(C-104) Multiscale Mechanical Characterization of Human Tooth With Combination of Scanning Electron Microscopy, Atomic Force Microscopy, And Finite Element Method

Introduction:: As dental hard tissue, including enamel and dentin, is the major structure withstanding biomechanical loading, knowledge of tooth mechanical properties has been determined in bulk and nano scales. Previous studies have elucidated Young’s modulus of enamel and dentin with a compression test and nanoindentation test [1–4]. However, the previous research only determined tooth mechanical properties without consideration of tooth microstructure. In this research, the microstructure of enamel and dentin was characterized with scanning electron microscopy (SEM) and atomic force microscopy (AFM), and Young’s modulus of enamel and dentin was determined through AFM. The enamel rod, dental tubules, and odontoblast processes were observed. Furthermore, the mechanical properties of dentin at the macro scale were determined by finite element method (FEM) simulation with a representative volume element (RVE) model. The influence of the dentin tubules on the mechanical properties of dentin was investigated.

Materials and Methods:: Extracted teeth was used. To prepare the tooth section, each tooth was embedded in resin for fixing. The fixed sample was machine-cut by a saw to get the occlusal section at the bottom of the tooth crown.

The SEM characterization of tooth occlusal cross-sections was conducted to examine the microstructures of enamel and dentin at the occlusal section. Enamel micro-feature and dentin microstructures were observed under 600× magnification to see the enamel rods, and 1200× magnification to observe the open dental tubules and odontoblast process.

For the AFM indentation test, the sample surface was first submerged in 1× phosphate buffered solution (PBS) for 30 minutes to avoid the effects of surface charge. The AFM test was conducted with JPK force Robot 300. RTESPA-150 (Bruker, Billerica, MA) probe was utilized for morphology characterization. RTESPA-525 probe was adopted for mechanical characterization. The force-indent curves were fitted with Hertz Model to get Young’s modulus at each point.

FEM simulation of dentin compression test was implemented to quantify the mechanical properties of dentin with the presence of dentin tubules. A 2-dimensional (2-D) RVE model with a size of 38.4 × 28.0 µm2 was reconstructed based on SEM image (Figure. 1). Plane stress was set as 2-D behavior with 3.84 µm thickness. The Young’s modulus of dentin were obtained from AFM test results. The boundary conditions were set as 2 nm downward displacement on the upper boundary and frictionless support on the lower boundary. The Young’s modulus of dentin, E_REV, was calculated from the simulation.

Results, Conclusions, and Discussions:: Results
The morphology of enamel and dentin characterized with SEM and AFM are shown in Fig. 2. The enamel rods and dentin tubules are observed (marked by white arrows in each figure). The dentin tubules observed by AFM scanning showed a consistence with the SEM image. The enamel rod had a diameter of ~5 µm, the tubules had a diameter of ~2 µm. The odontoblast process in dental tubules was also observed.

The Young’s modulus of enamel and dentin were quantified at six locations, as depicted in Fig. 3(a)&(b). The Young’s modulus of enamel showed an average value 49.6 ± 20.4 GPa. The Young’s modulus of dentin represented an average value of 35.4 ± 11.6 GPa. The representative Young’s modulus mapping images indicated that both enamel and dentin showed a highly heterogeneous behavior (Fig 3(c)&(d)).

The mechanical propertied of the dentin at the macro scale was determined by a compression simulation with an RVE model. Following simulation, the Young’s modulus (E_REV = 29.2 GPa) was determined by curve fitting of the extracted stress-strain curve from RVE model, which is 17.5% less than the value quantified with AFM (35.4 GPa) due to the presence of dentin tubules undertaking fewer load (Figure 4).

Conclusions
The average Young’s modulus of enamel and dentin is 49.6 ± 20.4 GPa and 35.4 ± 11.6 GPa respectively, which are corresponding to previous compression and nanoindentation test results ranging from 1.3 GPa to 98.3 GPa for enamel and 1.6 GPa to 100 GPa for dentin [4]. Enamel rods and dental tubule structures with the odontoblast processes can be observed with SEM. Enamel rods are perpendicular to the dentinal-enamel junction (DEJ) with a diameter of approximately 5 µm. The size of open tubules and the distance between tubules show consistency between AFM and SEM images. E_RVE is 17.24% less than that quantified by AFM due to the presence of dentin tubules. Combining the SEM, AFM, and FEM, we build a frame for multiscale mechanical characterization of the hard dental tissues considering the microstructures, which can be further applied to the study of enamel and the whole tooth.

Acknowledgements (Optional): : Thanks to Florida Institute of Technology core facilities for SEM and machine cutting.

References (Optional): : [1] Craig, R. G et al., J Dent Res, 40:936-945, 1961.
[2] Fong, H et al., Mater Sci Eng C, 7:119-128, 1999.
[3] He, L. H et al., Biomaterials, 27:4388-4398, 2006.
[4] Chun, K et al., J Dent Biomech, 5:5/0/1758736014520809, 2014.