Biomedical subjects

W H Douglas

Publications and source records attributed to W H Douglas.

At least 19 recordsLinked to original sources

PubMed2001

Nondestructive, in vitro quantification of crown margins.

STATEMENT OF PROBLEM: It is important that artificial crowns fit the prepared tooth accurately, as marginal deficiencies are predisposed to plaque accumulation and lead to increased risk of periodontal disease. Various methods of evaluation for marginal fit are described in the literature, but most approaches are limited by destructive methods of assessment and/or small points of measurement. PURPOSE: This study compared, in vitro, the marginal fit of 4 types of complete crowns on human premolar teeth with the use of nondestructive profilometry. This method determined whether fit was influenced by type of crown or surface morphology of the tooth, namely, grooved or ungrooved surfaces. MATERIAL AND METHODS: Four groups of specimens were prepared for complete crowns: group BA, bonding alloy with chamfer finish line; group G, gold alloy with chamfer finish line; group PC, porcelain with a chamfer finish line; and group PS, porcelain with a shoulder finish line. Two profiles of grooved mesial and ungrooved distal surfaces of the teeth were performed: (1) teeth prepared for each type of crown and (2) teeth with crowns seated but not cemented. Marginal fit (absolute marginal discrepancy) from the finish line edge of the tooth preparations to crown edges (CE) and leading edges (LE) of crowns were measured. RESULTS: A 2-way analysis of variance for crown type and tooth surface morphology revealed significant differences between crown types for all measurement parameters, except vertical LE. The effect of surface morphology was not significant, except for vertical LE (P<.05). For all parameters, except vertical LE, the ranking of marginal fit discrepancies from greatest to least was as follows: group PC, G, BA, and PS. For vertical LE distances, the ranking was PS, BA, G, and PC (P<.05). CONCLUSION: Profilometry was used as a nondestructive, accurate method of evaluating the absolute marginal fit of different types of crowns. Marginal fits varied continuously around the circumference of each crown and made clinical assessment of fit accuracy subjective and arduous.

Analysis of Variance↗

PubMed2001

Factors influencing optical 3D scanning of vinyl polysiloxane impression materials.

PURPOSE: Future growth in dental practice lies in digital imaging enhancing many chairside procedures and functions. This revolution requires the fast, accurate, and 3D digitizing of clinical records. One such clinical record is the chairside impression. This study investigated how surface angle and surface roughness affect the digitizing of vinyl polysiloxane impression materials. MATERIALS AND METHODS: Seventeen vinyl polysiloxane impression materials were digitized with a white light optical digitizing system. Each sample was digitized at 3 different angles: 0 degrees, 22.5 degrees, and 45 degrees, and 2 digitizer camera f-stops. The digitized images were rendered on a computer monitor using custom software developed under NIH/NIDCR grant DE12225. All the 3D images were rotated to the 0 degrees position, cropped using Corel Photo-Paint 8 (Corel Corp, Ottawa, Ontario, Canada), then saved in the TIFF file format. The impression material area that was successfully digitized was calculated as a percentage of the total sample area, using Optimas 5.22 image processing software (Media Cybernetics, LP, Silver Spring, MD). The dependent variable was a Performance Value calculated for each material by averaging the percentage of area that digitized over the 3 angles. New samples with smooth and rough surfaces were made using the 7 impression materials with the largest Performance Values. These samples were tested as before, but with the additional angle of 60 degrees. Silky-Rock die stone (Whip Mix Corp, Louisville, KY) was used as a control. RESULTS: The Performance Values for the 17 impression materials ranged from 0% to 100%. The Performance Values for the 7 best materials were equivalent to the control at f/11 out to a surface angle of 45 degrees; however, only Examix impression material (GC America Inc, Alsip, IL) was equivalent to the control at f/11/\16. At the 60 degrees surface angle with f/11/\16, the Performance Values were 0% for all the impression materials, whereas that for the control was 90%. The difference in the Performance Values for the smooth and rough surface textures was 7%, which was not significant. CONCLUSIONS: The digitizing performance of vinyl polysiloxane impression materials is highly material and surface angle-dependent and is significantly lower than the die stone control when angles to 60 degrees are included. It is affected to a lesser extent by surface texture.

Analysis of Variance↗

PubMed2000

In vitro inhibition of enamel demineralization by a polymerizable amphiphilic film.

An amphiphilic coating is configured as a substantive film that has a tendency for an in-plane two-dimensional polymerization. This coating is hypothesized to protect enamel from in vitro acid decalcification, assessed through the following artificial caries model. Three regions on labial enamel of eight bovine incisors were treated with an acid resistant varnish (A), the amphiphilic coating (B), or left undisturbed (C), and the teeth were immersed for 3 wk in lactic acid gel. Mineral loss (deltaZ-value) was determined by a cross-sectional microhardness technique. deltaZ-values (mean +/- SD; volume percent mineral-microm) were: -4 +/- 24 (A), 29 +/- 69 (B), and 7,372 +/- 1,766 (C). deltaZ-value of the uncoated enamel (C) was significantly different from the other groups. Scanning electron microscopy showed enamel etched pattern from citric acid, and the coating firmly attached on enamel surface. This amphiphilic coating can inhibit enamel decalcification under the present experimental condition.

Acrylates↗

PubMed2000

Atherosclerotic plaque rupture: a fatigue process?

The mechanism of atherosclerotic plaque rupture is not known. Current theories focus on the acute triggers of plaque rupture and myocardial infarction such as increased shear or circumferential stress, rupture of the vasa vasorum and vasospasm. We hypothesize that a critical mechanism causing plaque rupture is fatigue failure, the catastrophic rupture of a material following exposure to high-cycle, low-amplitude repetitive stress. Comparisons between material fatigue and plaque rupture demonstrate that this hypothesis is consistent with known physiologic and epidemiologic data on plaque rupture.

Arteriosclerosis↗

PubMed2000

Correlation of noncarious cervical lesion size and occlusal wear in a single adult over a 14-year time span.

STATEMENT OF PROBLEM: Noncarious cervical lesions are described as having a multifactorial cause, with occlusal trauma and toothbrush abrasion frequently mentioned as major factors. Finite element modeling studies have demonstrated a relocalization of occlusal stresses to the cervical area due to flexure of the crown. This may cause microcracking, especially under tensile stresses, that will lead to a loss of enamel and dentin in the cervical region. Clinical confirmation of an occlusal cause for noncarious cervical lesions has been difficult to obtain. PURPOSE: This study investigated whether occlusal wear was correlated with an increase in the size of noncarious cervical lesions. MATERIAL AND METHODS: Loss of contour at occlusal and cervical sites on 3 teeth of a single individual was measured using digital and visualization techniques at 3 time intervals over a 14-year time span. The 1983 baseline casts and 1991, 1994, and 1997 clinical impressions of a single adult patient with existing noncarious cervical lesions were replicated in epoxy. Surfaces of all replicas were digitized with a contact digitizing system. Sequential digitized surfaces were fit together and analyzed using AnSur-NT surface analysis software. Clinical losses of surface contour by volume and depth of the left mandibular first molar and first and second premolars were recorded. RESULTS: Nine measurements of cervical volume loss (range 0.9 to 11.5 mm(3)) and 9 corresponding measurements of occlusal volume loss (range 0.39 to 7.79 mm(3)) were made. The correlation between occlusal and cervical volume loss was strong (r(2)=0.98) and significant (P<.0001). CONCLUSION: For the single adult patient in this study, there was a direct correlation between occlusal wear and the growth of noncarious cervical lesions.

Adult↗

PubMed2000

Nominal shear or fracture mechanics in the assessment of composite-dentin adhesion?

This study addresses the anticipated problem of discriminating among high-performing dentin adhesives. The simplicity of the nominal shear bond test, despite being heavily criticized, has made it a routine procedure for the determination of bonding efficacy. A fracture mechanics approach has been suggested as a better assessment of bonding efficacy (Versluis et al., 1997). However, experimental complexity is a major limitation. It is hypothesized that a new, simplified interfacial fracture toughness test (Lin, 1994) will evaluate bonding agents differently if compared with the traditional shear bond test. Therefore, the objective of this study was to compare the performances of six dentin bonding agents subjected to the interfacial fracture toughness test (critical plane strain energy release rate) or to the nominal shear bond test (shear bond strength). Their performances were also characterized by scanning electron micrography of the fracture surfaces for evidence of dentin cohesive failure. Statistical analyses showed only marginal differences between these determinants of the two tests. However, when the analysis was applied only to the materials that had 100% frequency of dentin cohesive failure in shear testing, which also had high bonding efficacy, the difference in adhesive strengths between the two tests became significant. The reliability of the nominal shear test is questioned when dentin cohesive failure occurs, which usually is associated with high bonding efficacy. Since it is expected that bonding efficacy will increase further, the interfacial fracture toughness test is the preferred methodology to distinguish among high-performing dentin adhesives.

Adhesiveness↗

PubMed2000

Optical scattering power for characterization of mineral loss.

Mineral loss in early caries cannot be measured without invasive procedures. To quantify mineral loss without sectioning the tooth, one must determine the optical scattering of the enamel. Using enamel white-spot lesions, we hypothesize that the optical scattering power (Sp) of the demineralized enamel would provide a quantitative estimate of mineral loss. Enamel slabs were demineralized to produce artificial white spots. The data were acquired by means of a Charge-Coupled Device (CCD) camera and image-processing software. For the purpose of comparison, mineral loss (deltaZ) of the demineralized samples was determined by the use of a microhardness approach after the samples were sectioned. The scattering power correlated well with deltaZ (r2 = 0.82). In contrast, simple reflectance of the demineralized samples correlated poorly with deltaZ (r2 = 0.22). The validity of using scattering power to measure demineralization has been confirmed by a three-dimensional Monte Carlo Simulation.

Animals↗

PubMed2000

Dentine bonding systems: 1. Mode of action.

The achievement of a reliable bond to dentine has been more difficult than bonding to enamel, due principally to the anatomy of the dentine substrate. The mechanisms of bonding are now much better understood than a decade ago and clinically-viable bond strengths to dentine are now achieved with many dentine bonding systems.

Acid Etching, Dental↗

PubMed2000

Dentine bonding systems: 2. Clinical uses.

As the reliability of dentine bonding agents has improved, the clinical uses of these systems has increased and now include the bonding of directly and indirectly constructed restorations, amalgam bonding and treatment of dentinal sensitivity. Areas of potential use which require further evaluation include pulp capping and the attachment of fractured tooth fragments.

Adhesives↗

PubMed2000

Cumulative effects of successive restorative procedures on anterior crown flexure: intact versus veneered incisors.

OBJECTIVE: When successive restorative procedures (e.g., porcelain veneers, interdental resin composite restorations, and endodontic treatment) are carried out on the same tooth, significant effects on crown flexure can be expected. METHOD AND MATERIALS: Dentin-bonded porcelain veneers (experimental group) were assessed in vitro using functional and cyclic thermal loads. They were compared to natural teeth (control group) with respect to 2 parameters: coronal flexure (investigated using experimental strain gauges) and morphology of the tooth-restoration interface (scanning electron microscopic evaluation). For both veneered and natural teeth, crown deformation was recorded at 5 sequential experimental steps: intact tooth (baseline), Class III cavities, Class III resin composite restorations, endodontic treatment, and endodontic restoration (without posts). RESULTS: No significant differences in crown flexure were found between natural and veneered incisors when compared across experimental steps. The main effect for experimental steps was highly significant. When averaged across all specimens (natural and veneered teeth), the endodontic treatment step resulted in the highest crown flexure (1.55x the baseline value). The unrestored Class III cavities and the endodontic restoration were next highest (1.30x and 1.28x the baseline value, respectively). The lowest crown flexures were found after restoration of the Class III cavities (1.13x the baseline value). No measurable microleakage or gaps were detected at the ceramic-resin, resin-enamel, or resin-dentin interfaces (Optibond FL, Kerr). CONCLUSION: Each subsequent reduction in tooth structure resulted in a substantial increase in crown flexibility, even after restoration. Endodontic procedures were responsible for most of the loss in crown stiffness. Extensive proximal cutting and restorations seemed to minimally affect crown flexure. Porcelain veneers showed perfect biomimetic behavior, because cumulated restoration procedures had the same effect on natural and veneered incisors.

Analysis of Variance↗

PubMed1999

Retention of adhesive cement on the tooth surface after crown cementation.

STATEMENT OF PROBLEM: Adhesive cements increase crown retention, but it is unknown if traces of cement remain undetected on the tooth surface after clinical removal of excess cement, which could exacerbate plaque retention. PURPOSE: This study measured the surface area, volume, mean depth, and maximum depth of a resin composite and a compomer luting cement left adherent on the tooth surface after removal of excess cement, as judged clinically. METHODS AND MATERIAL: Four groups of specimens (n = 48) were prepared for full coverage crowns: group AC bonding alloy with chamfer finish line, group G gold alloy with chamfer finish line, group PC porcelain with a chamfer finish line, and group PS porcelain with a shoulder finish line. Two profiles of the mesial and distal surfaces of the teeth were carried out: (1) tooth with crown seated but not cemented and (2) tooth with the crown cemented in place. Two cements and 2 methods of cement removal were studied. RESULTS: A 4-way analysis of variance for cement, crown type, method of removal, and tooth surface morphology showed that significantly greater volumes and mean depth, but not surface areas, of resin composite cement remained adherent than compomer cement (P<.05). Among crown types, significant differences were found for cement volume (group G>AC, G>PC, G>PS), cement surface area (group AC>PC, G>PC, G>PS), and maximum cement depth (group G>AC). There was no significant difference between the 2 methods of cement removal. Significantly larger surface areas and maximum depths of cement were retained on the anatomically grooved mesial surface of the maxillary first premolars than on the ungrooved distal surface. CONCLUSION: Subclinical cement retention occurred after crown cementation, which was influenced by cement, crown type, and tooth surface morphology but not method of cement removal.

Adhesives↗

PubMed1999

Crack propensity of porcelain laminate veneers: A simulated operatory evaluation.

STATEMENT OF PROBLEM: Anterior teeth are especially subject to the thermal variations of ingested food and drinks. Postoperative cracks of porcelain laminates are considered a possible consequence of polymerization shrinkage, function, and thermocycling. PURPOSE: This investigation was conducted to define the parameters associated with the development of cracks in porcelain veneers using cyclic thermal fatigue. MATERIAL AND METHODS: Twenty-seven maxillary incisors were restored with porcelain laminate veneers and subjected to thermocycling (5 degrees C to 50 degrees C) for 1000 cycles. Ceramic cracks were reported for 11 of the 27 specimens. Teeth were sectioned and prepared for SEM analysis. Measurements of the ceramic and the luting composite thicknesses were performed for each specimen at different restoration locations (facial, incisal, and proximal). RESULTS: No significant differences in the ceramic or the luting composite thicknesses were observed between cracked and uncracked specimens. However, significant differences were observed in the ratio of the ceramic and luting composite thicknesses. Most cracked samples exhibited a ratio at the facial location below 3.0 (2.6 +/- 0.35), whereas most noncracked specimens were above this value (3.9 +/- 0.19). Incisal and especially proximal measurements alone were not significantly different between cracked versus uncracked specimens. Ceramic was slightly thinner in the facial aspect than in the proximal aspect, which was also thinner than the incisal aspect. Composite in the facial aspect was thinner in the cervical area than in the incisal third of the tooth. CONCLUSIONS: Significant cyclic temperature changes can induce the development of flaws in porcelain veneers. Control of tooth reduction and the application of die spacers during laboratory procedures undoubtedly represent key elements; a sufficient and even thickness of ceramic combined with a minimal thickness of luting composite will provide the restoration with a favorable configuration with regard to crack propensity, namely, a ceramic and luting composite thickness ratio above 3.

Analysis of Variance↗

PubMed1999

Effect of luting composite shrinkage and thermal loads on the stress distribution in porcelain laminate veneers.

STATEMENT OF PROBLEM: Cyclic thermal fatigue has demonstrated a significant influence of the thicknesses of luting composite and ceramic in crack propensity of porcelain laminates. PURPOSE: This study was conducted to define potentially involved parameters for crack development in porcelain laminates bonded to teeth. Finite element modeling was used to evaluate the respective effects of luting composite shrinkage and significant thermal changes. MATERIAL AND METHODS: A buccolingual cross-section of a maxillary incisor was digitized and used as a template to generate a single 2-dimensional mesh, including all the different restorative designs. Luting composite shrinkage was simulated at a baseline temperature of 20 degrees C. The effect of thermal loads from 20 degrees C to 5 degrees C and from 20 degrees C to 50 degrees C was assessed with and without preexisting composite shrinkage. RESULTS: Shrinkage of the luting composite alone generated important compressive forces on the ceramic, either at the restoration surface or interface. Compression intensity was related to geometry and ratio of thicknesses between the ceramic and luting composite (CER/CPR). Lower ratios produced higher compression forces in the ceramic. When thermal loads were combined to the composite shrinking forces, the stress pattern was significantly changed only for the experimental conditions with the lowest CER/CPR ratio. Temperature increase reduced compressive stresses and exacerbated tensile stresses. Thermal loads were simulated alone (situation of an "ideal nonshrinking" luting composite) and generated mainly tensile stresses in the ceramic, which intensity was again modulated by the CER/CPR ratio and the local geometry of composite and ceramic. Because of ceramic brittleness, these tensile forces were more detrimental than the high compression created by composite shrinkage alone. The stress pattern was not influenced by the incisal length of the veneer but rather by the facial thickness of ceramic. The worst record made with a shrinking luting agent (500 microm of luting composite, lowest CER/CPR ratio, 5 degrees C) was much less harmful than the worst record made with a hypothetical "nonshrinking" luting material. CONCLUSIONS: The ratio of the thickness of cement and luting composite appears to have a relevant influence on the stress distribution in porcelain laminates. Restorations that are too thin, combined with poor internal fit, resulted in higher stresses at both the surface and interface of the restoration. Because of its precompressed state given by composite shrinkage, ceramics performed better with regard to temperature-induced tensile forces.

Ceramics↗

PubMed1999

Rationalization of incisor shape: experimental-numerical analysis.

STATEMENT OF PROBLEM: Moving from the posterior segment in the anterior direction within the dental arch, the process of "incisivization" takes place. The occlusal table is gradually replaced by an incisal edge that has the function of cutting. PURPOSE: This study considers these genetically controlled changes by using strain gauge measurements and finite element analyses to rationalize the clinical and biologic advantages of incisal form. A direct clinical link in the common esthetic procedure of anterior veneering is expected. MATERIAL AND METHODS: Six maxillary incisors were mounted in a positioning device and equipped with 2 strain gauges bonded to the palatal surface: gauge 1 (G1) in the concavity and gauge 2 (G2) on the cingulum. A 50 N load was applied on the palatal side of the incisal edge, perpendicular to the long axis of the tooth. Displacement of the load tip and the palatal strain were recorded after successively removing one third, two thirds, and the total thickness of the facial enamel. The same experiment was reproduced with the finite element method (FEM). Four additional experimental designs were tested with the FEM by simulating the progressive thinning and elimination of palatal enamel and a thickened palatal lobe. Surface tangential stresses and local strain in the area corresponding to gauges 1 and 2 were calculated from the postprocessing files. RESULTS: The FEM was validated by experimental results considering both displacement of the load tip ( approximately 120 +/- 30 microm) and tangential surface strain at G1/G2. Recorded strains were always higher in the concavity when compared with the cingulum; high tensile strains were recorded at G1 after the total removal of the facial enamel. The entire facial surface was submitted to compressive forces. Subsequent compressive stresses were higher ( approximately 150 MPa) when facial enamel was thin or when the palatal enamel was removed. However, their absolute value never reached the elevated and potentially harmful tensile stresses measured in the palatal concavity, especially in the absence of facial enamel (272 MPa). Multiple experimental cracks were generated in the remaining palatal enamel as a consequence of stress redistribution. However, smooth and convex surfaces with local enamel bulk such as the cingulum, the marginal ridges, and the facial cervical third of the anatomic crown showed the lowest stress level. The optimal configuration with regard to the stress pattern was given by the modified natural tooth that exhibited thick palatal enamel and a mostly convex palatal surface. CONCLUSIONS: Palatal concavity that provides the incisor with its sharp incisal edge and cutting ability proved to be an area of stress concentration. This shortcoming can be compensated by specific areas that feature thick enamel such as the cingulum and the marginal ridges. When enamel is worn or removed from the facial surface, its replacement should be carried out by using materials with properties similar to enamel to restore the original biomechanical behavior of the tooth.

Compliance↗

PubMed1999

Fracture toughness of conventional, resin-modified glass-ionomer and composite luting cements.

OBJECTIVES: This study was conducted to determine if significant differences existed between the fracture toughness of three types of luting cement, and, if the method of mixing conventional glass-ionomer luting cements, hand-mixed or mechanically mixed, influenced the value obtained. METHODS: Three types of luting cement were investigated: conventional glass-ionomer cement (two handmixed and two capsulated cements, KetacCem, Fuji I and KetacCem Maxicap, Fuji Cap I), a resin-modified glass-ionomer cement (Vitremer Luting Cement) and a resin composite cement (Scotchbond Resin Cement). Eleven specimens of each of the six cements were fabricated to determine the plane strain fracture toughness using the chevron notch short rod technique. After seven days the specimens were loaded in a water bath, at a crosshead speed of 4 microns/s and the fracture toughness values calculated. RESULTS: ANOVA indicated significant differences between the cements (p < 0.0001) and each cement was compared with all others using Fishers PSLD test (p < 0.05). The rank order of results from highest fracture toughness value to lowest (mean +/- s.d.) was Scotchbond Resin Cement (1.31 +/- 0.17), Vitremer Luting Cement (1.08 +/- 0.1), Fuji Cap I (0.37 +/- 0.04), KetacCem Maxicap (0.37 +/- 0.05), Fuji I (0.34 +/- 0.04), KetacCem (0.27 +/- 0.03). SIGNIFICANCE: Of the cements tested, the resin composite cement is most likely to resist clinical failure by cement cohesive failure.

Analysis of Variance↗

PubMed1999

Comparison of two measurement techniques for clinical wear.

OBJECTIVES: Clinical wear of restorations is generally evaluated by marginal integrity over time. In this study, both a subjective and an objective method for wear assessment are compared, and the relative advantages and disadvantages of each are considered. METHODS: A surface evaluation technique for quantitative measurement of wear, as developed at the University of Minnesota (UMN) has been compared to the commonly employed method of wear assessment used by Leinfelder (LF). Measurements were made by comparing suitable casts of restored teeth before and after clinical function. Semi-quantitative wear assessment was studied in comparison with detailed quantitative information about the topology of the pre- and post-wear occlusal surfaces obtained from stylus profilometry and processed by imaging techniques. Fourteen model sets of baseline, 2, 3 and 5 year old Class II composite restorations from a Danish clinical trial were evaluated using both techniques. RESULTS: In general, after 5 years of function, the digitizing method (UMN) generated wear values that were twice as high, indicating that general wear of restorative materials is underestimated by the LF-method. CONCLUSIONS: The comparison clearly revealed the advantages and limitations of each technique. Evaluation of wear by means of the LF technique provides global semi-quantitative data on restoration margins relative to enamel, underestimating general wear of the restorative material. Advanced 3-D measuring techniques such as the UMN method provide extensive quantitative data regarding wear patterns of the entire occlusal surface, i.e. restoration and enamel. Such a highly accurate technique is capable of differentiation between wear behavior of restorative materials early on in clinical studies. In addition, through its digital alignment procedure, the UMN method provides data on accuracy of the replication process used in clinical studies.

Analysis of Variance↗

PubMed1999

Rationalization of esthetic restorative dentistry based on biomimetics.

UNLABELLED: The exponential progressivism that characterizes the current decade often comes with substantial financial implications. Dental care is not spared by this phenomenon. However, new generations of concepts emerging from biomimetics provide the operator with the ability to restore the biomechanical, structural, and esthetic integrity of teeth. The development of adhesion and the evolution of porcelain veneers constitute striking examples of this nascent process. Indications for bonding porcelain are extending to more perilous situations (crown-fractured incisors, nonvital teeth), resulting in considerable improvements, comprising both the medical-biologic aspect (economy of sound tissues and maintenance of tooth vitality) and the socioeconomical context (decrease of costs compared to traditional and more invasive prosthetic treatments). CLINICAL SIGNIFICANCE: In the bonded porcelain veneer and its extensions, restorative dentistry has found new solutions for the anterior segment that balance the need for functional and esthetic reconstruction. The optimal stiffness of porcelain in thin section, the ideal surface characteristics, and the biomechanical continuum achieved through high performance bonding mean the crown of the tooth as a whole can support incisal or masticatory function. By the same token, the conduction of optical effects from within the tooth combined with the ideal surface features of the porcelain veneer make this restorative approach the ultimate in esthetic satisfaction, for both the practitioner and the patient.

Cuspid↗

PubMed1999

Design optimization and evolution of bonded ceramics for the anterior dentition: a finite-element analysis.

OBJECTIVE: Finite-element method was used to explore the stress distribution of incisors restored with porcelain veneers. The design of the incisal palatal finish line was analyzed as a function of incisal overlap and initial tooth substance loss (coronal fractures). METHOD AND MATERIALS: The treatment of intact and fractured incisors was investigated using 8 different designs of porcelain veneer. The palatal finish line varied from butt margins to extended chamfers. The stress distribution was assessed in a 2-dimensional finite-element model, reproducing a buccolingual cross section of an incisor. A palatal 50-N horizontal force was applied to the incisal edge to simulate an extreme functional load. The palatal surface tangential stresses were calculated. RESULTS: Considerable differences were detected in the stresses at the level of the incisal-palatal restoration margin. The margins of restorations with limited incisal overlap (butt margin or minichamfer) showed low tensile stresses or even compressive stresses. Restorations with a long chamfer extending into the palatal concavity were subjected to the highest tensile stresses. In the presence of moderate crown fractures (incisal one third) or severe wear, butt margins limited the palatal extension of ceramic, thus reducing the amount of stress at the restoration interface. In the presence of severe crown fractures (incisal two thirds), the margins (either butt or chamfered) were subjected to low tensile forces when located in the smooth convex area of the cingulum. CONCLUSION: Because of the geometry and natural elastic modulus of mineralized tooth structures, a concentration of tensile stresses is formed at the palatal concavity of teeth restored with porcelain veneers. Long chamfers extending into the palatal concavity are unfavorable because thin extensions of ceramic are generated in an area of maximum tensile stresses. Minichamfers or butt margins are generally recommended, especially in the presence of moderate crown fractures or severe wear.

Compressive Strength↗