Carola A. Carrera

Degradation in the dentin-composite interface subjected to multi-species biofilm challenges

Oral biofilms can degrade the components in dental resin-based composite restorations, thus compromising marginal integrity and leading to secondary caries. This study investigates the mechanical integrity of the dentin-composite interface challenged with multi-species oral biofilms. While most studies used single-species biofilms, the present study used a more realistic, diverse biofilm model produced directly from plaques collected from donors with a history of early childhood caries. Dentin-composite disks were made using bovine incisor roots filled with Z100(TM) or Filtek(TM) LS (3M ESPE). The disks were incubated for 72 h in paired CDC biofilm reactors, using a previously published protocol. One reactor was pulsed with sucrose, and the other was not. A sterile saliva-only control group was run with sucrose pulsing. The disks were fractured under diametral compression to evaluate their interfacial bond strength. The surface deformation of the disks was mapped using digital image correlation to ascertain the fracture origin. Fracture surfaces were examined using scanning electron microscopy/energy-dispersive X-ray spectroscopy to assess demineralization and interfacial degradation. Dentin demineralization was greater under sucrose-pulsed biofilms, as the pH dropped <5.5 during pulsing, with LS and Z100 specimens suffering similar degrees of surface mineral loss. Biofilm growth with sucrose pulsing also caused preferential degradation of the composite-dentin interface, depending on the composite/adhesive system used. Specifically, Z100 specimens showed greater bond strength reduction and more frequent cohesive failure in the adhesive layer. This was attributed to the inferior dentin coverage by Z100 adhesive, which possibly led to a higher level of chemical and enzymatic degradation. The results suggested that factors other than dentin demineralization were also responsible for interfacial degradation. A clinically relevant in vitro biofilm model was therefore developed, which would effectively allow assessment of the degradation of the dentin-composite interface subjected to multi-species biofilm challenge.

The use of micro-CT with image segmentation to quantify leakage in dental restorations

OBJECTIVE To develop a method for quantifying leakage in composite resin restorations after curing, using non-destructive X-ray micro-computed tomography (micro-CT) and image segmentation. METHODS Class-I cavity preparations were made in 20 human third molars, which were divided into 2 groups. Group I was restored with Z100 and Group II with Filtek LS. Micro-CT scans were taken for both groups before and after they were submerged in silver nitrate solution (AgNO3 50%) to reveal any interfacial gap and leakage at the tooth restoration interface. Image segmentation was carried out by first performing image correlation to align the before- and after-treatment images and then by image subtraction to isolate the silver nitrate penetrant for precise volume calculation. Two-tailed Students t-test was used to analyze the results, with the level of significance set at p<0.05. RESULTS All samples from Group I showed silver nitrate penetration with a mean volume of 1.3 ± 0.7mm(3). In Group II, only 2 out of the 10 restorations displayed infiltration along the interface, giving a mean volume of 0.3 ± 0.3mm(3). The difference between the two groups was statistically significant (p<0.05). The infiltration showed non-uniform patterns within the interface. SIGNIFICANCE We have developed a method to quantify the volume of leakage using non-destructive micro-CT, silver nitrate infiltration and image segmentation. Our results confirmed that substantial leakage could occur in composite restorations that have imperfections in the adhesive layer or interfacial debonding through polymerization shrinkage. For the restorative systems investigated in this study, this occurred mostly at the interface between the adhesive system and the tooth structure.

Total and soluble fluoride content in commercial dentifrices in Chile

Abstract Objective. Regardless of the total amount of fluoride (F) in a dentifrice, only the soluble fraction presents anti-caries activity and its concentration must be above 1000 ppm. No information regarding the quality of F in Chilean toothpastes is available. The aim of this study, therefore, was to determine the F concentration present in commercially-available toothpastes in Chile. Materials and methods: Eighteen toothpastes of five different brands were tested, each purchased from three different chain drugstores in Chile. For each toothpaste, total, total soluble and ionic F concentrations were assessed and expressed as ppm of F (μg F/g). Results. Among the dentifrices evaluated, 77.8% contained NaF (sodium fluoride), 16.7% MFP (monofluorphosphate) and 5.6% both types of F salts; 77.2% of the dentifrices showed similar total F content to that declared by the manufacturer in the labeling with only five with lower concentration than expected, beyond a 5% disagreement tolerance range. Total soluble F ranged from 958.8–1496.3 ppm (mean ± SD: 1270.5 ± 183.7). Five toothpastes showed lower total soluble F than total F. Only one toothpaste showed soluble salt levels below the recommended dose for this type of dentifrice. Conclusion: Most Chilean dentifrices evaluated contained soluble F within the range of 1000–1500 ppm and, therefore, may have anti-caries activity.

Fluoride content in toothpastes commercialized for children in Chile and discussion on professional recommendations of use

BACKGROUND In Chile, no information is available regarding the soluble fluoride (F) content in the toothpastes commercialized for children and the countrys guidelines recommend the use of F in toothpastes in an age-dependent concentration. No global consensus has been reached on this subject. AIM To determine the soluble F concentration in dentifrices for children sold in Chile and to discuss Chilean guidelines and professional recommendations of use. DESIGN Three samples of twelve different dentifrices were purchased from drugstores. Toothpastes were analysed in duplicate using an ion-specific electrode. The concentrations of total F (TF) and total soluble F (TSF) were determined (μg F/g). RESULTS Measured TF was consistent with that declared by the manufacturer in eight products. Two dentifrices showed lower TF and two higher F concentrations than declared. A toothpaste, marketed as low-F (450 ppm), showed F concentration threefold higher. Most dentifrices exhibited TSF concentrations similar to the TF content, except one sample that displayed considerably lower TSF than TF. Recommendations on F toothpastes use in children widely vary from country to country. CONCLUSIONS Most dentifrices for children match F content in the labelling, but recommendations are not supported by the best evidence available on the benefit/risk of F toothpastes use.

Fatigue failure of dentin-composite disks subjected to cyclic diametral compression

OBJECTIVE Our aim was to establish the relationship between cyclic loading and fatigue life of the dentin-composite interface using the newly developed disk in diametral compression tests. The results were then used to estimate the fatigue life of restored teeth under occlusal loading. METHODS Disk specimens (5mm dia.×2mm thick) were prepared using bovine incisors and restored with either a methacrylate-based composite Z100™ with Adper Single Bond Plus (Z100) or silorane-based composite Filtek™ LS with LS System adhesive (LS). The dentin-composite disks were tested under cyclic diametral compression to determine the number of cycles to failure (Nf) at three load levels (n=3 per group). Finite element analysis (FEA) was used to calculate the interfacial stresses (σ) within the specimen, to establish the σ vs. Nf curves, and those within a restored tooth under normal chewing forces (15N maximum). These were then used to estimate the lifetime of the restored tooth for the two restorative systems. RESULTS The disks restored with LS had a higher fatigue resistance than those restored with Z100. The maximum interfacial stress in the restored tooth determined by FEA was ∼0.5MPa. Based on the estimate of 300,000 cycles of chewing per year, the predicted lifetime under occlusal loading for teeth restored with LS and Z100 was 33 and 10 years, respectively. SIGNIFICANCE The disk in cyclic diametral compression has been used successfully to provide fatigue data which allows the lifetime of composite-restored teeth under occlusal loading to be predicted using numerical simulation.

Dentin-composite bond strength measurement using the Brazilian disk test

OBJECTIVES This study presents a variant of the Brazilian disk test (BDT) for assessing the bond strength between composite resins and dentin. METHODS Dentin-composite disks (ϕ 5mm×2mm) were prepared using either Z100 or Z250 (3M ESPE) in combination with one of three adhesives, Adper Easy Bond (EB), Adper Scotchbond Multi-Purpose (MP) and Adper Single Bond (SB), and tested under diametral compression. Acoustic emission (AE) and digital image correlation (DIC) were used to monitor debonding of the composite from the dentin ring. A finite element (FE) model was created to calculate the bond strengths using the failure loads. Fracture modes were examined by scanning electron microscopy (SEM). RESULTS Most specimens fractured along the dentin-resin composite interface. DIC and AE confirmed interfacial debonding immediately before fracture of the dentin ring. Results showed that the mean bond strength with EB (14.9±1.9MPa) was significantly higher than with MP (13.2±2.4MPa) or SB (12.9±3.0MPa) (p<0.05); no significant difference was found between MP and SB (p>0.05). Z100 (14.5±2.3MPa) showed higher bond strength than Z250 (12.7±2.5MPa) (p<0.05). Majority of specimens (91.3%) showed an adhesive failure mode. EB failed mostly at the dentin-adhesive interface, whereas MP at the composite-adhesive interface; specimens with SB failed at the composite-adhesive interface and cohesively in the adhesive. CONCLUSIONS The BDT variant showed to be a suitable alternative for measuring the bond strength between dentin and composite, with zero premature failure, reduced variability in the measurements, and consistent failure at the dentin-composite interface. CLINICAL SIGNIFICANCE The new test could help to predict the clinical performance of adhesive systems more effectively and consistently by reducing the coefficient of variation in the measured bond strength.

Calibration of a lactic-acid model for simulating biofilm-induced degradation of the dentin-composite interface

OBJECTIVE To verify and calibrate a chemical model for simulating the degradation of the dentin-composite interface induced by multi-species oral biofilms in vitro. METHODS Dentin-composite disks (5-mm dia.×2-mm thick) were made from bovine incisor roots and filled with either Z100™ (Z100) or Filtek™ LS (LS) composite. The disks, which were covered with nail varnish, but with one of the dentin-composite margins exposed, were immersed in lactic acid solution at pH 4.5 for up to 48h. Diametral compression was performed to measure the reduction in bond strength of the dentin-composite disks following acid challenge. Scanning electron microscopy (SEM) was used to examine decalcification of dentin and fracture modes of the disks. To better understand the degradation process, micro-computed tomography, in combination with a radiopaque dye (AgNO3), was used to assess interfacial leakage in 3D longitudinally, while SEM was used to determine the path of leakage. One-way analysis of variance (ANOVA) was used to analyze the results, with the level of statistical significance set at p<0.05. The results were compared with those obtained previously using multi-species biofilms for verification and calibration purposes. RESULTS After 48h of acid challenge, the debonding load of both the LS- and Z100-filled disks reduced significantly (p<0.05). In the Z100-filled disks, debonding mostly occurred at the adhesive-dentin interface, while in the LS-filled disks, this happened at the adhesive-composite interface, instead. The degree of dentin demineralization, the reduction in debonding load and the modes of failure observed were very similar to those induced by multi-species oral biofilms found in the previous work. Leakage of AgNO3 occurred mainly along the hybrid layer. The specimens filled with Z100 had a thicker hybrid layer (∼6.5μm), which exhibited more interfacial leakage than those filled with LS. SIGNIFICANCE The chemical model with lactic acid used in this study can induce degradation to the dentin-composite interface similar to those produced by multi-species biofilms. With appropriate calibration, this could provide an effective in vitro method for ageing composite restorations in assessing their potential clinical performance.

Mineralization processes in hard tissues: Teeth

Abstract Teeth are unique and complex organs formed from different embryonic layers. Mature tooth comprises enamel, dentin, cementum, and dental pulp. During its embryonic development, soft tissues of the dental germs begin to mineralize through a series of highly synchronized spatiotemporal events until the tooth erupts into the oral cavity. Enamel contains an exquisitely intricate and ordered arrangement of densely packed mineral crystals. Enamel mineralization involves the participation of multiple specific proteins that modulate the processes of crystal secretion and maturation. Dentin, on the other hand, is a less mineralized tissue than enamel and contains a higher amount of proteins. Like enamel, dentin mineralization requires participation of several proteins, some of them not yet fully characterized. Highly intricate cellular and molecular processes are required to form mature dentin. By a thorough revision of the existent literature on the topic, this chapter presents a state-of-the-art summary of the current understanding of tooth biomineralization.

Comparison between two post-dentin bond strength measurement methods

The push-out (PO) test and the diametral compression (DC) test were performed to compare the merits of two post-dentin bond strength measurement methods. Compared with the push-out test, the disk in DC provided post-dentin bond strength measurements that were more precise. The load-displacement curves from the DC test were much smoother and more linear up to the point of fracture when compared to those from the PO test. Compared to the PO test, DC is easier to perform for determining the bond strength between posts and dentin. No specimen alignment is needed in the DC test, and it produces a smaller standard deviation in the measured bond strength. The main disadvantage of the DC test, however, is that finite element analysis (FEA) is required to calculate the bond strength. The shear bond strength given by the PO test based on the simple formula is not valid, though, and the peak failure load is dependent on friction at the post-dentin interface.

Interfacial degradation of adhesive composite restorations mediated by oral biofilms and mechanical challenge in an extracted tooth model of secondary caries

OBJECTIVE To study the combined effect of simulated occlusal loading and plaque-derived biofilm on the interfacial integrity of dental composite restorations, and to explore whether the effects are modulated by the incorporation of sucrose. METHODS MOD-class-II restorations were prepared in third molars. Half of the specimens (n=27) were subjected to 200,000 cycles of mechanical loading using an artificial oral environment (ART). Then, both groups of specimens (fatigued and non-fatigued) were divided into three subgroups for testing in CDC-reactors under the following conditions: no biofilm (Control), biofilm with no sucrose (BNS) and biofilm pulsed with sucrose (BWS). BNS and BWS reactors were incubated with a multispecies inoculum from a single plaque donor whereas the control reactor was not. The BWS reactor was pulsed with sucrose five times a day. The biofilm challenges were repeated sequentially for 12 weeks. pH was recorded for each run. Specimens were examined for demineralization with micro-CT and load capacity by fast fracture test. RESULTS Demineralization next to the restorations was only detectable in BWS teeth. Fracture loads were significantly reduced by the concomitant presence of biofilm and sucrose, regardless of whether cyclic mechanical loading was applied. Cyclic loading reduced fracture loads under all reactor conditions, but the reduction was not statistically significant. CONCLUSIONS Sucrose pulsing was required to induce biofilm-mediated degradation of the adhesive interface. We have presented a comprehensive and clinically relevant model to study the effects of mechanical loading and microbial challenge on the interfacial integrity of dental restorations.