The dentin organic matrix - limitations of restorative dentistry hidden on the nanometer scale

Abstract

The prevention and treatment of dental caries are major challenges occurring in dentistry. The foundations for modern management of this dental disease, estimated to affect 90% of adults in Western countries, rest upon the dependence of ultrafine interactions between synthetic polymeric biomaterials and nanostructured supramolecular assemblies that compose the tooth organic substrate. Research has shown, however, that this interaction imposes less than desirable long-term prospects for current resin-based dental restorations. Here we review progress in the identification of the nanostructural organization of the organic matrix of dentin, the largest component of the tooth structure, and highlight aspects relevant to understating the interaction of restorative biomaterials with the dentin substrate. We offer novel insights into the influence of the hierarchically assembled supramolecular structure of dentin collagen fibrils and their structural dependence on water molecules. Secondly, we review recent evidence for the participation of proteoglycans in composing the dentin organic network. Finally, we discuss the relation of these complexly assembled nanostructures with the protease degradative processes driving the low durability of current resin-based dental restorations. We argue in favour of the structural limitations that these complexly organized and inherently hydrated organic structures may impose on the clinical prospects of current hydrophobic and hydrolyzable dental polymers that establish ultrafine contact with the tooth substrate.

Fig. 1

Increasing complexity of the organizational hierarchy of collagen type I (modified from Orgel et al. [62]). (A) Collagen molecules are composed of three α polypeptide chains; one chain is shown. The repeating (X–Y–Gly)_n pattern in which the X and Y positions are frequently occupied by proline and 4-hydroxyproline residues is represented by X, Y and G. (B) A not to scale (shortened) illustration of the collagen monomeric molecular structure depicting the non-helical N- and C-telopeptides bordering the long, central, helical domain. (C) Molecules are approximately 303 nm long (the relaxed length is a straight line measurement from end to end). Four collagen–ligand binding sites are indicated. (D) Simplified collagen molecular lateral packing: each molecule is staggered from its neighbour by a multiple of ∼67 nm. The gap region is where there are four collagen molecular segments and the overlap region where there are five. (D-ii) Schematic two-dimensional representation of the lateral molecular packing (D) and microfibril topology (light grey) illustrating the quasi-hexagonal arrangement. The intermolecular separation is slightly more or slightly less than 1.3 nm inside the hydrated fibrils, yielding a molecular packing that is quasi-hexagonal. (D-ii) Each collagen molecule in the microfibril is coloured so that it is obvious that each D-period contains molecular segments from five different molecules. (E) Three interdigitated microfibrils where each red and grey microfibril bundle represents a single microfibril, as shown in D(ii), forming an intermolecular association that would resemble thinner microfibrillar bundles (provided that these are not a random disaggregation event) (F) The type I collagen fibril exhibits a characteristic periodic banded pattern originating from the presence of a gap (black) and an overlap region (white) in the collagen axial packing (D). (F-i) AFM micrograph of a collagen fibril. (F-ii) Lateral view of the molecular packing within a single fibril, where each circle represents the estimated position of each collagen molecule in cross-section (adapted from Hulmes et al. [71]).

Fig. 2

Topological features of collagen type I. (A) Tapping mode AFM image obtained in liquid of an individual gap zone of a dentin collagen fibril and the adjacent overlap zones. (B) Section analysis across the diameter of a fibril overlap zone reveals “bumps” at about 4 nm distance that have been associated with collagen microfibrils (A and B adapted from Habelitz et al. [37]). (C) Molecular model of the microfibrillar arrangement of collagen type I. (D) The same arrangement is shown in a freezefracture micrograph of hydrated, unfixed collagen type I from rat tail tendon with a horizontal field of view of 500 nm (C and D adapted from Ottani et al. [142]). (E) Schematic representation of the radial packing of collagen molecules (adapted from Hulmes et al. [71]) showing a fibril surrounded by the polymeric matrix illustrating the difficult hermetic enveloping of collagen by viscous monomers due to the presence of ∼4 nm surface “bumps”.

Fig. 3

Collagen fibril disaggregation unravelling thinner collagen internal substructural units. (A) SEM image of corneal collagen fibrils treated with acetic acid and dissociated into thinner (∼10 nm) fibrillar entities (no scale bar reported, ×79,000) (adapted from Yamamoto et al. [64]). (B) Demineralized dentin collagen fibrils treated with trypsin yielding an untwistedrope like appearance and unravelling ∼20 nm substructural fibrillar disaggregates (200 nm scale bar, ×160,000).

Fig. 4

Schematic depiction of a hierarchical view of a hybrid layer and its constituents. (A-i) composite resin, (A-ii) adhesive layer, and (A-iii) monomer infiltrated dentin substrate. (A)–(C) represent increasing magnifications of the currently accepted concept of hybridization, where the D-periodical ∼100 nm diameter dentin collagen fibrils represent the ultimate structures to be impregnated and enveloped (adapted from Powers and Sakaguchi [143]). (D) Collagen fibrils (adapted from Gautieri et al. [144]) interconnected by the proteoglycan decorin (a monomeric representation of the dimeric model based on the available crystal structure of the protein core of decorin [145]). (E) Collagen microfibrillar organization and structure where the C-axis has been compressed for easier visualization (adapted from Orgel et al. [50]). (E-i) Model showing the quasihexagonal lateral packing of the molecular segments. (E-ii) Conformation of the D-staggered collagen segments within a single microfibril. (E-iii) The molecular path of a collagen molecule through successive microfibrils. (E-iv) Enlarged view of the N- (bottom) and C-telopeptide (top) regions of type I collagen. (E-v) Taking several 1D staggered collagen molecules from the collagen packing structure (single molecule shown in C) it is possible to represent the collagen microfibril. (E-vi) Three microfibrils are shown side by side to indicate the probable binding relationship between them.

Fig. 5

Spacefilling longitudinal (top) and cross-sectional (bottom) representations of the progressive hydration of the Gly-Ala peptide as seen in the crystal structure of the collagen molecule . Each colour represents one peptide chain of the triple helix, whereas the water molecules are shown in blue. (A) A view of the molecule without water. Incorporation of the (B) first, (C) second and (D) third shells of water molecules. Water molecules are either directly hydrogen bonded to carbonyl, hydroxyl or even amide groups on the peptide surface or hydrogen bonded to the first or second shells of water molecules.

Fig. 6

MMP-1 as a model for MMP driven collagenolysis. (i) The MMP cleavage site is buried in a narrow cleft at the fibril surface. (ii) MMP access to collagen degradation is thought to require C-telopeptide removal for full enzyme access (top), as illustrated in the bottom image. Removal may not have to result in cleavage, however, it may be possible for the enzyme to squeeze into the cleft if the C-terminal region is moved due to extrinsic events affecting the packing arrangement of the fibril, such as in cases of thermal motion, bending of the fibril or putatively demineralization with strong acids, such as the phosphoric acid conditioner of adhesive systems. (iii) Longitudinal view of collagen molecular packing illustrating the MMP cleavage site (cyan) partially covered by the C-telopeptide region (green). (iv) Higher magnification view of (iii). (Modified from Orgel et al. .)

Fig. 7

Schematic representation of proteoglycan attached to the collagen surface (not to scale). (A) Monomeric model of decorin based on the available crystal structure of the dimeric protein core . The association of decorin with the collagen surface is based on a recently proposed model . The glycosaminoglycan side-chain, based on the structure of chondroitin 4-sulfate is positioned in a hypothetical region of the decorin protein core with associated ions. All molecular structures are available from the National Center for Biotechnology Information structure database (http://www.ncbi.nlm.nih.gov). (B) A not to scale schematic sketch of the interfibrillar supramolecular assemblies that interconnect collagen fibrils: (1) collagen fibril; (2) decorin protein core; (3) chondroitin 4-sulfate glycosaminoglycan. The known periodicity of these interfibrillar aggregates in register with the gap zones of collagen fibrils, present in most connective tissues, remains uncertain for mineralized tissues. (C) A high magnification image of a sample of acid-soluble collagen and decorin treated with cupromeronic blue, which reacts with glycosaminoglycans and demonstrates their assembly as interfibrillar co-aggregates (arrows) .