The Histopathology of Dental Erosion Lussi A, Ganss C (eds): Erosive Tooth Wear. Monogr Oral Sci. Basel, Karger, 2014, vol 25, pp 99–107 DOI: 10.1159/000359939

The Histological Features and Physical Properties of Eroded Dental Hard Tissues Carolina Ganss a  · Adrian Lussi b  · Nadine Schlueter a  

 

 

a Department of Conservative and Preventive Dentistry, Dental Clinic, Justus Liebig University Giessen, Giessen, Germany; b Department of Preventive, Restorative and Pediatric Dentistry, School of Dental Medicine, University of Bern, Bern, Switzerland  

Abstract Erosive demineralisation causes characteristic histological features. In enamel, mineral is dissolved from the surface, resulting in a roughened structure similar to an etching pattern. If the acid impact continues, the initial surface mineral loss turns into bulk tissue loss and with time a visible defect can develop. The microhardness of the remaining surface is reduced, increasing the susceptibility to physical wear. The histology of eroded dentine is much more complex because the mineral component of the tissue is dissolved by acids whereas the organic part is remaining. At least in experimental erosion, a distinct zone of demineralised organic material develops, the thickness of which depends on the acid impact. This structure is of importance for many aspects, e.g. the progression rate or the interaction with active agents and physical impacts, and needs to be considered when quantifying mineral loss. The histology of experimental erosion is increasingly well understood, but there is lack of knowledge about the histology of in vivo lesions. For enamel erosion, it is reasonable to assume that the principal features may be similar, but the fate of the demineralised dentine matrix in the oral cavity is unclear. As dentine lesions normally appear hard clinically, it can be assumed that it is degraded by the variety of enzymes present in the oral cavity. Erosive tooth wear may lead to the formation of reactionary or reparative dentine. © 2014 S. Karger AG, Basel

Erosive wear occurs from the interaction of dental hard tissue surfaces with the surrounding liquid phase and, in the oral environment, with various physical impacts. These interactions are complex and erosive loss is not simply increased by physical forces; rather, the histological changes occurring from acid impacts are the prerequisite that physiological forces (e.g. from chewing or normal physical impacts like adequate toothbrushing) can cause lesions of a clinically typical shape. The histological features of such lesions require specific strategies for prevention or non-invasive therapy that differ fundamentally from those of a carious lesion. Thus, understanding the character of eroded dental hard tissues is essential for applying and developing suitable causal and symptomatic measures. The histology of sound enamel has been extensively investigated [1]. It is a non-vital, densely packed mineralised structure which is mainly composed of calcium and phosphate in the form of a non-stoichiometric hydroxyapatite. The mineral is organised in rods of hexagonal structure. The dimension of these crystals is difficult to measure, but values for width and thickness in the order of 50–70 and 20–25 nm, respectively, have been published [2]. Other components of

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enamel are organic material (2 vol%) and water (11 vol%) [2], making up only a small portion of the tissue. The high mineral content makes the enamel resistant against physical impacts. The value for the hardness of enamel varies depending on the measuring system and the load applied [3] and is also different with respect to the region of the tooth crown as well as with respect to the distance from the surface [4] so that no fixed value exists. Overall, however, the hardness of enamel is sufficient to withstand the majority of physical forces occurring during physiological processes (e.g. chewing) or oral hygiene measures. Thus, it is assumed that even over-vigorous oral hygiene habits would not be relevant for the wear of sound enamel [5]. When erosive demineralisation occurs (see chapter by Shellis et al., this vol., pp. 163–179), mineral is dissolved from the surface, causing a rough irregular structure similar to the etching pattern known from adhesive dentistry. Little is known about how deep the partly demineralised zone reaches; values ranging between a few microns [6] up to around 100 µm [7] have been reported. When the acid exposure continues, bulk enamel loss occurs (fig.  1). On such demineralised enamel surfaces, the microhardness is re-

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duced. As a consequence, eroded enamel is less resistant against physical forces than the sound tissue and, at least under experimental conditions, enamel loss is distinctly increased under erosive/abrasive conditions compared to erosion or abrasion alone and the amount of abrasive wear is related to the loss of microhardness [8]. Though it has been speculated that the partly demineralised surface zone is easily removed by physical forces, scanning electron microscopy (SEM) studies revealed that signs of demineralisation or etched prism structures are still visible even after toothbrushing [9, 10] (fig. 2). The histology of experimental enamel erosion produced in in situ or in vitro models is quite well understood, but the erosive demineralisation is much more severe under such conditions compared to the in vivo situation. Respective experimental designs comprise single erosive challenges or cycles of erosion and intervention over one or few weeks [11], resulting in loss values distinctly higher than in vivo. It is therefore reasonable to assume that structural changes occurring under real life conditions are much less pronounced – all the more as in vivo erosive wear is not a straightforward process but consists of bursts and silent periods depending on habits, lifestyle and

Ganss · Lussi · Schlueter Lussi A, Ganss C (eds): Erosive Tooth Wear. Monogr Oral Sci. Basel, Karger, 2014, vol 25, pp 99–107 DOI: 10.1159/000359939

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Fig. 1. Cross section (embedded, cut and polished) of an enamel sample from an in vitro experiment [6]. Erosion was performed with 1% citric acid (6 × 5 min/day, 10 days). Substantial bulk mineral loss ­occurred; on the remaining surface, partial surface demineralisation in the order of approx. 20 µm in depth is clearly visible.

general health aspects. Correspondingly, an early replica study revealed different morphologies of erosive wear ranging from irregular and pitted structures to more or less smooth lesion surfaces. It was speculated that the former are related to active lesions whereas the latter characterise inactive stages [12]. So far, there is no information about the physical properties of in vivo lesions, particularly with respect to microhardness. The assumption that in vivo erosive demineralisation is much less severe, however, might also hold true for the loss of microhardness. Whether these differences between experimental and in vivo lesions are relevant for transposing experimental results to instructions for patients remains elusive. Dentine is structurally and biologically distinctly different from enamel (for normal histology, see e.g. [1]) as it is a vital and permeable tissue of complex structure. The mineral compound of dentine is also non-stoichiometric hydroxyapatite but, unlike enamel where large crystallites build densely packed prisms, the crystals are much smaller and are associated with the organic component of the tissue [13]. Bulk mineral solely

occurs around the tubules defined as peritubular dentine. Overall, the inorganic portion makes up only around 47 vol%, whereas organic material (mainly collagen) contributes to around 33 vol%. In addition, the amount of water in dentine is high (21 vol%) [2]. From this feature it is clear that the physical properties of sound dentine differ from those of enamel. The elastic modulus is much lower, as is the microhardness [4], the latter making dentine more prone to physical wear. As a consequence, abrasive lesions (e.g. from toothbrushing) occur in dentine rather than in enamel [5]. Experimental erosive demineralisation in dentine leads to a structure distinctly different from that in enamel. Acid impacts cause a rapid dissolution of peri- and intertubular mineral but the organic portion is not degraded [14–16]. The result is that there is no bulk loss; instead, there is a spongy, completely demineralised structure remaining, the surface of which keeps the same level as the original sound tissue as long as it stays hydrated. The border between the demineralised and the mineralised tissue can be sharp (fig. 3) or can consist of a zone of partial demineralisation

Histological Features of Erosive Wear Lussi A, Ganss C (eds): Erosive Tooth Wear. Monogr Oral Sci. Basel, Karger, 2014, vol 25, pp 99–107 DOI: 10.1159/000359939

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Fig. 2. Surface of an enamel sample from an in situ study [26]. Erosion was performed extra-orally with 0.5% citric acid (6 × 2 min/day, 7 days). Twice daily, the sample was exposed intra-orally to a saliva/NaF toothpaste mixture for 2 min and within this time brushed with a powered toothbrush for 5 s (load 250 g). The last intervention was brushing. The rough surface ­structure and prism-like pattern as distinct signs of erosive demineralisation are still clearly visible even after the brushing procedure.

Fig. 3. Cross section (fractured, critical point dried) of a dentine sample from an in vitro experiment [19]. Erosion was performed with HCl (6 × 2 min/day; 9 days) and brushed with a powered tooth brush (2 × 15 s/day, load 300 g). Though the sample was brushed, the fully ­demineralised organic matrix is still present. Though there are minor signs of partial demineralisation ­(arrows indicate partly demineralised peritubular dentine), there is a sharp demarcation against the ­underlying sound tissue.

covering the underlying sound tissue (fig. 4). This zone of demineralised organic matrix is of importance for several aspects. Firstly, as soon as it reaches a certain thickness, all chemical processes become diffusion controlled. In the case of continuing erosive demineralisation, this means that

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the mineral loss is not linear but decreases with increasing thickness of the organic surface material [17, 18]. Secondly, active ingredients also have to diffuse through this structure and so far it is not clear how relevant interactions between such substances and collagen are. Thirdly, it has

Ganss · Lussi · Schlueter Lussi A, Ganss C (eds): Erosive Tooth Wear. Monogr Oral Sci. Basel, Karger, 2014, vol 25, pp 99–107 DOI: 10.1159/000359939

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Fig. 4. Cross section (fractured and critical point dried) of a dentine sample from an in vitro experiment [27]. Erosive demineralisation was performed with 1% citric acid for 90 min. In the upper third, dentine is fully demineralised with enlarged tubules and a fluffy intertubular structure. In the middle third, the intertubular dentine appears more dense with signs of mineral, but the peritubular dentine is fully dissolved (broken arrow). Towards the lower third of the picture, the degree of mineralisation increases with some peritubular dentine preserved (dotted arrow); the lower part shows sound dentine with fully mineralised intertubular and completely preserved peritubular dentine (full arrow). The diameter of the tubules is much smaller than in the demineralised parts [in part from Lussi et al.; Caries Res 2011;45(suppl 1):2–12].

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been shown that the demineralised organic portion is strikingly resistant against abrasive forces [19] (fig. 3), which is relevant for designing erosion/abrasion experiments with dentine. Finally, this structure may impact the quantification of erosive mineral loss, at least when surface mapping methods are used [3]. Similar to in vivo enamel erosion, there is almost nothing known about the histology of dentine erosion developing in the oral cavity. From clinical experience, such lesions appear hard when scratched with a probe and are shiny, which is in contrast to experimental lesions which are resilient

and dull. As demineralised human collagen can be digested by collagenases and other proteolytic enzymes [18, 20, 21], it could be assumed that it does not survive in vivo. The rate of demineralisation is surely much slower than in experiments, so that intraoral proteolysis may be sufficient to remove such structures as soon as the mineral is dissolved. The SEM replica image of a subject with active erosion supports this assumption (fig. 5). If this is the case, solid/liquid interactions in vivo are, similar to enamel, surface controlled rather than diffusion controlled, which may be relevant for interpreting study results from in vitro and in situ experiments.

Histological Features of Erosive Wear Lussi A, Ganss C (eds): Erosive Tooth Wear. Monogr Oral Sci. Basel, Karger, 2014, vol 25, pp 99–107 DOI: 10.1159/000359939

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Fig. 5. SEM image from a replica of the palatal surface of an upper canine with exposed dentine. The patient suffered from a severe eating disorder with chronic vomiting for several hours per day. E = Proximal enamel rim, D = exposed dentine. a At higher magnification, signs of erosive demineralisation are clearly visible, indicating an active stage of the condition. b At higher magnification, patent tubules are visible (arrows). Peritubular dentine is preserved. The level of the surrounding intertubular dentine is only slightly above the peritubular dentine. This feature is distinctly different from experimental erosion and indicates that there is only a very thin, if any, layer of demineralised organic material on the lesion surface even in active stages [in part from Lussi et al.; Caries Res 2011;45(suppl 1):2–12].

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What has rarely been addressed so far is how the pulpo-dentinal complex may react to erosive wear and what kind of tertiary dentine is formed (fig. 6, 7). In principle, odontoblasts survive when mild injuries occur, whereas severe injuries may destroy the odontoblast layer subjacent to the affected dental hard tissue [22]. In the first case, the remaining odontoblasts are forming new (patent) tubules whereas in the second case pulpal cells pro-

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liferate, forming new reparative (tertiary) dentine. This dentine is often atubular but may also have some tubules. Patent tubules typical for reactionary dentine are responsible for pain patients may suffer when they experience erosive tooth wear. Normally, the outer ends of the coronal dentinal tubules are closed by enamel, but as soon as dentine is exposed to the oral cavity, pathways from and to the pulp potentially exist.

Ganss · Lussi · Schlueter Lussi A, Ganss C (eds): Erosive Tooth Wear. Monogr Oral Sci. Basel, Karger, 2014, vol 25, pp 99–107 DOI: 10.1159/000359939

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Fig. 6. a Incisor of the lower jaw with occlusal erosive tooth wear extending into dentine. b, c The histological section of the incisor of a shows involvement of dentine and a partial pulp sclerosis with formation of tertiary dentine (reactionary dentine). The erosive tooth wear caused a moderate irritation of the primary odontoblasts which produced reactionary dentine. The reactionary dentine is characterised by tubules which run without interruption towards the pulp (d).

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Mechanical impacts such as curetting or drilling may reduce permeability through creating a smear layer and smear plugs, and several agents, for instance against hypersensitivity, can do so via organic or inorganic precipitation. These protecting layers, however, may not be durable due to various chemical impacts (e.g. acid exposures from intrinsic or extrinsic acid sources). Lundy and Stanley [23] investigated the reactions of the pulp on dentine exposure. They prepared class V cavities in adult teeth scheduled for extraction for various reasons. The cavities were left unrestored up to several weeks and reactions to mechanical and thermal stimuli as well as histological changes of the pulp were investigated. Within the first days, the teeth became very sensitive and the histological examination revealed severe inflammation including pulpal abscesses.

After weeks, however, sensitivity decreased and the histological findings indicated a more chronic stage of inflammation and some healing. Bacterial invasion was also observed to a certain extent. The study indicates that there are pathways from the oral cavity to the pulp and that factors present in saliva can cause reactions of the pulpal tissue. These reactions, however, appear reversible to a certain extent. What has also been shown is that, similar to caries, a dentinal response to erosion and to other forms of wear is the formation of dentine sclerosis, reparative dentine and dead tracts [24]. This was also found in a study investigating teeth from animals and humans with dentine exposed from attrition. Reparative dentine was found in all permanent human teeth (all caries free and without restorations): bacteria were present in dentinal tubules in 82%, inflammation in 64%, extensive

Histological Features of Erosive Wear Lussi A, Ganss C (eds): Erosive Tooth Wear. Monogr Oral Sci. Basel, Karger, 2014, vol 25, pp 99–107 DOI: 10.1159/000359939

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Fig. 7. a Histological section of a primary canine with advanced occlusal erosive wear extending deep into dentine. b The enlargement of a shows the formation of tertiary dentine with tubules running without interruption through the tertiary dentine. c Patent tubules typical for reactionary dentine are responsible for pain patients may suffer when they experience erosive wear.

degenerative changes in 29% and pulpal necrosis in 7% [25]. Overall, however, limited knowledge is available about the reactions of the pulpo-dentinal complex to erosive wear. In view of the high prevalence of exposed dentine in the dentition, for example in cases of cupped cusps, cervical lesions or gingival recession, there must be sufficient defence mechanisms against the effects of bacterial invasion into or diffusion of inflammatory factors through the dentine. Clinically, there is no evidence that the exposure of dentine through erosive wear can cause serious damage to the pulp, at least given that the rate of erosive wear and the apposition of reparative dentine are balanced. In summary, the histology of the eroded dental hard tissues is characterised by surface mineral loss progressing in layers from the natural surface

towards the pulp, given that erosive conditions persist. The main difference between enamel and dentine erosion is that the organic portion of dentine is not degraded by clinically relevant acid impacts. The histology of erosive wear needs to be considered when developing preventive, non-invasive and restorative strategies as well as for experimental designs. Erosive lesions can be easily created experimentally and are in the meantime quite well understood. What warrants further investigation are the histological features of erosive wear in vivo to find out whether the erosive lesions ­created experimentally are meaningful for the in vivo situation, whether lesions should be created in a manner reflecting more closely the in vivo histology, and how this could be accomplished.

References

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Prof. C. Ganss Department of Conservative and Preventive Dentistry Dental Clinic, Justus Liebig University Giessen Schlangenzahl 14 DE–35392 Giessen (Germany) E-Mail [email protected]

The histological features and physical properties of eroded dental hard tissues.

Erosive demineralisation causes characteristic histological features. In enamel, mineral is dissolved from the surface, resulting in a roughened struc...
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