Future of dental biomaterials: Gazing into Bob's crystal ball.

In February 2019, the American Prosthodontics Society asked me to "ponder" the future of dental biomaterials. This talk was titled, "Gazing Into Bob's Crystal Ball". My basic approach was guided by the words of Confucius, "Study the past if you would define the future." As I have often studied and lectured on how many of our important materials got into dentistry, it was a natural assignment. In this article, I review how key materials came to be used in dental practice and organize these "sources" into an extended classification, assuming future materials will likely derive from one of these. Then, I identify important examples in each category and then expound on likely future developments, some already in progress. These include increasing the use of and benefiting from automated systems; better engineering with existing materials by means of failure analysis (from clinically retrieved specimens); iterative improvements in materials and practices, again based on observations from clinical behavior; the use of scaffolds for both hard- and soft-tissue repair; and the development of "smart" materials. I hope this contribution survives the test of time. Please note that, importantly, this list includes not just materials but often-overlooked steps in processing that will affect their properties and clinical durability.

Fatigue strength of bilayered ceramics under cyclic loading as a function of core veneer thickness ratios.

STATEMENT OF PROBLEM: Minimal evidence is available concerning the appropriate thickness of each layer in bilayered ceramic systems. PURPOSE: The purpose of this in vitro study was to examine the effect of core-veneer thickness ratios on the fatigue strength of a bonded bilayered ceramic system. MATERIAL AND METHODS: Specimens of Ivoclar Porcelain System (IPS) e.max lithium disilicate were fabricated with core/veneer thicknesses of 0.5/1.0 mm, 0.75/0.75 mm, 1.0/0.5 mm, and 1.5/0.0 mm. All specimens were cemented to bases of a dentin-like material. Each specimen was cyclically loaded by a 2-mm-diameter G10 piston in water. Loads ranging from 10 N to the target load were applied at a frequency of 20 Hertz for 500,000 cycles. If cracked, the next specimen was cycled at a lower load; if not cracked, at a higher load (step size of 25 N). RESULTS: Mean and standard deviations of fatigue loads for the different core thicknesses were 0.5-mm core 610.94 N ±130.11; 0.75-mm core 600.0 N ±132.80; 1.0-mm core 537.50 N ±41.67; a Nd 1.5-mm core 501.14 N ±70.12. All veneered groups were significantly stronger than the full thickness group (ANOVA, P<.001; 95% post hoc). Cone cracking was observed only in the 2 thinner core groups (χ(2) test, P<.05), possibly indicating residual stresses. CONCLUSIONS: Results indicate that the addition of veneering porcelain to lithium disilicate cores increases the fatigue strength of the biceramic system.

Porcelain-titanium bonding with a newly introduced, commercially available system.

STATEMENT OF PROBLEM: Titanium is widely used in implant dentistry because of its high strength, toughness, biocompatibility, and low cost. The fabrication of complex substructures is now possible with a number of automated systems. One major obstacle to broad clinical acceptance of porcelain-titanium prostheses is the poor ceramic-metal bonding. PURPOSE: The purpose of this study was to compare titanium bonding with a traditional bonder and a newly introduced titanium bonding system. Bonding was characterized by the delamination crack initiation stress per ISO 9693:2009. MATERIAL AND METHODS: Grade 4 titanium strips were laser machined to meet ISO 9693:2009 standards. Bonding and firing of opaque porcelain was accomplished by using a traditional titanium ceramic system and a newly introduced bonding system. Variables tested besides the bonding system included the length of the porcelain block (7 mm, 8 mm, 9 mm); the thickness of the block (1.0 mm, 1.6 mm); testing the specimens 1 mm off-center; and reusing the strips. Crack initiation stresses were calculated and compared by ANOVA and the Duncan test (α=.05). RESULTS: The newly introduced commercially available system gave the strongest bond (41.82 ±5.7 MPa), followed by the 9 mm specimens (32.2 ±3.9 MPa), then a statistically similar group of the 8 mm, 7 mm, and 1.6 mm thick specimens with new Ti (approximately 23 MPa), and last a similar group with 7 mm, 8 mm, and 1 mm offset loading on reused Ti (approximately 16 MPa). The lower limit under ISO 9693:2009 was 23 MPa. CONCLUSIONS: The newly introduced commercially available system achieved much higher bond strengths than traditional titanium systems, even better than those for many high-gold alloys.

The influence of different convergence angles and resin cements on the retention of zirconia copings.

PURPOSE: This in vitro study aimed to determine the ability of three resin cements to retain zirconia copings under two clinically simulated conditions. MATERIALS AND METHODS: Extracted human molars (72) were collected, cleaned, and divided into two groups. All teeth were prepared with a 15° total convergence angle for group 1 and a 30° total convergence angle for group 2, a flat occlusal surface, and approximately 4-mm axial length. Each group was divided by surface area into three subgroups (n = 12). All zirconia copings were abraded with 50-µm Al(2)O(3), then cemented using Panavia F 2.0 (PAN-1) (PAN-2) Rely X Unicem (RXU-1) (RXU-2), and Clearfil SA (CSA-1) (CSA-2). After cementation, the copings were thermocycled for 5000 cycles between 5°C and 55°C with a 15-second dwell time. Then the copings were subjected to dislodgment force in a universal testing machine at 0.5 mm/min. The force of removal was recorded, and the dislodgement stress was calculated. A Kruskal-Wallis test (nonparametric ANOVA) was used to analyze the data (α= 0.05), and the nature of failure was also recorded. RESULTS: The mean (SD) coping removal stresses (MPa) were as follows: PAN-1: 6.0 (1.3), CSA-1: 4.8 (1.4), RXU-1: 5.5 (2.3), PAN-2: 2.8 (1.1), CSA-2: 3.0 (1.25), and RXU-2: 2.6 (1.2). The Kruskal-Wallis test was significant. Mann-Whitney pairwise comparisons of the subgroups were significant (p < 0.05) for the comparisons between subgroups of group 1 and group 2. Mode of failure was mixed, with cement remaining principally on the tooth for PAN. For CSA and RXU, mode of failure was mixed with cement remaining principally on the zirconia copings. CONCLUSIONS: Retention values of zirconia copings with three different resin cements were not significantly different. Retention of zirconia copings cemented on the teeth with adequate resistance and retention form was higher than that cemented on teeth lacking these forms. The cement remained mostly on the tooth with the adhesive resin cement with a dentin bonding system. The cement remained mostly on the coping with the self-adhesive resin cement.

Computer-aided designed/computer-assisted manufactured (CAD/CAM) all-ceramic crowns appear to perform better than all-composite resin crowns following the first 3 years of placement.

ARTICLE TITLE AND BIBLIOGRAPHIC INFORMATION: Computer-aided designed/computer-assisted manufactured composite resin versus ceramic single-tooth restorations: a 3-year clinical study. Vanoorbeek S, Vandamme K, Lijnen I, Naert I. Int J Prosthodont 2010;23(3):223-30. REVIEWER: J. Robert Kelly, DDS, MS, DMedSc PURPOSE/QUESTION: To ascertain the 3-year success and survival rates of all-composite resin and all-ceramic single-tooth restorations manufactured with a CAD/CAM system SOURCE OF FUNDING: GC Corporation Europe and UNI-DENT University Hospitals Dental Laboratories, Leuven, Belgium, provided some support for this study TYPE OF STUDY/DESIGN: Cohort study LEVEL OF EVIDENCE: Level 2: Limited-quality, patient-oriented evidence STRENGTH OF RECOMMENDATION GRADE: Not applicable.

Development of a clinically validated bulk failure test for ceramic crowns.

STATEMENT OF PROBLEM: Traditional testing of ceramic crowns creates a stress state and damage modes that differ greatly from those seen clinically. There is a need to develop and communicate an in vitro testing protocol that is clinically valid. PURPOSE: The purpose of this study was to develop an in vitro failure test for ceramic single-unit prostheses that duplicates the failure mechanism and stress state observed in clinically failed prostheses. This article first compares characteristics of traditional load-to-failure tests of ceramic crowns with the growing body of evidence regarding failure origins and stress states at failure from the examination of clinically failed crowns, finite element analysis (FEA), and data from clinical studies. Based on this analysis, an experimental technique was systematically developed and test materials were identified to recreate key aspects of clinical failure in vitro. MATERIAL AND METHODS: One potential dentin analog material (an epoxy filled with woven glass fibers; NEMA grade G10) was evaluated for elastic modulus in blunt contact and for bond strength to resin cement as compared to hydrated dentin. Two bases with different elastic moduli (nickel chrome and resin-based composite) were tested for influence on failure loads. The influence of water during storage and loading (both monotonic and cyclic) was examined. Loading piston materials (G10, aluminum, stainless steel) and piston designs were varied to eliminate Hertzian cracking and to improve performance. Testing was extended from a monolayer ceramic (leucite-filled glass) to a bilayer ceramic system (glass-infiltrated alumina). The influence of cyclic rate on mean failure loads was examined (2 Hz, 10 Hz, 20 Hz) with the extremes compared statistically (t test; α=.05). RESULTS: Failure loads were highly influenced by base elastic modulus (t test; P<.001). Cyclic loading while in water significantly decreased mean failure loads (1-way ANOVA; P=.003) versus wet storage/dry cycling (350 N vs. 1270 N). G10 was not significantly different from hydrated dentin in terms of blunt contact elastic behavior or resin cement bond strength. Testing was successful with the bilayered ceramic, and the cycling rate altered mean failure loads only slightly (approximately 5%). CONCLUSIONS: Test methods and materials were developed to validly simulate many aspects of clinical failure.

A variety of patient factors may influence porcelain veneer survival over a 10-year period.

SUBJECTS: Clinical records were reviewed for more than 80,000 different patients from the General Dental Services (GDS) in England and Wales. A total of 2562 porcelain veneer restorations were placed in 1177 adult patients (> or =18 years old, 35% male and 65% female) over a period of 11 years (January, 1991, to March, 2002). KEY EXPOSURE/STUDY FACTOR: A number of factors were considered potentially capable of impacting the need for reintervention on teeth restored with porcelain veneers. These included a number of patient factors such as gender, age, and dental utilization practices. Other factors analyzed included the tooth position receiving veneers, dentists' qualifications, and where and when the veneer was placed (eg, month and geographic location). MAIN OUTCOME MEASURE: The main outcome was survival over 10 years between initial placement and the first date of intervention for each tooth treated with a porcelain veneer. Information was abstracted from clinical records to create an analytical dataset of porcelain veneers, with their dates of placement and their dates, if any, of reintervention. This dataset was analyzed using a modified Kaplan-Meier statistic and a Cox-regression model. MAIN RESULTS: Factors that were found to influence the survival of porcelain veneers included the following: gender, with veneers placed in male patients surviving for less time than those placed for female patients (P = .01); patient age, with poorer survival of porcelain veneers in older (60 years or older) patients (P = .003); changing dentist, with teeth of patients who change dentist surviving for shorter periods of time to reintervention than for those patients who continue in the care of the same dentist (P < .001); patient's treatment need, with patients whose gross annual fees on all dental treatment are higher having teeth with less time to reintervention after receiving veneers than those patients with lower annual fees (P < .001); patient paying status, with patients who are exempt from charges also having veneered teeth with shorter time to reintervention (P <.001). Nevertheless, the factors associated with tooth position or mouth quadrant and with the dentist (gender, age, years since graduation, and country of qualification) did not influence the survival rate. CONCLUSIONS: Although dentist factors do not appear to play a part, a variety of patient factors have been found to influence veneer survival to reintervention. Overall, 53% of porcelain veneers in the study survived without reintervention at 10 years.

Effect of occlusal contact size on interfacial stresses and failure of a bonded ceramic: FEA and monotonic loading analyses.

OBJECTIVE: This study investigated whether contact area (i.e. facet size) would influence the loads necessary for subsurface radial crack formation in porcelain specimens bonded to a dentin analog material. METHODS: Dental porcelain discs (0.5 mm, 1 mm, and 1.5 mm thick) were bonded to compliant bases simulating dentin, and loaded with either a 1 mm, 2 mm, or 3 mm diameter aluminum piston until fracture. Pop-in of the subsurface radial crack from the bonded interface was detected using the acoustic emission (AE) method. Pre-test and post-test finite element analysis (FEA) was used to model the experiment and to calculate subsurface failure stresses. RESULTS: There were significant differences in loads sustained before fracture according to both the ceramic thickness and the piston diameter (p<0.05; ANOVA and post-hoc Scheffe). Failure loads were found to be proportional to the square of the porcelain thickness. For all thicknesses, significantly higher loads were sustained beneath the 3mm piston than beneath the 1mm piston. FEA calculated failure strengths for the 1 mm thick porcelain (calculated from experimental mean loads) differed significantly for loading with the 1mm piston (168 MPa) or 3 mm piston (60 MPa). CONCLUSIONS: It appears that both ceramic thickness and contact facet size may be clinically controlled to increase load-bearing ability of all-ceramic crowns. Single value strengths may not accurately model bonded dental ceramics; adjustments such as with Weibull scaling may improve accuracy. These results further suggest that small spherical indenters do not create clinically analogous contact conditions.

State of the art of zirconia for dental applications.

Zirconia has been recently introduced in prosthetic dentistry for the fabrication of crowns and fixed partial dentures, in combination with CAD/CAM techniques. This review encompasses the specific types of zirconia available in dentistry, together with their properties. The two main processing techniques, soft and hard machining, are assessed in the light of their possible clinical implications and consequences on the long-term performance of zirconia. An update on the status of clinical trials occurring worldwide is provided.

Stabilized zirconia as a structural ceramic: an overview.

This review introduces concepts and background from the ceramics engineering literature regarding metastable zirconia ceramics to establish a context for understanding current and emerging zirconia-based dental ceramics.

The clinical success of all-ceramic restorations.

BACKGROUND: The authors conducted a comprehensive literature review to compile and compare clinical evidence for the treatment of teeth using all-ceramic restorations. TYPES OF STUDIES REVIEWED: The authors searched the English-language peer-reviewed literature using MEDLINE and PubMed with a focus on research published between 1993 and 2008. They also conducted a hand search of relevant dental journals. They reviewed randomized controlled trials, nonrandomized controlled studies, longitudinal experimental clinical studies, longitudinal prospective studies and longitudinal retrospective studies. RESULTS: Evidence suggests that for veneers, intracoronal restorations and complete-coverage restorations for single-rooted anterior teeth, clinicians may choose from any all-ceramic system on the basis of esthetic needs (many systems have had greater than 90 percent success at six years). Well-studied molar restorations include those made of alumina and, increasingly, zirconia and bonded lithium disilicate. Reasonable evidence has shown the effectiveness of anterior three-unit fixed partial dentures made of lithium disilicate, alumina and zirconia. For three-unit restorations involving a molar, expert consensus suggests that only zirconia-based systems are indicated. CLINICAL IMPLICATIONS: Available evidence indicates the effectiveness of many all-ceramic systems for numerous clinical applications. Bonding has been shown to increase clinical success. Studies of zirconia prostheses indicate problems with porcelain cracking.

Dental Ceramics for Restoration and Metal Veneering.

A survey of the development of dental ceramics is presented to provide a better understanding of the rationale behind the development and clinical indications of each class of ceramic material. Knowledge of the composition, microstructure, and properties of a material is critical for selecting the right material for specific applications. The key to successful ceramic restorations rests on material selection, manufacturing technique, and restoration design, including the balancing of several factors such as residual stresses, tooth contact conditions, tooth size and shape, elastic modulus of the adhesives and tooth structure, and surface state.

ADM guidance-Ceramics: guidance to the use of fractography in failure analysis of brittle materials.

OBJECTIVES: To provide background information and guidance as to how to use fractography accurately, a powerful tool for failure analysis of dental ceramic structures. METHODS: An extended palette of qualitative and quantitative fractography is provided, both for in vivo and in vitro fracture surface analyses. As visual support, this guidance document will provide micrographs of typical critical ceramic processing flaws, differentiating between pre- versus post sintering cracks, grinding damage related failures and occlusal contact wear origins and of failures due to surface degradation. RESULTS: The documentation emphasizes good labeling of crack features, precise indication of the direction of crack propagation (dcp), identification of the fracture origin, the use of fractographic photomontage of critical flaws or flaw labeling on strength data graphics. A compilation of recommendations for specific applications of fractography in Dentistry is also provided. SIGNIFICANCE: This guidance document will contribute to a more accurate use of fractography and help researchers to better identify, describe and understand the causes of failure, for both clinical and laboratory-scale situations. If adequately performed at a large scale, fractography will assist in optimizing the methods of processing and designing of restorative materials and components. Clinical failures may be better understood and consequently reduced by sending out the correct message regarding the fracture origin in clinical trials.

ADM guidance-Ceramics: all-ceramic multilayer interfaces in dentistry.

This guidance document describes the specific issues involved in dental multilayer ceramic systems. The material interactions with regard to specific thermal and mechanical properties are reviewed and the characteristics of dental tooth-shaped processing parameters (sintering, geometry, thickness ratio, etc.) are discussed. Several techniques for the measurement of bond quality and residual stresses are presented with a detailed discussion of advantages and disadvantages. In essence no single technique is able to describe adequately the all-ceramic interface. Invasive or semi-invasive methods have been shown to distort the information regarding the residual stress state while non-invasive methods are limited due to resolution, field of focus or working depth. This guidance document has endeavored to provide a scientific basis for future research aimed at characterizing the ceramic interface of dental restorations. Along with the methodological discussion it is seeking to provide an introduction and guidance to relatively inexperienced researchers.

Fatigue Behavior of Computer-Aided Design/Computer-Assisted Manufacture Ceramic Abutments as a Function of Design and Ceramics Processing.

PURPOSE: Zirconia is being widely used, at times apparently by simply copying a metal design into ceramic. Structurally, ceramics are sensitive to both design and processing (fabrication) details. The aim of this work was to examine four computer-aided design/computer-assisted manufacture (CAD/CAM) abutments using a modified International Standards Organization (ISO) implant fatigue protocol to determine performance as a function of design and processing. MATERIALS AND METHODS: Two full zirconia and two hybrid (Ti-based) abutments (n = 12 each) were tested wet at 15 Hz at a variety of loads to failure. Failure probability distributions were examined at each load, and when found to be the same, data from all loads were combined for lifetime analysis from accelerated to clinical conditions. RESULTS: Two distinctly different failure modes were found for both full zirconia and Ti-based abutments. One of these for zirconia has been reported clinically in the literature, and one for the Ti-based abutments has been reported anecdotally. The ISO protocol modification in this study forced failures in the abutments; no implant bodies failed. Extrapolated cycles for 10% failure at 70 N were: full zirconia, Atlantis 2 × 10(7) and Straumann 3 × 10(7); and Ti-based, Glidewell 1 × 10(6) and Nobel 1 × 10(21). Under accelerated conditions (200 N), performance differed significantly: Straumann clearly outperformed Astra (t test, P = .013), and the Glidewell Ti-base abutment also outperformed Atlantis zirconia at 200 N (Nobel ran-out; t test, P = .035). CONCLUSION: The modified ISO protocol in this study produced failures that were seen clinically. The manufacture matters; differences in design and fabrication that influence performance cannot be discerned clinically.

In Vivo Color Relationships Between the Maxillary Central Incisors and Canines as a Function of Age.

PURPOSE: It has long been taught that the hue of a patient can be taken from the canine and applied to other anterior teeth at a lower chroma. This concept does not appear to derive from published work. This study examined color relationships between in vivo maxillary central incisors and canines as a function of age. MATERIALS AND METHODS: The L*a*b* values and VITA Classical shades of the maxillary central incisor and canine of 62 subjects were determined using a handheld spectrophotometer. Linear regression analysis and t tests were used to describe the relationships of the L*a*b* values of these teeth within each patient and as a function of age. RESULTS: Linear regression demonstrated a significant decrease in ΔE with age (P = .056). Patient age was greater when ΔE (central-canine) > 3.3 (average age = 38.8 years) than when ΔE < 3.3 (average age = 58.8 years) (t test; P = .19). ΔC decreases significantly with age (P < .001). ΔH demonstrated a trend to decrease as a function of age (P = .2). ΔL remained the same over time (P = .21). Changes with age were due to central incisor differences, while the canine remained constant. CONCLUSION: ΔE (incisor-canine) significantly decreases with age; mostly due to ΔC. The majority of changes for all three color coordinates are due to alterations in the central incisor. The majority of the patients in this study were found to have a different shade family (VITA Classical) for the central incisor and canine.

Fractographic analyses of three ceramic whole crown restoration failures.

OBJECTIVES: Develop fractographic techniques to perform failure analyses of three ceramic whole-crown clinical failures. METHODS: Three clinical whole-crown failures of different ceramic material systems were studied. The systems included appropriate veneers applied to core materials of cold isostatically pressed alumina, injection molded alumina/alumina-magnesia spinel, and hot pressed lithium disilicate. The surface topography was examined for classical fractographic features, utilizing both optical and scanning electron microscopes independently as well as group examination by four fractographers. RESULTS: All three restorations displayed many common fractographic features, the most important of which was wake hackle. Wake hackle occurs when an approaching crack front sweeps by a small bubble or discontinuity in a brittle material, leaving a trail. The resulting trails can be used to trace back crack progression to an initial starting area. Wake hackle was found to be common in the veneer portions of the crowns, and the strategy of creating a wake hackle map enabled crack paths to be traced from a core area outward in all three restorations. The fracture origin areas were located where hoop stresses are high. Another helpful fracture feature was compression curl, an indication of flexural stress, most visible under low incident optical lighting. SIGNIFICANCE: This study indicates that fractographic analyses can be productively performed on ceramic whole-crown failures. Optical examination under low incident lighting and wake hackle mapping were developed as promising techniques. Hoop stress was implicated as a concern in the three studied specimens, and should be considered when analyzing other whole-crown failures.

Investigating failure behavior and origins under supposed "shear bond" loading.

PURPOSE: This study evaluated failure behavior when resin-composite cylinders bonded to dentin fractured under traditional "shear" testing. Failure was assessed by scaling of failure loads to changes in cylinder radii and fracture surface analysis. Three stress models were examined including failure by: bonded area; flat-on-cylinder contact; and, uniformly-loaded, cantilevered-beam. METHODS: Nine 2-mm dentin occlusal dentin discs for each radii tested were embedded in resin and bonded to resin-composite cylinders; radii (mm)=0.79375; 1.5875; 2.38125; 3.175. Samples were "shear" tested at 1.0mm/min. Following testing, disks were finished with silicone carbide paper (240-600grit) to remove residual composite debris and tested again using different radii. Failure stresses were calculated for: "shear"; flat-on-cylinder contact; and, bending of a uniformly-loaded cantilevered beam. Stress equations and constants were evaluated for each model. Fracture-surface analysis was performed. RESULTS: Failure stresses calculated as flat-on-cylinder contact scaled best with its radii relationship. Stress equation constants were constant for failure from the outside surface of the loaded cylinders and not with the bonded surface area or cantilevered beam. Contact failure stresses were constant over all specimen sizes. Fractography reinforced that failures originated from loaded cylinder surface and were unrelated to the bonded surface area. CONCLUSIONS: "Shear bond" testing does not appear to test the bonded interface. Load/area "stress" calculations have no physical meaning. While failure is related to contact stresses, the mechanism(s) likely involve non-linear damage accumulation, which may only indirectly be influenced by the interface.

Influence of the resin cement thickness on the fatigue failure loads of CAD/CAM feldspathic crowns.

OBJECTIVES: to evaluate the influence of the occlusal resin cement thickness on the cyclic loads-to-failure of feldspathic crowns and to compare the results to data from monotonic tests. A large range of cement thickness (50µm and 500µm) was tested, in order to better measure the influence of this variable. METHODS: Feldspathic ceramic crowns (Vita Mark II blocks, Vita Zahnfabrik) were bonded to dentin analog dies (G10 (NEMA grade G10, International Paper), with occlusal resin cement thicknesses of 50µm and 500µm (Multilink Automix, Ivoclar). The dies were prepared with microchannels for water transport to the cement layer. After 96-h water storage, the specimens (n=20) were submitted to cyclic loads (500,000 cycles at 20Hz; initial maximum load=40% of monotonic load, from previous data) following a staircase sensitivity design (step size=25N). Failure loads at 500,000 cycles were compared to monotonic failure loads (from a previous study with specimens produced by the same author, using the same materials, specimen configuration and cementation protocol). RESULTS: Crowns with an occlusal cement layer of 50µm were more resistant than those cemented with 500µm (246.4±22.9N vs. 158.9±22.9N), under wet cyclic testing conditions (p<0.001). The fatigue failure loads were reduced compared to monotonic loads: to 37% of monotonic for 50µm; to 53% of monotonic for 500µm. SIGNIFICANCE: An occlusal cement thickness of 50µm was more favorable for the structural performance of feldspathic crowns than was 500µm. Cyclic fatigue reduced failure loads well below those found under monotonic loading.