Ceramics in dentistry: historical roots and current perspectives

This article presents a brief history of dental ceramics and offers perspectives on recent research aimed at the further development of ceramics for clinical use, at their evaluation and selection, and very importantly, their clinical performance. Innovative ceramic materials and ceramics processing strategies that were introduced to restorative dentistry since the early 1980s are discussed. Notable research is highlighted regarding (1) wear of ceramics and opposing enamel, (2) polishability of porcelains, (3) influence of firing history on the thermal expansion of porcelains for metal ceramics, (4) machining and CAD/CAM as fabrication methods for clinical restorations, (5) fit of ceramic restorations, (6) clinical failure mechanisms of all-ceramic prostheses, (7) chemical and thermal strengthening of dental ceramics, (8) intraoral porcelain repair, and (9) criteria for selection of the various ceramics available. It is found that strong scientific and collaborative foundations exist for the continued understanding and improvement of dental ceramic systems.

In-Ceram failure behavior and core-veneer interface quality as influenced by residual infiltration glass

PURPOSE

Crown and fixed partial denture fabrication using In-Ceram all-ceramic restorative material (Vita Zahnfabrik, Bad Säckingen, Germany) involves the veneering of a glass-infiltrated alumina core with porcelain. The manufacturer instructs that excess infiltration glass be removed from all core surfaces before++ porcelain application; however, meticulous removal may not be practical. This study evaluates the failure behavior of two different types of In-Ceram structures with or without thin layers of excess infiltration glass left on core surfaces.

MATERIALS AND METHODS

Two groups of porcelain-veneered incisor crowns were fabricated having either (1) excess infiltration glass removed from the aluminous core (n = 10) or (2) a thin layer (0.1 to 0.3 mm) of excess glass remaining on the core (n = 10). Each crown was loaded on its incisal edge against a flat compression platen at 0.5 mm/min. Two groups of core disks (thickness, 1 mm; radius, 7.5 mm) underwent similar surface treatments; i.e., group A as recommended (n = 15) and group B having excess glass (n = 15). Disks were veneered with porcelain, polished to 1.5 mm, and loaded in biaxial flexure at 0.5 mm/min. All fracture surfaces were analyzed using light microscopy and a select sample were examined using scanning electron microscopy. A simple two-dimensional finite-element model was used to evaluate the stress state at the core-veneer interface of bend samples.

RESULTS

Failure loads were significantly higher for crowns having excess glass (Student's t test, P < .004). Weibull moduli were indistinguishable (m = 6.2) between crown groups, consistent with the microscopic impression that they shared a common failure mode. Microscopic evaluation of cross-sectioned crowns showed core-veneer interfaces with less porosity in the presence of excess infiltration glass. Failure loads for the disk groups were indistinguishable (Student's t test, P > .8). Scanning electron microscopic analysis of disk samples revealed that failures originated either at core-veneer interfaces or at lower disk surfaces, consistent with the finite-element analysis.

CONCLUSION

Excess infiltration glass on the core surface will not degrade the strength of In-Ceram structures. Central incisor crowns (as tested) were strengthened by excess infiltration glass, but disk samples (stressed differently) were not. Finite-element results suggest that disk failure originates from three possible sources.

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)

Perspectives on strength

Strength values are often relied upon as indicators of structural performance for brittle dental materials. Strength, however, is more of a "conditional" than an inherent material property, and strength data alone cannot be directly extrapolated to predict structural performance. Strength data are meaningful when placed into context via knowledge of material microstructure, processing history, testing methodology, testing environment and failure mechanism(s). Structure failure is determined by additional failure probability variables (in concert with strength) that describe stress distributions, flaw size distributions, and that can account for either single or multiple failure modes. Lifetime predictions require additional information about the time dependence of slow crack growth. Basic fracture mechanics principles and Weibull failure modeling will be reviewed for the perspective they provide in understanding strength and the data obtained using various laboratory tests. Examples will be given to demonstrate how failed specimens can provide crucial information to either validate or question the failure mechanisms invoked during laboratory testing. The role of interfacial stresses is discussed as applied to dental structures of current interest. Overall, it is emphasized that an understanding of actual clinical failure modes is absolutely necessary before results of in vitro strength testing can be considered to have clinical validity.

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.

Overview and patient recruitment in the Adult Day Health Care Evaluation Study

Although Adult Day Health Care (ADHC) is increasingly prominent in the continuum of long-term care services, the results from previous studies of the effects of ADHC are mixed. The objectives of the ADHC Evaluation Study were to determine the effect of ADHC on health status, utilization, and cost of care. The first phase was a randomized controlled trial evaluating ADHC provided directly by the VA. Patients at four medical centers (n = 826) were randomly assigned to receive either ADHC or customary care, and outcomes were compared for the two groups. The second phase was a prospective cohort study evaluating ADHC provided under contract to community agencies in which patients at four additional medical centers (n = 163) were assigned to contract ADHC programs. Outcomes were compared with those of similar patients in the randomized trial. Patients and care givers were assessed at intake and 6 and 12 months after intake. To be admitted to the study, patients must have met one of the following criteria: residence in a nursing home; dependence in ambulation, dressing, or toileting; bowel incontinence; or significant cognitive impairment. Patients at intake demonstrated major impairment in function and high levels of prior use of health care services.

The Adult Day Health Care Evaluation Study--impact on VA programs

The research study in Adult Day Health Care (ADHC) has had a fourfold impact on the Veterans Administration: 1) It defined the health care focus of ADHC and shaped consequent program design; 2) It forged a partnership between research and clinical staff in ADHC programmatic development; 3) It enhanced the perception of research in program development in long-term care; and 4) It provided VA with direction for program redesign to render ADHC more cost-effective. With careful planning, the program is expected to grow in the future as a part of VA's continuum of long-term care programs.

Summary and discussion of methods and results of the Adult Day Health Care Evaluation Study

This article summarizes the study results and presents an evaluative summary of the implementation of study methods designed to provide guidance in the degree of confidence with which the results may be accepted and generalized to other situations. Patients who were offered VA-ADHC services in the first phase of this study had significantly higher VA health care costs on average than patients assigned to customary care, with no apparent incremental health benefit to themselves or their care givers. One can have a high level of confidence in these results. The ADHC clinical services were implemented as planned, the randomized controlled trial was implemented successfully, and such threats to validity as insufficient numbers of patients and differential attrition were not present. Certain subgroups of patients assigned to VA-ADHC had VA costs of care that were not significantly higher than those assigned to customary care, although these results must be interpreted with caution. The findings of the second phase of the study evaluating contract ADHC provide no support for choosing to provide either contract ADHC or VA-ADHC over the other. The nonrandomized design and smaller sample size suggest that inferences from the contract ADHC evaluation should be drawn with more caution than those from the VA-ADHC evaluation.

Implications of the Adult Day Health Care Evaluation Study for program revision and research

With no additional effort to revise adult day health care (ADHC) services or the types of patients who receive them, it would appear that adding an ADHC program to a VA Medical Center would not achieve the desired objectives. The authors discuss here the advantages, disadvantages, and feasibility of 2 options for program revision. The first is to target ADHC to those types of patients who may be most likely to benefit. A targeting scheme should use the most objective criteria possible and may need to be implemented as part of a case-managed package of community-based services. The second option for program revision is to reduce the costs of ADHC services. A cost model developed as a part of the study demonstrated the effect of possible revisions, including increasing enrollment, reducing staffing costs, decreasing length of stay in ADHC, and increasing substitution of ADHC for other services. These changes differ in the level of administrative support and clinician behavior change needed for their implementation. This report then concludes with a discussion of the implications of the results for implementation of VA-ADHC versus contract ADHC, and a discussion of possible directions for future research.

Ordinary retirement: commonalities and continuity

Case studies of twenty-five recent retirees from a Midwest plant are the basis for a profile of ordinary retirement. The commonalities of preparation, satisfaction, activity patterns, and relationships are found to far outweigh differences. Exceptions are the widowed, divorced, and those with health disabilities.

Adult day health care evaluation study: methodology and implementation. Adult Day Health Care Evaluation Development Group

The Adult Day Health Care Evaluation Study was developed in response to a congressional mandate to study the medical efficacy and cost effectiveness of the Adult Day Health Care (ADHC) effort in the Department of Veterans Affairs (VA). Four sites providing ADHC in VA facilities are participating in an ongoing randomized controlled trial. Three years of developmental work prior to the study addressed methodological issues that were problematic in previous studies. This developmental work resulted in the methodological approaches described here: (1) a patient recruitment process that actively recruits and screens all potential candidates using empirically developed admission criteria based on predictors of nursing home placement in VA; (2) the selection and development of measures of medical efficacy that assess a wide range of patient and caregiver outcomes with sufficient sensitivity to detect small but clinically important changes; and (3) methods for detailed, accurate, and efficient measurement of utilization and costs of health care within and outside VA. These approaches may be helpful to other researchers and may advance the methodological sophistication of long-term care program evaluation.

A model for reducing the cost of care in VA medical centers that offer adult day health care

One approach to providing cost-effective adult day health care (ADHC) services is to guide both the inputs to ADHC services and the provision of other services so that substitution for institutional services can realistically take place. This approach has been used in a randomized clinical trial to evaluate the medical efficacy and cost of ADHC in the Department of Veterans Affairs (DVA). This article describes the strategies that were used to improve the cost effectiveness of ADHC during the evaluation. Cost and use estimates were developed based on the best data available from the DVA and previous research on the cost for patients' use of ADHC, nursing home, hospital, and ambulatory care. A cost workshop was attended by the ADHC managers to develop plans for controlling costs. Plans were identified that increase the likelihood that ADHC can be shown to be less costly than customary care.

Fracture surface analysis of dental ceramics: clinically failed restorations

Fractography was used to study all-ceramic restorations that had failed clinically. Some basic tenets of fracture mechanics and fractography are reviewed and related to the examination of clinically failed all-ceramic restorations. Dicor and Cerestore restorations that had failed either at trial placement or 17 to 36 months postcementation were evaluated. Failed Dicor restorations were studied to determine the origin of failure and calculate the intraoral stress at failure. Descriptive information regarding crack origin and crack path were obtained from failed restorations constructed of first-generation Cerestore. A majority of the crowns apparently failed from the internal surface, indicating this as the highest tensile surface and/or the location of the largest flaws. Flaws were identified as being related to fabrication or inherent in the ceramic microstructure.

Fracture-surface analysis of dental ceramics

This study demonstrated that quantitative fractography can be used to study failed aluminous and glass-ceramic central porcelains. Fracture surfaces of DICOR and Vitadur-N core porcelain modulous-of-rupture bars were studied to identify fracture mirror features useful in (1) locating the source of fracture and (2) calculating the stress at fracture in clinically failed restorations. The morphology of fracture surfaces results from events related to the initiation and propagation of the crack front during failure. Modulus-of-rupture testing was performed in four-point bending. Fracture surfaces were studied by scanning electron microscopy. The mean fracture stress for the Vitadur-N porcelain was 94.7 +/- 12.4 MPa (13,730 psi); for DICOR the fracture stress was 55.4 +/- 10.6 MPa (8,030 psi). The standard quantitative fractography relationship between in mirror radius and ln fracture stress was followed for both materials. This quantitative fractography relationship was used to calculate the in vivo stress at failure in a clinically fractured DICOR molar crown. Five clinically failed DICOR crowns were seen to fail from the internal surface.

Influence of surface preparation on the strength and surface microstructure of a cast dental ceramic

The effect of three surface preparations on the flexural strength of a cast glass-ceramic (Dicor) was studied. The microstructure of the external and fracture surfaces was evaluated using electron microscopy. It was found that there was no difference in strength for the "as-cast/cerammed," acid-etched, or glazed shading porcelain specimens. A significant increase in strength (twofold) was observed when the "ceram layer" was mechanically removed from the surface of the ceramic. Scanning electron microscopy revealed that the "ceram layer" was 25 to 100 microns thick and consisted of crystal "whiskers" oriented perpendicular to the external surfaces. It was observed in both the laboratory and the clinically failed specimens that a zone of strength-limiting porosity was associated with this "ceram layer." Potential origins of the crystal "whiskers" and the porosity were discussed.

Use of an elastomeric material to improve the occlusal seat and marginal seal of cast restorations

A method to improve the fit of castings by the use of a light-bodied condensation silicone impression material was tested. Eighteen-full coverage castings were made on individual resin dies and divided into two groups. The experimental group was adjusted internally to a uniform precementation space by using the silicone impression material as a disclosing agent for binding of the castings on the axial or occlusal walls. Measurements of marginal fit were made with a light microscope using a filar eyepiece both externally and internally by sectioning after cementation with a zinc phosphate cement. Results demonstrated a significant improvement in marginal seal and occlusal seating in the experimental group compared with the control group. The condensation silicone material proved to be an appropriate research tool for nondestructive, three-dimensional evaluation of the postcementation space and offers a new method of evaluation of cement thickness because seating was found to be not significantly different from that with zinc phosphate cement.

Nondestructive, three-dimensional internal fit mapping of fixed prostheses

Crown fit may be three-dimensionally mapped by using a nondestructive optical technique. Reflected light transmission through thicknesses of a colored impression material was shown to follow the Beer-Lambert relationship over a range of approximately 10 to 300 microns, a useful range of interest for study of cement layer thickness. Control thicknesses of impression material were first formed between the measuring platens of a micrometer, and light transmission values (relative reflections) were measured through these control thicknesses of impression material held against air-abraded, noncast gold alloy. Relative reflection values were similarly measured from impression material formed inside cast gold crowns seated on their respective dies and these values were plotted against direct thickness measurements by using the Beer-Lambert relationship. It was established that the micrometer-produced control specimens yielded a valid standard curve for use with the cast gold crowns. Precementation space measurements determined photometrically were found to correlate significantly (r = 0.72, p less than 0.001) with zinc phosphate cement thickness measured on subsequently cemented and cross-sectioned crowns. A three-dimensional representation of the precementation space inside one crown was developed after mapping of 45 points in less than 5 minutes.

Social integration, leisure activity, and life satisfaction in older adults: activity theory revisited

The relative contributions of objective integration, subjective integration, and total leisure activity to the life satisfaction of older adults are examined. A random sample of 400 persons ages forty through eighty-nine residing in a demographically typical midwestern city were interviewed by phone. Results show that Objective Integration does not contribute incrementally to Life Satisfaction except among males under age sixty-five. Subjective Integration, on the other hand, contributes significantly to Life Satisfaction of males and females under and over age sixty-five. Even when demographic variables, Objective Integration, and Subjective Integration are taken into account statistically, Total Leisure Activity contributes significantly to Life Satisfaction in all groups examined.

Evaluation of biangular reflection photometry for quantitative study of etched alloy surface roughness

Biangular reflection photometry was correlated with quantitative stereology and direct pit-depth measurements for an electrolytically etched nickel-chromium-beryllium alloy. Effects of viewing angle, viewing aperture, and plane polarization of incident and viewing light were also studied. The results showed that architectural changes in the etched metal surface could be quantitatively described with the use of reflection photometry. An off-specular peak, located at an angle of reflection considerably different from the angle of incidence, was observed to have an intensity comparable with that of the specular reflection peak. Viewing aperture was found to affect one's ability to distinguish among degrees of surface roughness.

Effect of the Today contraceptive sponge on growth and toxic shock syndrome toxin-1 production by Staphylococcus aureus

Toxic-shock syndrome toxin-1 (TSST-1)-producing Staphylococcus aureus MN8 was grown in dialyzed beef heart infusion broth, or dialyzed beef heart infusion broth supplemented with 5% human blood or with 5% human semen, in the presence and absence of Today contraceptive sponges. Early growth of the organism was inhibited by the presence of the sponges in aerobic, static aerobic, and anaerobic cultures. In aerobic cultures that contained the sponge, minimal TSST-1 was detected by immunodiffusion assay, whereas in control cultures that did not contain the sponge, extensive toxin production was seen. In static and anaerobic cultures, the presence of the contraceptive sponge resulted in toxin levels less than or equal to the low levels seen in control cultures. These results suggest that Today contraceptive sponges may inhibit TSST-1 production by S aureus.

Resin-bonded prostheses: microleakage and luting agent thickness of etched and cast-meshwork retainers

The purpose of this study was to compare in vitro marginal leakage, metal surface features, and luting resin film thickness between electrolytically etched and cast mesh resin-bonded retainer designs. The rate of microleakage along an etched metal-composite interface appears to be considerably slower than along a cast mesh-composite interface. Significant differences in surface area between these two retention systems, especially at the margin, is thought to be primarily responsible. Whether such a microleakage difference will be meaningful in terms of clinical longevity remains a matter for further study. Luting resin thickness was found to be significantly less for etched retainers. This may be due to characteristics of framework adaptation to the teeth under the conditions of this investigation.