Abstract
Objective
This study investigates monitored micro-crack growth and damage in the ceramic/enamel adhesive interface using the acoustic emission (AE) technique with optical coherence tomography (OCT) under fatigue shear testing.
Methods
Shear bond strength (SBS) was measured first with eight prepared ceramic/enamel adhesive specimens under static loads. The fatigue shear testing was performed with three specimens at each cyclic load according to a modified ISO14801 method, applying at 80%, 75%, 70%, and 65% of the SBS to monitor interface debonding. The number of cycles at each load was recorded until ceramic/enamel adhesive interface debonding occurred. The AE technique was used to detect micro-crack signals in static and fatigue shear bond tests.
Results
The results showed that the average SBS value in the static tests was 18.07 ± 1.72 MPa (mean ± standard deviation), expressed in Newton’s at 56.77 ± 5.40 N. The average number of fatigue cycles in which ceramic/enamel interface damage was detected in 80%, 75%, 70% and 65% of the SBS were 41, 410, 8141 and 76,541, respectively. The acoustic behavior varied according to the applied load level. Events were emitted during 65% and 70% fatigue tests. A good correlation was observed between the crack location in OCT images and the number of AE signal hits.
Significance
The AE technique combined with OCT images as a pre-clinical assessment tool to determine the integrity of cemented load bearing restored ceramic material. Sustainable cyclic load stresses in ceramic/enamel bonded specimens were substantially lower than the measured SBS. Predicted S–N curve showed that the maximum endured load was 10.98 MPa (about 34.48 N) passing 10 6 fatigue cyclic.
1
Introduction
The macro-retentive design is no longer a prerequisite if an adequate amount of tooth surface is available for bonding because adhesive resin cements have the ability to bond to both tooth structure and ceramic restoration . Clinicians are particularly interested in restoring minimal or absent macro-retentive preparations with extensive dentin- or enamel-bonded ceramic coverage . In recent years, tooth colored inlays, onlays, veneers and crowns can be constructed under demands for esthetic, metal-free restorations using a relatively simple approach using CAD/CAM techniques to prefabricate ceramics manufactured under controlled conditions .
However, challenges remain when working with extensive or total failure in large CAD/CAM ceramic restorations because of luting defects or resin cement wear between the ceramic restoration and tooth substance. Fatigue studies generally used repeated loads over many months or years as a likely mode of failure for bonds in the mouth to provide better insight into the in vivo performance and obtain more realistic sustainable stress values . However, fatigue micro-crack growth in a bonded adhesive layer is difficult to monitor from in vitro studies requiring sectioning or dissolving sample dental tissues to confirm failure paths . Consequently, the non-destructive acoustic emission (AE) technique has been used in dental materials to detect the fracture behavior and failure progression in structures such as ceramics , composites and porcelain .
The AE technique combined with optical coherence tomography (OCT) was proven to have potential in investigating micro-crack growth and damage in the ceramic/dentin adhesive interface under fatigue shear testing in our previous study . The results indicated that sustainable cyclic load stresses in ceramic/dentin bonded specimens were substantially lower than the measured shear bond strength (SBS). The predicted S–N curve showed that the maximum endured load was 4.18 MPa passing 10 6 fatigue cycles. Nevertheless, large CAD/CAM ceramic restorations usually attributed the adhesion complex to the bond formed between three different components: the tooth surface, the resin cement, and the ceramic surface . While bonding to enamel is dependent on the micromechanical retention to the etched substrate; that to dentin relies on hybridization with the exposed collagen mesh . The bond strength of ceramic to enamel is still superior compared to the bond strength of porcelain to dentin . However, limited information is available regarding the ability of enamel bonds to resist fatigue cycling forces.
This study applied the AE and OCT techniques to monitor the failure process in CAD/CAM ceramic block/enamel interfaces using the total-etch adhesive system under different cyclic load stages. The SBS and S–N curve results were compared with ceramic block/dentin specimens obtained from our previous study to understand the interfacial mechanics at adhesive layers.
2
Materials and methods
2.1
Specimen preparation
Similar specimen preparation procedures were described in a previous study . Specimen preparation is shown schematically in Fig. 1 . The enamel bonding sites were prepared by sectioning 16 caries-free, extracted human molars mesio-distally and then sectioning the crown portion to expose the enamel surface. The exposed enamel surface was milled and polished with a grinding machine (P20FR, Holy Instrument Co., Taipei, Taiwan), followed with 0.05 mm thickness vinyl tape drilled with a hole (2 mm in diameter) to place on the sample that exposed a similar enamel surface. An etch-and-rinse Variolink II adhesive system was applied to bond the enamel and ceramic. The exposed enamel was acid etched with 35% phosphoric acid gel and air-dried. Heliobond was uniformly applied to the enamel surface. The CAD/CAM ceramic blocks (Pro-CAD, Ivoclar Vivadent Inc., Schaan, Liechtenstein) were cut using a saw machine to prepare a series of ceramic pieces (2 mm × 2 mm × 2 mm). The ceramic piece was etched for 90 s with 6% hydrofluoric acid and cleaned with water spray. Light cured cement was applied for 40 s to bond the enamel and ceramic together ( Fig. 1 ).
2.2
Shear bond strength and cyclic load testing
The Instron E3000 (Instron, Canton, MA, USA) material testing machine was adopted to perform the shear bond strength (SBS) test with eight specimens randomly selected from all prepared specimens. The load was applied 0.5 mm away from the enamel substrate using a stainless steel knife-edged chisel with a crosshead speed of 0.5 mm/min until complete debonding occurred. The maximum shear bond loads/strengths were recorded ( Fig. 2 ).
The cyclic loads of the fatigue tests were set at 80%, 75%, 70% and 65% of the static maximum shear load. The R value ( F max / F min ) was set at 10. The test frequency was set at 1 Hz because the human mastication frequency was found from the literature to be 0.94–2.17 Hz . Three specimens were tested at each cyclic load. The number of cycles at each load was recorded until ceramic/enamel adhesive interface debonding occurred.
2.3
AE analysis and OCT scanning
The AE technique was used to monitor how the adhesive system failure process developed during the SBS and cyclic load tests. The signals were detected using the AE signal wide band transducer (Broadband sensor S9225, Physical Acoustic Corporation (PAC) Princeton Junction, NJ, USA) which was glued to the embedded resin block around the sample ( Fig. 2 ). The AE system parameters passed the signals through 40 dB gain preamplifiers with a band pass of 100 kHz to 2 MHz (Model 2/4/6, PAC) . The load versus AE hit in SBS testing and the number of fatigue test cycle loads for each specimen were recorded.
A swept-source OCT system (OCM1300SS, Thorlabs Inc., Newton, NJ) was used to monitor the micro-crack propagation in the adhesive interface. However, no visible debonding occurred in the ceramic/enamel interface after fatigue testing was needed as a requirement in the OCT scanning. Another 12 specimens were therefore prepared and divided into four groups to perform the fatigue tests under 80%, 75%, 70% and 65% of the static maximum shear load. The corresponding number of cycles at each load was set to half the average total number of cycles at which debonding occurred in the previous cyclic tests, i.e. 21 (41/2), 205 (410/2), 4071 (8141/2) and 38,271 (76,541/2) numbers for 80%, 75%, 70% and 65% of load, respectively ( Table 1 ). All specimens were scanned perpendicularly after testing using the OCT system. The swept-source OCT system had a median wavelength of 1310 nm, an axial resolution of around 10 μm in tissue, total power of 10 mW and an A-scan rate of 16 kHz . For each measurement, a 3D dataset covering a volume of 3 mm × 3 mm × 3 mm corresponding to 512 × 512 × 512 pixels was obtained in the X – Y – Z directions within 1 min. Three dimensional datasets of each specimen were constructed at each time point from several acquired two-dimensional (2D) OCT images.
| Load percentage | Cyclic numbers of debonding | Ave. total cycle | Initial signal (cycle) | Total hit |
|---|---|---|---|---|
| 80% | 10 (Sample 1) | 41 | 10 | 13 |
| 41 (Sample 2) | 41 | 7 | ||
| 73 (Sample 3) | 73 | 17 | ||
| 75% | 362 (Sample 1) | 410 | 188 | 6 |
| 188 (Sample 2) | 154 | 8 | ||
| 680 (Sample 3) | 89 | 7 | ||
| 70% | 7260 (Sample 1) | 8141 | 548 | 36 |
| 9031 (Sample 2) | 306 | 24 | ||
| 8133 (Sample 3) | 401 | 30 | ||
| 65% | 90,238 (Sample 1) | 76,541 | 995 | 101 |
| 52,433 (Sample 2) | 1242 | 155 | ||
| 86,952 (Sample 3) | 1071 | 134 | ||
2
Materials and methods
2.1
Specimen preparation
Similar specimen preparation procedures were described in a previous study . Specimen preparation is shown schematically in Fig. 1 . The enamel bonding sites were prepared by sectioning 16 caries-free, extracted human molars mesio-distally and then sectioning the crown portion to expose the enamel surface. The exposed enamel surface was milled and polished with a grinding machine (P20FR, Holy Instrument Co., Taipei, Taiwan), followed with 0.05 mm thickness vinyl tape drilled with a hole (2 mm in diameter) to place on the sample that exposed a similar enamel surface. An etch-and-rinse Variolink II adhesive system was applied to bond the enamel and ceramic. The exposed enamel was acid etched with 35% phosphoric acid gel and air-dried. Heliobond was uniformly applied to the enamel surface. The CAD/CAM ceramic blocks (Pro-CAD, Ivoclar Vivadent Inc., Schaan, Liechtenstein) were cut using a saw machine to prepare a series of ceramic pieces (2 mm × 2 mm × 2 mm). The ceramic piece was etched for 90 s with 6% hydrofluoric acid and cleaned with water spray. Light cured cement was applied for 40 s to bond the enamel and ceramic together ( Fig. 1 ).
2.2
Shear bond strength and cyclic load testing
The Instron E3000 (Instron, Canton, MA, USA) material testing machine was adopted to perform the shear bond strength (SBS) test with eight specimens randomly selected from all prepared specimens. The load was applied 0.5 mm away from the enamel substrate using a stainless steel knife-edged chisel with a crosshead speed of 0.5 mm/min until complete debonding occurred. The maximum shear bond loads/strengths were recorded ( Fig. 2 ).
