Abstract
Objective
This study aimed to evaluate the effects of the restorative material and cavity design on the facture resistance of inlay restorations under a compressive load using acoustic emission (AE) measurement.
Materials and methods
Two restorative materials, a composite resin (MZ100, 3M ESPE) and a ceramic (IPS Empress CAD, Ivoclar Vivadent), and two cavity designs, non-proximal box and proximal box, were studied. Thirty-two extracted human third molars were selected and divided into 4 groups. The restorative materials and cavity designs used for the four groups were: (1) composite and non-proximal box; (2) ceramic and non-proximal box; (3) composite and proximal box; (4) ceramic and proximal box. The restored molars were loaded in a MTS machine via a loading head of diameter 10 mm. The rate of loading was 0.1 mm/min. During loading, an AE system was used to monitor the debonding and fracture of the specimens. The load corresponding to the first AE event, the final maximum load sustained, as well as the total number of AE events recorded were used to evaluate the fracture resistance of the restored teeth.
Results
For the initial fracture load, Group 2 (236.15 N) < Group 1 (428.14 N) < Group 4 (441.24 N) < Group 3 (540.06 N). The same trend was found for the final load, i.e., Group 2 (1594.68 N) < Group 1 (2003.82 N) < Group 4 (2004.89 N) < Group 3 (2057.53 N). For the total number of AE events, Group 4 (2135) > Group 2 (1685) > Group 3 (239) > Group 1 (221). The differences from pairwise comparisons in the initial fracture load and final load were mostly insignificant statistically ( p > 0.05), the only exception being that between Groups 2 and 3 in the initial fracture load ( p = 0.039). For the total number of AE events, statistically significant differences ( p < 0.05) were found between all group pairs that involved different materials, with the composite groups giving much fewer AE events than the ceramic groups. Conversely, no statistically significant difference in the AE results was found between groups with the same material, irrespective of the cavity design.
Significance
For teeth restored with MOD inlays, the use of composite resin as the restorative material may provide higher fracture resistance than using ceramic. Using a proximal box design for the cavity may further improve the fracture resistance of the inlay restoration, although the improvement was not statistically significant under axial compression.
1
Introduction
Inlays are important intra-coronal restorations for restoring damaged teeth, especially those requiring a large restoration. With increasing patients’ demand for esthetics, the restorative materials used for making inlays need to have improved optical properties. As tooth-colored materials, ceramics and, more recently, resin-based composites play a significant role in chair-side computer-aided design and computer-aided manufacturing (CAD/CAM) systems that are employed widely in the design and fabrication of dental prostheses . A debate is currently taking place on whether ceramics or composite resins should be selected for CAD/CAM inlays.
The fracture resistance of inlays is one of the most important factors which can influence their rate of survival. Many efforts have been made to compare the fracture resistance of resin composite inlays against that of ceramic inlay restorations. An in vitro study , which subjected inlays to simulated pre-cementation functional occlusal tapping, showed that inlays made of lithium disilicate glass ceramic had higher fracture resistance than those made of resin composite or feldspathic porcelain. Using a compressive load, St-Georges et al. found no significant differences in fracture resistance between teeth restored with ceramic (Vitablock Mark II) and those with composite resin (Paradigm MZ100). Resin-based composite inlays have also been reported to perform equally well as porcelain inlays based on a three-year clinical investigation .
The wear resistance of a restorative material is another factor that needs to be considered when choosing a suitable material for inlays. The low wear resistance of composite resins is a major reason why most dentists choose ceramics rather than composites for inlays. On the other hand, because of its low wear resistance, composite restorations appear to be less abrasive to the opposing dentitions.
Using 3D finite element (FE) models, Dejak et al. and Jiang et al. studied the stress levels of composite and ceramic inlays in molars under occlusal loads. A lower stress level was found within the composite inlay due to its lower elastic modulus. On the other hand, when considering the stresses in the layer of luting cement, which were relevant to interfacial debonding, Dejak et al. found that those with a ceramic inlay were lower than those with a composite resin inlay. They did not, however, consider the residual stresses caused by the polymerization shrinkage of the luting cement.
Although the marginal gaps of inlays are initially filled with luting cement, marginal deficiencies are expected due to polymerization shrinkage of the cement and degradation through aging. Under heavy occlusal loading, marginal fractures of the restoration may also occur. The subsequent reduced support at the margins can result in the cohesive fracture of the inlay restoration . The occurrence of these marginal failures will also lead to secondary caries and pulpitis . There have been few studies on the initial fracture or interfacial debonding of inlay restorations. An example is provided by Ereifej et al. , who studied the edge strength of restorations made of different materials and found that indirect composite samples had higher edge strength than ceramic ones.
The geometry of the cavity preparation is another critical factor for the longevity of restorations. Magne et al. found that thick CAD–CAM resin composite overlays increased the fatigue resistance of endodontically treated premolars when compared to thin ones. Using 3D finite element analysis, Yamanel et al. examined the stresses in inlays and onlays made of two different resin composites and two different all-ceramic materials under oblique loading. The all-ceramic restorations were found to transfer less stress to the tooth structures in comparison with the composite ones. On the effect of the cavity design, the onlay design was more efficient in protecting the tooth structures than the inlay. Cavity preparations with or without a proximal box have been used for MOD inlays. However, their influence on the fracture resistance of the restoration is still not clear.
The measurement of acoustic emission (AE) is a non-destructive method which is widely 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 measurement has proved to be an efficient method for studying the fracture and interfacial debonding of different dental structures .
The aim of this paper was to use the AE measurement method to evaluate the fracture resistance of inlay restorations constructed with different restorative materials and cavity designs. Both the initial fracture/debonding and the subsequent development of further cracking were evaluated. Two inlay restorative materials, a composite resin and a ceramic, and two cavity designs, a proximal box and a non-proximal box, were considered to study their influence on the inlay’s fracture resistance.
2
Materials and methods
2.1
Specimens preparation
Thirty-two extracted human third molars with almost the same morphology and without decay and wear were selected. The maximum width and length of each tooth was measured to within 1 mm. These teeth were cleaned and stored in saturated thymol solution at 4 °C for about one month prior to preparation. Before preparation, they were rinsed under tap water and placed into deionized water at room temperature for 24 h. The teeth were then randomly divided into four groups of 8. A high-speed handpiece with a diamond bur was used to prepare the inlay cavities according to the standard operating procedures. The two cavity designs with dimensions are shown in Fig. 1 . All specimens were prepared by the same operator to eliminate inter-operator differences.
The two inlay restorative materials used were the composite resin block MZ100 (3M ESPE) and the ceramic block IPS Empress CAD (Ivoclar Vivadent). The E4D CAD/CAM system (D4D, USA) was used to mill the restorations following the manufacturer’s instructions. The intaglio surfaces were sandblasted with 50 μm aluminum oxide for 1 min, cleaned in an ultrasonic oscillator (Giltron Inc., USA) for 30 s and dried for 10 s.
2.2
Adhesive procedure
The cleaned composite resin restorations were silanized using Rely X Ceramic Primer (3M ESPE, St. Paul, USA) for 60 s and dried for 5 s. The intaglio surfaces were then coated with a thin layer of Adper™ Single Bond Plus agent (3M ESPE, St. Paul, USA). For the ceramic restorations, the intaglio surfaces were etched by hydrofluoric acid (VITA Ceramics Etch, Vita Zahnfabrik, Germany) for 20 s and then thoroughly rinsed. This was followed by oil-free air-drying. The etched surfaces were then silanized and applied with a bonding agent the same way as the composite restorations.
The prepared tooth samples were cleaned using ethanol pads, washed under running tap water thoroughly and dried with compressed air. The intaglio surfaces of the teeth were treated with a total-etch adhesive (Adper™ Single Bond Plus, 3M ESPE, St. Paul, USA) which was photocured for 10 s. The resin composite Z100 (3M ESPE, St. Paul, USA) was preheated at 170 °F for 10–15 min and used as a luting cement to fill up the gap between the restoration and tooth cavity. Excessive resin cement was removed and the remaining material in the gap was cured with a blue light (ESPE Elipar ® Trilight) for 40 s, buccally and lingually.
The restored tooth specimens were then mounted into Teflon rings using an orthodontic resin (DENTSPLY International Inc., USA), as shown in Fig. 2 (b) . The depth of the orthodontic resin was such that the tooth root was covered up to a position of about 2 mm below the cementum–enamel junction.
2.3
Fracture test and AE measurement
The fracture tests were conducted on a MTS machine (810 Material Testing System, MTS, USA), as shown in Fig. 2 . An axial compressive load was applied to each of the specimens from the top via a 10 mm-diameter stainless steel sphere at a crosshead speed of 0.1 mm/min. The load was continuously increased until the whole specimen fractured or the maximum limit of the load cell, 2200 N, was reached. A 1.0 mm-thick polyethylene film (Copyplast 1.0, Scheu Dental, Germany) was placed between the sample and the loading head to avoid local stress concentration by dispersing the load.
A two-channel AE system (PCI-2, Physical Acoustic Corporation, USA) was used to monitor the fracture and debonding. The AE operational settings were: 40 dB pre-amplification, 100 kHz–2 MHz pass band and 32 dB threshold. An AE sensor was attached to the loading head ( Fig. 2 b) with cyanoacrylate adhesive (Super Bond, Staples Inc., USA). The recordings of AE and mechanical load were synchronized so that the two sets of data could be correlated according to time. The compressive force associated with the first AE event was considered as the initial fracture or debonding load. The total number of AE events was used to evaluate the development of cracking during the entire loading process.
2.4
Statistical analysis
Non-parametric Mann–Whitney U test were conducted using SPSS (Version 13.0) to examine the statistical significance of differences among the groups in the initial fracture load, final load and total number of AE events, as well as the correlations between the initial fracture load, total number of AE events and final load ( Table 1 ).
| Group | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Number of specimens | 8 | 6 a | 8 | 8 |
| Cavity shape | Non-proximal box | Non-proximal box | Proximal box extended gingivally | Proximal box extended gingivally |
| Material | MZ100 | IPS Empress CAD | MZ100 | IPS Empress CAD |
a The results for 2 specimens from Group 2 could not be used in the analysis because the AE and load records were not synchronized.
2
Materials and methods
2.1
Specimens preparation
Thirty-two extracted human third molars with almost the same morphology and without decay and wear were selected. The maximum width and length of each tooth was measured to within 1 mm. These teeth were cleaned and stored in saturated thymol solution at 4 °C for about one month prior to preparation. Before preparation, they were rinsed under tap water and placed into deionized water at room temperature for 24 h. The teeth were then randomly divided into four groups of 8. A high-speed handpiece with a diamond bur was used to prepare the inlay cavities according to the standard operating procedures. The two cavity designs with dimensions are shown in Fig. 1 . All specimens were prepared by the same operator to eliminate inter-operator differences.
