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Biomedical subjects

A Versluis

Publications and source records attributed to A Versluis.

At least 19 recordsLinked to original sources
PubMed2006

How does canal taper affect root stresses?

AIM: To examine the effect of specific tapers on root stresses and thus vertical root fracture. METHODOLOGY: The effect of taper on root stresses was calculated during simulated warm vertical compaction of gutta-percha in a straight rooted premolar for three tapers (0.04, 0.06 and 0.12 mm mm(-1)) using finite element analysis. Stresses in the dentine were observed whilst the root was filled with three subsequent gutta-percha increments. Each increment was compacted at 10 or 15 N and the gutta-percha cooled down to 37 degrees C. After filling, composite was polymerized in the access space. A functional occlusal load of 50 N was then applied on the buccal cusp incline. The stress distribution in the root during the occlusal loading was compared with the stresses during filling. RESULTS: During filling, the highest stresses were found: (a) at the canal surface; (b) using the smallest taper; (c) in the apical third; and (d) during the first gutta-percha increment. The root stress distribution changed when the functional post-filling load was applied. It generated the highest stresses at the external root surface, with a tensile stress concentration at the lingual surface of the cervical third. Since the stresses during simulated masticatory loading concentrated on the external surface, an increased taper size caused only slightly higher root stress levels. CONCLUSIONS: With increasing taper, root stresses decreased during root filling but tended to increase for masticatory loading. Root fracture originating at the apical third is likely initiated during filling, whilst fracture originating in the cervical portion is likely caused by occlusal loads.

Computer Simulation↗
PubMed2006

Changes in compaction stress distributions in roots resulting from canal preparation.

AIM: To examine if canal enlargement with instruments of controlled taper leads to more uniform stress distributions within a root, thereby reducing fracture susceptibility. METHODOLOGY: Finite element models of a mandibular incisor were constructed with round and oval canal profiles, based on measurements from extracted teeth. The shapes of rotary nickel-titanium instruments (ProTaper F1, F2, and F3 and ProFile size 30, 0.04 taper and size 30, 0.06 taper; Dentsply Maillefer) were superimposed on the canals. Equivalent stresses and circumferential stresses in the root were calculated for a compaction load. RESULTS: The highest stresses were found at the canal wall. Round canals showed lower uniform distributions, whilst oval canals showed uneven distributions with high concentrations at the buccal and lingual canal extensions and greater stresses in the coronal and middle thirds than in the apical third. Preparation of round canals introduced only small circumferential stress increases in the apical half; preparation of oval canals produced substantial reductions where the canal was enlarged to a smooth round shape. Even where fins were not completely eliminated, the maximum stresses were still reduced by up to 15%. External distal and mesial surfaces of roots with oval canals showed moderate stress concentrations that were minimally affected by preparations, whilst stress concentrations emerged on roots with round canals when preparation sizes increased. CONCLUSIONS: The potential for reducing fracture susceptibility exists as a result of round canal profiles achieved and smooth canal taper. Even when fins were not contacted by the instrument, stresses within the root were lower and more evenly distributed than before preparation.

Compressive Strength↗
PubMed2003

Fracture toughness and microhardness of a composite: do they correlate?

OBJECTIVES: Chipping and bulk fracture are major contributors in clinical failures of composite restorations. Fracture toughness (K(Ic)) quantifies susceptibility for fracture, but experimental determination is complicated. It would be beneficial for the dental community if a relatively simple experiment, such as microhardness (HK), could be used to screen composites for fracture resistance. This study explores a possible correlation between K(Ic) and HK. METHODS: Composite cylinders (4mm diameter and approximately 7 mm long) were cured for five combinations of light intensity (I, microm W/cm(2)) and curing time (T, s) to achieve a range of different total light energy densities (I x T=100 x 10, 100 x 20, 300 x 20, 300 x 40, and 700 x 60 microm W s/cm(2)). A chevron-notch was cut in the median plane of the cylinders for the fracture toughness test, which was executed in a displacement control mode at 6 micro m/s cross-head speed (sample size 4). Knoop hardness was determined at the median plane of the cylinders (sample size 6). The tests were performed 15 min and 24h after curing. RESULTS: Both the K(Ic) and HK increased with increased light energy density and storage time. Linear regression analysis indicated a strong correlation between HK and K(Ic) tested at the same time period (R(2)=0.97 and 0.90 for 15 min and 24h, respectively). The correlation became weaker between the different storage times (R(2)=0.71), indicating a change in fracture toughness and/or microhardness mechanisms. CONCLUSION: Fracture toughness of a composite cannot be simply extrapolated from microhardness.

Algorithms↗
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

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↗
PubMed1999

Effect of washers on reverse torque displacement of dental implant gold retaining screws.

STATEMENT OF THE PROBLEM: Gold screw loosening is a frequent problem that affects dental implants. PURPOSE: This study determined the effect of spring washers on gold screw displacements during applied removal torque in dental implants. MATERIAL AND METHODS: A 3-unit fixed partial denture was cast in semiprecious alloy and fixed to 2 dental implants with gold-retaining screws. Implants were embedded in acrylic resin and restrained from movement. A miniature load sensor was attached to a torque controller handpiece to accurately measure the time and torque needed to completely loosen the gold screws. Rotational displacements were calculated for 4 experimental setups, involving washers placed on (a) both, (b) the mesial, (c) the distal, and (d) no implants. Screw displacement data were compared between implants in all 4 combinations by means of paired t tests. RESULTS: Gold screws with underlying conical spring washers underwent on average up to 35% (14.1 microm) more rotational displacement during applied removal torque than those without washers. CONCLUSION: Inclusion of a conical spring washer significantly increased the amount of rotational displacement needed to completely loosen an implant gold retaining screw.

Dental Prosthesis, Implant-Supported↗
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

Theoretical considerations of contraction stress.

Polymerization shrinkage of composite restorative materials causes clinical concerns because it introduces residual stresses in restored teeth. These stresses can, among other things, propagate enamel cracks, bring about microleakage, and cause postoperative sensitivity. The amount of shrinkage stress does not depend only on how much a composite contracts, but also on the elastic modulus ("stiffness") of the composite, the shape of the cavity, the established bonding between the tooth and restoration, etc. The relationships between these many factors can be described by universal physical laws. To analyze shrinkage stresses, theoretical models must be used that relate the various shrinkage properties and clinical conditions via physical laws. Models will assist us to rethink and optimize established paths to clinical success.

Composite Resins↗
PubMed1998

Do dental composites always shrink toward the light?

Many of the current light-curing composite restorative techniques are rationalized in compliance with the theory that composite shrinks toward the light. Shrinkage directed toward the margins is believed to be responsible for the observed improved marginal properties. However, the dental literature does not consistently support this theory. Experimental determination of contraction patterns is very difficult. In this study, a finite element technique is used to analyze the direction of composite shrinkage as it cures. The process of polymerization can be characterized by pre- and post-gel phases. The stress developed in a restoration can be relieved quickly by the flow of material still in the pre-gel phase. Residual stresses arise after gelation. Both auto- as well as photo-curing composites were analyzed. In photo-curing composites, the gel-point varies throughout the material with the intensity of the light. Experimentally determined light transmittance data for different materials were used in the simulation. Degree of cure and time-dependent shrinkage properties were also included from experimental measurements. The analysis showed that the shrinkage direction was not significantly affected by the orientation of the incoming curing light, but instead was mostly determined by the bonding of the restoration to the tooth and by the free surfaces. Consequently, differences between the contraction patterns of auto- and photo-cure were minimal. It was concluded that composite does not shrink toward the light, but that the direction is predominantly determined by cavity shape and bond quality. Improved marginal properties should be pursued by the optimization of other factors, such as the polymerization process, the curing procedure, and the bond quality. The direction of shrinkage vectors in response to light position does not seem to be an appropriate criterion for the optimization of marginal quality.

Animals↗
PubMed1997

Forces and moments generated at the dental incisors during forceful biting in humans.

A miniature load sensor capable of measuring all forces and all moments simultaneously at a single location in space was used to assess the magnitude and direction of loads that affect the dental incisors during forceful, static biting. While prior approaches have not measured all necessary six degrees of freedom during biting, the complete set of loads is needed to serve as realistic boundary conditions for analytical or computational models of mandibular mechanics. Four subjects were asked to perform controlled and repetitive edge-to-edge incisal biting activities. Customized devices were used to rigidly hold the load sensor in place at pre-specified tooth separations of less than 1 mm. The results yielded force resultants with a magnitude range of 24.5 to 28.4 N. This range was intentionally limited in magnitude to avoid damage to the internal strain gauge assembly of the sensor. In all cases, the highest force component was oriented upwards. An additional simultaneous moment resultant (range: 8.9-17.0 N cm) with a main moment component oriented backwards and downwards towards the oral cavity was also detected. These data suggest that in order for the biting loads to be composed of six DOF, the remaining forces acting on the mandibular force system (i.e. muscular and/or articular) may indeed be non-coplanar and non-concurrent. Although useful for static biting activities, the bulk of the sensor would probably preclude meaningful measurements during dynamic events such as chewing or swallowing.

Adult↗
PubMed1997

Analysis of strain gage method for measurement of post-gel shrinkage in resin composites.

OBJECTIVE: The objective of this study was to refine a strain gage method for measuring polymerization contraction of resin composites and to isolate the net post-gel contraction by identifying factors contributing to the measured strains. The hypothesis to be tested was that carefully controlled strain gage measurements of composite polymerization could isolate post-gel contraction events. METHODS: Composite was placed on a biaxial strain gage and light-cured. This method enabled real-time registration of the progress of shrinkage strain, corresponding to elastic modulus development. Strain from the two axes of the strain gage were averaged and plotted as a function of time. A representative curve was calculated from the mean of ten measurements. The following factors influencing the total contraction measurement were evaluated: thermal expansion of the gage, thermal expansion of the composite due to the exothermic reaction and exposure to the curing light, and adhesion of the composite to the gage. These parameters were measured so that the net deformation of the composite during polymerization could be calculated. RESULTS: Parametric studies of pre-cured and photointiator-free materials confirmed the hypothesis that strain gages measure post-gel contraction. Thermal artifacts were measured and subtracted from the total strain output. SIGNIFICANCE: Strain gages are suitable for measuring the clinically significant phase of composite polymerization contraction.

Adhesiveness↗
PubMed1997

Inhibitive effect of a resin-modified glass ionomer cement on remote enamel artificial caries.

Glass ionomer cements (GICs) demonstrate the inhibition of caries lesions formed immediately adjacent to the restoration. This in vitro study was conducted to evaluate the distance at which a resin-modified GIC is able to exert its cariostatic effect on artificial enamel lesions ('remote effect'). Resin-modified GIC or bis-GMA resin was applied on the cervical third of the labial surface of 10 paired halves of bovine incisors. Specimens were separately immersed for 3 weeks in lactic acid gel which was changed every other day to reduce fluoride accumulation. Artificial lesions were examined by the cross-sectional microhardness (MHN) method. Volume percent mineral, mineral loss (delta Z value) and change in mineral content (delta M) were computed for each MHN profile, performed at distances of 0.2, 0.5, 1.0, 2.0, 4.0 and 7.0 mm from the edge of the materials. delta Z values of the resin-modified GIC group were significantly lower than those of the bis-GMA control group at all remote sites (t test, p < 0.05). The delta M caused by resin-modified GIC was more pronounced within 1.0 mm from the material which suggested that the demineralization inhibition can be divided into the near effect (< 1.0 mm). In this in vitro study, resin-modified GIC provided caries resistance in bovine enamel located at a considerable distance from the margin of the material.

Animals↗
PubMed1997

Why do shear bond tests pull out dentin?

It is widely accepted that a dentin shear bond test which pulls out dentin must mean that the adhesive strength is superior to the cohesive strength of the dentin. Using numerical modeling techniques, Van Noort et al. (1988, 1989) and DeHoff et al. (1995) alerted the scientific community that there were massive stress concentrations in the familiar dentin bond test. It is not inconceivable that these localized high tensile stresses could initiate cracks which diverge monolithically into dentin, leaving the interface unchallenged. To test this hypothesis, we developed a failure accumulation simulation program which determined localized failure interactively "on the fly" with a finite element solver, and also included brittle behavior, adhesive and cohesive failure, stochastic response, and dynamic remeshing. All of the familiar dentin bond variables were included in the simulation. A parallel experimental dentin bond test validation was run, and the fractography was examined in the scanning electron microscope for mode of failure. The simulation confirmed the tensile monolithic fracture hypothesis. It is also confirmed that dentin pull-out was partly due to the biomechanics of the test and did not necessarily mean superior adhesive strength or even that the cohesive strength of the dentin was reduced. There is clear need for a new technology for the evaluation of biological interfaces, and the present work has shown the vital role of numerical modeling in the interpretation of such experimental procedures.

Adhesiveness↗
PubMed1997

Modeling the mechanical behavior of the jaws and their related structures by finite element (FE) analysis.

In this paper, we provide a review of mechanical finite element analyses applied to the maxillary and/or mandibular bone with their associated natural and restored structures. It includes a description of the principles and the relevant variables involved, and their critical application to published finite element models ranging from three-dimensional reconstructions of the jaws to detailed investigations on the behavior of natural and restored teeth, as well as basic materials science. The survey revealed that many outstanding FE approaches related to natural and restored dental structures had already been done 10-20 years ago. Several three-dimensional mandibular models are currently available, but a more realistic correlation with physiological chewing and biting tasks is needed. Many FE models lack experimentally derived material properties, sensitivity analyses, or validation attempts, and yield too much significance to their predictive, quantitative outcome. A combination of direct validation and, most importantly, the complete assessment of methodical changes in all relevant variables involved in the modeled system probably indicates a good FE modeling approach. A numerical method for addressing mechanical problems is a powerful contemporary research tool. FE analyses can provide precise insight into the complex mechanical behavior of natural and restored craniofacial structures affected by three-dimensional stress fields which are still very difficult to assess otherwise.

Anatomy, Cross-Sectional↗
PubMed1996

The use of drug information sources by physicians: development of a data-generating methodology.

The aim of this study was: (1) to develop and evaluate a methodology to determine hospital physician's use of personal drug information sources; (2) a preliminary insight into the use personal drug information sources. Written case simulations appeared to be the most appropriate method. To construct the written case simulations a step-wise procedure was developed. In the first stage 5 internists formulated 35 complex cases from their daily practice in which they consulted drug information sources; after an evaluation by experts 20 cases were left over. Next, using a written questionnaire, these 20 cases were evaluated in a random sample of 50 internists according to criteria concerning aspects of the process, the contents and the context. Finally, we analyzed these ratings using an elimination-by-aspects decision rule, with the dominant criterion 'need for consultation'. After this selection programme, two cases for each stage in the decision-making process of hospital physicians were selected which met the criteria. In general the colleague internist was the most frequently mentioned information source. Subspecialists and supporting specialists were considered less often and varied per stage in the decision-making process of physicians. The hospital pharmacist was hardly mentioned as a possible information source. The representatives of the pharmaceutical industry were not mentioned at all by the respondents.

Decision Making↗
PubMed1996

Thermal expansion coefficient of dental composites measured with strain gauges.

OBJECTIVES: A simple test method was developed to determine the coefficient of thermal expansion of prevailing restorative resin composites and to study the transient behavior as a function of temperature and repeated thermocycles. METHODS: Strain gauges were used to determine the thermal expansion for seven commonly used restorative resin composites by measuring the instantaneous strain along with temperature change. The temperature was measured by means of a thermocouple, the tip of which was embedded in the composite. The differences among the test groups were analyzed using ANOVA, followed by Scheffé's multiple comparisons test. RESULTS: The coefficient of thermal expansion determined for the composites tested was: 22.5 +/- 1.4 x 10(-6)/degree C (Z-100), 23.5 +/- 1.4 x 10(-6)/degree C (P-50), 32.6 +/- 1.6 x 10(-6)/degree C (Herculite XR), 34.1 +/- 1.8 x 10(-6)/degree C (APH), 35.4 +/- 1.4 x 10(-6)/degree C (Conquest), 41.6 +/- 1.5 x 10(-6)/degree C (Silux Plus), 44.7 +/- 1.2 x 10(-6)/degree C (Heliomolar). The coefficient was almost linear in the considered temperature range (26-75 degrees C) for all composites (r > 0.99) and decreased with each consecutive thermocycle (p < 0.1). SIGNIFICANCE: Thermally induced loads, introduced into restored teeth by the mismatch of the coefficient of thermal expansion of the tooth and the restorative material, may be related to microleakage and wear problems. A highly filled hybrid composite such as Z-100 had a coefficient of thermal expansion closest to that of the tooth crown, confirming other studies which demonstrated the benefits of high filler loading in matching the properties of the dental hard tissues.

Acrylic Resins↗