While relaxing in my recliner enjoying a large container of seasoned popcorn this past holiday, I experienced that sickening sound of catastrophic tooth failure. My tongue quickly ascertained that the lingual cusp of my maxillary right second premolar was no longer one with the buccal cusp. It was now a loose appendage, retained only by a sliver of attached gingiva. A few frantic phone calls and an endodontic procedure later, I was walking around with one-half of a crown, but I am still faced with the dilemma of what material I should choose to make myself whole again. This is a dilemma that patients are faced with every day and one that they depend greatly on our knowledge, experience, and advice to help them resolve. Unfortunately, the practice of dentistry lives almost entirely within the world of medical devices. We rely heavily on things rather than therapies. Our most coveted products require precious little clinical validation of effectiveness, and we often are forced to rely on empirical observation or laboratory study results to guide our choices. This is a long stretch from the world of therapeutics, in which drugs undergo years of exhaustive and expensive safety and efficacy trials to demonstrate their safety and use. But the evidence is what it is, and too often we assume that the laboratory evidence we see reported in advertising and throwaway journals is a suitable substitute for clinical outcomes.There are times when we need to step back and challenge some of our basic assumptions regarding the evidence we take as being meaningful. There are times when we need to step back and challenge some of our basic assumptions regarding the evidence we take as being meaningful. There are also times when we need to step back and challenge some of our basic assumptions regarding the evidence we take as being meaningful. Take the case of the sacred cow we call polymerization shrinkage. For decades, dentistry has lived under the universal assumption that polymerization shrinkage leads to a whole host of clinical sequelae that result in increased recurrent disease and degraded restoration performance. Dentistry has invested millions of dollars into research and development of low-shrinkage materials, assuming that success would translate into a new era of clinical performance. The authors of 2 articles, however, finally have called this assumption into question. The authors of the first article reviewed both laboratory and clinical evidence related to shrinkage defects and clinical performance.1Ferracane J.L. Hilton T.J. Polymerization stress: is it clinically meaningful?.Dent Mater. 2016; 32: 1-10Abstract Full Text Full Text PDF Scopus (136) Google Scholar Their conclusions were that, although there was extensive laboratory evidence of shrinkage-induced defects, there was little or no evidence linking shrinkage to clinical deficiencies, such as recurrent caries or postoperative sensitivity. These conclusions were supported by the authors of the second article, who reported a 15-year randomized clinical trial and compared a reduced shrinkage composite to a higher shrinkage control.2van Dijken J.W. Lindberg A.A. 15-year randomized controlled study of a reduced shrinkage stress resin composite.Dent Mater. 2015; 31: 1150-1158Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar They found no performance differences between the 2 materials in all the measured clinical outcomes. There are many other sacred cows that should be called into question. I am embarrassed to admit the number of articles and abstracts on microleakage or bond strength that I have coauthored, even though neither property has been shown to have much, if any, correlation with clinical outcomes.3Ferracane J.L. Resin-based composite performance: are there some things we can’t predict?.Dent Mater. 2013; 29: 51-58Abstract Full Text Full Text PDF PubMed Scopus (262) Google Scholar, 4Heintze S.D. Zimmerli B. Relevance of in vitro tests of adhesive and composite dental materials: a review in 3 parts—part 3, in vitro tests of adhesive systems.Schweiz Monatsschr Zahnmed. 2011; 121: 1024-1040Google Scholar, 5Van Meerbeek B. Peumans M. Poitevin A. et al.Relationship between bond-strength tests and clinical outcomes.Dent Mater. 2010; 26: e100-e121Abstract Full Text Full Text PDF PubMed Scopus (531) Google Scholar, 6Dennison J.B. Sarrett D.C. Prediction and diagnosis of clinical outcomes affecting restoration margins.J Oral Rehabil. 2012; 39: 301-318Crossref PubMed Scopus (61) Google Scholar So if in vitro evidence is such a poor predictor of clinical performance, where does that leave us? Fortunately, we do not have a complete void of clinical evidence. Each year in my review of the dental materials literature for the Academy of Restorative Dentistry, I find that I can rely more and more on in vivo studies, and that the length of many of these clinical trials has increased. I rarely reference trials less than 2 years in length, and many run 10 years or longer. Although the quality of these studies may not be high or consistent, they are improving over time as investigators are adopting better and more consistent criteria for study design. The number of systematic reviews and meta-analyses also is increasing. We definitely have a long way to go before we can rely confidently on clinical evidence, and the call for more and better quality studies must continue. We also need to recognize that laboratory studies always will be a necessity for the cost-effective and timely screening of new materials. They are an essential component of materials development, but we need to recognize the distinction between development and demonstration. One promising area of materials evaluation has been the application of engineering modeling and forensic failure evaluation. Aeronautical engineers cannot test critical flight components by flying them until failure. Instead, they model conditions through computer simulations by using tools such as finite element analysis, and if parts fail in use, they subject them to forensic analysis to determine how, where, and why they failed. More and more we are seeing these same tools being applied to dental materials. These tools allow us to model multiple properties and parameters of a material or restorative system under loading conditions that simulate oral function before ever placing that material in a patient.7May L.G. Kelly J.R. Bottino M.A. Hill T. Effects of cement thickness and bonding on the failure modes of CAD/CAM ceramic crowns: multi-physics FEA modeling and monotonic testing.Dent Mater. 2012; 28: e99-e109Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar We also have better tools and methods to determine the failure process once clinical failures appear.8Scherrer S.S. Quinn J.B. Quinn G.D. Kelly J.R. Failure analysis of ceramic clinical cases using qualitative fractography.Int J Prosthodont. 2006; 19: 185-192PubMed Google Scholar Forensic analysis of failed restorations has provided insights into why many of our laboratory tests have been such poor predictors, and forensic analysis has guided the development of vastly improved test methods.9Kelly J.R. Clinically relevant approach to failure testing of all-ceramic restorations.J Prosthet Dent. 1999; 81: 652-661Abstract Full Text Full Text PDF PubMed Scopus (549) Google Scholar More and more materials in the laboratory are being subjected to microbiological challenges to simulate oral conditions. Even with these improvements, however, in vitro testing can never replace long-term, well-designed, and properly executed clinical trials. The best laboratory simulation simply cannot anticipate and incorporate all the environmental challenges that occur clinically. So back to my dilemma. I cannot place the blame of my tooth failure on an inadequately tested restorative material, because this particular premolar had no prior restoration. Like many other parts of my aging anatomy, this tooth had reached what we engineers would call its service life. What I can do is look to the literature on which full-coverage materials and systems have the largest, longest, and best quality of evidence related to clinical outcomes. I can go to sources such as the Cochrane Library, PubMed, and the American Dental Association Center for Evidence-Based Dentistry. The key to this strategy is to focus on clinical outcomes, ignoring the charts, graphs, and claims of the glossy ads for the latest and greatest. I am not going to disclose the results of my search, because perhaps this may tease your curiosity just enough to do one of your own. What I will tell you is that my search provided enough evidence based on clinical performance to make a comfortable decision. I can only hope the same holds true as my other parts reach their service life. Dr. Eichmiller is the vice president and science officer, Delta Dental of Wisconsin, 2801 Hoover Rd., Box 828, Stevens Point, WI 54481.

ABSTRACT The erupted tusk of the narwhal exhibits sensory ability. The hypothesized sensory pathway begins with ocean water entering through cementum channels to a network of patent dentinal tubules extending from the dentinocementum junction to the inner pulpal wall. Circumpulpal sensory structures then signal pulpal nerves terminating near the base of the tusk. The maxillary division of the fifth cranial nerve then transmits this sensory information to the brain. This sensory pathway was first described in published results of patent dentinal tubules, and evidence from dissection of tusk nerve connection via the maxillary division of the fifth cranial nerve to the brain. New evidence presented here indicates that the patent dentinal tubules communicate with open channels through a porous cementum from the ocean environment. The ability of pulpal tissue to react to external stimuli is supported by immunohistochemical detection of neuronal markers in the pulp and gene expression of pulpal sensory nerve tissue. Final confirmation of sensory ability is demonstrated by significant changes in heart rate when alternating solutions of high‐salt and fresh water are exposed to the external tusk surface. Additional supporting information for function includes new observations of dentinal tubule networks evident in unerupted tusks, female erupted tusks, and vestigial teeth. New findings of sexual foraging divergence documented by stable isotope and fatty acid results add to the discussion of the functional significance of the narwhal tusk. The combined evidence suggests multiple tusk functions may have driven the tooth organ system's evolutionary development and persistence. Anat Rec, 297:599–617, 2014. © 2014 Wiley Periodicals, Inc.

In nature, an interface between dissimilar tissues is often bridged by a graded zone, and provides functional properties at a whole organ level. A perfect example is a “biological interphase” between stratified cementum and dentin of a narwhal tooth. This study highlights the graded structural, mechanical, and chemical natural characteristics of a biological interphase known as the cementum–dentin junction layer and their effect in resisting mechanical loads. From a structural perspective, light and electron microscopy techniques illustrated the layer as a wide 1000–2000 μm graded zone consisting of higher density continuous collagen fiber bundles from the surface of cementum to dentin, that parallels hygroscopic 50–100 μm wide collagenous region in human teeth. The role of collagen fibers was evident under compression testing during which the layer deformed more compared to cementum and dentin. This behavior is reflected through site-specific nanoindentation indicating a lower elastic modulus of 2.2 ± 0.5 GPa for collagen fiber bundle compared to 3 ± 0.4 GPa for mineralized regions in the layer. Similarly, microindentation technique illustrated lower hardness values of 0.36 ± 0.05 GPa, 0.33 ± 0.03 GPa, and 0.3 ± 0.07 GPa for cementum, dentin, and cementum–dentin layer, respectively. Biochemical analyses including Raman spectroscopy and synchrotron-source microprobe X-ray fluorescence demonstrated a graded composition across the interface, including a decrease in mineral-to-matrix and phosphate-to-carbonate ratios, as well as the presence of tidemark-like bands with Zn. Understanding the structure–function relationships of wider tissue interfaces can provide insights into natural tissue and organ function.

Narwhal tusks, although well described and characterized within publications, are clouded by contradictory references, which refer to them as both incisors and canines. Vestigial teeth are briefly mentioned in the scientific literature with limited descriptions and no image renderings. This study first examines narwhal maxillary osteoanatomy to determine whether the erupted tusks are best described as incisiform or caniniform teeth. The study also offers evidence to support the evolutionary obsolescence of the vestigial teeth through anatomic, morphologic, and histologic descriptions. Examination of 131 skull samples, including 110 museum skull specimens and 21 harvested skulls, revealed the erupted tusks surrounded by maxillary bone over the entire length of their bone socket insertion, and are thus more accurately termed caniniform or canine teeth. The anatomy, morphology, and development of vestigial teeth in five skull samples are more fully described and documented. Vestigial tooth samples included 14 embedded pairs or individual teeth that were partially exposed or removed from the maxillary bone. Their location was posterior, ventral, and lateral to the tusks, although male vestigial teeth often exfoliate in the mouth lodging between the palatal tissue and underlying maxillary bone. Their myriad morphologies, sizes, and eruption patterns suggest that these teeth are no longer guided by function but rather by random germ cell differentiation and may eventually cease expression entirely. The conclusions reached are that the narwhal tusks are the expression of canine teeth and that vestigial teeth have no apparent functional characteristics and are following a pattern consistent with evolutionary obsolescence. Anat Rec, 2012. © 2012 Wiley Periodicals, Inc.

Introduction The purpose of this study was to evaluate the direct pulp capping response to a novel resin-based calcium phosphate cement (RCPC). Methods The RCPC was placed in contact with the exposed healthy pulps of dog teeth and in a follow-up study on the healthy or inflamed pulps of ferret teeth. The inflamed ferret teeth had reversible pulpitis induced with Salmonella typhimurium lipopolysaccharides. After direct pulp capping with RCPC or visible light-curing resin-modified calcium hydroxide material (VLCCH) as a control, the restorations were bonded using a composite resin. The pulp responses and dentin repair were evaluated histologically in dog teeth after 7, 28, or 90 days and in ferret teeth after 45 days. Results Most of the RCPC-treated healthy pulps and 75% of the RCPC-treated inflamed ferret teeth had dentin healing and repair, whereas those teeth treated with VLCCH had minimal healing and dentin repair. Conclusions The direct pulp capping of ferret and dog teeth with RCPC was associated with superior healing in comparison to VLCCH.

Interdisciplinary studies of narwhal cranial and tooth anatomy are combined with Inuit traditional knowledge to render a more complete description of tooth-related structures and to propose a new hypothesis for tusk function in the adult mate. Gross anatomy findings from computed tomography (CT) and magnetic resonance (MR) imaging and dissections of an adult male and female and one fetus, four to six months in development, were documented. Computed tomography scans rendered images of the tusks and vestigial teeth and their shared sources of innervation at the base of the tusks. Paired and asymmetrical tusks and vestigial teeth were observed in all three samples, and their relative positions reversed during development. Vestigial teeth shifted anteriorly during growth, and the developing tusks moved posteriorly as they developed. Examination of tusk micro-anatomy revealed the presence of a dentinal tubule network with lumena approximately 2 micrometers in diameter and 10-20 micrometers apart over the pulpal and erupted tusk surfaces. Orifices were present on the cementum surface indicating direct communication and sensory capability from the environment to the inner pulpal wall. Flexural strength of 95 MPa at mid tusk and 165 MPa at the base indicated resistance to high flexural stresses. Inuit knowledge describes a tusk with remarkable and combined strength and flexibility. Elder observations of anatomy are described by variable phenotypes and classified by skin coloration, sex, and tusk expression. Behavioral observations of males leading seasonal migration groups, nonaggressive tusk encounters, and frequent sightings of smaller groups separated by sex add to the discussion of tusk function.

One possible reason for the sealing ability of mineral trioxide aggregate (MTA) is its slight expansion upon setting. Both gray mineral trioxide aggregate (GMTA) and white mineral trioxide aggregate (WMTA) are composed of approximately 75% Portland cement (PC). WMTA differs from GMTA in its lower content of tetracalcium aluminoferrite. This difference in composition may affect setting expansion. The purpose of this study was to compare the hydroscopic linear setting expansions of GMTA, WMTA, and PC with a new device. Materials were mixed with water, placed into a cylindrical mold, and covered with sterile water or Hank's balanced salt solution (HBSS). Expansion changes were measured by using a linear variable displacement transformer dilatometer. One-way analysis of variance and post hoc tests (alpha = 0.05) showed the mean expansion at 24 hours was 1.02% for GMTA, 0.29% for PC, and 0.08% for WMTA in water immersion and 0.68% for GMTA and 0.11% for WMTA in HBSS immersion. GMTA expanded significantly more than WMTA in either water or HBSS immersion.

The narwhal, Monodon monoceros, is a marine mammal that moves seasonally in Arctic waters. In males, left upper canines grow to between 2 and 3 metres in length as tapered tusks (Figs 1,2). Occasionally, tusks grow on the right side in males, but they are generally short.

This study measured root surface temperature changes when ultrasonic vibration, with and without irrigation, was applied to cemented endodontic posts. Twenty-six, extracted, single-rooted premolars were randomly divided into two groups. Root lengths were standardized, canals instrumented, obturated, and posts cemented into prepared spaces. Thermocouples were positioned at two locations on the proximal root surfaces. Samples were embedded in plaster and brought to 37 degrees C in a water bath. Posts were ultrasonically vibrated for 4 minutes while continuously measuring temperature. Two-way ANOVA compared effects of water coolant and thermocouple location on temperature change. Root surface temperatures were significantly higher (p < 0.001) when posts were instrumented dry. A trend for higher temperatures was observed at coronal thermocouples of nonirrigated teeth and at apical thermocouples of irrigated teeth (p = 0.057). Irrigation during post removal with ultrasonics had a significant impact on the temperature measured at the external root surface.

OBJECTIVE:Whiskers were recently used to reinforce dental composites to extend their use to large stress-bearing restorations. The aim of this study was to investigate the effects of different types of whiskers on composite properties.METHODS:Silicon nitride and silicon carbide whiskers were each mixed with silica particles at whisker/silica mass ratios of 0:1, 1:5, 1:2, 1:1, 2:1, 5:1, and 1:0, and thermally treated. The composite was heat-cured at 140 degrees C. Strength and fracture toughness were measured in flexure, while elastic modulus and hardness were measured with nano-indentation.RESULTS:Both whisker type and whisker/silica ratio had significant effects on composite properties (two-way ANOVA; p<0.001). Silicon nitride whiskers increased the composite strength and toughness more than did silicon carbide. Silicon carbide whiskers increased the modulus and hardness more than silicon nitride did. The silicon nitride whisker composite reached a strength (mean+/-SD; n=6) of 246+/-33 MPa at whisker/silica of 1:1, while the silicon carbide whisker composite reached 210+/-14 MPa at 5:1. Both were significantly higher than 114+/-18 MPa of a prosthetic control and 109+/-23 MPa of an inlay/onlay control (Tukey's multiple comparison test; family confidence coefficient=0.95). Fracture toughness and work-of-fracture were also increased by a factor of two. Higher whisker/silica ratio reduced the composite brittleness to 1/3 that of the inlay/onlay control.SIGNIFICANCE:Whisker type and whisker/silica ratio are key microstructural parameters that determine the composite properties. Reinforcement with silica-fused whiskers results in novel dental composites that possess substantially higher strength and fracture toughness, and lower brittleness than the non-whisker control composites.

The mechanical properties of dental resin composites need to be improved in order to extend their use to high stress-bearing applications such as crown and bridge restorations. Recent studies used single crystal ceramic whiskers to reinforce dental composites. The aim of this study was to investigate the effects of thermal cycling on whisker-reinforced composites. It was hypothesized that the whisker composites would not show a reduction in mechanical properties or the breakdown of whisker–resin interface after thermal cycling. Silicon carbide whiskers were mixed with silica particles, thermally fused, then silanized and incorporated into resin to make flexural specimens. The filler mass fraction ranged from 0% to 70%. The specimens were thermal cycled in 5 °C and 60 °C water baths, and then fractured in three-point bending to measure strength. Nano-indentation was used to measure modulus and hardness. No significant loss in composite strength, modulus and hardness was found after 105 thermal cycles (family confidence coefficient=0.95; Tukey's multiple comparison test). The strength of whisker composite increased with filler level up to 60%, then plateaued when filler level was further increased to 70%; the modulus and hardness increased monotonically with filler level. The strength and modulus of whisker composite at 70% filler level were significantly higher than the non-whisker controls both before and after thermal cycling. SEM revealed no separation at the whisker–matrix interfaces, and observed resin remnants on the pulled-out whiskers, indicating strong whisker–resin bonding even after 105 thermal cycles. In conclusion, novel dental resin composites containing silica-fused whiskers possessed superior strength and modulus compared to non-whisker composites both before and after thermal cycling. The whisker–resin bonding appeared to be resistant to thermal cycling in water, so that no loss in composite strength or stiffness occurred after prolonged thermal cycling.

Dental resin composites need to be strengthened in order to improve their performance in large stress-bearing applications such as crowns and multiple-unit restorations. Recently, silica-fused ceramic whiskers were used to reinforce dental composites, and the whisker-to-silica ratio was found to be a key microstructural parameter that determined the composite strength. The aim of this study was to further investigate the effects of whisker-to-silica ratio on the fracture toughness, elastic modulus, hardness and brittleness of the composite. Silica particles and silicon carbide whiskers were mixed at whisker:silica mass ratios of 0:1, 1:5. 1:2, 1:1, 2:1, 5:1, and 1:0. Each mixture was thermally fused, silanized and combined with a dental resin at a filler mass percentage of 60%. Fracture toughness was measured with a single-edge notched beam method. Elastic modulus and hardness were measured with a nano-indentation system. Whisker:silica ratio had significant effects on composite properties. The composite toughness (mean+/-SD; n = 9) at whisker:silica = 2:1 was (2.47+/-0.28) MPa m(1/2), significantly higher than (1.02+/-0.23) at whisker:silica = 0:1, (1.13+/-0.19) of a prosthetic composite control, and (0.95+/-0.11) of an inlay/onlay composite control (Tukey's at family confidence coefficient = 0.95). Elastic modulus increased monotonically and hardness plateaued with increasing the whisker:silica ratio. Increasing the whisker:silica ratio also decreased the composite brittleness, which became about 1/3 of that of the inlay:onlay control. Electron microscopy revealed relatively flat fracture surfaces for the controls, but much rougher ones for the whisker composites, with fracture steps and whisker pullout contributing to toughness. The whiskers appeared to be well-bonded with the matrix, probably due to the fused silica producing rough whisker surfaces. Reinforcement with silica-fused whiskers resulted in novel dental composites that possessed fracture toughness two times higher than, and brittleness less than half of current dental composites.