Clinical implications of the response of enamel and dentin to masticatory loads

The dynamic interplay of dietary acid pH and concentration during early-stage human enamel and dentine erosion

Dental erosion continues to be a significant global health concern affecting nearly 30% of adults world-wide. With increasing soft drink consumption predominantly driving its prevalence, strategies for prevention and control are often implemented when erosion is severe, or rates are high in the populace. While factors affecting dental erosion such as pH on enamel has received much attention, the effect of dietary acid concentration when factored out to a commercially available pH has yet to be determined. Furthermore, understanding these effects on dentine, which is known to be more susceptible to erosion than enamel can unravel structure property relationships between acid characteristics and hard tissue types. This study aimed to develop structure-property relationships between dietary acid concentration, and pH, on the nano-textural and nano-mechanical properties of human enamel and dentine during short-term simulated drinking. To achieve this, a novel sample preparation methodology and analysis approach was developed by applying atomic force microscopy (AFM) in quantitative imaging mode. This enabled simultaneous measurement of enamel and dentine morphology and mechanical properties. Flow-cells were used to simulate drinking, exposing polished and smear layer free human enamel and dentine to 30 s repeated cycles of unbuffered citric acid 6% (pH = 1.88) and 1% (w/v) (pH = 2.55) and commercially available buffered pH = 3.8 states, for up to 180 s. The same 50 µm × 50 µm area of specimen morphology was analysed using in-house developed nanotextural analysis using the bearing area curve (BAC) with a focus on roughness (Ra), normalised peak (PA) and valley areas (VA). Mechanical properties were simultaneously measured for stiffness (N/m) after each 30 s. While all studies agree pH is a major factor in the erosion of enamel, here its dominance over the treatment time varied, with concentration surpassing the importance of pH after initial acid contact. Conversely, dentine erosion showed concentration-dependent changes in morpho-mechanical properties only. These results not only highlight the dynamic process of erosion, but how the interplay between acid characteristics and dental tissue type impact the progression of very early-stage erosion.

Evaluation of cavity size, kind, and filling technique of composite shrinkage by finite element

Background:

Cavity preparation reduces the rigidity of tooth and its resistance to deformation. The purpose of this study was to evaluate the dimensional changes of the repaired teeth using two types of light cure composite and two methods of incremental and bulk filling by the use of finite element method.

Materials and Methods:

In this computerized in vitro experimental study, an intact maxillary premolar was scanned using cone beam computed tomography instrument (SCANORA, Switzerland), then each section of tooth image was transmitted to Ansys software using AUTOCAD. Then, eight sizes of cavity preparations and two methods of restoration (bulk and incremental) using two different types of composite resin materials (Heliomolar, Brilliant) were proposed on software and analysis was completed with Ansys software.

Results:

Dimensional change increased by widening and deepening of the cavities. It was also increased using Brilliant composite resin and incremental filling technique.

Conclusion:

Increase in depth and type of filling technique has the greatest role of dimensional change after curing, but the type of composite resin does not have a significant role.

Abfraction: A review

Abfraction (AF) is the pathological loss of tooth substance caused by biomechanical loading forces that result in flexure and failure of enamel and dentin at a location away from the loading. The theory of AF is based primarily on engineering analyses that demonstrate theoretical stress concentration at the cervical areas of the teeth. However, the theory is not yet proven and there are explanations against it. AF lesions present primarily at the cervical region of the dentition and are typically wedge-shaped, with sharp internal and external line angles. Diagnosis of AF plays a very important role in the management of patients. The aim of the present narrative review is to comprehensively discuss the historical, etiopathogenetic, clinical, and diagnostic and treatment aspects of AF.

In Vitro Performance of Class I and II Composite Restorations: A Literature Review on Nondestructive Laboratory Trials—Part I

Posterior adhesive restorations are a basic procedure in general dental practices, but their application remains poorly standardized as a result of the number of available options. An abundant number of study hypotheses corresponding to almost unlimited combinations of preparation techniques, adhesive procedures, restorative options, and materials have been described in the literature and submitted to various evaluation protocols. A literature review was thus conducted on adhesive Class I and II restorations and nondestructive in vitro tests using the PubMed/Medline database for the 1995-2010 period. The first part of this review discusses the selected literature related to photoelasticity, finite element analysis (FEM), and microleakage protocols. Based on the aforementioned evaluation methods, the following parameters proved influential: cavity dimensions and design, activation mode (light or chemical), type of curing light, layering technique, and composite structure or physical characteristics. Photoelasticity has various limitations and has been largely (and advantageously) replaced by the FEM technique. The results of microleakage studies proved to be highly inconsistent, and the further use of this technique should be strictly limited. Other study protocols for adhesive Class II restorations were also reviewed and will be addressed in part II of this article, together with a tentative relevance hierarchy of selected in vitro methods.

Tensile strength and durability of bovine dentin

This study investigated the effects of thermal cycling on the tensile strength of dentin. Bovine dentin were divided into 10 groups, which were then subjected to various conditions: intact after preparation, thereby serving as a control; heating in boiling water for 45 minutes; 10,000 thermal cycles in water; 10,000 thermal cycles in PBS; storage in water at 5, 23, or 55 degrees C for two weeks; and storage in PBS at 5, 23, or 55 degrees C for two weeks. Subsequently, bovine dentin were trimmed into dumbbell-shaped specimens and the tensile test performed in distilled water at 37 degrees C. Mean tensile strengths were compared statistically by one-way ANOVA and Fisher's PLTD test (p<0.05). Fracture surfaces were observed by scanning electron microscopy, and reliability of the results was analyzed with Weibull distribution. Tensile strength did not significantly change after thermal cycling or storage in water and PBS at all temperatures tested (71.2-77.0 MPa) but decreased after treatment with boiling water (65.5 MPa).

The biomechanics of abfraction

The role of occlusal loading in the development of non-carious cervical lesions is becoming increasingly prominent. It is suggested that high occlusal loads result in large stress concentrations in the cervical region of the teeth. These stresses may be high enough to cause disruption of the bonds between the hydroxyapatite crystals, eventually resulting in the loss of cervical enamel.

This article reviews the evidence to support the thesis that occlusal loading can contribute to the process of non-carious cervical tooth loss or abfraction. It also reviews the potential interactions between occlusal loading and erosion that may contribute to non-carious cervical tooth loss.

Abfraction Lesions: Myth or Reality?

Loss of tooth substance in the cervical region is usually attributed to toothbrush abrasion, erosion, or a combination of both factors. Recently the role of occlusal loading has become increasingly prominent. It is suggested that high occlusal loads cause large cervical stress concentrations, resulting in a disruption of the bonds between the hydroxyapatite crystals and the eventual loss of cervical enamel. This process has been called noncarious cervical tooth loss or abfraction. This article reviews the available evidence to support the thesis that occlusal loading can contribute to the process of abfraction. It also reviews the potential interactions between occlusal loading and erosion that may contribute to abfraction lesion formation.

CLINICAL SIGNIFICANCE

It is important to recognize the potential role of occlusal loading in the loss of cervical tooth tissue so that management of the occlusion can be incorporated into a treatment plan for a patient with abfraction lesions.

Noncarious cervical lesions and abfractions: a re-evaluation

BACKGROUND

The concept of abfraction is controversial. The authors present the fundamental basis of abfraction and review studies that describe the condition.

TYPES OF STUDIES REVIEWED

The authors used data on masticatory forces, enamel and dentin properties, as well as stress studies related to abfraction, for background information. They also analyzed the genesis of the abfraction theory, experimental evidence, case presentations, clinical investigations and restorative studies.

RESULTS

The theory of abfraction is based primarily on engineering analyses that demonstrate theoretical stress concentration at the cervical areas of teeth. While some recent stress studies support earlier findings, others have provided significant deviating information. Few controlled studies exist that demonstrate the relationship between occlusal loading and abfraction lesions. The role of occlusal loading in noncarious cervical lesions (as shown by clinical data) appears to be part of a multifactorial event that may not necessarily follow the proposed classic abfraction mechanism, and other mechanisms or factors may explain cervical restoration failure just as well.

CLINICAL IMPLICATIONS

There is little direct evidence supporting the theory of abfraction as the primary factor in the formation of noncarious cervical lesions. Controlled clinical trials are necessary to elucidate more fully the etiology of those lesions.

Strains in the marginal ridge during occlusal loading

BACKGROUND

The marginal ridge is considered fundamental to the ability of the tooth to resist functional and parafunctional occlusal loads without damage. Despite this role, very little is known of patterns of stress and strain in marginal ridges under load. This study investigated strains in proximal enamel of mandibular premolars using finite element analysis (FEA) and strain gauge measurements.

METHODS

A three-dimensional FEA model of a human mandibular premolar was developed using commercial FEA software, and strains were computed in response to loads simulating clenching and chewing functions. Strains were measured in extracted premolars using strain gauges mounted on the proximal surfaces, under similar occlusal loading conditions.

RESULTS

Strains in the vicinity of contact areas and marginal ridges were lower than near the cemento-enamel junction and on buccal and lingual surfaces. The magnitude of proximal strains increased with oblique loading on cuspal inclines. Finite element analysis results correlated well with strain gauge measurements and can be used to predict strain directions and magnitudes.

CONCLUSIONS

At least for mandibular premolars, the marginal ridges are not highly stressed areas during simulated occlusal loading.

The effect of normal occlusal forces on fluid movement through human dentine in vitro

Receptors inside human incisors appear to respond to stress (comparable to pressure as opposed to force) on the crown. This ability may be used to limit the stress applied to teeth or to discriminate between the hardness of objects clenched between upper and lower teeth. Here the hypothesis that these receptors are activated by fluid squeezed out of dentinal tubules when the loaded tooth is stressed was tested. Vertically compressing the crowns of extracted human teeth with loads of from 20 to 120 N, similar to those used in other studies and during natural chewing, did indeed displace fluid into the pulp. The fluid was displaced away from the crown immediately after the tooth had been loaded and moved back into the crown when the load was removed. The volume ranged from 3.5 to 22.2x10(3) pl, similar to that known to stimulate single pulpal nerve fibres. Thus, normal chewing forces could displace sufficient fluid out of dentine to excite putative mechanoreceptors somewhere inside the dentine/pulp complex.

Strain patterns in cervical enamel of teeth subjected to occlusal loading

OBJECTIVES

This study was planned to investigate the variations in strains in enamel under different patterns of occlusal loading, using three-dimensional finite element analysis (3D FEA) and strain gage measurements in extracted teeth.

METHODS

A 3D FEA model of a mandibular second premolar was used to investigate effects of occlusal load on enamel surface strains, particularly in response to oblique directions of cuspal loading. Point loads of 100 N were applied axially and at 45 degrees from the vertical on the buccal or lingual incline of the buccal cusp, either in the bucco-lingual plane or at varying mesio-distal angulations (up to +/- 20 degrees). Patterns of strain observed in the FEA model were confirmed experimentally using strain gages on extracted premolars mounted in a servohydraulic testing machine.

RESULTS

Strains predicted from the FEA model were in excellent agreement with the strain gage measurements. Strains were concentrated near the cementoenamel junction (CEJ) regardless of load direction. A vertical load on the buccal cusp tip resulted in compressive strains on the buccal surface but small tensile strains in lingual cervical enamel. Strains resulting from oblique loads on buccal cusp inclines were complex and asymmetric, with either tension or compression occurring in any location depending on the site and angle of loading.

SIGNIFICANCE

The magnitude, direction and character of strains in cervical enamel are highly dependent on patterns of loading. The asymmetric pattern of strains in buccal cervical enamel in response to oblique occlusal forces is consistent with the common clinical picture of asymmetric non-carious cervical lesions.

Effects of loading on the biomechanical [correction of biochemical] behavior of molars of Homo, Pan, and Pongo

In a previous study, we found systematic differences in the biomechanical behavior of modern human molars using finite element stress analyses (FESA), which led us to propose that molars are adapted to differently-directed loads depending on their position within the mouth (Spears and Macho [1998] Am. J. Phys. Anthropol. 106:467-482). While the FESA results thus derived have not been verified experimentally, such an interpretation seemed reasonable. To refine the model previously presented, this study assessed the effects of 1) food particle size on the biomechanical behavior of molars, and those of 2) differences in morphology, particularly enamel thickness, on stress distribution. In order to appraise the evolutionary significance of the findings, the FESA results for modern humans were subsequently compared with those obtained for molars of one individual of Pan and Pongo, respectively. Bearing in mind limitations imposed by the FESA models created and analyzed in this study, constant cleavage-type loads and cuspal tip loads at different directions were employed on all teeth: this facilitated comparisons of patterns of stress distribution across molars and species. In Pan and Homo, cleavage-type loads exerted by big food particles tended to be better dissipated anteriorly than posteriorly, although trends in Pongo were less clear-cut. Furthermore, similar to modern humans, the buccal cusps of mandibular molars appeared to be able to dissipate the loads associated with a pestle-type action, while maxillary molars were better designed to dissipate the loads which would result if they acted as mortars against which the food is crushed/ground. While increases in enamel thickness lowered the overall stress values in teeth only slightly, changes in outer morphology could have a more profound effect on these stress levels. Overall, Pan appeared to be most generalized, while Homo and Pongo showed a number of unique specializations, which are in accordance with what is currently understood about their respective masticatory apparatus and dietary niche.

Biomechanical behaviour of modern human molars: implications for interpreting the fossil record

Finite-element models of 29 intact molars were created and subjected to cleavage-type loads in order to assess differences in the biomechanical behaviour of molars. A simulated food particle, which was one-third the size of the intercuspal distance and had the properties of a Mezzettia seed, was pushed onto the occlusal basin of these models at various angles, resulting in either both or one particular cusp being preferentially loaded. In all cases, the maximum tensile stresses occurred in enamel at the intercuspal fissure. With regard to first maxillary molars, supporting (functional) and guiding (nonfunctional) cusps apparently dissipate loads equally well, whereas, in second and third maxillary molars, the guiding cusps are better designed to resist loads. Overall, lingual cusps of maxillary posterior molars dissipate loads poorly. Conversely, loads exerted toward supporting cusps of mandibular molars are consistently well dissipated, regardless of position along the tooth row. Because the directions of loads to which these teeth are best adapted change along the tooth row, it seems reasonable to suggest that these may correlate with the well-documented structural and functional orofacial complex. This study indicates that the biomechanical behaviour of molars and the orofacial skeleton are likely to have undergone complementary directional changes during evolution. Consequently, caution must be exercised in making inferences about dietary adaptations of extinct species on the basis of isolated teeth or fragmentary gnathic remains without proper regard of the orofacial skeleton as a whole.

Effect of cavity depth on stresses in a restored tooth

Restorative procedures commonly replace lost tooth structure, but redistribution of functional stresses after treatment is not fully understood. Many restorative methods are dictated by the integrity of the remaining tooth structure, because sparse tooth structure can lead to fracture. It is essential to prevent fractures by having a clear concept of the designs for cavity preparations, and to anticipate the stresses of mastication on the remaining tooth structure. Knowledge of various internal parameters of cavity designs would facilitate selection of the appropriate cavity preparation for a specific clinical situation. Three cavity designs and restorations were examined in this study for stresses using the finite element technique. After placement of restorative materials, the dentin experienced a dramatic change in stress gradient immediately below the pulpal wall, and this response was magnified in deeper cavity preparations. Enamel also exhibited major alterations in the stress gradient in all three designs of cavity preparations. The combination of the changes can cause cracks in the remaining tooth structure, leading to cusp fracture immediately adjacent to the deepest portion of the cavity.

Stresses at the dentinoenamel junction of human teeth--a finite element investigation

A three-dimensional, linear, elastic finite element model of a maxillary first premolar from longitudinal ground sections was developed to investigate stress variation in the enamel and dentin adjacent to the dentinoenamel junction (DEJ). The effect of regional variation in the contour of the DEJ on the stress patterns for enamel and dentin was also analyzed. The normal (compressive or tensile) and shear stresses in the dentin and enamel surfaces of the DEJ were computed for a vertical load of 170 N acting on the entire occlusal surface of the model. The normal stresses in dentin and enamel were maximum on the occlusal surface of the model and diminished along the buccal and lingual surfaces of the DEJ. However, the magnitude of the normal stresses increased at the cervical enamel, which also showed increased values for shear stress distribution. The normal and shear stresses were markedly affected by the contour of the DEJ and the thickness of enamel in the occlusal third on the buccal and lingual surfaces. The results suggested that because the mechanical interlocking between enamel and dentin in the cervical region is weaker than in other regions of the DEJ, enamel in this region may be susceptible to belated cracking that could eventually contribute to the development of cervical caries.