Three-dimensional finite element analysis of glass-ceramic dental crowns

Because of the improved esthetic potential of glass-ceramic crowns as dental restorations, they are sometimes preferred over metal-ceramic crowns for restoration of anterior teeth. Because of their relatively high strength, these ceramic crowns are also frequently used for restoration of posterior teeth. However, due to the larger magnitude of biting forces on posterior teeth, intraoral fracture of all-ceramic crowns tends to occur more frequently in posterior crowns (Moffa, 1988). The objective of this study was to determine the relative influence of load orientation and the occlusal thickness of posterior ceramic crowns on the stress distribution which develops under these loading and design conditions. Three-dimensional finite element models for a molar crown were developed to determine the stress distribution under simulated applied loads. Glass-ceramic crowns with occlusal thicknesses of 0.5, 1.5, and 3.0 mm were considered. The largest principal tensile stresses induced in ceramic due to a distributed load of 600 N applied in a cuspal region were approximately 12 and 182 MPa for vertical and horizontal loading orientations, respectively. Stresses which developed in the facial and lingual marginal regions were primarily compressive under vertical loads. However, tensile stresses developed when the load was applied horizontally. Differences in stress distribution within crowns with the three occlusal thicknesses occurred only near the site of loading. Because of the relatively large failure rates of ceramic crowns in the posterior regions, these restorations should be strengthened by improvement in design, composition, and thermal processing conditions. Before any significant progress is made in these areas, these restorations should be used for the anterior teeth. The results of this study suggest that orientation of the applied load has a more important effect on development of large tensile stresses than the occlusal thickness of ceramic.     

Contact fracture of full-ceramic crowns subjected to occlusal loads

The mechanisms contributing to failure of full dental ceramic crowns under occlusal loads were studied using a unique optical approach. Model specimens comprising triple-layered crowns (veneer, core and substrate) were developed with both flat and curved occlusal surfaces and then subjected to simulated quasi-static occlusal loading using a spherical indenter. Deformation within the specimens during loading was analyzed by means of digital image correlation (DIC). Finite element models were also developed and used to examine the mechanics of contact. Results of the experiments with flat dental crowns indicated three typical modes of failure, i.e. cone cracks, plastic yielding and radial cracks. Fracture of the specimens with curved dental crowns was complicated by contributions from competing and multiple modes of failure. Both experimental and numerical results conclude that the dominant fracture mode in the full-ceramic crowns was radial cracking in the core beneath the contact area. However, displacement fields obtained using DIC showed that debonding developed near the shoulder of the crown, particularly during off-axis loading, and initiated under substantially lower occlusal loads than those required for crack initiation.     

A novel computational method to determine subject-specific bite force and occlusal loading during mastication

The evaluation of three-dimensional occlusal loading during biting and chewing may assist in development of new dental materials, in designing effective and long-lasting restorations such as crowns and bridges, and for evaluating functional performance of prosthodontic components such as dental and/or maxillofacial implants. At present, little is known about the dynamic force and pressure distributions at the occlusal surface during mastication, as these quantities cannot be measured directly. The aim of this study was to evaluate subject-specific occlusal loading forces during mastication using accurate jaw motion measurements. Motion data was obtained from experiments in which an individual performed maximal effort dynamic chewing cycles on a rubber sample with known mechanical properties. A finite element model simulation of one recorded chewing cycle was then performed to evaluate the deformation of the rubber. This was achieved by imposing the measured jaw motions on a three-dimensional geometric surface model of the subject’s dental impressions. Based on the rubber’s deformation and its material behaviour, the simulation was used to compute the resulting stresses within the rubber as well as the contact pressures and forces on the occlusal surfaces. An advantage of this novel modelling approach is that dynamic occlusal pressure maps and biting forces may be predicted with high accuracy and resolution at each time step throughout the chewing cycle. Depending on the motion capture technique and the speed of simulation, the methodology may be automated in such a way that it can be performed chair-side. The present study demonstrates a novel modelling methodology for evaluating dynamic occlusal loading during biting or chewing.     

The use of functionally graded dental crowns to improve biocompatibility: a finite element analysis

In post-core crown restorations, the significant mismatch between stiffness of artificial crowns and dental tissues leads to stress concentration at the interfaces. The aim of the present study was to reduce the destructive stresses by using a class of inhomogeneous materials called functionally graded materials (FGMs). For the purpose of the study, a 3-dimentional computer model of a premolar tooth and its surrounding tissues were generated. A post-core crown restoration with various crown materials, homogenous and FGM materials, were simulated and analyzed by finite element method. Finite element and statistical analysis showed that, in case of oblique loading, a significant difference (p < 0.05) was found at the maximum von Mises stresses of the crown margin between FGM and homogeneous crowns. The maximum von Mises stresses of the crown margin generated by FGM crowns were lower than those generated by homogenous crowns (70.8 vs. 46.3 MPa) and alumina crown resulted in the highest von Mises stress at the crown margin (77.7 MPa). Crown materials of high modulus of elasticity produced high stresses at the cervical region. FGM crowns may reduce the stress concentration at the cervical margins and consequently reduce the possibility of fracture.     

Three-dimensional finite element analysis of the composite and compomer onlays in primary molars

Resin onlay restoration is an esthetic alternative technique used for restoring extensively damaged primary molars. Understanding the behavior of materials under repeated functional stress and how the stress is transmitted to the remaining tooth structure is important. The aim of this study was to compare stresses in primary molars restored with indirect composite and compomer onlay. 3D frame models of the right mandibular and maxillary primary molars and the alveolar bone were created using computerized tomography images of a six-year-old girl. The enamel and dentine layers above the cement layer were unified to generate onlay restoration, and composite and compomer were used as restorative materials. The vertical occlusal load (100?N) was applied to the teeth in the occlusal contact areas. The von Mises stress distributions and normal stress distributions of the y-axis (parallel to the long axis of tooth) were evaluated. The occlusal stress is transmitted to the cervical part of healthy teeth by spreading it through the enamel layer. The composite and compomer restorative materials exhibited similar stress distribution patterns. An indirect technique creates a structure similar to the original morphological form, and it allows restorations to distribute high occlusal stresses and to minimize possible breakages.     

Digital image correlation analysis of the load transfer by implant-supported restorations

This study compared splinted and non-splinted implant-supported prosthesis with and without a distal proximal contact using a digital image correlation method. An epoxy resin model was made with acrylic resin replicas of a mandibular first premolar and second molar and with threaded implants replacing the second premolar and first molar. Splinted and non-splinted metal-ceramic screw-retained crowns were fabricated and loaded with and without the presence of the second molar. A single-camera measuring system was used to record the in-plane deformation on the model surface at a frequency of 1.0Hz under a load from 0 to 250N. The images were then analyzed with specialist software to determine the direct (horizontal) and shear strains along the model. Not splinting the crowns resulted in higher stress transfer to the supporting implants when the second molar replica was absent. The presence of a second molar and an effective interproximal contact contributed to lower stress transfer to the supporting structures even for non-splinted restorations. Shear strains were higher in the region between the molars when the second molar was absent, regardless of splinting. The opposite was found for the region between the implants, which had higher shear strain values when the second molar was present. When an effective distal contact is absent, non-splinted implant-supported restorations introduce higher direct strains to the supporting structures under loading. Shear strains appear to be dependent also on the region within the model, with different regions showing different trends in strain changes in the absence of an effective distal contact.     

全瓷冠、高嵌体与覆盖体对大面积牙体缺损后牙修复效果及牙周组织的影响

摘要 目的：探讨全瓷冠、高嵌体与覆盖体对大面积牙体缺损后牙修复效果及牙周组织的影响。方法：选择2015年6月到2018年5月选择在本院诊治的大面积牙体缺损后牙患者144例作为研究对象,根据随机抽签原则将其分为全瓷冠组、高嵌体组与覆盖体组各48例。全瓷冠组给予二氧化锆全瓷冠修复治疗,高嵌体组给予高嵌体修复治疗,覆盖体组给予覆盖体修复治疗,观察随访患者的预后情况。结果：三组修复后3个月的牙龈指数（GI）与探诊出血（BOP）阳性率低于修复前,高嵌体组、覆盖体组低于全瓷冠组（P<0.05）。全瓷冠组、高嵌体组、覆盖体组修复后3年的龋齿、牙周炎症、牙体修复体折断等并发症发生率为2.1 %、16.7 %、8.3 %,对比有差异（P<0.05）。高嵌体组、覆盖体组随访3年的修复体固位、修复体外形、修复体边缘适合性评分都高于全瓷冠组（P<0.05）。随访3年,三组修复体固位对比无差异（P>0.05）,但修复体外形、修复体边缘适合性对比有差异（P<0.05）。全瓷冠组、高嵌体组、覆盖体组随访3年的满意度分别为87.5 %、97.9 %和100.0 %,对比有差异（P<0.05）。结论：全瓷冠、高嵌体与覆盖体在大面积牙体缺损后牙修复的应用均具有一定的效果,对牙周组织也有一定的影响,在临床上要根据患者实际情况合理选择修复方法。     

Occlusion in an Adult Male Gorilla with a Supernumerary Maxillary Premolar

Supernumerary teeth, or teeth that develop in addition to the normal number of deciduous and permanent dentition, have been widely described in human and nonhuman primates. Most studies have focused on the morphology and on the etiology of supernumerary teeth, and little is known about their occlusal relationships with adjacent and antagonistic teeth, and their effects on individuals’ masticatory efficiency. We analyzed the occlusal wear pattern of an adult male Western lowland gorilla (Gorilla gorilla gorilla) with a fully erupted extra maxillary right premolar. We used a virtual method, occlusal fingerprint analysis, to reconstruct the major mandibular occlusal pathways responsible for the creation of wear facets on the tooth crowns. This approach is based on analysis of facet parameters such as inclination, directions, and areas, all measured using high-resolution 3-D virtual models of dental crowns. The results show unusual wear patterns in the supernumerary premolar and on its antagonist contacts (lower P4 and M1) that cannot be associated with a normal masticatory behavior. Occlusal simulation and kinematic analyses reveal a high level of directional overlapping combined with the absence of common occlusal contacts. This indicates a case of malocclusion that must have caused discomfort in this gorilla when biting or chewing, and may represent the first evidence of bruxism (grinding the teeth and clenching the jaw) in wild great apes.     

Salivary levels of mutans streptococci associated with restorations: a case-control study

Our aim was to estimate whether restorative therapy with amalgam and composite resin could decrease salivary mutans streptococcal level, thus also decreasing the risk for other caries development. We selected a case group of 93 children with detectable salivary mutans levels (i.e., at least 1x10(4) cfu/ml), and a control group (n=93 subjects) with undetectable levels. Children had the same age (12 years), no extracted teeth, crowns, temporary fillings, and restorations other than amalgam and composite resin, and the two groups had similar gender distribution. We clinically examined children and recorded active caries, restorations and oral hygiene level by means of gingival bleeding on probing; we also investigated sucrose intake at breakfast. The case group had statistically significant higher prevalence of restorations (36.6% vs. 18.3%), active caries (44.1% vs. 12.9%), and bad oral hygiene (84.9% vs. 68.8%) than the control group. However, the logistic regression analysis showed that presence of active caries was the only significant variable associated with mutans streptococci (OR=4.0; p=0.0002), while the effects of sucrose intake and of restorations were marginally significant. This apparent contrast between statistical analyses was due to the concomitant presence, in children with detectable mutans streptococci level, of restorations and decayed teeth at the same time, and, on the basis of the multivariate analysis, presence of mutans streptococci in these children was explained by the presence of active caries, more than restorations.     

Quantitative approach for the prediction of tooth movement during orthodontic treatment

The orthodontic treatment is aimed to displace and/or rotate the teeth to obtain the functionally correct occlusion and the best aesthetics and consists in applying forces and/or couples to tooth crowns. The applied loads are generated by the elastic recovery of metallic wires linked to the tooth crowns by brackets. These loads generate a stress state into the periodontal ligament and hence, in the alveolar bone, causing the bone remodeling responsible for the tooth movement. The orthodontic appliance is usually designed on the basis of the clinical experience of the orthodontist. In this work, a quantitative approach for the prediction of the tooth movement is presented that has been developed as a first step to build up a computer tool to aid the orthodontist in designing the orthodontic appliance. The model calculates the tooth movement through time with respect to a fixed Cartesian frame located in the middle of the dental arch. The user interface panel has been designed to allow the orthodontist to manage the standard geometrical references and parameters usually adopted to design the treatment. Simulations of specific cases are reported for which the parameters of the model are selected in order to reproduce forecasts of tooth movement matching data published in experimental works.     

FEA analysis of Normofunctional forces on periodontal elements in different angulations

In the literature, the periodontal tissue reaction to dissimilar occlusal stress has been described, including clinical and histologic changes caused by stresses in periodontal structures. With respect to occlusal forces, periodontal assembly demonstrates varying adaptive capacity from individual to individual and period to period within the same individual. Unfortunately, these occlusal stresses are yet to be quantified. As a result, determining the effect of normal occlusal force on periodontal elements in various angulations is of interest. Based on CBCT images, one FEA of the maxillary First molar was created, consisting of tooth pulp, periodontal ligament (PDL), and alveolar bone; the effect of normal occlusal force on the pdl in alternate angulations was assessed. Occlusion will occur at three contact areas representing the centric occlusion contact points, each of which will share a 150 N force. The analysis was performed for four force inclinations (0, 22.5°, 45°, and 90°). Maximum stresses are observed in cases of 90-degree loading. These stresses, however, are insignificant and will not cause the periodontal components to rupture. These tensile stresses, which are concentrated in the apical and cervical regions, may obstruct blood flow, resulting in tooth decay or, in some cases, periodontal breakdown in PDL. There have been attempts to express numerical data of stress to be provided for normal and hyper function loads to simulate occlusal situations at various angulations that are known to be accountable for healthy and diseased periodontium.     

Functional significance of cortical bone distribution in anthropoid mandibles: an in vitro assessment of bone strain under combined loads

Local variation in cortical bone thickness in the postcanine mandibular corpus appears to be stereotypical among anthropoids. Specifically, at sections under the molars, lingually situated cortical bone is typically thinner than that along the lateral aspect. This pattern applies despite phylogenetic, dietary, and allometric differences among the anthropoids sampled to date. Demes et al. (Food Acquisition and Processing in Primates [1984] New York: Plenum Press, p. 369-390) employed a theoretical analysis of mastication in Gorilla and Homo to argue that this pattern could be explained with reference to biomechanical stresses. Specifically, they proposed that the combined effects of torsion and direct shear on the working-side corpus create a condition in which net stresses and strains are reduced along the lingual cortical plate. Demonstration of this effect would suggest a functional linkage between localized differences in bone mass and strain gradients in the facial skeleton. We conducted an empirical evaluation of the effects of the combined loads of torsion and direct shear in vitro on a sample of formalin-fixed human mandibles. Rosette strain gages were affixed to the lateral and medial aspects of the corpus in each specimen, and surface strains were recorded separately under controlled torsional and occlusal loads, and under simultaneous application of these loads. The hypothesis that lingual strains are reduced under combined twisting and occlusal loads was generally supported; however, we observed reduction in surface strains at some sites along the lateral aspect of the corpus under these combined loads as well. These unexpected findings are attributable to unanticipated loading conditions imposed by occlusal forces, which result from sources of stress in addition to direct shear. These experiments provide provisional support for the hypothesis that superposed sources of bone strain produce large strain gradients between buccal and lingual aspects of the mandibular corpus, and that local variation in bone mass may be associated with these gradients.     

Finite element analysis of a new customized composite post system for endodontically treated teeth

This paper investigated the mechanical behavior of a new customized post system built up with a composite framework presently utilized for crowns, bridges, veneers and inlay/onlay dental restorations. The material has been shaped so to follow perfectly the profile of the root canal in order to take advantage of the better mechanical properties of composites with respect to metallic alloys commonly used for cast posts.

The analysis has been carried out with 3D finite element models previously validated on the basis of experimental work. The new post system has been compared to a variety of restorations using either prefabricated or cast posts. The structural efficiency of the new restoration has been evaluated for an upper incisor under different loading conditions (mastication, bruxism, impact).

Results prove that maximum stress values in restored teeth are rather insensitive to post types and materials. However, the new customized composite restoration allows to reduce significantly the stresses inside the dentinal regions where conservative clinical interventions are not possible.     

Movement behavior of teeth and dental implants in periotest measurement in occlusion--an in vitro study]

Early diagnosis of periodontal disease is essential for the planning of restorative, prosthetic and surgical treatment. The usual clinical methods are mostly subjective. Although the Periotest device has been specially developed for the diagnosis of periodontal disease, it can also be used to aid occlusal adjustment after insertion of inlays or artificial crowns. To investigate this application of the Periotest, a jaw model with idealized dental crowns incorporating motion and force sensors has been constructed. Motion and force are recorded during measurements with the Periotest. The maximum amplitudes in the apical (vertical) direction obtained with unpolished occlusal points increased with increasing occlusal load. With polished contact points, the maximum values in the apical direction revealed no dependence on occlusal load. The maximum amplitudes of movement and force in the oral (horizontal) direction decrease with increasing occlusal load both with polished and unpolished contact points. With the first, the teeth appear to slide one upon the other such that the overall distance between them remains unchanged, and no additional apical forces develop. In the case of rough contact points, additional forces develop, and falsify Periotest measurements.     

Quantitative Approach for the Prediction of Tooth Movement During Orthodontic Treatment

The orthodontic treatment is aimed to displace and/or rotate the teeth to obtain the functionally correct occlusion and the best aesthetics and consists in applying forces and/or couples to tooth crowns. The applied loads are generated by the elastic recovery of metallic wires linked to the tooth crowns by brackets. These loads generate a stress state into the periodontal ligament and hence, in the alveolar bone, causing the bone remodeling responsible for the tooth movement. The orthodontic appliance is usually designed on the basis of the clinical experience of the orthodontist. In this work, a quantitative approach for the prediction of the tooth movement is presented that has been developed as a first step to build up a computer tool to aid the orthodontist in designing the orthodontic appliance. The model calculates the tooth movement through time with respect to a fixed Cartesian frame located in the middle of the dental arch. The user interface panel has been designed to allow the orthodontist to manage the standard geometrical references and parameters usually adopted to design the treatment. Simulations of specific cases are reported for which the parameters of the model are selected in order to reproduce forecasts of tooth movement matching data published in experimental works.     

Numerical simulation on the biomechanical interactions of tooth/implant-supported system under various occlusal forces with rigid/non-rigid connections

The aim of this study was to analyze the biomechanics in an implant/tooth-supported system under different occlusal forces with rigid/non-rigid connectors by adopting a 3D non-linear finite element (FE) approach. A 3D FE model containing one Frialit-2 implant splinted to the mandibular second premolar was constructed. Contact elements (frictional surface) were used to simulate the realistic interface condition within the implant system and the sliding keyway stress-breaker function. The stress distributions in the splinting system and dissimilar mobility between natural tooth and implant with rigid and non-rigid connectors were observed for six loading types. The simulated results indicated that the lateral occlusal forces significantly increased the implant (sigma(I, max)), alveolar bone (sigma(AB, max)) and prosthesis (sigma(P, max)) stress values when compared with the axial occlusal forces. The sigma(I, max) and sigma(AB, max) values did not exhibit significant differences regardless of the connector type used. However, the sigma(P, max) values with a non-rigid connection increased more than two times those of the rigid connection. The sigma(I, max), sigma(AB, max) and sigma(P, max) stress values were significantly reduced in centric or lateral contact situations once the occlusal forces on the pontic were decreased. Moreover, the vertical-tooth-to-implant displacement ratios with a non-rigid connection were 23 and 9.9 times that for axial and lateral loads, respectively, applied on the premolar. However, the compensated non-rigid connector capabilities were not significant when occlusal forces acted on the complete prosthesis. The non-rigid connector (keyway device) only significantly exploited its function when the occlusal forces acted on a natural tooth. Minimizing the occlusal loading force on the pontic area through occlusal adjustment procedures to redistribute stress in the maximum intercuspation or lateral working position for an implant/tooth-supported prosthesis is recommended.     

Effects of occlusal attrition and continuous eruption on odontometry of rat molars

Recurrent reports by others of posteruptive dimensional increase of the crowns of rat molar teeth were analyzed in the context of our present study of occlusal attrition, continuous eruption and alteration of the occlusal planes of rat maxillary molar teeth with age. Marked attrition of the anatomical crowns occurs, together with a considerable continuous eruption that increasingly brings the markedly convex mesial root of the maxillary first molar into the clinical crown. Further, a slight change in occlusal plane occurs. Previous workers used standardized planes of orientation and of registration prior to measurement, which masked the phenomena mentioned above. They thus mistakenly reported increased coronal dimensions.     

Predicting lower body power from vertical jump prediction equations for loaded jump squats at different intensities in men and women

The purpose of this study was to determine the efficacy of estimating peak lower body power from a maximal jump squat using 3 different vertical jump prediction equations. Sixty physically active college students (30 men, 30 women) performed jump squats with a weighted bar's applied load of 20, 40, and 60% of body mass across the shoulders. Each jump squat was simultaneously monitored using a force plate and a contact mat. Peak power (PP) was calculated using vertical ground reaction force from the force plate data. Commonly used equations requiring body mass and vertical jump height to estimate PP were applied such that the system mass (mass of body + applied load) was substituted for body mass. Jump height was determined from flight time as measured with a contact mat during a maximal jump squat. Estimations of PP (PP(est)) for each load and for each prediction equation were compared with criterion PP values from a force plate (PP(FP)). The PP(est) values had high test-retest reliability and were strongly correlated to PP(FP) in both men and women at all relative loads. However, only the Harman equation accurately predicted PP(FP) at all relative loads. It can therefore be concluded that the Harman equation may be used to estimate PP of a loaded jump squat knowing the system mass and peak jump height when more precise (and expensive) measurement equipment is unavailable. Further, high reliability and correlation with criterion values suggest that serial assessment of power production across training periods could be used for relative assessment of change by either of the prediction equations used in this study.     

Clinically acceptable restorations may be a hotbed for cariogenic microbes

doi: 10.1111/j.1741‐2358.2011.00571.x Clinically acceptable restorations may be a hotbed for cariogenic microbes Objective: The aim of this study was to investigate the cross‐sectional association of dental restorations with salivary cariogenic pathogens among the elderly to establish effective parameters of caries risk for this population. Materials and methods: Stimulated whole saliva was collected from 289 community‐dwelling older adults (66.2 ± 3.9 years old) who had 20 or more teeth. Salivary levels of three cariogenic bacteria (Streptococcus mutans, Streptococcus sobrinus and lactobacilli) were estimated using quantitative polymerase chain reaction (real‐time PCR) method. Results: The mean number of residual teeth was 26.4, and restored teeth with crowns, inlays and composite resin were 7.35, 3.88 and 0.68, respectively. The number of crowns correlated positively with salivary S. mutans, S. sobrinus and lactobacilli. Multiple linear regression analyses showed that the number of restored teeth with crowns was independently associated with salivary S. mutans, S. sobrinus and lactobacilli after controlling for age, gender, number of residual teeth and salivary flow rate. Salivary flow rate was independently associated with salivary S. mutans and lactobacilli. Conclusion: The number of crowns had an association with salivary levels of cariogenic bacteria, suggesting that this parameter may be a caries risk indicator for the elderly population.     

Comparison of stress distribution in dental crown with different cusp angles: 3D finite element analysis

The purpose of this study is to measure the failure risk of a crown depending on the cusp angle. Three all-ceramic crown models consisting of C_H (high incline), C_M (middle incline), and C_L (low incline) are designed. Stress is applied to the crown with Loading case-1 (top of cusp tip) and Loading case-2 (middle of cusp ridge) with the use of FEA software. In Loading case-1 and case-2, the C_H showed the highest Maximum Principal Stress (MPS) while the CL showed the lowest MPS. The cusp angle is an influential factor affecting stress distribution in dental crowns.