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.

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.

Digital dentistry: The new state of the art - Is it disruptive or destructive?

OBJECTIVE: Summarizing the new state of the art of digital dentistry, opens exploration of the type and extent of innovations and technological advances that have impacted - and improved - dentistry. The objective is to describe advances and innovations, the breadth of their impact, disruptions and advantages they produce, and opportunities created for material scientists. METHODS: On-line data bases, web searches, and discussions with industry experts, clinicians, and dental researchers informed the content. Emphasis for inclusion was on most recent publications along with innovations presented at trade shows, in press releases, and discovered through discussions leading to web searches for new products. RESULTS: Digital dentistry has caused disruption on many fronts, bringing new techniques, systems, and interactions that have improved dentistry. Innovation has spurred opportunities for material scientists' future research. SIGNIFICANCE: With disruptions intrinsic in digital dentistry's new state of the art, patient experience has improved. More restoration options are available delivering longer lifetimes, and better esthetics. Fresh approaches are bringing greater efficiency and accuracy, capitalizing on the interest, capabilities, and skills of those involved. New ways for effective and efficient inter-professional and clinician-patient interactions have evolved. Data can be more efficiently mined for forensic and epidemiological uses. Students have fresh ways of learning. New, often unexpected, partnerships have formed bringing further disruption - and novel advantages. Yes, digital dentistry has been disruptive, but the abundance of positive outcomes argues strongly that it has not been destructive.

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.

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.

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.

Computer-aided design and fabrication of dental restorations: current systems and future possibilities.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.