an indirect restoration . Current procedures for tooth preparation now aim to preserve as much dental hard tissue as possible , hence, restoration retention mainly relies on adhesion to the prepared tooth . Therefore, adhesion is crucial and should be thoroughly evaluated in order to examine and understand bond durability.
To begin with, there are adhesive interfaces. In a traditional indirect restoration adhesive system, there are always two adhesive interfaces: ceramic to resin cement and tooth structure to resin cement . Various studies have evaluated the ceramic to resin and tooth to resin bonds in order to better understand and enhance the adhesive strength . Other than the adhesive interface, the bonding mechanism has also been studied extensively. There are two mechanisms involved in ceramic to resin cement bonding, namely, micromechanical interlocking and chemical bonding . To create a micromechanical interlock, hydrofluoric acid etching or, sometimes, grit-blasting of the ceramic surface are the usual methods. Application of a silane coupling agent on etched glass ceramic surface is mandatory to create durable chemical bonding .
As mentioned, to determine whether a restoration is successful, not only needs the individual performance of ceramic and resin cement to be considered, but also the actual bonding mechanisms between the ceramic and resin cement must also be studied. To evaluate durable bonding in a laboratory setting, various bond strength tests have been developed . The strength to hold the adherend components together is denoted as the bond strength (adhesion strength). A typical model for bond strength testing involves either a (pre-)treated tooth or ceramic specimen joined to a resin composite block (specimen) with resin-based luting cement . The bond strength is calculated by dividing the maximum load to break the bonded specimen by the actual bonding area . Bond strength tests can be categorized into two main types: tensile and shear , depending on the primary stress applied to the interface. Nevertheless, bearing in mind that there is no genuine shear strength test in existence in dentistry .
Debate on bond strength tests has continued, with some researchers challenging the validity of these tests with strong criticism to the experimental methodologies . Numerous studies have been performed, not only to evaluate the bond strength, but also to verify the test methods . Among the various laboratory bond strength test methods, the microtensile bond strength (MTBS, μTBS) test is the most popular technique to test ceramic to resin cement bonding . Before the microtensile bond strength test was developed , the tensile bond strength (TBS) test was available. The only difference between the micro- and tensile bond strength test is the bonding area, otherwise the test conditions are almost identical. Indeed, for the microtensile bond strength test, which was first proposed by Sano et al. in 1994, the bonding area was set below 1.0 mm 2 so that stress distribution across the bonded interface was suggested to be distributed more evenly . It was also suggested that compared with the traditional tensile bond strength test method, the microtensile bond strength test results in more adhesive failures, i.e., adhesive failures may reveal the ‘true’ bond strength. On the other hand, the microshear bond strength test is the most recently accepted method to test tooth to resin composite cement adhesion , e.g., a resin composite component joined to a pre-treated tooth surface. Nevertheless, it seems that the word “micro” is a matter of absolutely arbitrary terminology for the sake of appearance only; it has no real meaning!
Four-point bending test indeed is one of the newest methods to access bond strength, such that only few studies were published to date . Although interfacial tension test has been advocated for measuring bond strength, the stress distribution at interface is indeed complex and the specimen preparation as well as the alignment is not as simple as that of the other methods . More importantly, four-point bending test has the maximum tensile stress on the convex surface and removed the stress concentration at the surface of adhesive , which deemed to be more clinical relevant than direct tension test. However, different from the tensile bond strength test, the four-point bend test requires the specimen to be placed horizontally with the adhesive joint placed centrally where the stress is placed at the adhesive joint and the specimen supported at both ends using a fixed distance. The load leads to bending of the specimen and creates a combination of stresses, namely tension and compression, and therefore, interpretation can be more difficult. At the same time, it is achievable to have most jointed specimens fail adhesively but no further research has been undertaken to better understand this method and its applicability to clinical outcomes. This said, direct comparison between four-point bending and tensile bond strength tests are impossible, since the test conditions, e.g. the stress application along the adhesive interface and generated outcomes are different. Certain test configuration and conditions are necessary for bending test, such as the ratio between span width and specimen depth should be greater than 10 (based on the requirement for testing dental ceramics ), in order to prevent shear stresses within the adhesive joint , otherwise correction is necessary.
Furthermore, another factor of a bond test that has been often overlooked is the failure mode. It is a crucial parameter to judge whether a bond strength test method is reliable. An adhesive failure represents failure at the bonded interface while cohesive failures in the parts of the test set-up represent failure found outside the bonded interface , i.e., the failure originates from either adherend components or adhesive. It is universally believed that a true bond strength can only be determined from an adhesive failure mode . Therefore, it is not a surprise that the microtensile bond strength is claimed to be reliable as most joints fail adhesively . In fact, study has shown that it is achievable to have adhesive failure in the majority of specimens with a four-point bending test, i.e., similar outcomes as with the microtensile bond strength test can be achieved. This raises the question that four-point bend tests may also be a very reliable test method.
Nowadays, researchers still attempt to optimize bond strength tests , and so far, no wide agreement has been reached. Some claim the microtensile bond strength test method is the most reliable, others insist on using a microshear bond strength test method . Therefore, the aim of this laboratory study was to compare and contrast the two bond test methods: the four-point bending test and the tensile bond test. In this research, the interest of the bond strength test only focuses on ceramic to resin cement bonding. Given this, it is suggested to establish a test model with two ceramic components being joined with a resin composite cement , so that fracture can only be generated at the bonded interface or cohesively from the ceramic component or resin composite cement. In addition, the influences of HF etching time and storage conditions were studied.
Materials and Methods
The materials used, together with the batch numbers and manufacturers are listed in Table 1 .
|Materials||Brand||Manufacturer||Composition a||Batch number|
|Glass ceramics||IPS e.max CAD||Ivoclar Vivadent AG, Schaan, Liechtenstein||SiO 2 , Li 2 O, K 2 O, P 2 O 5 , ZrO 2 , ZnO, Al 2 O 3 , MgO, Coloring Oxides||S10602|
|Resin cement||Variolink II ®||Ivoclar Vivadent AG, Schaan, Liechtenstein||bis -GMA, TEGDMA, UDMA, inorganic fillers, catalyst and stabilizer, pigment||S01620|
|Silane coupling agent||Vitasil ®||Vita Zahnfabrik, Bad Sackingen, Germany||Ethanol, 3-trimethoxysilylpropyl methacrylate||44690|
|Hydrofluoric acid (HF)||IPS ceramic etching gel||Ivoclar Vivadent AG, Schaan, Liechtenstein||Hydrofluoric aicd||33780|
Ceramic beam preparation
Ceramic ingots (IPS e.max CAD, Ivoclar Vivadent AG, Schaan, Liechtenstein) were manually polished using 180-, 400-, 600-, and finally 1200-grit SiC papers under running tap water. The polished ingots were sectioned with a diamond wafering blade (Buehler, Lake Bluff, IL, USA) with a high speed cutting machine (Isomet 5000, Buehler, Lake Bluff, IL, USA), yielding slices with dimensions of 14 × 12 × 2 mm 3 that were then further sectioned to give 14 × 2 × 2 mm 3 beams. The beams were cut manually to approximately 12.6 mm long.
The beams were fired in an oven, i.e., for the final crystallization of the ceramic. The firing procedure was carried out by using a pre-set program in a Programat CS ceramic furnace (Ivoclar Vivadent AG, Schaan, Liechtenstein). In brief, the furnace was pre-heated to 403 °C, and then ramped up to 820 °C for 10 s at the rate of 90 °C/min with vacuum on from 550 °C to 820 °C. Then, the temperature was further ramped to 840 °C for 7 min with vacuum at the rate of 30 °C/min. Finally, the temperature was cooled to 700 °C at 20 °C/min before natural cooling. Each end of the fired beams was manually polished with 1200-grit SiC paper in order to achieve a length exactly 12.45 mm.
The beams were ultrasonically cleansed in 70% ethanol for 5 min and allowed to air dry. The ends were etched with 4.7% hydrofluoric acid (HF, IPS Ceramic etching gel, Ivoclar Vivadent AG, Schaan, Liechtenstein) for 20 s, 40 s, or 60 s, depending on the test group, rinsed with tap water for 60 s to remove excess acid gel and finally rinsed with distilled water ultrasonically for 5 min. Following HF etching, silanization was performed by applying a silane coupling agent (Vitasil, Vita Zahnfabrik, Bad Sackingen, Germany) on the etched surfaces for 60 s and air dried.
Equal amounts of base and catalyst pastes (Variolink II, Ivoclar Vivadent AG, Schaan, Liechtenstein) were mixed in 1:1 ratio which is according to manufacturer’s instructions, and applied on silanized surfaces. Two ceramic beams were joined and placed into a 2 × 2 × 25 mm 3 custom-made mould, in order to standardize the luting cement thickness. Excess resin composite cement was removed manually with a scalpel before complete setting of the cement. The luted specimens were light cured (ESPE Elipar™, 3M, USA, 430–480 nm utilizable wavelength, 1200 mW cm −2 light intensity) on two sides of the adhesive joint for 20 s (total 40 s).
Specimens were divided into two groups: the first group was tested one day after the bonding procedure and stored in 37° C dry chamber; the second group was stored for 4 weeks in distilled water at 37° C.
Tensile bond strength test
After storage, the beams were subjected to testing with the cross-sectional area measured with a dial calliper (1143 M, Moore & Wright, Sheffield, UK). For the tensile bond test both ends of each bonded assembly were embedded in cold-cured poly(methyl methacrylate) (ProBase Cold, Ivoclar Vivadent, Schaan, Liechtenstein) to enable specimen mounting in a custom-made jig attached to the universal testing machine (ElectroPuls™ E3000, Instron, MA USA). The bond was stressed at a cross-head speed of 1 mm/min until bond rupture.
The maximum load at failure was divided by the cross-sectional area to give the tensile bond strength:
σ t = M a x . L o a d A r e a
σ t equals to the tensile bond strength calculated in MPa; Maximum Load = measured in Newtons; Area = calculated in mm 2 .
Four-point bend test
Different from the tensile bond test, not only the bonded area (cross-sectional area at adhesive joint) was measured, but also the length of each beam. The specimens were mounted with the adhesive joint centralized between the upper loading and lower supporting span on the same universal testing machine. The distance between the centers of loading rollers is 10.0 mm, and the distance between the centers of supporting rollers is 20.0 mm. A load was applied via loading rollers of 3.0 mm diameter at a cross-head speed of 0.5 mm/min on the universal testing machine. The flexural strength was calculated as:
σ f = 3 × M a x . L o a d × S p a n L e n g t h 4 × W i d t h × T h i c k n e s s 2