Hardness of the Root Dentine: A Comparative Study of Human Primary and Permanent Mandibular Incisors Tilen Šušterčič, DMD1, Taja Pogorelčnik, DMD2, Iztok Štamfelj, DMD, PhD1,3 1Department of Dental Diseases and Dental Morphology, Faculty of Medicine, University of Ljubljana; 2Lux Dental d.o.o., Slovenia; 3Centre for Operative Dentistry and Endodontics, University Clinical Centre Ljubljana 2 AIM: To compare hardness of the dentine forming roots of intact primary and permanent teeth and to identify possible differences in relation to distance from dentine-cementum junction (DCJ) and between root surfaces. 3 MATERIAL AND METHODS: 11 primary and 14 permanent caries-free mandibular incisors from collection of extracted teeth. Primary teeth with preserved at least cervical third of the root. Roots sectioned transversely below the cemento-enamel junction. Embedded in epoxy resin (EpoFix, Struers, Copenhagen, Denmark) and polished metalographically. Universal indenter (Shimadzu, Kyoto, Japan), load of 100 g for 10 s. 400x magnification. Measurements were made in the middle of each root surface (Figure 1). The first indentation was made 100 µm from the CDJ and the following indentations at regular intervals of 200 µm (Figure 2). Statistical analysis was performed in IBM SPSS 21; Student t-test for independent samples, one-way ANOVA, post-hoc Tukey’s (HSD) procedure. 4 RESULTS: On average, the dentine was harder in permanent than in primary teeth (68.9 HV vs. 62.1 HV; p < 0.001). Furthermore, the results show that hardness of the dentine tends to decrease with proximity of the pulp, both in primary and permanent teeth (Figures 3 and 4). Figure 1: Horizontal cut through cervical part of the tooth root (left lower permanent second incisor). Vickers pyramid indentations can be seen along the midline of each side of the root surface from DCJ to root canal (K). La – labial, Li – lingual, M – mesial, D – distal, UM – epoxy resin. Figure 2: Mesial root surface (left lower permanent second incisor). The bigger the indentation is, the smaller the microhardness is. Numbers next to indentations indicate microhardness in Vickers hardness units (HV). ↑ - crack in dentine. Figure 4: Box-plot diagram for dentine hardness in relation to distance from DCJ for 14 permanent teeth. Figure 3: Box-plot diagram for dentine hardness in relation to distance from DCJ for 11 primary teeth. 1 INTRODUCTION: Dentine is a mineralized tissue which forms most of the tooth structure and hardness is its key physical property. Hardness of crowns in permanent teeth is the area which we know most about, but not much is known about deciduous teeth, root dentine and differences between dentine layers and root surfaces. The hardness of the surface layer (100 µm below the CDJ) deviated from this rule; in both groups of teeth it was lower than for the outer third of the dentine (p < 0.001) and in primary teeth even lower than for the middle third of the dentine (p < 0.001) (Figures 5 and 6). Figure 6: Box-plot diagram for dentine hardness in relation to layers of root dentine for 14 permanent teeth. SL – surface layer (100 µm below the DCJ), OUT/MID/INN – outer/middle/inner third of the dentine. Dentine layers marked with different letters are statistically significantly different. Figure 5: Box-plot diagram for dentine hardness in relation to layers of root dentine for 11 primary teeth. A B C D In permanent teeth, the dentine was harder on approximal root surfaces (mesially 71.0 HV, distally 74.1 HV) than on labial (67.1 HV) and lingual surfaces (66.0 HV) (p < 0.001) (Figures 7 and 8). Figure 7: Box-plot diagram for dentine hardness in relation to root surface for 11 primary teeth. La – labial, Li–lingual, D – distal, M – mesial. There were no statistically significant differences between root surfaces. Figure 8: Box-plot diagram for dentine hardness in relation to root surface for 14 permanent teeth. La – labial, Li–lingual, D – distal, M – mesial. Dentine layers marked with different letters are statistically significantly different. A A B B 5 CONCLUSIONS: Hardness of the dentine in primary and permanent teeth decreases in pulp direction. This could be attributed to increasing density and diameter of dentinal tubules and reduced hardness of intertubular dentine. On the contrary hardness of the surface dentine layer is not the highest because of the less mineralized interglobular areas. Average hardness of all root dentine layers is lower in primary than in permanent teeth, which reflects difference in mineralization. In permanent teeth hardness of the dentine at approximal root surfaces is higher than at other root surfaces. Finding could be connected to faster progression of dentine sclerosation at approximal root surfaces. In primary teeth functional time is too short for such differences in hardness to appear. ISDM-IAPO-2017: 17th International Symposium od Dental Morphology (ISDM) & 2nd congress of International Association for Paleodontology (IAPO); Bordeaux, France; October 4-7, 2017