Enamel microstructure and microstrain in the fracture of human and pig molar cusps T.E. Popowics a, *, J.M. Rensberger b , S.W. Herring c a Department of Oral Biology, University of Washington, Seattle, Washington, DC 98195, USA b The Burke Museum and Department of Earth and Space Sciences, University of Washington, Seattle, Washington, DC 98195, USA c Department of Orthodontics, University of Washington, Seattle, Washington, DC 98195, USA Accepted 26 January 2004 Introduction The relationship between enamel structure and tooth fracture is critical to understanding the feed- ing ecology of mammalian species and the oral health of modern humans. High stresses are con- centrated on tooth cusps in order to break down food, but may also fracture dental tissues. For most mammals, the extent to which the dental tissues, especially the brittle enamel, resist fracture deter- mines the degree of hardness or toughness of the diet. Matching the physical properties of food with tooth strength is crucial for a species’ exploitation of diverse food resources, and hard or tough foods are expected to result in selection for adaptations that improve tooth strength. Modern humans gen- erally bypass mechanical limitations of the denti- tion by reducing the fracture resistance of the diet through food preparation. Tooth fracture remains a risk, however, as occasional hard food particles can generate high stresses during a bite; these stresses may produce enamel fractures, often in regions of Archives of Oral Biology (2004) 49, 595—605 KEYWORDS Enamel fracture; Molar cusp; Strain gauge; Enamel microstructure Summary The role of microstructure in enamel strain and breakage was investigated in human molar cusps and those of the pig, Sus scrofa. Rosette strain gauges were affixed to cusp surfaces (buccal human M 3 , n ¼ 15, and lingual pig M 1 , n ¼ 13), and a compressive load was applied to individual cusps using an MTS materials testing machine. Load and strain data were recorded simultaneously until cusp fracture, and these data were used to estimate enamel stresses, principal strains, and stiffness. Fractured and polished enamel fragments were examined in multiple planes using scanning electron microscopy (SEM). Human cusp enamel showed greater stiffness than pig enamel (P ¼ 0:02), and tensile stress at yield was higher (17.9 N/mm 2 in humans versus 8.9 N/mm 2 in pigs, P ¼ 0:006). SEM revealed enamel rod decussation in both human and pig enamel; however, only pig enamel showed a decussation plane between rod and inter-rod crystallites. Human inter-rod enamel was densely packed between rods, whereas in pig enamel, inter-rod enamel formed partitions between rows of enamel rods. Overall, human enamel structure enabled molar cusps to withstand horizontal tensile stress during both elastic and plastic phases of compressive loading. In contrast, pig cusp enamel was less resistant to horizontal tensile stresses, but appeared to fortify the enamel against crack propagation in multiple directions. These structural and biomechanical differences in cusp enamel are likely to reflect species- level differences in occlusal function. ß 2004 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: þ1-206-543-5477; fax: þ1-206-685-3162. E-mail address: popowics@u.washington.edu (T.E. Popowics). 0003–9969/$ — see front matter ß 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.archoralbio.2004.01.016