Special report: NIST workshop on reference data for the properties of biomaterials.

A workshop on Reference Data for Biomaterials was held at the National Institute of Standards and Technology (NIST) on July 27, 2000. The primary purpose of the workshop was to determine whether needs existed for the establishment of reference data (RD) databases on the properties of biomaterials. Special attention was given to critiqued RD such as those traditionally found in databases that are established within the NIST Standard Reference Data Program. Critiqued data are data that have been critically evaluated for issues dealing with components of uncertainty, experimental design (details, descriptions, and appropriateness), measurement methods (appropriateness), conclusions drawn from the data, and so forth. Among the workshop's 65 registrants were representatives from industry, the National Institutes of Health (NIH), the Food and Drug Administration (FDA), and academia. These joined with NIST staff to address reference biomaterial property database needs within five categories: orthopedic, cardiovascular, ophthalmologic, tissue-engineered, and dental biomaterials. A general session on other issues focused specifically on database accesses (portals), contents, and maintenance. While the workshop's intended focus was on critiqued RD, it was suggested that closely related issues be considered as well. In this way, a more comprehensive assessment of opportunities for the cooperation of NIST with the biomaterials community might be developed. As a result, the needs for noncritiqued data and for reference materials (RMs), useful for developing data, also became a part of the focus of the workshop. Hence, this article presents the results from the breakout sessions of the workshop according to two categories: reference data and databases, and reference materials. In the following summary, the workshop is presented in the following order: An introduction to databases, resource presentations, action items identified in breakout sessions, assessment of resources (personnel and monetary) needed to work on action items, and portals for databases. Except for the individual concurrent breakout sessions themselves, all other sessions of the workshop were participated in fully by those in attendance.

Simulation of transient thermal stress in gypsum-bonded investment.

The aim of this study was to clarify the effect of heating rate on the development of both transient and residual stresses in investment molds. Solid, cylindrical, gypsum-based molds were modeled and theoretical calculations made of temperature distributions with a constant heating rate. These calculations used experimental thermal diffusivity data obtained with a laser flash method. The simulations calculated transient thermal stresses during heating and at the end of heating for two surface conditions: an unrestricted surface, and a surface restricted by a casting ring. The simulation model developed nonuniform strains and stresses at casting temperature; tangential stress was compressive at the surface and tensile at the mid-point for the unrestricted surface model. The surface restricted model developed compressive tangential stresses throughout the mold at the casting temperature. This resulted in significant thermal strain differences compared to the magnitudes of expansion of the mold due to heating.

Failure of all-ceramic fixed partial dentures in vitro and in vivo: analysis and modeling.

Hertzian cone cracks visible at the loading site of 20 all-ceramic fixed partial dentures (FPDs), tested in vitro, led to the hypotheses that failure was due to the propagation of localized contact damage crack systems (Hertzian stress state) and that such damage was an unlikely clinical failure mode. Fractographic analysis of the 20 laboratory-failed and nine clinically-failed all-ceramic FPDs allowed for definitive testing of these hypotheses and a comparison between in vitro and in vivo failure behavior. In all cases, failure occurred in the FPD connectors (none from contact damage), with approximately 70 to 78% originating from the interface between the core and veneer ceramics. The coincidence between failure origins provides strong evidence that the in vitro test modeled aspects of structural behavior having clinical importance. The fractographic observations, coupled with the in vitro failure load data, furnished very specific boundary conditions which were applied to constrain mathematical models of FPD connector failure. Finite element analysis (FEA) of the laboratory FPDs found that maximum principal tensile stresses would occur at locations consistent with the fractographic observations only if: (1) there were appropriate elastic moduli differences between the ceramics; and (2) a small amount of abutment rotation was allowed. Weibull failure probability (Pf) calculations, incorporating FEA stress profiles, very closely replicated the laboratory failure distribution only when: (1) the veneer ceramic was much weaker than the core ceramic; and (2) the Weibull modulus of the core-veneer interface was much lower than that for the free veneer surface (i.e., the interface is of lower quality with regard to defects).(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro fracture behavior of ceramic and metal-ceramic restorations.

PURPOSE: Failed crowns and failure load data were studied to gain insights into the fracture behavior of prostheses under incisal-directed, load-to-failure testing. MATERIALS AND METHODS: Incisor crowns (n = 68) were fabricated: two all-ceramic groups (feldspathic veneer on high-strength core), differing in core design, and two metal-ceramic groups, differing in metal oxidation time (30 seconds v 3 minutes). Crowns were loaded to failure on their incisal edge. Gross visual, microscopic, and elemental microprobe analyses of failed crowns were coupled with Weibull analysis of the failure load data. RESULTS: Failure loads were higher for the normal oxidation time (TN) than for the extended oxidation time (TE) metal-ceramic crowns (P < .02), but both groups had indistinguishable Weibull moduli indicating the possibility of a common failure origin. Fracture behavior and Weibull results both implicated the oxide layer as being the origin of failure. The ratio of fracture loads (TE/TN) corresponded well with calculated oxide-volume ratios. Failure loads were lower for the all-ceramic than for the metal-ceramic crowns (P < .001). Fifty percent of the all-ceramic crowns failed by delamination of veneering glass alone, leaving a thin layer of residual glass on the core surface. Scanning electron microscope views showed that delamination occurred 10 to 50 microns away from the core-veneer interface. Electron microprobe elemental analysis of the core-veneer interface showed that residual core infiltration glass was not present on the core surface and that chemical alterations in the veneering glass were apparently limited to less than a 2- to 3-microns thick layer. CONCLUSIONS: Failure for both restorative systems involved interfacial stresses with crack propagation occurring at or near the core-veneer interface. The weaker interface in the metal-ceramic system probably resulted from an increase in surface oxide volume, irrespective of any change in its adherence or physical properties. For the ceramic crowns, delamination crack fronts appeared to propagate through chemically unaltered veneering porcelain. Both the Weibull moduli and characteristic strengths were indistinguishable between either of the two ceramic core designs or between groups failing from delamination with or without core cracking/failure. This is consistent with delamination being the primary fracture process during failure. Clinical implications should not be drawn from results of this study because no correlation is known to have ever been established between clinical behavior and incisal load-to-failure results.

Influence of tempering method on residual stress in dental porcelain.

The porcelain component of a porcelain-fused-to-metal restoration is strengthened by residual (tempering) stresses which are induced by cooling procedures followed in dental laboratories. The thermophysical properties of materials and cooling rate are the main factors which determine the residual stress. In this paper, the temperatures in the midplane of body-porcelain disks were measured from a heat-soak temperature (1000 degrees C) to room temperature during two different cooling procedures: slow cooling in air and forced-air cooling. Experimental results approximated exponential cooling wherein the cooling rates could be represented by a linear equation of temperature. Residual stresses, as affected by the tempering method and thickness of a porcelain disk, were calculated by computer simulation for regions away from the edges. The following temperature-dependent factors were incorporated into the simulation: elastic modulus, viscosity, and coefficient of thermal expansion. The cooling rate dependencies of the glass transition temperature and the temperature distribution during cooling were also included. The cooling rates used in this simulation were derived from the tempering data. The agreement between development of transient and residual stresses--calculated by computer simulation for various cooling methods, and the tendency toward failures of porcelain disks subjected to the tempering processes--was examined. Simulated residual stresses were also in good agreement with those measured by the indentation fracture method of Marshall and Lawn (1977) and Anusavice et al. (1989).

Visco-elastic deformation of dental porcelain and porcelain-metal compatibility.

A computer simulation using a visco-elastic stress analysis was conducted to clarify the effect of the heating rate on deformation temperature of dental porcelain during firing. In this simulation, the following temperature-dependent factors were incorporated: elastic modulus, viscosity, and coefficient of thermal expansion. The cooling/heating rate dependencies of both the glass-transition temperature and the temperature distribution in the slab were also included. Thermal expansion curves of porcelain with an applied load at various heating rates were computed. Effects of the applied stress and the heating rate on the deformation temperature of porcelain were revealed. The results suggest that the temperature where the incompatibility stress develops in the porcelain-fused-to-metal strips during cooling can be estimated closely from the deformation point of the heating curve of the porcelain with an applied stress of about 1.2 approximately 3.1 MPa. A method for measuring temperature dependence of viscosity as represented by an Arrhenius equation is proposed.

Multidimensional internal setting expansion of a phosphate-bonded casting investment measured with strain gauges.

Strain gauges were used to determine the setting expansion of phosphate-bonded casting investments. The gauges were placed at the approximate site of the wax pattern. The isotropy of the expansion was evaluated with multidirectional strain gauges that measured the difference in expansion in two orthogonal directions. A large difference was found between the setting expansion measured externally (1% to 1.5%) and the method described here (0.01%). For casting rings lined with a nonasbestos layer, no statistically significant differences were found in the setting expansion parallel to the long axis of the ring and the expansion in the direction perpendicular to the long axis.

Transient and residual stress in a porcelain-metal strip.

Porcelain-fused-to-metal (PFM) restorations may develop cracks during processing or in-mouth service if the relative physico-mechanical properties of the porcelain and metal are highly mismatched. Precise conditions when this might occur are not known. Many processing and property variations can affect the stresses developed throughout a porcelain-metal system. To understand this, we conducted a computer simulation of stress developed in a PFM beam. The simulation considers cooling from temperatures higher than the porcelain sagpoint. The following temperature-dependent factors were incorporated: the elastic modulus, shear viscosity (porcelain), and coefficients of thermal expansion. The cooling rate dependencies of the glass transition temperature, (Tg), and the temperature distribution during cooling were also included. The results suggest that transient tensile stress at the porcelain alloy interface may result in cracks in the porcelain during cooling. Occlusal forces may set up stresses to cause cracking at the surface of the porcelain if the compressive residual stress is not high enough. PFM restorations with an alloy of high thermal expansion coefficient require rapid cooling; on the contrary, PFM restorations with the alloys of lower coefficients require slow cooling. A high cooling rate can make up for thermal expansion mismatches between the alloy and the porcelain up to 2 x 10(-6)/degrees C. Finally, the results indicated that curvature was not a sensitive indication of stress for a multimaterial beam when visco-elastic relaxation and high cooling rates are involved. For the case modeled here, curvature varied inversely with a 1/2 to 1/7th power of the stress.

Mesh monitor casting of Ni-Cr alloys: element effects.

A mesh monitor has previously been used for quantitative evaluation of the casting of dental alloys. A castability value, Cv was defined. For curve-fitting, a transformed castability value, Cv,t, was used. A series of alloys was selected so that effects of major elements on Cv, t and, hence, on Cv could be determined. Compositions were chosen so that correlated effects would be avoided. Assuming a linear dependence on the concentrations of individual elements, one may use the following equation to describe Cv,t: (Formula: see text) where each term employs an elemental concentration in weight percent, (Ei), and a coefficient, Ki, for the ith term. Because Ei's are constant for each alloy, Ki = fi (TA, TM) = gi (TC, TM). The temperature-dependent coefficients, Ki, were determined for seven elements and for the (Ni/(Cr) ratio. It was also found that Si and Be produce a synergistic effect. The results help our understanding of the casting behavior of Ni-Cr dental alloys; this approach may be useful in the design of dental alloys.

Elastic constants of three Ni-Cr dental alloys at room and elevated temperatures.

Dental porcelains are fused to metals at elevated temperatures. Therefore, the development of stress that affects the stability of porcelain-fused-to-metal (PFM) systems may be influenced by the temperature dependence of the elastic constants of both materials. The sonic resonance technique was used to determine the elastic moduli for three nickel-based dental alloys. In addition to room temperature data, the temperature dependencies of the Young's modulus (Y) and the shear modulus (G) were determined up to 600 degrees C (873 degrees K). The measured values for the resonant frequencies decreased with increasing temperatures, which resulted in correspondingly low moduli. The decrement of the elastic constants over the temperature range considered is less than 10%. The change of Poisson's ratio as a function of temperature for the alloys considered is presented in this paper.

Transient and residual stresses in dental porcelains as affected by cooling rates.

The development of either transient or residual stress in a slab of dental porcelain during cooling was simulated by use of a super-computer. The temperature dependences of the elastic modulus, the thermal expansion coefficient, and the shear viscosity, and the cooling rate dependence of the glass transition temperature, Tg, were considered in this calculation. Internal stress and viscoelastic creep were computed for several cooling rates. Calculated results display stress profiles which agree reasonably well with reported measured profiles in quenched, tempered glasses. The calculated residual surface stress, sigma, could be fit by the following empirical formula, sigma = kl2(q/q0)n, q is the cooling rate, q0 is a reference cooling rate and l is the half-thickness of the porcelain. The method by which residual stress develops is also discussed. This discussion suggests a method for strengthening of the porcelain by the development of high-compressive residual stress on the surface.

Dental research at the National Bureau of Standards: how it changed the practice of dental health service.

A history of dental research at the National Bureau of Standards since its inception in 1919 is presented. The initial thrust on dental amalgam by the US Army Dental Corps, the assignment of Dr. William Souder to the project, and subsequent developments are traced. Difficulties in obtaining support for the early stages of the program following World War I are described. The involvement of the American Dental Association in 1928, issuance of the first ADA specification on dental amalgam, and the ultimate ramifications on dental (and medical) standards programs throughout the world are described. Benefits to patients, dentists, and taxpayers from support of the dental research program have been calculated as exceeding the combined budgets of the currently supporting institutions--NBS, ADA, and National Institute of Dental Research.

Internal setting expansion of a dental casting investment measured with strain gauges.

A study was undertaken to investigate the feasibility of using strain gauges to evaluate the internal setting expansion of dental casting investments. A strain gauge was invested in the center of a casting ring to determine the linear setting expansion at the site of the wax pattern. The findings suggest that strain gauges can be used to measure setting expansion internally more accurately than volumetric or external surface measurement techniques. It was also found that winged strain gauges were more stable in the investment than wingless strain gauges.

A technique for characterizing casting behavior of dental alloys.

A technique for characterizing casting behavior of dental alloys has been developed and tested. The method employs easily reproducible specimen patterns and uses equipment and procedures generally available in dental prosthetic laboratories. A castability value is arrived at by counting complete segments of a cast alloy grid. Test results indicate that the castability evaluation procedure described can be a valuable aid in the laboratory for both selecting and using dental materials for producing cast restorations. The test method would also appear useful for the evaluation of products and techniques during research and development. An absolute minimum for the castability value determined relative to clinical and laboratory efficacy has yet to be established.