Thomas J. Hilton

Effect of energy density on properties and marginal integrity of posterior resin composite restorations

OBJECTIVES The purpose of this study was to determine the minimal extent of cure required by the base of a Class 2 resin composite restoration (Z250, 3M ESPE, St Paul, MN, USA) that allows it to support the rest of the restoration and maintain its marginal seal under simulated clinical conditions. METHODS Resin composite (Z250, 3M ESPE, St Paul, MN, USA) was placed incrementally or in bulk into Class 2 preparations in extracted human molar teeth and exposed to various light-curing energy densities. The restorations were subjected to 1000 thermal cycles (5-55 degrees C) and 500,000 fatigue cycles from 18 to 85 N using a stainless-steel sphere. Marginal integrity was evaluated using visual rating (ridit analysis) and microleakage. Degree of conversion (DC) and Knoop hardness (KHN) were determined at the occlusal and gingival surfaces using a reusable tooth template with identical preparation dimensions. Percentage of maximum DC and KHN were determined. Mechanical properties were tested in resin composite bars having similar KHN values as the resin composite at the gingival margins. RESULTS Energy density had a significant effect on gingival marginal defects as determined by ridit analysis but not on microleakage. Water had a significant dissolving effect on gingival margin integrity at very low degrees of conversion and energy densities (4000 mJ/cm2). There was no overall significant effect of thermal-mechanical stressing on gingival marginal defects or microleakage. SIGNIFICANCE Based on ridit analysis, a recommended lower limit of gingival margin acceptability in the bulk-filled Z250 resin composite restoration was created by 80% of maximum conversion, 73% of maximum hardness and approximately 70% of maximum flexural strength and modulus in the gingival marginal area.

Comparison of CaOH with MTA for Direct Pulp Capping A PBRN Randomized Clinical Trial

This practice-based, randomized clinical trial evaluated and compared the success of direct pulp capping in permanent teeth with MTA (mineral trioxide aggregate) or CaOH (calcium hydroxide). Thirty-five practices in Northwest PRECEDENT were randomized to perform direct pulp caps with either CaOH (16 practices) or MTA (19 practices). Three hundred seventy-six individuals received a direct pulp cap with CaOH (n = 181) or MTA (n = 195). They were followed for up to 2 yrs at regular recall appointments, or as dictated by tooth symptoms. The primary outcomes were the need for extraction or root canal therapy. Teeth were also evaluated for pulp vitality, and radiographs were taken at the dentist’s discretion. The probability of failure at 24 mos was 31.5% for CaOH vs. 19.7% for MTA (permutation log-rank test, p = .046). This large randomized clinical trial provided confirmatory evidence for a superior performance with MTA as a direct pulp-capping agent as compared with CaOH when evaluated in a practice-based research network for up to 2 yrs (ClinicalTrials.gov NCT00812887).

Polymerization stress - Is it clinically meaningful?

OBJECTIVES The objective of this article is to discuss the evidence for polymerization shrinkage and shrinkage stress of dental composite restoratives in terms of its potential relevance to the clinical situation METHODS Articles relating to the issue of polymerization contraction stress generation in dental composite materials, and the factors that influence it, were reviewed and included. Particular attention was paid to evidence derived from clinical studies. Articles were identified through PubMed and through the bibliographies of other articles. RESULTS There is extensive evidence for the presence of polymerization contraction stress in dental composites, as well as evidence for its deleterious effects, which include marginal leakage, gap formation, cuspal deflection, tooth cracking, reduced bond strength and lowered mechanical properties of the restorative. There is little, if any, direct evidence for the clinical effect of these contraction stresses. No study has directly established a link between these stresses and enhanced postoperative sensitivity or recurrent caries, for example. However, the concern over these stresses and the manner in which they influence the placement of current composite materials demonstrates that they are considered to be very important. CONCLUSION Though no direct evidence exists to prove that the generation of contraction stress in dental composite restorations causes reduced clinical longevity, the indirect evidence from numerous in vitro studies and the concern over controlling their effects proves that they are clinically relevant.

Cracked Teeth: A Review of the Literature

Although cracked teeth are a common problem for patients and dentists, there is a dearth of evidence-based guidelines on how to prevent, diagnose, and treat cracks in teeth. The purpose of this article is to review the literature to establish what evidence exists regarding the risk factors for cracked teeth and their prevention, diagnosis, and treatment.

Bioactive glass fillers reduce bacterial penetration into marginal gaps for composite restorations

OBJECTIVE Bioactive glass (BAG) is known to possess antimicrobial and remineralizing properties; however, the use of BAG as a filler for resin based composite restorations to slow recurrent caries has not been studied. Accordingly, the objective of this study was to investigate the effect of adding 15wt% BAG to a resin composite on bacterial biofilms penetrating into marginal gaps of simulated tooth fillings in vitro during cyclic mechanical loading. METHODS Human molars were machined into approximately 3mm thick disks of dentin and 1.5-2mm deep composite restorations were placed. A narrow 15-20 micrometer wide dentin-composite gap was allowed to form along half of the margin by not applying dental adhesive to that region. Two different 72wt% filled composites were used, one with 15wt% BAG filler (15BAG) and the balance silanated strontium glass and one filled with aerosol silica and silanated strontium glass without BAG (0BAG-control). Samples of both groups had Streptococcus mutans biofilms grown on the surface and were tested inside a bioreactor for two weeks while subjected to periods of cyclic mechanical loading. After post-test biofilm viability was confirmed, each specimen was fixed in glutaraldehyde, gram positive stained, mounted in resin and cross-sectioned to reveal the gap profile. Depth of biofilm penetration for 0BAG and 15BAG was quantified as the fraction of gap depth. The data were compared using a Students t-test. RESULTS The average depth of bacterial penetration into the marginal gap for the 15BAG samples was significantly smaller (∼61%) in comparison to 0BAG, where 100% penetration was observed for all samples with the biofilm penetrating underneath of the restoration in some cases. SIGNIFICANCE BAG containing resin dental composites reduce biofilm penetration into marginal gaps of simulated tooth restorations. This suggests BAG containing composites may have the potential to slow the development and propagation of secondary tooth decay at restoration margins.

Tooth wear: prevalence and associated factors in general practice patients

OBJECTIVES To estimate the prevalence of tooth wear and to investigate factors associated with tooth wear in patients from general practices in the Northwest United States. METHODS Data on the diagnosis and treatment of oral diseases during the previous year were collected in a survey with a systematic random sample of patients (n= 1530) visiting general dentists from the Northwest Practice-based REsearch Collaborative in Evidence-based DENTistry (PRECEDENT) (n=80). Prevalence ratios (PRs) of moderate to severe occlusal and incisal tooth wear by patient characteristics were estimated using cluster-adjusted multiple binomial regression for adults (18+ years) and children/adolescents (3-17 years). RESULTS For adults, the mean number of teeth with wear facets was 5.4 [95% confidence interval (CI) =4.6-6.2] and 51% of the adults had four or more teeth with wear. Participants 45-64 and 65+ years old were 1.3 (95% CI=1.1-1.6) and 1.4 (95% CI=1.1-1.8) times as likely to have 4+ teeth with moderate to severe wear facets as participants 18-44 years old. Adult males had a 20% (PR=1.2; 95% CI=1.1-1.4) higher prevalence of wear than adult females. Adults who were using, or had ever used occlusal splints had higher prevalence of tooth wear compared to those who never used such appliances (PR=1.3; 95% CI=1.0-1.5). Adults with any periodontal bone loss also had a 20% higher prevalence of wear than adults without periodontal disease (PR=1.2; 95% CI=1.0-1.4). For children/adolescents, the mean number of teeth with moderate to severe wear facets was 1.6 (95% CI=0.9-2.6) and 31% of the children had one or more teeth with wear facets. The adjusted prevalence ratio of tooth wear (1+ teeth with wear facets) for boys was 1.6 times as high (95% CI=1.1-2.4) as compared with girls. The prevalence of wear for children 12+ years old was 50% (PR=0.5; 95% CI=0.3-0.8) lower than that of children <12 years old. Angles class II was associated with higher tooth wear prevalence (PR= 1.8; 95% CI=1.3-2.6) than class I. Children with posterior or anterior open bite had lower prevalence of wear than their counterparts (PR=0.6; 95% CI=0.3-1.0). No associations were observed between tooth wear and orthodontic treatment, missing teeth, and race/ethnicity. CONCLUSION Tooth wear is a prevalent condition in this population. Among adults, higher prevalences of tooth wear were observed among those who were older, males, had used occlusal splints and had periodontal disease. Among children, higher prevalences were associated with younger age, male gender, class II malocclusion and the absence of open bite. Submitted on behalf of the Northwest PRECEDENT network, with support from NIDCR grants DE016750 and DE016752.

Cyclic mechanical loading promotes bacterial penetration along composite restoration marginal gaps

OBJECTIVES Secondary caries is the most common reason for composite restoration replacement and usually forms between dentin and the filling. The objective of this study was to investigate the combined effect of cyclic loading and bacterial exposure on bacterial penetration into gaps at the interface between dentin and resin composite restorative material using a novel bioreactor system and test specimen design. METHODS Human molars were machined into 3mm thick disks with 2mm deep × 5 mm diameter cavity preparations into which composite restorations were placed. A ∼ 15-30 μm (small) or ∼ 300 μm wide (large) marginal gap was introduced along half of the interface between the dentin and restoration. Streptococcus mutans UA 159 biofilms were grown on each sample prior to testing each in a bioreactor both with and without cyclic loading. Both groups of samples were tested for 2 weeks and post-test biofilm viability was confirmed with a live-dead assay. Samples were fixed, mounted and cross-sectioned to reveal the gaps and observe the depth of bacterial penetration. RESULTS It was shown that for large gap samples the bacteria easily penetrated to the full depth of the gap independent of loading or non-loading conditions. The results for all cyclically loaded small gap samples show a consistently deep bacterial penetration down 100% of the gap while the average penetration depth was only 67% for the non-loaded samples with only two of six samples reaching 100%. SIGNIFICANCE A new bioreactor was developed that allows combining cyclic mechanical loading and bacterial exposure of restored teeth for bacterial biofilm and demineralization studies. Cyclic loading was shown to aid bacterial penetration into narrow marginal gaps, which could ultimately promote secondary caries formation.

Mechanical performance of novel bioactive glass containing dental restorative composites

OBJECTIVES Bioactive glass (BAG) is known to possess antimicrobial properties and release ions needed for remineralization of tooth tissue, and therefore may be a strategic additive for dental restorative materials. The objective of this study was to develop BAG containing dental restorative composites with adequate mechanical properties comparable to successful commercially available composites, and to confirm the stability of these materials when exposed to a biologically challenging environment. METHODS Composites with 72 wt% total filler content were prepared while substituting 0-15% of the filler with ground BAG. Flexural strength, fracture toughness, and fatigue crack growth tests were performed after several different soaking treatments: 24h in DI water (all experiments), two months in brain-heart infusion (BHI) media+Streptococcus mutans bacteria (all experiments) and two months in BHI media (only for flexural strength). Mechanical properties of new BAG composites were compared along with the commercial composite Heliomolar by two-way ANOVA and Tukeys multiple comparison test (p≤0.05). RESULTS Flexural strength, fracture toughness, and fatigue crack growth resistance for the BAG containing composites were unaffected by increasing BAG content up to 15% and were superior to Heliomolar after all post cure treatments. The flexural strength of the BAG composites was unaffected by two months exposure to aqueous media and a bacterial challenge, while some decreases in fracture toughness and fatigue resistance were observed. The favorable mechanical properties compared to Heliomolar were attributed to higher filler content and a microstructure morphology that better promoted the toughening mechanisms of crack deflection and bridging. SIGNIFICANCE Overall, the BAG containing composites developed in this study demonstrated adequate and stable mechanical properties relative to three successful commercial composites.

Detection of dentinal cracks using contrast-enhanced micro-computed tomography

A new technique using contrast enhanced micro-computed tomography (micro-CT) was developed to improve the ability to detect dentinal cracks in teeth and assess associated risks to oral health. Extracted, whole human molars that exhibited visual evidence of external cracks following extraction and machined, partially fractured elephant dentin specimens were labeled by BaSO(4) precipitation and imaged by micro-CT. Contrast-enhanced micro-CT was demonstrated in vitro to enable non-destructive, 3-D imaging of the presence, morphology and spatial location of dentinal cracks in whole human molars and machined specimens. BaSO(4) staining provided enhanced contrast for the detection of cracks that could not be detected prior to staining. Backscattered SEM micrographs showed that BaSO(4) was precipitated on the surfaces of dentinal cracks and within adjacent tubules. The new methods demonstrated in this study are expected to be useful for clinical and scientific studies investigating the etiology and treatment of dentinal cracks in teeth.

Academy of Dental Materials guidance—Resin composites: Part I—Mechanical properties

OBJECTIVE The objective of this project, which was initiated from the Academy of Dental Materials, was to review and critically appraise methods to determine fracture, deformation and wear resistance of dental resin composites, in an attempt to provide guidance for investigators endeavoring to study these properties for these materials. METHODS Test methods have been ranked in the priority of the specific property being tested, as well as of the specific test methods for evaluating that property. Focus was placed on the tests that are considered to be of the highest priority in terms of being the most useful, applicable, supported by the literature, and which show a correlation with clinical findings. Others are mentioned briefly for the purpose of being inclusive. When a standard test method exists, including those used in other fields, these have been identified in the beginning of each section. Also, some examples from the resin composite literature are included for each test method. RESULTS The properties for evaluating resin composites were ranked in the priority of measurement as following: (1) Strength, Elastic Modulus, Fracture toughness, Fatigue, Indentation Hardness, Wear-abrasion (third body) and Wear-attrition (contact/two body), (2) Toughness, Edge strength (chipping) and (3) Wear determined by toothbrush. SIGNIFICANCE The following guidance is meant to aid the researcher in choosing the proper method to assess key properties of dental resin composites with regard to their fracture, deformation and wear resistance.