This study aims to assess the viability of using the acoustic emission (AE) measurement technique to detect and monitor in situ the interfacial debonding in resin composite restorations due to build-up of shrinkage stresses during polymerization of the composite.
Materials and methods
The non-destructive testing technique that measures acoustic emission (AE) was used to detect and monitor the interfacial debonding in resin composite during curing of the composite. Four groups of specimens, n = 4 each, were tested: (1) intact human molars with Class-I cavities restored with the composite Z100 (3M ESPE, USA); (2) intact human molars with Class-I cavities restored with the composite Filtek™ P90 (3M ESPE, USA); (3) ring samples prepared from the root of a single bovine tooth and ‘restored’ with Z100; (4) freestanding pea-size specimens of Z100 directly placed on the AE sensor. The restorations in Groups (1)–(3) were bonded to the tooth tissues with the adhesive Adper™ Scotchbond™ SE Self-Etch (3M ESPE, USA). The composites in all the specimens were cured with a blue light (3M ESPE, USA) for 40 s. The AE signals were recorded continuously for 10 min from the start of curing. Non-destructive 3D imaging was performed using X-ray micro-computed tomography (micro-CT) to examine the bonding condition at the tooth–restoration interface.
The development of AE events followed roughly that of the shrinkage stress, which was determined separately by the cantilever beam method. The number of AE events in the real human tooth samples was more than that in the ring samples, and no AE events were detected in the pea-size specimens placed directly on the AE sensor. The number of AE events recorded in the specimens restored using Z100 was more than that found in specimens restored with Filtek P90. The micro-CT imaging results showed clear interfacial debondings in the tooth specimens restored with Z100 after curing, but no clear debonding was found in the P90 specimens.
The AE technique is an effective tool for detecting and monitoring in situ the interfacial debonding of composite restorations during curing. It can potentially be employed to evaluate the development of shrinkage stress and the quality of interfacial bonds in teeth restored with different composite materials, cavity geometries, and restorative techniques.
Dental composites are widely used to repair decayed or damaged tooth structures because of their superior esthetics, ease of use and ability to bond to tooth tissues. However, during setting, the resin composite shrinks, producing shrinkage stresses within the tooth and composite . This is a very quick process which normally takes place within 30–50 s. The volumetric contraction that accompanies polymerization is typically on the order of 1.5–5% . If the internal shrinkage stress is high enough, debonding between the tooth and restoration will occur, causing problems such as reduced fracture resistance and increased micro-leakage. The latter will ultimately lead to secondary caries . Clinical studies have identified the loss of interfacial integrity as one of the main causes for replacement of composite restorations . Many factors can influence the initial quality and subsequent degradation of bonding at the tooth–restoration interface, e.g. properties of the composite and adhesive materials, cavity geometry, restorative techniques, thermal and mechanical loading .
It is difficult to measure or predict the shrinkage stress within a real tooth restoration because of the small but complicated geometry and rapidly changing material properties involved. Experimental methods using simple specimens have therefore been developed to estimate the shrinkage stress that can develop in dental composites during polymerization . However, the results are sensitive to the testing configuration and procedures, such as the instrument’s compliance, direction of light application and specimen shape. In addition to the shrinkage strain, the development of shrinkage stresses depends also on the viscosity, Young’s modulus and Poisson’s ratio of the composite which all change rapidly with time during polymerization. The accurate evaluation of these properties and the subsequent prediction of the shrinkage stress using a suitable mathematical model are therefore not trivial .
The bond strength between the tooth and restoration is also difficult to determine precisely. The micro-tensile and micro-shear tests have been widely used for bond strength testing since they can overcome some of the limitations of conventional tensile and shear bond strength tests . However, the results are highly variable, being dependent on the test devices and specimen size used . Specifically, the micro bond strengths are usually higher than the macro values. This makes it difficult to predict whether debonding will take place for a particular composite system even if the shrinkage stress can be accurately predicted or measured.
Traditional methods for studying interfacial bonding/debonding of dental restorations include optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) . However, the major disadvantage of these methods is that they are limited to essentially surface examination; no bulk measurement can be provided. They also require destructive sectioning of the specimens. Recently, researchers have begun to use non-destructive methods, such as X-ray micro-computed tomography (micro-CT), to study interfacial bonding/debonding of composite restorations . Micro-CT is a computer-aided 3D reconstruction of a structure or material that can be sliced virtually along any direction to gain accurate information on their internal geometric properties and structural parameters. But because of its lower resolution, it cannot detect debonding at a submicron level. Most of all, none of the imaging techniques mentioned above can be used to monitor debonding as it happens.
The acoustic emission (AE) measurement technique is also a non-destructive method. It is normally used to monitor the integrity of structures by providing real-time information of the fracture or damage process. It uses transducers or sensors to detect the high-frequency sound waves produced as a result of the strain energy released within a material following fracture. AE technology has been widely used in research and industry to monitor the development of crack growth, wear, fiber-matrix debonding in composites, and phase transformation . AE measurement has also been used to detect the fracture of different dental structures. For example, Ereifej et al. used the AE technique to detect the initial fracture of ceramic crowns, Vallittu used it to study the fracture of a composite veneer reinforced by woven glass fibers, and Kim and Okuno used it to study the micro-fracture behavior of composite resins containing irregular-shaped fillers.
However, to date, no research has been reported on the use of AE techniques for studying the interfacial debonding of composite restorations during polymerization of the composite resin. This paper presents exactly such a study completed recently by the authors. Different kinds of specimen were utilized to verify that the development of AE events was a direct consequence of interfacial debonding caused by the shrinkage of the composite resin during polymerization. In the process, the debonding behaviors of restorations containing composite resins with different shrinkage properties were evaluated and compared. Finally, 3D Micro-CT imaging was used to provide visual evidence for the interfacial debonding indicated by the AE measurement.
Materials and methods
Specimens and materials
Three types of specimens were prepared ( Fig. 1 ): (a) intact human molars with a Class-I restoration, (b) ring specimens prepared from the root of a single bovine tooth and (c) free standing pea-size specimens of composite placed directly on the AE sensor.
For the whole tooth specimens, 8 intact human molars with similar dimensions, which were extracted and stored in saturated thymol solution at 4 °C for less than 1 month, were selected and randomly divided into 2 groups of 4. Standard Class-I cavities were prepared on these teeth by a single operator following clinical procedures with a high-speed handpiece and dental cutting burs; see Fig. 1 (a). Each tooth specimen was treated with a bonding agent to the cavity surface and then restored with a composite resin, which was cured with a 550 mW/cm 2 blue light (Elipar TriLight, 3M ESPE, USA) for 40 s. The bonding agent used for all the specimens was Adper™ Scotchbond™ SE Self-Etch (3M ESPE, USA), while the composite resins used for the first and second group were Z100™ (3M ESPE, USA) and Filtek™ P90 (3M ESPE, USA), respectively. The whole tooth specimens were considered to be the most representative of those in real clinical situations.
Four ring specimens were cut from the root of a single bovine tooth. They were, therefore, similar in material properties and structural anatomy; see Fig. 1 (b). The central holes of the specimens, which were originally the root canal, were enlarged to 3 mm in diameter with a high-speed handpiece. They were then treated with the bonding agent Adper™ Scotchbond™ SE Self-Etch, and restored with the composite Z100, which was cured with a blue light for 40 s. The larger free-to-bonded surface ratio of the ring specimens allowed the effect of the so-called C -factor on interfacial debonding to be investigated.
The final group of specimens consisted of free-standing pea-size blobs of Z100 composite of about 5 mm in diameter. They were directly placed on the AE sensor without using any adhesive material, as shown in Fig. 1 (c). Each of these specimens was also cured with a blue light for 40 s. Testing with these specimens helped to verify that free shrinkage of the composite resin itself did not induce any AE event.
In total, four groups of specimens were prepared: (Group-I) 4 intact human molars with Class-I cavities restored with Z100; (Group-II) 4 intact human molars with Class-I cavities restored with Filtek P90; (Group-III) 4 ring specimens prepared from the root of a bovine tooth and restored with Z100; (Group-IV) 4 freestanding pea-size blobs of Z100 directly placed on the AE sensor.
Shrinkage stress measurement
Before measuring the interfacial debonding by AE method, the developments of shrinkage stress for Z100 and Filtek P90 during curing were measured using a tensometer (American Dental Association Foundation) . This device is based on the basic engineering cantilever beam bending theory. The tensile force generated by the shrinking composite was calculated from the beam deflection based upon a previously obtained calibration constant. Before restore the composite material, the end surfaces of two glass rods were first polished with 600-grit sandpaper and then silanized with porcelain primer (Bisco Inc., Schaumburg, IL, USA), followed by application of a layer of adhesive Scotchbond Multi-purpose (3M, St. Paul, MN, USA). Detail of the measuring method refer to Ref. . The dimensions of the composite specimens were 6 mm in diameter and 2 mm in height. Three specimens were tested for each material.
Assessment of interfacial debonding by AE measurement
A 2-channel AE system (PCI-2, Physical Acoustic Corporation, USA) was used in this study for AE data acquisition and digital signal processing. Fig. 2 shows a schematic diagram of the AE test system. The AE sensor/transducer used for detecting interfacial debonding was S9225 (Physical Acoustic Corporation, USA), which had a resonance frequency of 250 kHz. For the whole tooth and ring specimens, the AE sensor was attached to their outer surfaces using superglue. The signals acquired with the sensor were amplified by a preamplifier with 20/40/60 dB gains. The parameters selected for the signal acquisition were: a 40 dB gain for the preamplifier, a 100 kHz to 2 MHz band pass and a 32 dB threshold.
At the start of each test, the AE system and the blue curing light were turned on simultaneously. The composite resin was cured for 40 s, while the AE system recorded data continuously for 10 min. The curves of instantaneous and accumulated AE events against time were used to study the curing behavior of the different specimens.
The integrity of the tooth–restoration interface within the whole tooth specimens after curing was examined using a micro-CT machine (XT H 225, X-TEK Systems Ltd.). Four specimens, two from Group-I and two from Group-II, were selected for micro-CT examination. The specimens were first scanned immediately after placement of the composites to see how well they had adapted to the cavity walls. After curing and AE measurement, they were scanned for the second time to look for any detachment of the restoration from the cavity walls. In order to ensure the same position and orientation for the two scans to facilitate “same-slice” comparison, each of the specimens was mounted into a Teflon ring using an orthodontic resin (DENTSPLY International Inc., USA). During scanning, the teeth were covered by wet paper tissue to avoid cracking through dehydration.
The solidification of the composite within the tooth samples after curing was assessed by using micro-hardness tests. Two specimens, one from Group-I and one from Group-II, were selected to do the micro-hardness test. After curing and AE measurement, the two specimens were cut along the vertical central line with a 102 mm dia. × 0.3 mm diamond blade (Buehler, USA). The Vickers hardness was then measured with a micro-hardness testing machine (Micromet 5104, Buehler, USA). The indenter load was 100 g and the load-holding time was 10 s. The measurement points were located along 4 vertical lines within the restoration, which were schematically shown in Fig. 5 c. The measurement began from the top surface and continued until the bottom interface.