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.

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: 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.

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.

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.

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.

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.

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.

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.

Fracture of a narrow-diameter roxolid implant: clinical and fractographic considerations.

PURPOSE: Narrow-diameter implants have become popular, as bone augmentation procedures can frequently be avoided. In order to provide implants with sufficient strength, a titanium-zirconium (Ti-Zr) alloy has recently been introduced purportedly showing improved mechanical properties as compared to commercially pure titanium (cpTi). This paper reports on the clinical and fractographic aspects of a fractured Ti-Zr implant, generally considered a rare event. MATERIALS AND METHODS: A narrow-diameter Ti-Zr implant was placed in a patient aged 66 years in the area of the maxillary right canine and used to support a removable partial denture retained by cylindrical telescopic crowns placed on the implant and on the maxillary left canine. Eleven months after successful osseointegration, the patient presented with a horizontally fractured implant. The fractured part was retrieved for fractographic analysis. RESULTS: The implant fractured just apical to the abutment screw, where the wall thickness of the implant is minimal. Scanning electron microscopy (SEM) analysis revealed the starting point of the fracture at the palatal aspect of the implant. No fatigue striations could be observed, indicating a brittle fracture atypical for Ti-based alloys. CONCLUSIONS: The fractographic observations correlate with the given loading situation, which is characterized by high levels of moment loading being imposed on the implant by a rigidly retained cantilever prosthesis. Mechanical overload going beyond the approved indications for Roxolid implants (Straumann) has to be seen as primary cause for this fracture.

Influence of thermal gradients on stress state of veneered restorations.

OBJECTIVES: To assess transient and residual stresses within the porcelain of veneered restorations (zirconia and metal) as a result of cooling rate and porcelain thickness. METHODS: Porcelain-on-zirconia (PZ) and porcelain-fused-to-metal (PFM) crowns were fabricated with 1 or 2mm of porcelain. Thermocouples were attached both internally and externally to the crowns to record transient temperatures. For fast cooling, the furnace was opened after the holding time and switched off. Slow cooling was accomplished by opening the furnace at 50°C below the glass transition temperature (Tg) of the material. An axially symmetric FEA model simulated thermal stresses. Time-dependent temperature equations from thermocouple readings were set as boundary conditions. Framework materials and the porcelain below Tg were considered to behave elastically. Visco-elastic behavior was assumed for porcelain above the Tg modeling properties as dependent on cooling rate. RESULTS: Differences in residual stress were found for fast and slow cooled PZ and PFM crowns. Significant transient stress waves were observed within the porcelain when fast cooling through Tg. They are believed to be related to non-uniform volumetric changes originated from thermal gradients. Results were confirmed by modeling and physical testing of crowns containing a defect. SIGNIFICANCE: Residual stresses do not distinguish PZ from PFM. High magnitude transient stresses observed within the porcelain during fast cooling may explain clinical fractures involving internal defects. Stress waves may also originate internal micro-cracking which could grow under function. Therefore, slow cooling, especially for all-ceramic crowns with thick porcelain, is important to prevent thermal gradients and high-magnitude transient stresses.

Systematic review of the survival rate and incidence of biologic, technical, and esthetic complications of single implant abutments supporting fixed prostheses.

PURPOSE: To assess the 5-year survival rate and number of technical, biologic, and esthetic complications involving implant abutments. MATERIALS AND METHODS: Electronic (Medline) and hand searches were performed to assess studies on metal and ceramic implant abutments. Relevant data from a previous review were included. Two reviewers independently extracted the data. Failure and complication rates were analyzed, and estimates of 5-year survival proportions were calculated from the relationship between event rate and survival function. Multivariable robust Poisson regression was used to compare abutment characteristics. RESULTS: The search yielded 1,558 titles and 274 abstracts. Twenty-four studies were selected for data analysis. The survival rate for ceramic abutments was 97.5% (95% confidence interval [CI]): 89.6% to 99.4%) and 97.6% (95% CI: 96.2% to 98.5%) for metal abutments. The overall 5-year rate for technical complications was 11.8% (95% CI: 8.5% to 16.3%), 8.9% (95% CI: 4.3% to 17.7%) for ceramic and 12.0% (95% CI: 8.5% to 16.8%) for metal abutments. Biologic complications occurred with an overall rate of 6.4% (95% CI: 3.3% to 12.0%), 10.4% (95% CI: 1.9% to 46.7%) for ceramic, and 6.1% (95% CI: 3.1% to 12.0%) for metal abutments. CONCLUSIONS: The present meta-analysis on single-implant prostheses presents high survival rates of single implants, abutments, and prostheses after 5 years of function. No differences were found for the survival and failure rates of ceramic and metal abutments. No significant differences were found for technical, biologic, and esthetic complications of internally and externally connected abutments.

Cyclic fatigue testing of denture teeth for bulk fracture.

OBJECTIVES: Clinical experience with implant-supported dentures indicates that fracture and chipping of teeth are becoming an issue. Tooth fracture and chipping rates of approximately 2.5% per year are being experienced at one university. There has been no standardized test developed for bulk fracture or chipping of denture teeth. Such a test would aid in the development of improved teeth and in their evaluation. METHODS: Central incisor teeth were embedded in acrylic and loaded on incisal edges at 90° to their long axes. Teeth tested included ones commercially available and two sets made from improved materials designed to increase toughness. Cyclic loading was done at 5Hz from 20N to 135N, 150N, 175N and 200N. Data was analyzed using lifetime analysis software fit at each of the accelerated loads and then extrapolated to clinical use loads (Alta 7, Reliasoft Corp.) Clinical use loads were derived from extrapolation of probability of failure (Pf) data to 2.5% Pf. RESULTS: When carefully embedded, teeth could be reproducibly loaded to failure by bulk fracture involving a failure mode similar to that seen clinically. Clinical use loads were calculated to be in the range of 70N. Results from accelerated loading could be fit to similar probability of failure distributions allowing extrapolation to clinical use loads. SIGNIFICANCE: This work was able to develop a clinically valid bulk fracture test for the fatigue failure of incisor denture teeth. It appears that teeth fabricated with improved materials will be expected to perform better clinically. Thus both the null hypotheses were rejected.

Influence of resin cement polymerization shrinkage on stresses in porcelain crowns.

OBJECTIVE: The aim of this study was to analyze the influence of polymerization shrinkage of the cement layer on stresses within feldspathic ceramic crowns, using experimentally validated FEA models for (1) increasing occlusal cement thickness; and, (2) bonded versus non-bonded ceramic-cement interfaces. METHODS: 2-D axial symmetric models simulated stylized feldspathic crowns (1.5mm occlusal thickness) cemented with resin-cement layers of 50-500µm on dentin preparations, being loaded (500N) or not. Ceramic-cement interface was either bonded or not. Cement was bonded to the dentin in all models. Maximum axial shrinkage of 0%, 1%, 2%, 3%, 4% and 4.65% were simulated. The first principal stresses developing in the cementation surface at the center and at the occluso-axial line-angle of the crown were registered. RESULTS: Polymerization shrinkage of the cement increased tensile stresses in the ceramic, especially in loaded non-bonded crowns for thicker cement layers. Stresses in loaded non-bonded crowns increased as much as 87% when cement shrinkage increased from 0% to 4.65% (100-187MPa), for a 500µm-thick cement. Increasing polymerization shrinkage strain raised the tensile stresses, especially at the internal occlusal-axial line-angle, for bonded crowns. SIGNIFICANCE: Changes in the polymerization shrinkage strain (from 0% to 4.65%) have little effect on the tensile stresses generated at the cementation surface of the ceramic crowns, when the occlusal cement thickness is thin (approx. 50µm for bonded crowns). However, as the cement becomes thicker stresses within the ceramic become significant.

Comparison of an absolute and surrogate measure of relative translucency in dental ceramics.

PURPOSE: The objective was to compare absolute translucency with a frequently used surrogate measure of relative translucency (contrast ratio) from 14 all-ceramic materials having a wide range of translucencies. MATERIALS AND METHODS: Standardized disks were fabricated from fourteen ceramics, varying both thickness (0. 3mm, 0.5 mm, 1.0 mm, and 1.5 mm) and chroma (high and low). Absolute translucency (percent transmission) was measured using a spectroradiometer with an integrating sphere. Relative translucency was measured using a spectrophotometer as contrast ratio (contrast ratio; ratio of L* values recorded on black and white backgrounds). Non-linear regression was used to compare measurements of absolute versus relative values for each of the spectrophotometers. RESULTS: Contrast ratio was not able to characterize translucency across the range of materials studied and became less sensitive with increasing opacity. A non-linear correlation was found between percent transmission and contrast ratio down to 50% transmission (r(2)=0.97) and contrast ratio was insensitive to transmission differences below 50% transmission. CONCLUSION: Contrast ratio is not a direct measure of translucency and cannot be used below 50% transmission.

Assessment of the primary stability of dental implants in artificial bone using resonance frequency and percussion analyses.

PURPOSE: There is no quantitative gold standard instrumentation to assess the quality of implant osseointegration. The purpose of this exploratory study was to evaluate the response of two devices (one based on resonance frequency analysis, the Osstell device, and another that analyzes the percussion energy response, the Periometer) to assess the primary stability of implants embedded in artificial bone models. MATERIALS AND METHODS: Standard implants were placed into polyurethane blocks of varying densities, and the two mechanical devices were challenged to test the specimen block series. Both analysis of variance and regression analysis were used to examine the output from each device over each series of specimen blocks as well as to directly compare outputs between the two devices. RESULTS: The stability of the implants increased with the foam density for solid block specimens. Linear regression analysis showed significant correlation between the two instruments for testing with monolithic blocks ( r2 = 0.984). Both devices also indicated that a hybrid block with the greatest density at the top provided the best implant stability versus a hybrid block with relatively low density at the top of the block. However, resonance frequency analysis readings seemed to be more dependent on the density of the top layer of the hybrid blocks. CONCLUSION: Osstell and Periometer readings were in good agreement for monolithic blocks, and they were reasonably consistent when blocks of hybrid density were tested.

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.