Structure, chemical composition and mechanical properties of human and rat cementum and its interface with root dentin

Abstract

This work seeks to establish comparisons of the physical properties of rat and human cementum, root dentin and their interface, including the cementum-dentin junction (CDJ), as a basis for future studies of the entire periodontal complex using rats as animal models. In this study the structure, site-specific chemical composition and mechanical properties of cementum and its interface with root dentin taken from 9- to 12-month-old rats were compared to the physiologically equivalent 40- to 55-year-old human age group using qualitative and quantitative characterization techniques, including histology, atomic force microscopy (AFM), micro-X-ray computed tomography, Raman microspectroscopy and AFM-based nanoindentation. Based on results from this study, cementum taken from the apical third of the respective species can be represented as a woven fabric with radially and circumferentially oriented collagen fibers. In both species the attachment of cementum to root dentin is defined by a stiffness-graded interface (CDJ/cementum-dentin interface). However, it was concluded that cementum and the cementum-dentin interface from a 9- to 12-month-old rat could be more mineralized, resulting in noticeably decreased collagen fiber hydration and significantly higher modulus values under wet conditions for cementum and CDJ (E(rat-cementum)=12.7+/-2.6 GPa; E(rat-CDJ)=11.6+/-3.2 GPa) compared to a 40- to 55-year-old human (E(human-cementum)=3.73+/-1.8 GPa; E(human-CDJ)=1.5+/-0.7 GPa). The resulting data illustrated that the extensions of observations made from animal models to humans should be justified with substantial and equivalent comparison of data across age ranges (life spans) of mammalian species.

Fig. 1

(a) Schematic of a molar (not drawn to scale). (b) Top: mandible from a Sprague Dawley rat; bottom: half-mandible with flesh removed. The units on the ruler are in centimeters. (c) Top and side views of the jaw illustrating the ultrasectioned surface and sectioned jaw mounted on an AFM stub.

Fig. 2

Polarized micrographs illustrating secondary cementum and CDJ (white asterisks) in rat (a, c) and human (b, d) molars. Notice the woven fabric-like cementum structure (white arrows) and collagen-fiber bridges (white asterisks) defining CDJ in both species.

Fig. 3

AFM micrographs of secondary human cementum illustrating radial and circumferential collagen fibers (red arrows) (a) and collagen fibers (red asterisks) in the longitudinal (Z) direction (b). A schematic representing secondary cementum as a woven fabric-like structure due to collagen fibers in the radial, circumferential and longitudinal directions (c).

Fig. 4

(a) Reflectance light micrographs of an ultrasectioned surface block illustrating the mineralized tissues of interest. (b) Composite AFM micrographs of cementum (C) and root dentin (D) under dry conditions illustrating the porous interface, including the collagen fibers: a 5 μm × 5 μm region illustrating collagen fiber bundle at the cementum–dentin junction (CDJ). (c) 100 μm × 100 μm AFM micrographs of cementum (C) and root dentin (D) under wet conditions illustrating the partially swollen collagen fibers and interface.

Fig. 5

(a) A radiograph of the ground section taken at ×20 using MicroXCT. (b, c) Transverse and longitudinal slices taken from different areas of tomograph illustrating the bulk structure of CDJ (white arrows in (b)), including other interfaces such as the periodontal ligament space (PLS) and periodontal ligament (PL) between alveolar bone (AB) and cementum (C). The noticeable black regions are considered to be voids either representative of lacunae or pores due material/region architecture. (d) A tomograph of the red region marked in (c) illustrating the CDJ (black arrows). RCS: root canal space; RD: root dentin.

Fig. 6

(a) Longitudinal slices taken from a tomograph illustrating mantle dentin (MD),which is a combination of a Tomes granular layer (TG, most often observed in ground sections) and a structureless layer (SLL) between human lamellar cementum and tubular root dentin. (b) The width of the CDJ between bulk cementum with lacunae (dark regions) and tubular dentin. (c) A tomograph illustrating radially oriented collagen fibers within the CDJ in between tubular dentin and cementum.

Fig. 7

(a) Light microscopy area images of a 400 μm × 129 μm region bracketing the CDJ. (b) 3-D surface plots of the region illustrating a variation in inorganic, organic and a ratio of organic to inorganic contents. C: cementum; D: dentin.

Fig. 8

(a) Topographical map of an indented area illustrating indents (white spots). (b) Representative graph illustrating variation of reduced elastic modulus from cementum to dentin under dry and wet conditions along a line of indents. The lowest modulus values under dry and wet conditions were observed at the interface. Additionally, under wet conditions a gradual increase in modulus was observed when compared to dry conditions. The horizontal lines represent the average reduced elastic modulus values of dry cementum (1), wet cementum (2), dry dentin (3) and wet dentin (4) from all specimens (n=5), with corresponding standard deviation bars.