Effect of margin design on fracture load of zirconia crowns

Zirconia‐based restorations are showing an increase as the clinicians’ preferred choice at posterior sites because of the strength and esthetic properties of such restorations. However, all‐ceramic restorations fracture at higher rates than do metal‐based restorations. Margin design is one of several factors that can affect the fracture strength of all‐ceramic restorations. The aim of this study was to assess the effect of preparation and crown margin design on fracture resistance. Four groups of bilayer zirconia crowns (with 10 crowns in each group) were produced by hard‐ or soft‐machining technique, with the following four different margin designs: chamfer preparation (control); slice preparation; slice preparation with an additional cervical collar of 0.7 mm thickness; and reduced occlusal thickness (to 0.4 mm) on slice preparation with an additional cervical collar of 0.7 mm thickness. Additionally, 10 hard‐machined crowns with slice preparation were veneered and glazed with feldspathic porcelain. In total, 90 crowns were loaded centrally in the occlusal fossa until fracture. The load at fracture was higher than clinically relevant mastication loads for all preparation and margin designs. The crowns on a chamfer preparation fractured at higher loads compared with crowns on a slice preparation. An additional cervical collar increased load at fracture for hard‐machined crowns.publishedVersio

Fractographic analysis of 35 clinically fractured bi-layered and monolithic zirconia crowns

Objectives The aim of this retrieval study was to analyze the fracture features and identify the fracture origin of zirconia-based single crowns that failed during clinical use. Methods Thirty-five fractured single crowns were retrieved from dental practices (bi-layered, n = 15; monolithic, n = 20). These were analyzed according to fractographic procedures by optical and scanning electron microscopy to identify fracture patterns and fracture origins. The fracture origins were closely examined. The crown margin thickness and axial wall height were measured. Results Three types of failure modes were observed: total fractures, marginal semilunar fractures, and incisal chippings. Most of the crowns (23) had fracture origins at the crown margin and seven of them had defects in the fracture origin area. The exact fracture origin was not possible to identify due to missing parts in four crowns. The crown wall thickness was 20% thinner and wall height 30% shorter in the fracture origin area compared to the opposite side. Conclusions The findings in this study show that fractography can reveal fracture origins and fracture modes of both monolithic and bi-layered dental zirconia. The findings indicate that the crown margin on the shortest axial wall is the most common fracture origin site. Clinical significance Crown design factors such as material thickness at the margin, axial wall height and preparation type affects the risk of fracture. It is important to ensure that the crown margins are even and flawless.publishedVersio

Monolithic zirconia crowns in the aesthetic zone in heavy grinders with severe tooth wear – An observational case-series

Objectives The aim of this study was to assess the clinical outcomes and patient satisfaction with monolithic zirconia crowns in patients with severe tooth wear (≥1/3 of the tooth crown) in the aesthetic zone. Methods The historical prospective study sample consisted of 13 patients previously treated with a total of 84 monolithic zirconia crowns. The patients had been treated in a private clinic in Bergen, Norway, in the period 2012 to 2014. All patients were men, aged 35–67 years (mean age 56.3 years) and had been in need of prosthetic rehabilitation because of severe tooth wear in the aesthetic zone. Technical complications as well as biologic findings were registered when the crowns had been in function one to three years (mean 20 months). The patients completed a self-administered questionnaire regarding satisfaction with aesthetic and function. Results No biological complications were registered in 79 of the crowns (94%), and technical complications were registered in only two patients. All patients were satisfied with the aesthetic and function of the monolithic zirconia crowns and would choose the same treatment modality if they were to be treated again. Conclusions Within the limitations of this study, we conclude that the rate of clinical complications was low and that the patients were satisfied with the aesthetic as well as the function of the monolithic zirconia crowns. Clinical significance Monolithic zirconia crowns may provide a valid treatment modality in the aesthetic zone in patients with severe tooth wear.publishedVersio

Monolithic zirconia crowns – wall thickness, surface treatment and load at fracture

Purpose: The aim of this study was to evaluate the effect of wall thickness on load at fracture of monolithic zirconia dental crowns after aging. Materials and Methods: Seventy translucent monolithic zirconia crowns (DD Bio ZX2, Dental Direkt GmbH) were produced to fit a second upper premolar preparation with a circumferential shallow chamfer. Thirty crowns had a minimum wall thickness of 0.4 mm and 40 had 0.8 mm. TwentyAQ4 of the thick-walled crowns were glazed. The remaining crowns were polished. Ten crowns from each group functioned as controls, while the remaining were subjected to an aging procedure of alternation between dynamic loading and autoclaving. The surviving crowns were assessed for margin damages and surface wear before beeing subjected to quasi-static loading until fracture. All fractures were analyzed by fractographic methods. Results: There were statistically significant differences among the test groups concerning the effects of aging and surface wear. All thick-walled and eight of the thin-walled crowns survived the aging procedure. All fracture origins both from dynamic and quasi-static loading were located in the cervical margin with crack propagation corresponding to cervical hoop stress as observed in clinical failures. Conclusions: Thin-walled translucent monolithic zirconia crowns were more affected by the aging procedure than thick-walled crowns.publishedVersio

Toxic and essential trace elements in human primary teeth: A baseline study within The MoBaTooth Biobank and The Norwegian Mother, Father and Child Cohort Study (MoBa)

The Norwegian Mother, Father and Child Cohort Study (MoBa) includes a nation-wide collection of deciduous teeth located in the MoBaTooth biobank. The aim of the present study is to create a baseline for early-life metal exposure using dentine biomarkers. Deciduous teeth were collected in the MoBaTooth biobank, a sub-study of the MoBa-study. This study uses 94 primary teeth from children with no known medical conditions at the age of 6 months, a normal birth weight (2500-4500g) and an equal number of teeth shed between 2008-2013 and 2014-2019. A total of 48 girls and 46 boys are included to create a baseline to characterise retrospective exposure to toxicants during multiple early-life developmental periods. Estimates of weekly prenatal and postnatal exposure to 18 metals by measuring dentine concentrations have been made using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Temporal trends in dentine levels differed from metal to metal. Girls had higher postnatal dentine levels of Mn and Zn, compared to boys (p = 0.020 for postnatal Mn-levels, and p = 0.011 for postnatal Zn-levels). Deciduous teeth provide retrospective information on the intensity and timing of early-life metal exposure at weekly temporal resolution. Creating a baseline, future studies can use outcomes of conditions and illness in children in case-control-studies aiming at prevention. Using deciduous teeth, a novel noninvasive biomarker, characterising early-life exposure to 18 metals in approximately weekly increments during sensitive developmental periods extending from the second trimester to 4 months postnatally has been performed.publishedVersio

Wear of human teeth: a tribological perspective

The four main types of wear in teeth are attrition (enamel-on-enamel contact), abrasion (wear due to abrasive particles in food or toothpaste), abfraction (cracking in enamel and subsequent material loss), and erosion (chemical decomposition of the tooth). They occur as a result of a number of mechanisms including thegosis (sliding of teeth into their lateral position), bruxism (tooth grinding), mastication (chewing), toothbrushing, tooth flexure, and chemical effects. In this paper the current understanding of wear of enamel and dentine in teeth is reviewed in terms of these mechanisms and the major influencing factors are examined. In vitro tooth wear simulation and in vivo wear measurement and ranking are also discussed

A comparative study of ultrasonic direct contact, immersion, and layer resonance methods for assessment of enamel thickness in teeth

Wear of dental enamel is a growing problem, but is clinically difficult to diagnose and monitor. An accurate and easy-to-use non-destructive method for the measurement of enamel thickness would be useful for early diagnosis of enamel loss and for monitoring progression. Ultrasound has been identified by several researchers as a potential tool suitable for enamel thickness measurement. However, in vitro studies have shown that while the method is feasible, it suffers from wide variability. The methods proposed to date rely on the measurement of the time of flight of an ultrasonic pulse through the enamel layer. This requires the operator to locate the enamel-dentine junction. In this work, three methods are evaluated to try to reduce this variability and to investigate some practicalities of the approach. Time-of-flight methods using both contact and immersion transducers were used. Immersion transducers gave the most accurate results, within 10-15 per cent of values deduced from tooth sections, but would be harder to arrange for in vivo measurements. Preliminary studies have also shown that it is possible to achieve a resonance in the enamel layer and to measure thickness that way. While this approach needs further experimental refinement, it has the potential to be used for much thinner enamel layer thicknesses

Fluoride content and recharge ability of five glassionomer dental materials

<p>Abstract</p> <p>Background</p> <p>The relationship between fluoride content and fluoride release for glass-ionomer cements is not well understood. The aim of this laboratory study was: to determine the fluoride concentrations at the surfaces of glass-ionomer materials with respect to different storage media and different pH environments; to examine the recharge ability of the materials after NaF immersion; and to assess the morphological changes at the material surfaces using scanning electron microscope and energy dispersive spectroscopic techniques (SEM/EDS).</p> <p>Methods</p> <p>Five glass-ionomer materials, Fuji Triage (FT), Fuji II LC (FII), Fuji VIII (FVIII), Fuji IX GP (FIX), and Ketac N100 (KN), were analyzed in this study. Resin-based fluoride releasing material Helioseal F (HSF) was used as a comparison material. The sample consisted of 120 cured cement disks (n = 20 disks of each tested material, 10 × 1.5 mm). Five disks of each material were stored in 4 different storage media (I- saline, II- acidic solution ph = 2.5, III- acid solution ph = 5.5, IV- NaF solution (c = 500/106). After 7 days, two disks of each material were transferred from media I, II and III to the NaF solution for 3 min. EDS analysis was conducted in 3 randomly selected spots of each experimental disk. SEM was used to determine morphological characteristics of the material surface. Differences between the experimental groups have been analyzed using Student's t-test with the level of significance set at p < 0.001.</p> <p>Results</p> <p>FT showed the highest fluoride content at the surface of the material. The lowest amounts of fluoride ions were detected at the surfaces of the FT disks stored at low pH environments, and this difference was statistically significant (p < 0.001). Glass-ionomers showed significantly higher fluoride concentrations when compared to the HSF (p < 0.001). After immersion in the NaF solution, fluoride concentrations at the surfaces of the disks increased when compared with previous storage media (FT>FVIII>KN>FII>FIX). SEM analysis of the surface morphology revealed numerous voids, cracks and microporosities in all experimental groups, except for KN and HSF. More homogenous material structure with more discrete cracks was observed in samples stored at neutral pH environment, compared to disks stored in acidic solutions.</p> <p>Conclusion</p> <p>The tested materials could be considered as promising dental materials with potential prophylactic characteristics due to their relatively high fluoride content, but also the ability to extensively reabsorb fluoride ions, especially in acidic environments.</p

The oral microbiome – an update for oral healthcare professionals

For millions of years, our resident microbes have coevolved and coexisted with us in a mostly harmonious symbiotic relationship. We are not distinct entities from our microbiome, but together we form a 'superorganism' or holobiont, with the microbiome playing a significant role in our physiology and health. The mouth houses the second most diverse microbial community in the body, harbouring over 700 species of bacteria that colonise the hard surfaces of teeth and the soft tissues of the oral mucosa. Through recent advances in technology, we have started to unravel the complexities of the oral microbiome and gained new insights into its role during both health and disease. Perturbations of the oral microbiome through modern-day lifestyles can have detrimental consequences for our general and oral health. In dysbiosis, the finely-tuned equilibrium of the oral ecosystem is disrupted, allowing disease-promoting bacteria to manifest and cause conditions such as caries, gingivitis and periodontitis. For practitioners and patients alike, promoting a balanced microbiome is therefore important to effectively maintain or restore oral health. This article aims to give an update on our current knowledge of the oral microbiome in health and disease and to discuss implications for modern-day oral healthcare