Securing Public Trust: Dentistry, SARS-CoV-2, and "Testing for Tomorrow"

The SARS-CoV-2 pandemic created societal upheaval well beyond what anyone, but the oldest of Americans, has seen in their lifetime. As the pandemic begins to subside, it is leaving behind a legacy of permanently changed practices, including enhanced environmental controls in clinical settings, reconsideration of modes of personal protective equipment outsourcing, changes and/or reinterpretation of dental practice acts, and entirely new approaches to testing and vaccine design, among many others. This article focuses on one change that the authors hope will prevail: greater trust in the dental profession as a valuable public resource during healthcare crises. The article cites the initial low perception of dentistry as an important component of a health surge response by public health authorities, then describes how a group of eight institutions came together to form the "Testing for Tomorrow Collaborative" to help each other and the dental profession identify modes of testing and practice conduct that make dentistry safer to practice. The importance of the public's trust in the profession is underscored, and pathways to improving that trust are proffered.

What constitutes an ideal dental restorative material?

Intense environmental concerns recently have prompted dentistry to evaluate the performance and environmental impact of existing restoration materials. Doing so entices us to explore the 'what if?' innovation in materials science to create more ideal restorative materials. Articulating a specification for our design and evaluation methods is proving to be more complicated than originally anticipated. Challenges exist not only in specifying how the material should be manipulated and perform clinically but also in understanding and incorporating implications of the skill of the operator placing the restoration, economic considerations, expectations patients have for their investment, cost-effectiveness, influences of the health care system on how and for whom restorations are to be placed, and global challenges that limit the types of materials available in different areas of the world. The quandary is to find ways to actively engage multiple stakeholders to agree on priorities and future actions to focus future directions on the creation of more ideal restorative materials that can be available throughout the world.

Priorities for future innovation, research, and advocacy in dental restorative materials

Innovations in materials science, both within and outside of dentistry, open opportunities for the development of exciting direct restorative materials. From rich dialog among experts from dental and non-dental academic institutions and industry, as well as those from policy, research funding, and professional organizations, we learned that capitalizing on these opportunities is multifactorial and far from straightforward. Beginning from the point when a restoration is needed, what materials, delivery systems, and skills are needed to best serve the most people throughout the world's widely varied economic and infrastructure systems? New research is a critical element in progress. Effective advocacy can influence funding and drives change in practice and policy. Here we articulate both research and advocacy priorities, with the intention of focusing the energy and expertise of our best scientists on making a difference, bringing new innovations to improve oral health.

Conventional and modified veneered zirconia vs. metalloceramic: fatigue and finite element analysis

PURPOSE

The purpose of this study was to test the hypothesis that all-ceramic crown core-veneer system reliability is improved by modifying the core design and as a result is comparable in reliability to metal-ceramic retainers (MCR). Finite element analysis (FEA) was performed to verify maximum principal stress distribution in the systems.

MATERIALS AND METHODS

A first lower molar full crown preparation was modeled by reducing the height of proximal walls by 1.5 mm and occlusal surface by 2.0 mm. The CAD-based preparation was replicated and positioned in a dental articulator for specimen fabrication. Conventional (0.5 mm uniform thickness) and modified (2.5 mm height, 1 mm thickness at the lingual extending to proximals) zirconia (Y-TZP) core designs were produced with 1.5 mm veneer porcelain. MCR controls were fabricated following conventional design. All crowns were resin cemented to 30-day aged composite dies, aged 14 days in water and either single-loaded to failure or step-stress fatigue tested. The loads were positioned either on the mesiobuccal or mesiolingual cusp (n = 21 for each ceramic system and cusp). Probability Weibull and use level probability curves were calculated. Crack evolution was followed, and postmortem specimens were analyzed and compared to clinical failures.

RESULTS

Compared to conventional and MCRs, increased levels of stress were observed in the core region for the modified Y-TZP core design. The reliability was higher in the Y-TZP-lingual-modified group at 100,000 cycles and 200 N, but not significantly different from the MCR-mesiolingual group. The MCR-distobuccal group showed the highest reliability. Fracture modes for Y-TZP groups were veneer chipping not exposing the core for the conventional design groups, and exposing the veneer-core interface for the modified group. MCR fractures were mostly chipping combined with metal coping exposure.

CONCLUSIONS

FEA showed higher levels of stress for both Y-TZP core designs and veneer layers compared to MCR. Core design modification resulted in fatigue reliability response of Y-TZP comparable to MCR at 100,000 cycles and 200 N. Fracture modes observed matched with clinical scenarios.

Performance of dental ceramics: challenges for improvements

The clinical success of modern dental ceramics depends on an array of factors, ranging from initial physical properties of the material itself, to the fabrication and clinical procedures that inevitably damage these brittle materials, and the oral environment. Understanding the influence of these factors on clinical performance has engaged the dental, ceramics, and engineering communities alike. The objective of this review is to first summarize clinical, experimental, and analytic results reported in the recent literature. Additionally, it seeks to address how this new information adds insight into predictive test procedures and reveals challenges for future improvements.

Modified Y-TZP core design improves all-ceramic crown reliability

This study tested the hypothesis that all-ceramic core-veneer system crown reliability is improved by modification of the core design. We modeled a tooth preparation by reducing the height of proximal walls by 1.5 mm and the occlusal surface by 2.0 mm. The CAD-based tooth preparation was replicated and positioned in a dental articulator for core and veneer fabrication. Standard (0.5 mm uniform thickness) and modified (2.5 mm height lingual and proximal cervical areas) core designs were produced, followed by the application of veneer porcelain for a total thickness of 1.5 mm. The crowns were cemented to 30-day-aged composite dies and were either single-load-to-failure or step-stress-accelerated fatigue-tested. Use of level probability plots showed significantly higher reliability for the modified core design group. The fatigue fracture modes were veneer chipping not exposing the core for the standard group, and exposing the veneer core interface for the modified group.

Damage and reliability of Y-TZP after cementation surface treatment

Zirconia-based restorations are widely used in prosthetic dentistry, but their susceptibility to post-sintering cementation surface treatments remains controversial. We hypothesized that grinding (600-grit) and alumina abrasion (50 microm, 5 sec, 0.5 MPa) affect the damage modes and reliability of zirconia core material. Monolithic CAD/CAM-machined and sintered Y-TZP plates (0.5 mm thickness) were adhesively cemented to dentin-like composite substrates. Uni-axial mouth-motion cyclic contact was applied through a tungsten carbide spherical indenter (r = 3.18 mm). Results showed that zirconia core ceramic is vulnerable to lower surface radial fracture after grinding or alumina abrasion, while the as-received control chiefly fractured from load-application surface cone fracture. Significantly lower reliability of ground and alumina-abraded compared with the as-received zirconia core ceramic can be attributed to damage induced on the cementation surface. Clinical relevance concerning surface treatment protocols for zirconia framework materials prior to cementation is addressed.

Concerns of hydrothermal degradation in CAD/CAM zirconia

Zirconia-based restorations are widely used in prosthetic dentistry; however, their susceptibility to hydrothermal degradation remains elusive. We hypothesized that CAD/CAM machining and subsequent surface treatments, i.e., grinding and/or grit-blasting, have marked effects on the hydrothermal degradation behavior of Y-TZP. CAD/CAM-machined Y-TZP plates (0.5 mm thick), both with and without subsequent grinding with various grit sizes or grit-blasting with airborne alumina particles, were subjected to accelerated aging tests in a steam autoclave. Results showed that the CAD/CAM-machined surfaces initially exhibited superior hydrothermal degradation resistance, but deteriorated at a faster rate upon prolonged autoclave treatment compared with ground and grit-blasted surfaces. The accelerated hydrothermal degradation of CAD/CAM surfaces is attributed to the CAD/CAM machining damage and the absence of surface compressive stresses in the fully sintered material. Clinical relevance for surface treatments of zirconia frameworks in terms of hydrothermal and structural stabilities is addressed.

Design features of a three-dimensional molar crown and related maximum principal stress. A finite element model study

OBJECTIVE

To evaluate the effects of clinically relevant variables on the maximum principal stress (MPS) in the veneer layer of an anatomically correct veneer-core-cement-tooth model.

METHODS

The average dimensions of a mandibular first molar crown were imported into CAD software; a tooth preparation was modeled by reducing the proximal walls by 1.5 mm and the occlusal surface by 2.0 mm. 'Crown systems' were composed by varying characteristics of a cement layer, structural core, and veneer solid, all designed to fit the tooth preparation. The main and interacting effects of proximal wall height reduction, core material, core thickness, cement modulus, cement thickness, and load position on the maximum stress distribution were derived from a series of finite element models and analyzed in a factorial analysis of variance.

RESULTS

The average MPS in the veneer layer over the 64 models was 488 MPa (range = 248-840 MPa). MPS increased significantly with the addition of horizontal load components and with increasing cement thickness. In addition, MPS levels varied as a function of interactions between: proximal wall height reduction and load position; load position and cement thickness; core thickness and cement thickness; cement thickness and proximal wall height reduction; and core thickness, cement thickness and proximal wall height reduction.

CONCLUSION

Rational design of veneered structural ceramics must consider the complex geometry of the crown-tooth system and integrate the influence of both the main effects and interactions among design parameters.

Laboratory simulation of Y-TZP all-ceramic crown clinical failures

Clinically, zirconia-supported all-ceramic restorations are failing by veneer-chipping without exposing the zirconia interface. We hypothesized that mouth motion step-stress-accelerated fatigue testing of standardized dental crowns would permit this previously unrecognized failure mode to be investigated. Using CAD software, we imported the average dimensions of a mandibular first molar crown and modeled tooth preparation. The CAD-based tooth preparation was rapid-prototyped as a die for fabrication of zirconia core porcelain-veneered crowns. Crowns were bonded to aged composite reproductions of the preparation and aged 14 days in water. Crowns were single-cycle-loaded to failure or mouth-motion step-stress- fatigue-tested. Finite element analysis indicated high stress levels below the load and at margins, in agreement with only single-cycle fracture origins. As hypothesized, the mouth motion sliding contact fatigue resulted in veneer chipping, reproducing clinical findings allowing for investigations into the underlying causes of such failures.

Competition of fracture mechanisms in monolithic dental ceramics: flat model systems

Monolithic (single layer) glass-ceramic restorations often fail from chipping and fracture. Using blunt indentation of a model flat porcelain-like brittle layer bonded onto a dentin-like polymer support system, a variety of fatigue fracture modes has been identified and analyzed: outer cone, inner cone, and median cracks developing in the near-contact region at the occlusal surface; radial cracks developing at the internal cementation surface along the loading axis. Our findings indicate that monolithic glass-ceramic layers are vulnerable to both occlusal surface damage and cementation internal surface fracture. Clinical issues in the longevity of ceramic restorations are discussed in relation to biting force, physical properties of ceramic crowns and luting cement, and thicknesses of ceramic and cement layers.

Fracture Modes in Curved Brittle Layers Subject to Concentrated Cyclic Loading in Liquid Environments

Damage response of brittle curved structures subject to cyclic Hertzian indentation was investigated. Specimens were fabricated by bisecting cylindrical quartz glass hollow tubes. The resulting hemi-cylindrical glass shells were bonded internally and at the edges to polymeric supporting structures and loaded axially in water on the outer circumference with a spherical tungsten carbide indenter. Critical loads and number of cycles to initiate and propagate near-contact cone cracks and far-field flexure radial cracks to failure were recorded. Flat quartz glass plates on polymer substrates were tested as a control group. Our findings showed that cone cracks form at lower loads, and can propagate through the quartz layer to the quartz/polymer interface at lower number of cycles, in the curved specimens relative to their flat counterparts. Flexural radial cracks require a higher load to initiate in the curved specimens relative to flat structures. These radial cracks can propagate rapidly to the margins, the flat edges of the bisecting plane, under cyclic loading at relatively low loads, owing to mechanical fatigue and a greater spatial range of tensile stresses in curved structures.

MicroCT analysis of hydroxyapatite bone repair scaffolds created via three-dimensional printing for evaluating the effects of scaffold architecture on bone ingrowth

Recent studies have shown that it is now possible to construct tissue-engineered bone repair scaffolds with tight pore size distributions and controlled geometries using 3-D Printing techniques (3DP). This study evaluated two hydroxyapatite (HA) 8-mm diameter discs with controlled architectures in a rabbit trephine defect at 8 and 16 weeks using a 2 x 2 factorial design. Input parameters were time and scaffold void volume at two levels. Three output variables were extracted from MicroCT data: bone volume ingrowth with respect to total region of interest, bone volume ingrowth with respect to available ingrowth volume, and soft tissue volume. The experiment measured two groups--Group 1: 500-microm x 500-microm channels parallel to the scaffold's long axis and penetrating up 3-mm from the bottom. Group 2: 800-microm x 800-microm struts spaced 500 microm apart set perpendicularly to each other in each printed layer. Rendered 3-dimensional MicroCT scans and undecalcified histological slides of implants revealed good integration with the surrounding tissue, and a sizeable amount of bone ingrowth into the device. Factorial analysis revealed that the effects of time were the greatest determinant of soft tissue ingrowth, while time and its interaction with void volume were the greatest determinants of bone volume ingrowth with respect to both total and available volume.

In vivo bone response to 3D periodic hydroxyapatite scaffolds assembled by direct ink writing

The in vivo bone response of 3D periodic hydroxyapatite (HA) scaffolds is investigated. Two groups of HA scaffolds (11 mm diameter x 3.5 mm thick) are fabricated by direct-write assembly of a concentrated HA ink. The scaffolds consist of cylindrical rods periodically arranged into four quadrants with varying separation distances between rods. In the first group, HA rods (250 microm in diameter) are patterned to create pore channels, whose areal dimensions are 250 x 250 microm(2) in quadrant 1, 250 x 500 microm(2) in quadrants 2 and 4, and 500 x 500 microm(2) in quadrant 3. In the second group, HA rods (400 microm in diameter) are patterned to create pore channels, whose areal dimensions of 500 x 500 microm(2) in quadrant 1, 500 x 750 microm(2) in quadrants 2 and 4, and 750 x 750 microm(2) in quadrant 3. Each group of scaffolds is partially densified by sintering at 1200 degrees C prior to being implanted bilaterally in trephine defects of skeletally mature New Zealand White rabbits. Their tissue response is evaluated at 8 and 16 weeks using micro-computed tomography, histology, and scanning electron microscopy. New trabecular bone is conducted rapidly and efficiently across substantial distances within these patterned 3D HA scaffolds. Our observations suggest that HA rods are first coated with a layer of new bone followed by subsequent scaffold infilling via outward and inward radial growth of the coated regions. Direct-write assembly of 3D periodic scaffolds composed of micro-porous HA rods arrayed to produce macro-pores that are size-matched to trabecular bone may represent an optimal strategy for bone repair and replacement structures.

All hazards training: incorporating a catastrophe preparedness mindset into the dental school curriculum and professional practice

Catastrophic preparedness should be incorporated into the dental school curriculum. The experience at New York University College of Dentistry is that a combination of catastrophic preparedness elements integrated within existing courses with a short, meaningful capstone course dedicated to all hazards preparedness can be accomplished successfully and meet proposed competencies for training in the dental curriculum. The roles and responsibilities in catastrophic response preparedness and response of dentists are actively being discussed by the dental profession. An element of that discussion has to include the "what" and "how" of education and training for dentists at the predoctoral level and after dental school graduation. The concepts presented in this article should be debated at all levels of the profession.

Modeling of water absorption induced cracks in resin-based composite supported ceramic layer structures

Cracking patterns in the top ceramic layers of the modeled dental multilayers with polymer foundation are observed when they are immersed in water. This article developed a model to understand this cracking mechanism. When water diffuses into the polymer foundation of dental restorations, the foundation will expand; as a result, the stress will build up in the top ceramic layer because of the bending and stretching. A finite element model based on this mechanism is built to predict the stress build-up and the slow crack growth in the top ceramic layers during the water absorption. Our simulations show that the stress build-up by this mechanism is high enough to cause the cracking in the top ceramic layers and the cracking patterns predicted by our model are well consistent with those observed in experiments on glass/epoxy/polymer multilayers. The model is then used to discuss the life prediction of different dental ceramics.

Damage accumulation and fatigue life of particle-abraded ceramics

PURPOSE

This investigation compared initial and fatigue strengths of particle-abraded ceramics to those of as-polished alumina and zirconia ceramics in crown-like layer structures.

MATERIALS AND METHODS

Alumina or zirconia plates bonded to polycarbonate substrates were subjected to single-cycle and multi-cycle contact (fatigue) loading. Cementation surfaces of the ceramic were damaged by controlled particle abrasion, indentation with a sharp diamond at low load, or a blunt indenter at high load. The stresses needed to initiate radial fractures were evaluated.

RESULTS

The strengths of specimens were lowered by fatigue loading. After the equivalent of 1 year of occlusal contacts, the strengths of undamaged specimens degraded to approximately half of their single-cycle values. In particle-abraded specimens, an additional 20% to 30% drop in strength occurred after several hundred load cycles. Particle abrasion damage was approximately equivalent to damage from sharp indentation at low load or blunt indentation at high load.

CONCLUSION

Damage from particle abrasion, not necessarily immediately apparent, compromised the fatigue strength of zirconia and alumina ceramics in crown-like structures. In fatigue, small flaws introduced by particle abrasion can outweigh any countervailing strengthening effect from compression associated with surface damage or, in the case of zirconia, with phase transformation.

Computer-aided design and fabrication of dental restorations

BACKGROUND

For more than 20 years, researchers have been trying to automate conventional manual processes in dental technology with the hope of producing higher- and more uniform-quality materials, standardizing manufacturing processes and reducing production costs.

METHODS

The authors review existing computer-aided design (CAD)/computer-aided manufacturing (CAM) systems, describing the components of CAD/CAM technologies and addressing the limitations of current systems, and suggest possibilities for future systems.

CONCLUSIONS

Existing dental CAD/CAM systems vary dramatically in their capabilities; each has distinct advantages and limitations. None can yet acquire data directly in the mouth and produce the full spectrum of restoration types (with the breadth of material choices) that can be created by traditional techniques. Emerging technologies may expand dramatically the capabilities of future systems, but they also may require a different type of training to use them to their full effectiveness.

CLINICAL IMPLICATIONS

In the future, automatically fabricated, fully esthetic restorations might be produced more quickly and have longer lifetimes than restorations currently produced with CAD/CAM systems.

Proposed educational objectives for hospital-based dentists during catastrophic events and disaster response

The purpose of this project was to define education and training requirements for hospital-based dentists to efficiently and meaningfully participate in a hospital disaster response. Eight dental faculty with hospital-based training and/or military command and CBRNE (chemical, biological, radiological, nuclear, and explosive) expertise were recruited as an expert panel. A consensus set of recommended educational objectives for hospital-based dentists was established using the following process: 1) identify assumptions supported by all expert panelists, 2) determine current advanced dental educational training requirements, and 3) conduct additional training and literature review by various panelists and discussions with other content and systems experts. Using this three-step process, educational objectives that the development group believed necessary for hospital-based dentists to be effective in treatment or management roles in times of a catastrophic event were established. These educational objectives are categorized into five thematic areas: 1) disaster systems, 2) triage/medical assessment, 3) blast and burn injuries, 4) chemical agents, and 5) biological agents. Creation of training programs to help dentists acquire these educational objectives would benefit hospital-based dental training programs and strengthen hospital surge manpower needs. The proposed educational objectives are designed to stimulate discussion and debate among dental, medical, and public health professionals about the roles of dentists in meeting hospital surge manpower needs.

Introducing a senior course on catastrophe preparedness into the dental school curriculum

This article describes an integrated fourth‐year course in catastrophe preparedness for students at the New York University College of Dentistry (NYUCD). The curriculum is built around the competencies proposed in “Predoctoral Dental School Curriculum for Catastrophe Preparedness,” published in the August 2004 Journal of Dental Education. We highlight our experience developing the program and offer suggestions to other dental schools considering adding bioterrorism studies to their curriculum.

Introducing a senior course on catastrophe preparedness into the dental school curriculum

This article describes an integrated fourth-year course in catastrophe preparedness for students at the New York University College of Dentistry (NYUCD). The curriculum is built around the competencies proposed in "Predoctoral Dental School Curriculum for Catastrophe Preparedness," published in the August 2004 Journal of Dental Education. We highlight our experience developing the program and offer suggestions to other dental schools considering adding bioterrorism studies to their curriculum.

Factorial analysis of variables influencing stress in all-ceramic crowns

OBJECTIVE

The objective of this investigation was to evaluate the relative contribution of variables in the crown-cement-tooth system that can influence magnitude of maximum principal stress in all-ceramic crowns. METHODS.: Factorial analysis was performed to calculate the coefficients of main and interactive effects of seven variables on maximum stress distribution in a series of finite element models of an axisymmetric stylized ceramic crown-cement-tooth system. Variables investigated at two levels were selected for their clinical relevance and included those relating to crown material and geometry (thickness and cuspal incline), cement modulus and thickness, supporting tooth core, and position at which the occlusal load was applied.

RESULTS

The average principal stress for all combinations of all variables was 135.1 MPa but stress magnitude ranged from 73.7 to 214.0 MPa. Crown material and thickness are of primary importance in stress magnitude but other variables (cement modulus, load position, and supporting tooth core) also contribute to the stress magnitude. Interactions between these variables can have an important influence, particularly since the stress in the crown is not necessarily sensitive to the same factors for all crown material systems.

SIGNIFICANCE

Comprehensive evaluation of stress in a crown of a crown-cement-tooth system must integrate the influence of single variables and their interactions.

The Medical Reserve Corps. an opportunity for dentists to serve

The response to the events of Sept. 11, 2001, relied on local resources and personnel. Aware of how important their contribution could be, many people are now inspired to volunteer during times of crisis. The Medical Reserve Corps is a community-based volunteer network of health professionals that trains to respond to large-scale emergencies.

Emergency preparedness in the dental office

BACKGROUND

Terrorist activities now can be added to the list of possible man-made and nature-induced health and safety disasters that can affect a community. There are two basic responses that people can choose to protect themselves during these events. One is to evacuate the area, the other is to shelter in place.

CONCLUSIONS

The authors provide an overview of the issues, present basic principles and increase the awareness of the dental profession to the various responses available in an emergency. The key issue is that families, dental offices and communities should plan ahead. Dentists should be cognizant of their professional role and help educate the public in regard to emergency issues.

PRACTICE IMPLICATIONS

The uncertainties and stress of a potential terrorist attack can be mitigated somewhat by planning. These plans can be fairly basic, involving minimum equipment and supplies; however, they may go a long way to protect dental staff members, patients and families.

Effect of sandblasting on the long-term performance of dental ceramics

A study has been made of the effects of sandblasting on the strength of Y-TZP and alumina ceramic layers joined to polymeric substrates and loaded at the top surfaces by a spherical indenter, in simulation of occlusal contact in ceramic crowns on tooth dentin. The sandblast treatment is applied to the ceramic bottom surface before bonding to the substrate, as in common dental practice. Specimens with polished surfaces are used as a control. Tests are conducted with monotonically increasing (dynamic) and sinusoidal (cyclic) loading on the spherical indenter, up to the point of initiation of a radial fracture at the ceramic bottom surface immediately below the contact. For the polished specimens, data from the dynamic and cyclic tests overlap, consistent with a dominant slow crack growth mode of fatigue. Strengths of sandblasted specimens show significant reductions in both dynamic and cyclic tests, indicative of larger starting flaws. However, the shift is considerably greater in the cyclic data, suggesting some mechanically assisted growth of the sandblast flaws. These results have implications in the context of lifetimes of dental crowns.

Performance of hydroxyapatite bone repair scaffolds created via three-dimensional fabrication techniques

The current study analyzes the in vivo performance of porous sintered hydroxyapatite (HA) bone repair scaffolds fabricated using the TheriForm solid freeform fabrication process. Porous HA scaffolds with engineered macroscopic channels had a significantly higher percentage of new bone area compared with porous HA scaffolds without channels in a rabbit calvarial defect model at an 8-week time point. An unexpected finding was the unusually large amount of new bone within the base material structure, which contained pores less than 20 microm in size. Compared with composite scaffolds of 80% polylactic-co-glycolic acid and 20% beta-tricalcium phosphate with the same macroscopic architecture as evaluated in a previous study, the porous HA scaffolds with channels had a significantly higher percentage of new bone area. Therefore, the current study indicates that scaffold geometry, as determined by the fabrication process, can enhance the ability of a ceramic material to accelerate healing of calvarial defects.

Predoctoral dental school curriculum for catastrophe preparedness

Preparing for catastrophic events, both human-made and natural, is in the national interest and has become a priority since catastrophic events in Oklahoma City, Washington, DC, and New York City. Dentists are a large source of non-physician health manpower that could contribute to the public welfare during catastrophic events that require additional public health human resources. Dentists, by virtue of their education, understand biomedical concepts and have patient care skills that can be directly applied during a catastrophic event. Dentists also can provide training for other types of health care workers and can supervise these individuals. In this article, we propose that dentistry can make a significant contribution as part of a national response before, during, and after a catastrophic event or at the time of a public health emergency. We describe the potential collaboration among a dental school, city and state health departments, law enforcement, the military, and others to develop a curriculum in catastrophe preparedness. Then we describe one dental school's effort to build a catastrophe preparedness curriculum for our students. The competencies, goals and objectives, and sources of content for this catastrophe preparedness curriculum are described as well as suggestions for sequencing instruction.

Meeting a disaster's medical surge demand: can dentists help?

No one can deny that these are extraordinary times in world affairs. This is true not because there is turmoil in the world, which tends to be the norm, but because the problems in the world are dominated not by countries or religions, but by individuals and radical sects for whom life seems far too expendable. It is a time for each of us to determine how to respond to help protect our families. It is also a time for dentists to assess how they should respond to meet the additional demands placed upon them by way of their professional licenses and underlying responsibility to society. This article frames a set of commitments that the profession should consider assuming. It is related from the perspective of an institution that has taken an active role in societal protection from before Sept. 11, 2001, to the present. The discussion includes a description of the nature of medical surge demand and why the dental profession is uniquely positioned to assist in meeting this demand. The skill set of the dentist is highlighted in terms of triage ability, a role in bio-surveillance, the capacity to calm the "worried well," and community responsiveness. In addition, concepts like shelter in place and the Medical Reserve Corps are explained, and valuable reference sources and websites are provided.

Preparing for a terrorist event: a scenario-driven approach

Since September 11, 2001, government agencies on all levels have focused on planning and preparing to respond to another possible terrorist attack. In addition to emergency and medical issues, these agencies must be concerned about the public's behavior and psychological response when they plan the management of a bioterrorist event. We present readers with one such possible incident, a radiological bomb scenario, with the aim of educating dentists and communicating the risks involved.

Materials design in the performance of all-ceramic crowns

Results from a systematic study of damage in material structures representing the basic elements of dental crowns are reported. Tests are made on model flat-layer specimens fabricated from various dental ceramic combinations bonded to dentin-like polymer substrates, in bilayer (ceramic/polymer) and trilayer (ceramic/ceramic/polymer) configurations. The specimens are loaded at their top surfaces with spherical indenters, in simulation of occlusal function. The onset of fracture is observed in situ using a video camera system mounted beneath the transparent polymer substrate. Critical loads to induce fracture and deformation at the ceramic top and bottom surfaces are measured as functions of layer thickness and contact duration. Radial cracking at the ceramic undersurface occurs at relatively low loads, especially in thinner layers. Fracture mechanics relations are used to confirm the experimental data trends, and to provide explicit dependencies of critical loads in terms of key variables: material-elastic modulus, hardness, strength and toughness; geometric-layer thicknesses and contact radius. Tougher, harder and (especially) stronger materials show superior damage resistance. Critical loads depend strongly (quadratically) on crown net thickness. The analytic relations provide a sound basis for the materials design of next-generation dental crowns.

Performance of degradable composite bone repair products made via three-dimensional fabrication techniques

This study analyzed the in vivo performance of composite degradable bone repair products fabricated using the TheriForm process, a solid freeform fabrication (SFF) technique, in a rabbit calvarial defect model at 8 weeks. Scaffolds were composed of polylactic-co-glycolic acid (PLGA) polymer with 20% w/w beta-tricalcium phosphate (beta-TCP) ceramic with engineered macroscopic channels, a controlled porosity gradient, and a controlled pore size for promotion of new bone ingrowth. Scaffolds with engineered macroscopic channels and a porosity gradient had higher percentages of new bone area compared to scaffolds without engineered channels. These scaffolds also had higher percentages of new bone area compared to unfilled control defects, suggesting that scaffold material and design combinations could be tailored to facilitate filling of bony defects. This proof-of-concept study demonstrated that channel size, porosity, and pore size can be controlled and used to influence new bone formation and calvarial defect healing.

Engineered cellular response to scaffold architecture in a rabbit trephine defect

Tight control of pore architecture in porous scaffolds for bone repair is critical for a fully elucidated tissue response. Solid freeform fabrication (SFF) enables construction of scaffolds with tightly controlled pore architecture. Four types of porous scaffolds were constructed using SFF and evaluated in an 8-mm rabbit trephine defect at 8 and 16 weeks (n = 6): a lactide/glycolide (50:50) copolymer scaffold with 20% w/w tri-calcium phosphate and random porous architecture (Group 1); another identical design made from poly(desaminotyrosyl-tyrosine ethyl ester carbonate) [poly(DTE carbonate)], a tyrosine-derived pseudo-polyamino acid (Group 2); and two poly(DTE carbonate) scaffolds containing 500 microm pores separated by 500-microm thick walls, one type with solid walls (Group 3), and one type with microporous walls (Group 4). A commercially available coralline scaffold (Interpore) with a 486-microm average pore size and empty defects were used as controls. There was no significant difference in the overall amount of bone ingrowth in any of the devices, as found by radiographic analysis, but patterns of bone formation matched the morphology of the scaffold. These results suggest that controlled scaffold architecture can be superimposed on biomaterial composition to design and construct scaffolds with improved fill time.

Bioterrorism and catastrophe response: a quick-reference guide to resources

BACKGROUND

Dentists' responses to catastrophe have been redefined by bioterrorism. Informed response requires accurate information about agents and diseases that have the potential to be used as weapons.

METHODS

The authors reviewed information about the most probable bioterrorist weapons (those from the Center for Disease Control and Prevention's Category A) from the World Wide Web and print journals and distilled it into a resource list that is current, relevant to dentistry and noncommercial. The Web sites cited include those sponsored by federal agencies, academic institutions and professional organizations. The articles cited include those published in English within the last six years in refereed journals that are available in most higher education institutions.

RESULTS

The authors present the information in a table that provides a quick-reference guide to resources describing agents and diseases with the greatest potential for use as weapons: anthrax, botulism, plague, smallpox, tularemia and viral hemorrhagic fevers. This article presents Web site and journal citations for background and patient-oriented information (fact sheets), signs and symptoms, and prophylactic measures and treatment for each of the agents and diseases. The table facilitates quick access to this information, especially in an emergency. This article also points out guidelines for response should a suspected attack occur.

CONCLUSIONS

Armed with information about biological weapons, dentists can provide faster diagnosis, inform their patients about risks, prophylaxis or treatment and rethink their own role in terrorism response.

CLINICAL IMPLICATIONS

Fast, accurate diagnosis limits the spread of exceptionally contagious diseases. Providing accurate information to patients minimizes misinformation and the associated public fear and panic that, unchecked, could overwhelm health care systems.

Enhancing medical and public health capabilities during times of crisis

Terrorist attacks and other catastrophic events will create demands that severely challenge the capacity of the medical/public health system. To meet the surge, a cadre of professionals should be trained to operate around the nucleus of medical/public health officials. At New York University, an inter-institutional team is considering specific roles for and an approach to training dentists to enable these health care professionals to supplement medical/public surge needs based upon informatics systems that provide critical information.

Materials design of ceramic-based layer structures for crowns

Radial cracking has been identified as the primary mode of failure in all-ceramic crowns. This study investigates the hypothesis that critical loads for radial cracking in crown-like layers vary explicitly as the square of ceramic layer thickness. Experimental data from tests with spherical indenters on model flat laminates of selected dental ceramics bonded to clear polycarbonate bases (simulating crown/dentin structures) are presented. Damage initiation events are video-recorded in situ during applied loading, and critical loads are measured. The results demonstrate an increase in the resistance to radial cracking for zirconia relative to alumina and for alumina relative to porcelain. The study provides simple a priori predictions of failure in prospective ceramic/substrate bilayers and ranks ceramic materials for best clinical performance.

Mechanical characterization of dental ceramics by hertzian contacts

Hertzian indentation testing is proposed as a protocol for evaluating the role of microstructure in the mechanical response of dental ceramics. A major advantage of Hertzian indentation over more traditional fracture-testing methodologies is that it emulates the loading conditions experienced by dental restorations: Clinical variables (masticatory force and cuspal curvature) identify closely with Hertzian variables (contact load and sphere radius). In this paper, Hertzian responses on four generic dental ceramics systems-micaceous glass-ceramics, glass-infiltrated alumina, feldspathic porcelain, and transformable zirconiaare presented as case studies. Ceramographic sectioning by means of a "bonded-interface" technique provides new information on the contact damage modes. Two distinct modes are observed: "brittle" mode, classic macroscopic fracture outside the contact (ring, or cone cracks), driven by tensile stresses; and "quasi-plastic" mode, a relatively new kind of deformation below the contact (diffuse microdamage), driven by shear stresses. A progressive transition from the first to the second mode with increasing microstructural heterogeneity is observed. The degree of quasi-plasticity is readily apparent as deviations from ideal linear elastic responses on indentation stress-strain curves. Plots of threshold loads for the initiation of both fracture and deformation modes as a function of indenter radius constitute "damage maps" for the evaluation of prospective restoration damage under typical masticatory conditions. The degree of damage in both modes evolves progressively with load above the thresholds. Strength tests on indented specimens quantify sustainable stress levels on restoration materials after damage. The most brittle responses are observed in the fine glass-ceramics and porcelain; conversely, the most quasi-plastic responses are observed in the coarse glass-ceramics and zirconia; the medium glass-ceramics and alumina exhibit intermediate responses. Implications of the results in relation to future materials characterization, selection, and design are considered in the clinical context.

Indentation damage and mechanical properties of human enamel and dentin

Understanding the mechanical properties of human teeth is important to clinical tooth preparation and to the development of "tooth-like" restorative materials. Previous studies have focused on the macroscopic fracture behavior of enamel and dentin. In the present study, we performed indentation studies to understand the microfracture and deformation and the microcrack-microstructure interactions of teeth. It was hypothesized that crack propagation would be influenced by enamel rods and the dentino-enamel junction (DEJ), and the mechanical properties would be influenced by enamel rod orientation and tooth-to-tooth variation. Twenty-eight human third molars were used for the measurement of hardness, fracture toughness, elastic modulus, and energy absorbed during indentation. We examined the effect of enamel rod orientation by propagating cracks in the occlusal surface, and in the axial section in directions parallel and perpendicular to the occlusal surface. The results showed that the cracks in the enamel axial section were significantly longer in the direction perpendicular to the occlusal surface than parallel. The cracks propagating toward the DEJ were always arrested and unable to penetrate dentin. The fracture toughness of enamel was not single-valued but varied by a factor of three as a function of enamel rod orientation. The elastic modulus of enamel showed a significant difference between the occlusal surface and the axial section. It is concluded that the cracks strongly interact with the DEJ and the enamel rods, and that the mechanical properties of teeth are functions of microstructural orientations; hence, single values of properties (e.g., a single toughness value or a single modulus value) should not be used without information on microstructural orientation.

Enamel subsurface damage due to tooth preparation with diamonds

In clinical tooth preparation with diamond burs, sharp diamond particles indent and scratch the enamel, causing material removal. Such operations may produce subsurface damage in enamel. However, little information is available on the mechanisms and the extent of subsurface damage in enamel produced during clinical tooth preparation. The aim of this study, therefore, was to investigate the mechanisms of subsurface damage produced in enamel during tooth preparation by means of diamond burs, and to examine the dependence of such damage on enamel rod orientation, diamond particle size, and removal rate. Subsurface damage was evaluated by a bonded-interface technique. Tooth preparation was carried out on two enamel rod orientations, with four clinical diamond burs (coarse, medium, fine, and superfine) used in a dental handpiece. The results of this study showed that subsurface damage in enamel took the form of median-type cracks and distributed microcracks, extending preferentially along the boundaries between the enamel rods. Microcracks within individual enamel rods were also observed. The median-type cracks were significantly longer in the direction parallel to the enamel rods than perpendicular to the rods. Preparation with the coarse diamond bur produced cracks as deep as 84 +/- 30 microns in enamel. Finishing with fine diamond burs was effective in crack removal. The crack lengths in enamel were not significantly different when the removal rate was varied. Based on these results, it is concluded that subsurface damage in enamel induced by tooth preparation takes the form of median-type cracks as well as inter- and intra-rod microcracks, and that the lengths of these cracks are sensitive to diamond particle size and enamel rod orientation, but insensitive to removal rate.

Force degradation in elastomeric chains

Elastomeric chains are a frequent choice for delivering forces required to close spaces orthodontically. Unfortunately, these forces degrade over time. Open and closed chains from six orthodontic suppliers were evaluated over time. For both types and for all suppliers, the greatest loss of force occurred within the first hour. During the next 2 to 4 days, forces delivered continued to fall but at a slower rate. Beyond that time, in general, forces delivered remain nearly constant but at a level lower than originally available. The amount of the force delivered at 28 days ranged from 85% to 30% of that available at the time of placement. At 28 days, gray chains from all suppliers delivered forces greater than 100 g.

Confirmation of Leinfelder clinical wear standards

OBJECTIVES

Accuracy of composite wear studies based on Leinfelder standards has been disputed. There are differences with other well-calibrated systems such as the M-L and Vivadent wear standards. The objective of this study was to reevaluate the margin height at key regions along the restoration margins for each of the 6 Leinfelder standards using laser profiling techniques.

METHODS

The Leinfelder standards were profiled in parallel paths 100 microns apart and measured in x-y-z position every 20 microns along those paths using a laser profilometer.

RESULTS

Rounding of cavosurface enamel margins from intraoral wear greatly increased the uncertainty of the true enamel margin location and step height measurements, precluding unequivocal measurements for standards #2 and #3. Values for other standards for the original report, newly measured means and standard errors, and measured ranges were: #4 (322 microns, 333 +/- 34 microns, 171-507 microns), #5 (382 microns, 459 +/- 44 microns, 202-649 microns), and #6 (493 microns, 584 +/- 91 microns, 315-1022 microns). There were no statistically significant differences (p < or = 0.10) between these and original values. Large standard errors may have obscured small differences that may exist. The Leinfelder cast conversion scale seems to be the correct relative magnitude.

SIGNIFICANCE

Differences between Leinfelder casts and other standards may be due to differences in shadow production. Clinical wear may be systematically underestimated by other cast evaluation methods that have well-defined margins. This emphasizes the need for standard casts with margin morphology similar to the clinical casts being evaluated for wear.

High-technology innovations--and limitations--for restorative dentistry

Technology has brought some exciting advances to dentistry--restorations that can be produced in one appointment and fit as well as cast restorations; laser procedures that arrest early caries, etch enamel, and that the patient finds more comfortable; and improved diagnostic capabilities through computer-enhanced images. Dental applications of these technologies are in their infancy and most of the limitations discussed in this article will be eliminated in the near future. More and more applications and possibilities are continually evolving. Technology promises to enhance dentistry for the clinician and patient alike.