The Effects of Cyclic Loading on Fracture Modes in Brittle Layer Structures: Relevance to Failure of Dental Crowns

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1 Advanced Materials Research Vols (2008) pp online at (2008) Trans Tech Publications, Switzerland Online available since 2008/Apr/08 The Effects of Cyclic Loading on Fracture Modes in Brittle Layer Structures: Relevance to Failure of Dental Crowns Sarah Lam 1, a, Kimble Halliday 1, b, Tarek Qasim 2, c 1 School of Dentistry. Oral Health Centre, The University of Western Australia 17 Monash Avenue, Nedlands, WA School of Mechanical Engineering, The University of Western Australia 35 Stirling Highway, Crawley, WA 6009 Phone: Fax: a lams02@student.uwa.edu.au, b khallid@ohcwa.uwa.edu.au, c Tarek.Qasim@uwa.edu.au Keywords: Cyclic loading; All ceramic dental crowns; Brittle coatings; Radial cracks Abstract. The effects of cyclic loading on contact damage in curved bi-layer systems are investigated. Dome structures consisting of glass shells, filled with epoxy resin, simulate the essential structure of monolithic all-ceramic dental crowns on natural tooth dentine. Cyclic loading, with only a vertical component, was carried out with the Multi-Functional Chewing Simulator (Willytec. Munich, Germany). The specimens were tested by indentation with hard spheres of tungsten carbide, with the load applied axially at the apex of the dome. This project reports some new results on the effect of cyclic loading on curved bi-layer systems. In addition, the effect of aqueous environments is addressed. In both air and water tests, observations taken throughout the cyclic loading course indicated that the outer cone cracks inhibited the propagation of radial cracks. Results confirm that crack initiation occurred more rapidly in wet conditions of testing, emphasizing the influences of the moist environment of the oral cavity. Furthermore, the experiments took into account the evolution of inner cone cracks observed in wet cyclic loading tests. The new results are important since nearly all-dental crowns exhibit some curvature. The implications of the results on the failure of dental crowns are discussed. Introduction While natural teeth exhibit micro-cracks (craze lines) [1] in its outermost layer (dental enamel), the failure of the natural tooth crown is uncommon, unless the tooth has already been compromised (due to caries, complex restoration, restoration failure, trauma etc) [2]. This displays the ability of natural tooth structure to disperse stresses: effectively distributing the occlusal stresses to the underlying dentine, which provides an elastic foundation for enamel [3]. But when the failure of costly restorations (dental crowns), continues at a rate of about 3% per year [4], the need to understand the mechanics of failure are more evident in the design of ceramicbased dental crowns and restorations [5, 6]. Ceramics are materials of choice for crowns because of an increasing demand for aesthetics in the mouth, as well as for biocompatibility and inertness. Typically, molar crowns, where chewing stresses are highest, must be designed to withstand biting loads of 100 N to 1000 N for more than a million cycles in aqueous environments. Hence the current need for further research is indicated [7,8]. Factors affecting crack initiation and propagation in dental restorations must be taken into consideration. Some these factors include: How does the crown system behave under occlusal stress? Can it resist fracture under load? How does the system behave under cyclic loading (a normal chewing pattern) or mono-loading (e.g. biting on a grain of sand/hard object)? Will off-axis loading affect it much? How will the system behave under certain environmental conditions (aqueous environments, differing temperatures)? What was the veneer and core of the crown made of? All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, (ID: /07/09,09:53:23)

2 22 Structural Integrity and Failure A complete analysis of the failure of crown systems would need to incorporate all these factors. Rekow s review (2007) displays the studies undertaken to investigate many of these factors in recent times. [9] Controlled experiments examining one parameter at the time would be appropriate to understand the individual effects of each factor. This simplistic approach enables identification of basic failure modes with minimum complication. For the purpose of this study, an all-ceramic crown structure on top of natural tooth dentine is represented with a bi-layer system: a three-dimensional domed glass shell filled with epoxy resin. The glass has an elastic modulus of about 73GPa, which is similar to that of tooth enamel or dental porcelain (See Table 1). Epoxy resin is a compliant polymeric substrate and has been used in numerous previous research studies to represent dentine. For studies on bi-layer systems, with a hard indenter, three main damage modes have been identified [10,11]. They are quasi-plasticity, cone cracks (inner, Hertizan and outer cone cracks) and radial cracks, as depicted in Fig. 1. Radial cracks, considered by many [11,12] as the principal mode of clinical failure, initiates from a starting flaw at the coating under-surface below the indentation region. This is due to the build up of tensile stresses (caused by the bending of the coating but allowed by compliant substrate). In 2005, a study [13] on curved glass-polymer bi-layers showed radial cracks growing steadily from the coating under-surface up towards the top surface. Upon reaching the top surface, the crack would accelerate abruptly towards the margin. Indenter Coating Substrate 2a r i Load (P) Cone cracks: Hertzian Outer Inner Radial cracks Substrate Quasi- Plasticity Fig. 1 Representation of Hertzian indentation showing cone cracking, radial cracking and substrate quasiplasticity (plastic deformation). r i indenter radius, a indentation radius. Fundamentally, the clinician aims for long term success and performance of dental crowns. Understanding relevant damage modes is an important factor leading towards a better design of dental crowns. Clinical factors, manufacturing processes and design must be taken into consideration. Today, majority of previous studies have focussed on single loading. However, in the mouth, cyclic loading is the main mode of loading. This study aims to study the effects of cyclic loading on the failure modes of all-ceramic dental crowns in service, on a convex/dome bi-layer of glass-epoxy resin. The multi-functional chewing simulator developed by Willytech was employed to carry this out in both dry and wet conditions. Table 1 Modulus of elasticity values for selected dental materials Material Modulus of Elasticity (GPa) Dentine *o 20 Enamel * (Axial-Occlusal) Porcelain *o Zirconia Y-TZP Borosilicate Glass 72.9 Epoxy Resin 3.4 * Values from Anusavice (2003). º Values from Ferracane (2001) + Values from 3M ESPE, Nobel Biocare

3 Advanced Materials Research Vols Specimen fabrication The glass shells were produced in the glassblowing workshop of the School of Biomedical, Biomolecular and Chemical Sciences, University of Western Australia. They were produced from 14mm outer diameter 1mm thick borosilicate glass tubes (Schott Duran, Germany). The specimens were formed on a lathe. A test tube was formed and a glass rod was attached to its dome end. The test-tube was heated at a region near the dome, and the dome was draw away, thinning the glass out. The end of the tube attached to the glass rod was pulled off, allowing the glass to close up. The closed up end was broken open, leaving a hollow domed shell of glass. The ends of the shell were then lightly flamed, smoothing out any roughness. The glass rods were detached from the glass domes. Following this, the inner concave surfaces of the glass shells were given an abrasion treatment with 50µm sand particles. This was conducted with a dental sandblast machine (P-G4000 Harnish and Reith, Czech Republic). Sandblasting reduces the strength of the glass close to that of porcelain (to best simulate the clinical crown). Also, it is often used in preparation of dental crowns. [14] Besides providing a more uniform density of surface flaw distribution, the procedure also facilitates bonding of the epoxy resin to the glass shell [14, 12]. The glass shells were placed into a mould. Subsequently, each concave interior filled with epoxy resin (R2512, ATL Composites, Australia) in layers of 3-4mm with a drying time of 24 hours allowed between each layer. This was done to avoid residual stress in the glass [14]. A final layer of approximately 6mm above the rim of the specimen was added in one addition. The specimen was sunk in diestone to stabilize and adapt it for testing. An inverted plastic cup was used to hold the specimen adaptor within the sample chamber. The diestone was vacuum-mixed to prevent air traps under the specimen that could negatively affect testing. Metal wire loops were inserted into the diestone to facilitate the ease of removal of the specimen from the plastic cup where it was firmly placed into (See insertion Fig. 2). Experimental Testing Developed by Willytec (Munich, Germany), the multi-functional chewing simulator is situated at the research lab of the Oral Health Centre of Western Australia (OHCWA), Fig. 2. It has eight identical sample chambers and two stepper motors which allow vertical and horizontal movements between two antagonistic specimens in each sample chamber through the movement of the upper and lower crossbeams. Each sample chamber has a plastic sample holder. The machine has been used to evaluate veneer materials [15], implant suprastructure testing [16] as well as recent endodontic work by postgraduates in OHCWA. In this study, for the sake of simplicity, only the vertical load component is considered. The lower crossbeam is motionless, holding the specimens in place, while the upper crossbeam holds the indenter holder and moves vertically downwards during testing. Test parameters were input into the computer unit: Vertical movement = 6 mm; Horizontal movement = 0.0 mm; Rising speed = 55 mm/sec; Descending speed = 30 mm/sec; Cyclic frequency = 1.3 Hz. Cycles were run to 40,000 cycles and above, with an observatory focus on crack initiation, propagation and failure of the specimen. Indenter adapter Fig. 2 Set up of the chewing simulator machine at OHCWA Specimen adapter

4 24 Structural Integrity and Failure Each specimen was run individually, due to video surveillance constraints and specific specimen observations. Crown systems are subjected to intra-oral cyclic loading to the magnitude of approximately P = N at the rate of about 1.5 Hz in a wet environment. [8] The loads used were 8kg, 12kg and 16kg (corresponding to 78N, 118N, 156N respectively), given the available weights present for the machine at OHCWA. Care was taken to align the indenter and specimens so as to ensure axisymmetric indentation as well as to calibrate the machine. A light source was placed behind the specimens to optimize crack visibility, as with similar experiments run by Qasim et al [11,12,14]. Results and Discussion Crack initiation refers to when the first radial crack is seen, while failure refers to when a situation arises that any increase in number of cycles does not change the failure pattern; severe local damage observed directly under the indenter. Fig. 3 shows a specimen at 60,000 cycles, under loading of 19 kg at 30mm/sec downward speed in a wet environment, and Fig. 4 shows a specimen at 40,000 cycles, under a loading of 16kg at 30mm/sec downward speed in a wet environment. The wet specimen had a marked reduction in the number of cycles taken to reach failure relative to the dry specimen. (a) (b) Fig. 4(a) Specimen at 60,000 cycles. (b) Specimen at 40,000 cycles. 16 kg ( 156 N); 30 mm/sec; Dry 16 kg ( 156 N); 30 mm/sec; Wet. The effects the wet loading conditions had on crack initiation are noted. The number of cycles to crack initiation was significantly lesser than in the dry specimen, especially in the 8kg loading. However, when the load was increased to 16kg (156N), there seemed to be no effect on the number of cycles to crack initiation regardless of dry/wet conditions, as shown in Fig. 5. With increased load, the number of cycles to initiation of radial cracks decreased, as is predictable. Observation of crack evolution in the wet environment was almost impossible, as the specimen was submerged and the lighting was inadequate. Number of cycles to initiate radia cracks, n I Dry Wet Fig. 5 Graph showing the number of cycles to initiation of radial cracks (dry vs. wet) Maximum load, P m (Kg)

5 Advanced Materials Research Vols Fig. 6 displays indentation with hard indenter in single cycle loading [11]. In comparison with the results obtained from cyclic loading of this study, it can be noted that it is the same as far as radial cracks initiation goes. However, the final stages of failure pattern are completely different, as there was no sudden extension of the cracks. In the cyclic loaded specimen, steady propagation of cracks occurred slowly to a minimal extent, but the outer cone cracks inhibited the radial cracks extension. Conclusions (a) (b) Fig. 6 Single cycle loading, initial and failure [11]. (a) Radial crack initiation; (b) Specimen failure In this study a simple bi-layer system limited to a perfect dome structure has been chosen as representative to simulate the essential of monolithic all-ceramic dental crowns on natural tooth dentine. The results from this study give a general indication of failure trends in more complicated 3-D crown structures. Some additional calculations incorporating the brittle cement layer between the crown and the natural tooth in the form of a tri-layer structure are currently under investigation. The issue of the shock absorbing effect of the supportive alveolar bone and periodontal ligament has not been considered in the present study to simplify the current research. These factors will influence the results, but are believed to be secondary relative to the coating and substrate (the main structure components). However, these factors and other variables will be considered in future studies. The results of aqueous environment testing demonstrate the way in which mouth environment (wet) can enhance fracture, and accelerate damage. The number of cycles to crack initiation was significantly less than in the dry specimen, especially in the 8kg (78N) loading. However, when the load was increased to 16kg (156N), there seemed to be no effect on the number of cycles to crack initiation regardless of dry/wet conditions; note that 156 N is above the critical load for crack initiation in mono-loading [11]. In comparison between the results obtained from cyclic loading with the previous results from mono-loading, it can be noted that it is the same as far as radial cracks initiation goes, the later stage of crack propagation to failure shows a distinct behaviour: in mono-loading with increasing load, the radial cracks demonstrate initial steady propagation then followed by an un-steady and sudden propagation to the edges of the specimen. This can be attributed to continuous deflection of the coating (see Fig. 6). In all the specimens tested in this study in both wet and dry conditions, the outer cone cracks inhibited the propagation of radial cracks as can be seen from Figs. 4 and 5. Acknowledgments This work is supported by a grant from the Australian Dental Research Foundation, and the Australian Research Council (DP ). References [1] Abou-Rass M. Crack lines: the precursor of tooth fractures their diagnosis and treatment. Quintessence Int 1983;4:

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