A simple model for enamel fracture from margin cracks Herzl Chai a , James J.-W. Lee b , Jae-Young Kwon c , Peter W. Lucas d , Brian R. Lawn b, * a School of Mechanical Engineering, Faculty of Engineering, Tel Aviv University, Tel Aviv, Israel b Ceramics Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8520, USA c School of Nano and Advanced Materials Engineering, Changwon National University, Changwon, Kyung-Nam, Republic of Korea d Department of Anthropology, George Washington University, Washington, DC 20052, USA Received 8 September 2008; received in revised form 10 October 2008; accepted 13 November 2008 Available online 3 December 2008 Abstract We present results of in situ fracture tests on extracted human molar teeth showing failure by margin cracking. The teeth are mounted into an epoxy base and loaded with a rod indenter capped with a Teflon insert, as representative of food modulus. In situ observations of cracks extending longitudinally upward from the cervical margins are recorded in real time with a video camera. The cracks appear above some threshold and grow steadily within the enamel coat toward the occlusal surface in a configuration reminiscent of channel-like cracks in brittle films. Substantially higher loading is required to delaminate the enamel from the dentin, attesting to the resilience of the tooth structure. A simplistic fracture mechanics analysis is applied to determine the critical load relation for traversal of the margin crack along the full length of the side wall. The capacity of any given tooth to resist failure by margin cracking is predicted to increase with greater enamel thickness and cuspal radius. Implications in relation to dentistry and evolutionary biology are briefly considered. Published by Elsevier Ltd. on behalf of Acta Materialia Inc. Keywords: Dental enamel; Fracture modes; Margin cracks; Channel cracks; Occlusal loading 1. Introduction Teeth are brittle, but resilient. Their capacity to with- stand high loads over a lifetime of stringent mastication is of particular concern in dentistry and evolutionary biol- ogy [1–8]. Tooth enamel provides mechanical protection for the pulp–dentin interior, and limits access of bacterial products to it. Enamel is harder and stiffer than dentin, and thereby shields the tooth interior from external loads. But enamel is also considerably less tough than dentin, meaning that it is relatively susceptible to crack propaga- tion. Anecdotal evidence in the dental and anthropological literature suggests that cracks are a regular occurrence in mature human enamel, generally developing from the cementum–enamel margins and extending longitudinally toward the occlusal surface, as depicted schematically in Fig. 1. There is some suggestion that such ‘‘margin” cracks may evolve within the enamel from ‘‘tuft”-like defects ema- nating from the dentin–enamel junction (DEJ) [2,9,10]. Margin cracks are also reported to be a source of failure in all-ceramic dental crowns, with the cracking manifested as a spall on the side walls adjacent to the cervical base of the crown [11–13]. Interestingly, attempts in the biome- chanics literature to quantify margin failure processes have been restricted to post-mortem examinations of extracted teeth in overload tests [14,15], with little or no effort to identify how the cracks initiate and evolve through the tooth structure en route to failure. In this communication we present results of in situ fail- ure tests on extracted molar teeth from human subjects to confirm the importance of margin cracks as a potential source of tooth failure [16]. We mount the teeth roots into an epoxy base and load the cusps with a disk indenter, to simulate an occlusal contact. A layer of Teflon is inserted between indenter and tooth, to simulate biting on soft food. Such a soft contact generally suppresses any compet- ing top-surface fracture modes [11]. A video camera is used 1742-7061/$ - see front matter Published by Elsevier Ltd. on behalf of Acta Materialia Inc. doi:10.1016/j.actbio.2008.11.007 * Corresponding author. E-mail address: brian.lawn@nist.gov (B.R. Lawn). Available online at www.sciencedirect.com Acta Biomaterialia 5 (2009) 1663–1667 www.elsevier.com/locate/actabiomat