DBM0010.1177/1758736014520809Journal of Dental BiomechanicsChun et al. 520809research-article2014 Original Article Journal of Dental Biomechanics 5: 1758736014520809 Comparison of mechanical © The Author(s) 2014 Reprints and permissions: property and role between enamel sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1758736014520809 and dentin in the human teeth dbm.sagepub.com KJ Chun1, HH Choi1, and JY Lee2 Abstract The mechanical properties of enamel and dentin were studied using test specimens having the same shape and dimensions because these properties might vary with the experimental conditions and specimen shapes and dimensions. Healthy human teeth were used as specimens for mechanical tests. The stress (MPa), strain (%), and elastic modulus (E, MPa) of the specimens were obtained from compression tests. The maximum stresses of the enamel, dentin, and enamel–dentin specimens were 62.2 ± 23.8, 193.7 ± 30.6, and 126.1 ± 54.6 MPa, respectively. The maximum strains of the enamel, dentin, and enamel–dentin specimens were 4.5 ± 0.8%, 11.9 ± 0.1%, and 8.7 ± 2.7%, respectively. The elastic moduli of the enamel, dentin, and enamel–dentin specimens were 1338.2 ± 307.9, 1653.7 ± 277.9, and 1628.6 ± 482.7 MPa, respectively. The measured hardness value of enamel specimens (HV = 274.8 ± 18.1) was around 4.2 times higher than that of dentin specimens (HV = 65.6 ± 3.9). Judging from the measured values of the stress and strain of enamel specimens, enamel tended to fracture earlier than dentin; therefore, it was considered more brittle than dentin. However, judging from the measured hardness values, enamel was considered harder than dentin. Therefore, enamel has higher wear resistance, making it suitable for grinding and crushing foods, and dentin has higher force resistance, making it suitable for absorbing bite forces. The different mechanical roles of enamel and dentin may arise from their different compositions and internal structures, as revealed through scanning electron micrographs of enamel and dentin. Keywords Mechanical properties, compression tests, hardness tests, enamel, dentin Received: 5 December 2013; accepted: 9 May 2013 Introduction Dental hard tissue comprises a combination of enamel and Dentin consists of the mineral hydroxyapatite (70%), dentin, both of which have different compositions and organic material (20%), and water (10%).6 Dentin is harder structures. Enamel is mainly made of the mineral than bone but softer than enamel, and it is mostly made of hydroxyapatite, which is crystalline calcium phosphate.1 It phosphoric apatite crystallites.7 Tooth decay leads to the has a glossy surface and varies in color from light yellow formation of cavities in the tooth. This process starts with to grayish white. It is the hardest tissue in the human body a small hole on the surface of the hard enamel; however, because it contains almost no water. Structurally, enamel once the decay passes through the enamel, it spreads wider covers the entire anatomic crown of the tooth above the gum and protects the dentin. During mastication, enamel comes in direct contact with food.2,3 Enamel comprises 1Department of Advanced BioMedical Enginnering, Korea Institute of millions of trichome-like hexagonal-prism-shaped enamel Industrial Technology, Cheonan-si, Republic of Korea 2Department of Dentistry, Samsung Medical Center, Seoul, Republic of rods having a diameter of 3–5 µm, which is 1/100th the Korea thickness of hair. Tooth enamel does not regenerate once abraded or eroded.4 Enamel varies in thickness and density Corresponding author: KJ Chun, Department of Advanced BioMedical Engineering, Korea over the tooth surface, being thickest and hardest at the Institute of Industrial Technology, 35-3, Hongcheon-ri, Ipjang-myeon, cusps or biting edges. The enamel of deciduous teeth is Seobuk-gu, Cheonan-si, Chungnam 331-825, Republic of Korea. less hard than and half as thick as that of permanent teeth.5 Email: [email protected] 2 Journal of Dental Biomechanics and deeper because the softer dentin is generally easily water-spray-cooled carborundum wheel (PSI-140-5L, decayed.2,3 Odontoblasts, which initiate dentin formation, Eagle Superabrasives, Inc, USA) in a micro diamond saw are located around the pulp chamber as a single layer. The machine (RB216 Culux; R&B Inc., Korea) that can cut at calcium ions of dentin and odontoblasts are connected by a feeding speed of 0.005–5 mm/s and 200–5000 r/min. channels within dentinal tubules.6,7 Pain, pressure, and Each specimen was prepared for testing after it was wet- temperature are delivered to the nerves in the dentinal sand polished (METPOL2; R&B Inc., Korea) using a tubules via the channels of odontoblasts.8,9 #3000 wet sanding polisher at 45 r/min, the speed of which Previous studies have elucidated the mechanical prop- ranged from 25 to 300 r/min. After polishing, each speci- erties—specifically, the elastic modulus—of enamel and men was viewed at 40× magnification to check for the dentin through compression tests, nanoindentation tests, presence of cracks and to identify potential damages that transmitted light microscopy (TLM) observation, and may have occurred during polishing using a microscope nanohardness tests (NHT).10–21 However, as enamel and (IX71; Olympus, Japan) with a 10× objective lens and 4×, dentin are not homogeneous and isotropic materials, they 10×, 20×, and 40× eyepiece lenses. No cracks were found do not obey Hook’s law. Therefore, a comparison of in most of the specimens. Finally, the specimens were kept mechanical properties based on the elastic modulus alone in normal saline at room temperature (22°C) immediately is not feasible; it is essential to also consider the stress before testing. and strain values. Furthermore, these studies did not Here, the above machine-cut and polishing process can investigate the mechanical properties of enamel and den- be new techniques to prepare the enamel and dentin speci- tin using test specimens having the same shapes and mens with the shape of a rectangular parallelepiped along dimensions. the force direction which have a very small geometric Now, these properties may vary depending on the error (±0.02 mm) without cracks. experimental conditions and specimen shapes and dimen- sions. Therefore, this study aims to determine the mechan- Hardness test. After epoxy resin molding, the extracted ical properties and primary roles of enamel and dentin by mandibular canine (#43, Fédération dentaire internationale using test specimens having the same shapes and dimen- (FDI) tooth-numbering system) was cross-sectioned by sions under the same experimental conditions. Specimens using a micro diamond saw machine (RB216 Culux) at a of dental hard tissues—enamel, dentin, and enamel–dentin feeding speed of 0.2 mm/s and 1000 r/min to maximize the (ED) composite—were used for compression tests, and the square measurement area of the sectioned surface and to obtained data were analyzed to determine the mechanical obtain dental hard tissue specimens for hardness tests (Fig- properties with respect to the bite force. Furthermore, the ure 2). Two cross-sectioned specimens were prepared for hardness of enamel and dentin was tested and analyzed the hardness test, and the sectioned surfaces were wet-sand with respect to the wear resistance. polished with #400, #1200, and #3000 (in order) polishers (METPOL2) at 45 r/min. Materials and methods Testing of mechanical properties Specimen preparation Compression test. A micro-load system (Universal Testing Compression tests. Five healthy human canines and 10 first System; R&B Inc., Korea) was used for the compression premolars (age = 19.3 ± 4.1 years) were extracted and used tests (Figure 3(a)). Its maximum force capacity was 5 kN, as dental hard tissue specimens for the mechanical tests. its crosshead loading speed ranged from 0.0001 to 1600 Protocols were approved by the Institutional Review mm/min, and it had ±0.5% operating accuracy. The volume Board of the Samsung Medical Center for human research, of the load cell was decided by considering the failure loads and all individuals gave informed consent (IRB No. of the specimens. The load cell was installed in the upper KBC12150). The extracted teeth were fixed in epoxy resin jig of the micro-load system to measure the load during the (ELR-3200; SPC Inc., Korea) moldings (epoxy resin:epoxy compression tests (Figure 3(b)). A 100-kgf load cell hardener = 40:10) and then cross-sectioned, as shown in (SM603; MScell Inc., Korea) was used for the enamel, den- Figure 1(a). The cross-sectioned teeth were machine-cut to tin, and ED specimens. A prepared specimen was placed on obtain 10 dentin specimens, 10 enamel specimens, and 10 the lower jig for the compression test (Figure 3(c)). Subse- ED specimens having width, height, and length of 1.2, 1.2, quently, the upper jig was moved toward the lower jig, and and 3.0 mm, respectively, for the compression tests, as then, the displacement and force were reset to zero when shown in Figure 1(b) and (c). Here, the geometric error the force measured in the load cell was 0.05 N. was ±0.02 mm for the machine-cut process described In all compression tests, the loading speed was set at 0.1 below. Furthermore, the specimens were machine-cut mm/min. Compressive stresses (σ in MPa) and strains (ε in along the direction of the force applied. %) of the dental hard tissues and dental materials were The teeth were cross-sectioned and machine-cut at a respectively calculated using equations (1) and (2), and the feeding speed of 0.2 mm/s and 1000 r/min by using a elastic modulus was calculated using equation (3) Chun et al. 3 Figure 1. Specimen preparation: (a) cross-sectioned after fixing a tooth within epoxy resin, (b) dimensions of enamel and dentin specimens, and (c) dimensions of ED specimens.