The Role of Process-Directing Agents on Enamel Lesion Remineralization: Fluoride Boosters.

The aim of this study was to investigate the effects of two process-directing agents (polyaspartic acid and osteopontin) used in a polymer-induced liquid-precursor (PILP) process on the remineralization of bacteria-induced enamel demineralization. Enamel demineralization lesions (depths of about 180-200 µm) were created and exposed to Streptococcus mutans, cultured with a 10% sucrose solution for 21 days, and remineralized using a PILP process (pH = 7.4, 14 days) with a calcium phosphate solution containing either polyaspartic acid or osteopontin in the presence or absence of fluoride (0.5 ppm). The specimens were examined under scanning electron microscopy. The fluoride was successfully incorporated into the PILP remineralization process for both polyaspartic acid and osteopontin. When the fluoride was added to the PILP remineralization solution, there was more uniform remineralization throughout the lesion than with either polyaspartic acid or osteopontin alone. However, in the absence of these process-directing agents, fluoride alone showed less remineralization with the formation of a predominantly surface-only layer. The PILP remineralization process relies on the ability of process-directing agents to stabilize calcium phosphate ions and holds promise for enamel lesion remineralization, and these agents, in the presence of fluoride, seem to play an important role as a booster or supplement in the continuation of remineralization by reducing the mineral gains at the surface layer.

Enhanced silver diamine fluoride therapy using the PILP method -A nanoindentation study.

The aim of this study was to evaluate the feasibility of applying the polymer-induced liquid-precursor (PILP) method to enhance silver diamine fluoride (SDF) therapy. One hundred forty micrometer deep artificial caries lesions were treated with (A) 38% SDF solution and (B) 38% SDF containing poly-L-aspartic acid (pASP). Changes in the nanomechanical profile across the lesion were evaluated. Hydrated artificial lesions had a low reduced elastic modulus (0.3 GPa) and nanohardness (0.02 GPa) region extending about 100 µm into the lesion, with a gradual linear increase to about 168 µm where the values plateaued to around 18 GPa/1.0 GPa. Topical application of SDF resulted in significantly recovered properties (p<0.001). SDF containing pASP resulted in greater nanomechanical properties compared to SDF alone, showing similar sloped regions up to 96 µm, then SDF alone dropped while SDF containing pASP continued at a modest slope until reaching normal at 144 µm. This nanoindentation study shows enhanced SDF therapy using the PILP method.

The Academy of Dental Materials: Providing roots and wings.

OBJECTIVES: The long history of the Academy of Dental Materials (ADM) is documented with its strategies (a) to rapidly communicate science among its members, (b) to establish special awards to stimulate new science, and (c) to develop new dental materials scientists. METHODS: We searched the history of the last 35 years of the ADM newsletters, transactions, journals, and officer notes. We document the (a) presidents, (b) meeting history, (c) membership growth, and (d) development of special awards through 2019 with the recent creation of the ADM Marshall Post-Doctoral Award. RESULTS: There are 36 years of recent ADM history, 42 international meetings, membership growth to 400 individuals from 15 countries, service of 19 presidents, Paffenbarger annual Awardees since 1989, induction of >200 fellows, and recognition of the first winner of Marshall Post-Doctoral Award in 2018. New directions for recruiting members are suggested. Three potential new thrusts for the organization are presented: artificial intelligence, genetic engineering, and intensive member mentoring. SIGNIFICANCE: These suggestions for the ADM provide a path for the ADM to continue to adapt to the ever changing scientific landscape.

Integrating the PILP-mineralization process into a restorative dental treatment.

The addition of charged polymers, like poly-aspartic acid (pAsp), to mineralizing solutions allows for transport of calcium and phosphate ions into the lumen of collagen fibrils and subsequent crystallization of oriented apatite crystals by the so-called Polymer-Induced Liquid Precursor (PILP) mineralization process, leading to the functional recovery of artificial dentin lesions by intrafibrillar mineralization of collagen. OBJECTIVE: To evaluate the feasibility of applying the PILP method as part of a restorative treatment and test for effectiveness to functionally remineralize artificial lesions in dentin. MATERIALS AND METHODS: Two methods of providing pAsp to standardized artificial lesions during a restorative procedure were applied: (A) pAsp was mixed into commercial RMGI (resin modified glass ionomer) cement formulations and (B) pAsp was added at high concentration (25mg/ml) in solution to rehydrate lesions before restoring with a RMGI cement. All specimens were immersed in simulated body fluid for two weeks to allow for remineralization and then analyzed for dehydration shrinkage, integrity of cement-dentin interface, degree of mineralization, and changes in the nanomechanical profile (E-modulus) across the lesion. RESULTS: After the remineralization treatment, lesion shrinkage was significantly reduced for all treatment groups compared to demineralized samples. Pores developed in RMGI when pAsp was added. A thin layer at the dentin-cement interface, rich in polymer formed possibly from a reaction between pAsp and the RMGI. When analyzed by SEM under vacuum, most lesions delaminated from the cement interface. EDS-analysis showed some but not full recovery of calcium and phosphorous levels for treatment groups that involved pAsp. Nanoindentations placed across the interface indicated improvement for RMGI containing 40% pAsp, and were significantly elevated when lesions were rehydrated with pAsp before being restored with RMGI. In particular the most demineralized outer zone recovered substantially in the elastic modulus, suggesting that functional remineralization has been initiated by pAsp delivery upon rehydration of air-dried demineralized dentin. In contrast, the effectiveness of the RMGI on functional remineralization of dentin was minimal when pAsp was absent. SIGNIFICANCE: Incorporation of pAsp into restorative treatments using RMGIs promises to be a feasible way to induce the PILP-mineralization process in a clinical setting and to repair the structure and properties of dentin damaged by the caries process.

The role of protease inhibitors on the remineralization of demineralized dentin using the PILP method.

Mineralized and sound dentin matrices contain inactive preforms of proteolytic enzymes that may be activated during the demineralization cycle. In this study, we tested the hypothesis that protease inhibitors (PI) preserve demineralized collagen fibrils and other constituents of the dentin matrix and thereby affect the potential for remineralization. Artificial carious lesions with lesion depths of 140 µm were created with acetate buffer (pH = 5.0, 66 hours), and remineralized using a polymer-induced-liquid-precursor (PILP) process (pH = 7.4, 14 days) containing poly(aspartic acid) (pAsp) as the process-directing agent. De- and remineralizing procedures were performed in the presence or absence of PI. Ultrastructure and mechanical recovery of demineralized dentin following PILP remineralization were examined and measured in water with atomic force microscopy (AFM) and nanoindentation. Nanomechanical properties of hydrated artificial lesions had a low elastic modulus (ER <0.4 GPa) extending about 100 µm into the lesion, followed by a sloped region of about 140 µm depth where values reached those of normal dentin (18.0-20.0 GPa). Mapping of mineral content by both micro-FTIR and micro x-ray computed tomography correlated well with modulus profiles obtained by nanoindentation. Tissue demineralized in the presence of PI exhibited higher elastic moduli (average 2.8 GPa) across the lesion and comprised a narrow zone in the outer lesion with strongly increased modulus (up to 8 GPa; p < 0.05), which might be related to the preservation of non-collagenous proteins that appear to induce calcium phosphate mineral formation even under demineralizing physical-chemical conditions. However, mechanical aspects of remineralization through the elastic modulus change, and the micromorphological aspects with SEM and TEM observation were almost identical with PILP treatments being conducted in the presence or absence of PI. Thus, the application of the protease inhibitors (PI) seemed to be less effective in promoting the remineralization of demineralized dentin.

Using biomimetic polymers in place of noncollagenous proteins to achieve functional remineralization of dentin tissues.

In calcified tissues such as bones and teeth, mineralization is regulated by an extracellular matrix, which includes non-collagenous proteins (NCP). This natural process has been adapted or mimicked to restore tissues following physical damage or demineralization by using polyanionic acids in place of NCPs, but the remineralized tissues fail to fully recover their mechanical properties. Here we show that pre-treatment with certain amphiphilic peptoids, a class of peptide-like polymers consisting of N-substituted glycines that have defined monomer sequences, enhances ordering and mineralization of collagen and induces functional remineralization of dentin lesions in vitro. In the vicinity of dentin tubules, the newly formed apatite nano-crystals are co-aligned with the c-axis parallel to the tubular periphery and recovery of tissue ultrastructure is accompanied by development of high mechanical strength. The observed effects are highly sequence-dependent with alternating polar and non-polar groups leading to positive outcomes while diblock sequences have no effect. The observations suggest aromatic groups interact with the collagen while the hydrophilic side chains bind the mineralizing constituents and highlight the potential of synthetic sequence-defined biomimetic polymers to serve as NCP mimics in tissue remineralization.

Strontium effects on root dentin tubule occlusion and nanomechanical properties.

OBJECTIVES: Dentin hypersensitivity often is treated by promotion of dentin tubule occlusion. In this in vitro study we evaluated nanomechanical properties and degree of tubule occlusion conferred to sound and demineralized human root dentin following treatment with a 10% (w/w) strontium acetate solution and its relation to the treatment duration and delivery method. METHODS: 24 human cervical root dentin disks (8 groups of 3) were polished through 0.25 µm. 12 disks were subjected to an acid challenge (1% citric acid, pH 3.8) for 2 min. The specimens were incubated in artificial saliva, treated by soaking or brushing with deionized (DI) water or a solution of 10% strontium acetate for 2 min twice a day for 28 days. The occlusion percent and nanomechanical properties were determined at the baseline, 5, 14 and 28 days. Cross-sectioned specimens were prepared to evaluate the depth affected by strontium acetate / dentin interaction by SEM. Statistical analysis was performed using linear mixed effects models. RESULTS: A 10% strontium acetate treatment over 5-28 days significantly increased tubule occlusion for normal root dentin and to a lesser extent for demineralized dentin and increased the AFM based nanomechanical properties of demineralized dentin. Brushing was more effective than soaking in recovery of properties of demineralized dentin when treated with strontium. No difference in tubuleocclusion was found between the two delivery methods. SIGNIFICANCE: Strontium acetate itself proved to have the ability to occlude dentin tubules and result in small changes in the mechanical properties of dentin.

In vitro evaluation of adhesive characteristics of 4-META/MMA-TBB resin with organic filler.

OBJECTIVES: A commercial restorative material, BondfillSB (BF), is a modification of 4-META/MMA-TBB resin cement. BF uses a self-etching primer and added pre-polymerized organic fillers. We compared BF with another self-etching system, EasyBond (EB), in shear bond strength, bonded interface characteristics to human dentin and contraction gap when used in bulk-filling. METHODS: Shear bond strength of BF and EB + Z100 (Z), bonded by different experience-level operators, was evaluated. Bonded interfaces were characterized by SEM, AFM, and AFM based nano-indentation. Contraction gaps (CG) at 0h and 24h after polymerization were evaluated for BF or EB bulk filled class I cavities. To meet the clinical recommendation, BF's powder was replaced by experimental radioopaque powder (BFO) for the CG study. EB was used with Z (EBZ) or with a resin marketed for bulk-fill base (SureFil-SDR-flow (EBSF)). RESULTS: Shear bond strengths (Mean ± Standard Deviation (S.D.)) of BF (37.4 ± 2.6 MPa; n=36) were higher and less variable than EBZ (18.2 ± 7.6 MPa; n=36) (p<0.0001, One-way ANOVA). Weibull characteristic strength (η) differed significantly between materials (p<0.0001) but not between operators (p=0.90). EBZ often had non-uniform interfaces and a wider band of reduced elastic modulus (E) of greater than 20 µm across the interface. BF had uniform interfaces and a smaller width of affected dentin under the interface (∼1 µm). There was a difference in dentin-E between EBZ and BF up to 9 µm from the interface (mixed-effects model; P=0.03). A stratified linear regression model used for CG. EBSF and BFO showed significantly smaller CG than EBZ at time 0. None of three combinations showed any significant change between 0h-CG and 24h-CG. SIGNIFICANCE: BF possessed bonding characteristics required to serve as a restorative.

Immobilization of phosphate monomers on collagen induces biomimetic mineralization.

BACKGROUND: Immobilization of phosphoproteins on type-I collagen via covalent binding may induce extra- and intrafibrillar mineralization. OBJECTIVE: This study tested the hypothesis that methacrylate phosphate esters immobilized on reconstituted type-I collagen can mimic the nucleating role of phosphoproteins. METHODS: Three functional monomers (MDP, GPDM and Phenyl-P) that differed in chemical structure and steric hindrances around the phosphate moiety were evaluated. Reconstituted type-I collagen was either left untouched (control) or treated by 5% monomer/ethanol for 20 s. All samples were incubated in simulated dentinal fluid as mineralizing medium at 37°C for 7 or 14 days. The extra- and intrafibrillar mineralization were examined by SEM and TEM/SAED crystallography, respectively. RESULTS: FT-IR spectroscopy showed that the phosphate groups were incorporated on reconstituted collagen, irrespective of their chemical structure. MDP immobilization induced dense growth of extrafibrillar mineral over time, while with GPDM- and Phenyl-P-immobilized collagen, mineralization was moderate and sparse, respectively. TEM/SAED evidence disclosed that intrafibrillar minerals exclusively occurred in MDP-immobilized collagen. CONCLUSIONS: Immobilization of MDP, which had the lowest steric hindrance, could induce significant biomimetic extra- and intrafibrillar mineralization; resembling the lowest level of hierarchy organization of dentin.

Analysis of interfacial structure and bond strength of self-etch adhesives.

PURPOSE: To determine the bond strength, nanoleakage and interfacial morphology of four self-etch adhesives bonded to superficial dentin. METHODS: Microtensile (MT) (n= 15) and single plane shear (SP) (n= 8) bond tests were performed using human dentin polished through 320-grit SiC paper. Clearfil Protect Bond (PB), Clearfil S3 Bond (S3), Prompt L-Pop (PLP) and G-Bond (GB) were used according to their manufacturers' instructions. Composite was applied as cylinders with a thickness of 4 mm with a 1 mm diameter and stored in water at 370C for 24 hours. Specimens were debonded with a testing machine at a cross-head speed of 1 mm/minute. Means and standard deviations of bond strength were calculated. Data were analyzed using ANOVA. Fisher's PLSD intervals were calculated at the 0.05 level of significance. Failure modes were determined at x100. The hybrid layer was revealed by treatment with 5N HC1/5% NaOCl or fractured perpendicular to the interface and sputter coated with gold. Specimens were viewed at x1,000, x2,500, and x5,000 in a field emission SEM at 15 kV. Teeth (n=2) sectioned into 0.9 mm-thick slabs were immersed in ammoniacal silver nitrate solution for 24 hours, rinsed and immersed in photo-developing solution for 8 hours. Specimens were sectioned (90 nm-thick) and observed under TEM. RESULTS: Means ranged from 25.0 to 73.1 MPa for MT and from 15.5 to 56.4 MPa for SP. MT values were greater than SP, but were highly correlated (R2 = 0.99, P= 0.003) and provided the same order for the systems studied. Fisher's PLSD intervals (P< 0.05) for bond strength techniques and adhesives results were 1.7 and 2.3 MPa, respectively. Failures sites were mixed. TEM showed that hybrid layers were -0.5 pm for PB, GB and S3 and approximately 5 microm for PLP. SEM showed morphologic differences among adhesives. Silver nitrate deposits were observed within the interfaces for all adhesive systems.

Functional remineralization of dentin lesions using polymer-induced liquid-precursor process.

It was hypothesized that applying the polymer-induced liquid-precursor (PILP) system to artificial lesions would result in time-dependent functional remineralization of carious dentin lesions that restores the mechanical properties of demineralized dentin matrix. 140 µm deep artificial caries lesions were remineralized via the PILP process for 7-28 days at 37°C to determine temporal remineralization characteristics. Poly-L-aspartic acid (27 KDa) was used as the polymeric process-directing agent and was added to the remineralization solution at a calcium-to-phosphate ratio of 2.14 (mol/mol). Nanomechanical properties of hydrated artificial lesions had a low reduced elastic modulus (E(R) = 0.2 GPa) region extending about 70 µm into the lesion, with a sloped region to about 140 µm where values reached normal dentin (18-20 GPa). After 7 days specimens recovered mechanical properties in the sloped region by 51% compared to the artificial lesion. Between 7-14 days, recovery of the outer portion of the lesion continued to a level of about 10 GPa with 74% improvement. 28 days of PILP mineralization resulted in 91% improvement of E(R) compared to the artificial lesion. These differences were statistically significant as determined from change-point diagrams. Mineral profiles determined by micro x-ray computed tomography were shallower than those determined by nanoindentation, and showed similar changes over time, but full mineral recovery occurred after 14 days in both the outer and sloped portions of the lesion. Scanning electron microscopy and energy dispersive x-ray analysis showed similar morphologies that were distinct from normal dentin with a clear line of demarcation between the outer and sloped portions of the lesion. Transmission electron microscopy and selected area electron diffraction showed that the starting lesions contained some residual mineral in the outer portions, which exhibited poor crystallinity. During remineralization, intrafibrillar mineral increased and crystallinity improved with intrafibrillar mineral exhibiting the orientation found in normal dentin or bone.

Nanomechanical properties of endodontically treated teeth.

INTRODUCTION: Although it is apparent that teeth become more susceptible to fracture after root canal treatment, the contributing factors for this are not completely established. The purpose of this study was to determine whether there are changes in nanomechanical properties of dentin in root canal-treated teeth compared with non-root canal-treated control teeth. METHODS: Atomic force microscopy-based nanoindentation testing was performed on root canal-treated teeth and age- and type-matched control teeth. Radicular intertubular dentin was indented in 6 locations, and triplicate measurements were averaged. Paired t tests were used to compare root canal-treated teeth with control teeth. RESULTS: The moduli of elasticity were 17.8 ± 2.9 GPa and 18.9 ± 2.9 GPa for root canal-treated teeth and controls, respectively; the hardness values for the 2 groups were 0.84 ± 0.25 GPa and 0.84 ± 0.18 GPa, respectively. Neither the modulus of elasticity nor the hardness differed between groups (P > .05). CONCLUSIONS: It appears that root canal treatment does not result in nanomechanical changes to radicular intertubular dentin.

The ionic products of bioactive glass particle dissolution enhance periodontal ligament fibroblast osteocalcin expression and enhance early mineralized tissue development.

This study resulted in enhanced collagen type 1 and osteocalcin expression in human periodontal ligament fibroblasts (hPDLF) when exposed to bioactive glass conditioned media that subsequently may promote early mineralized tissue development. Commercial Bioglass™ (45S5) and experimental bioactive coating glass (6P53-b), were used to make a glass conditioned media (GCM) for comparison to control medium. ICP-MS analysis showed increased concentrations of Ca(2+), PO(4) (3-), Si(4+), and Na(+), for 45S5 GCM and Mg(2+), K(+), Ca(2+), PO(4)(3-), Si(4+), and Na(+) for 6P53-b GCM (relative to control medium). Differentiating hPDLF cultures exposed to 45S5 and 6P53-b GCM showed enhanced expression of collagen type 1 (Col1α1, Col1α2), osteocalcin, and alkaline phosphatase gene expression. These GCM also enhanced osteocalcin protein expression. After 16 d of culture, 45S5 and 6P53-b GCM treated cells showed regions of deep red Alizarin staining, indicating increased Ca within their respective extracellular matrices (ECM), while control-treated cells did not exhibit these features. SEM analysis showed more developed ECM in GCM treated cultures, indicated by multiple tissue layering and abundant collagen fiber bundle formation, while control treated cells did not exhibit these features. SEM analysis showed polygonal structures suggestive of CaP in 45S5 GCM treated cultures. These results indicate the osteogenic potential of bioactive coating glass in periodontal bone defect filling applications.

Nano- and micromechanical properties of dentine: Investigation of differences with tooth side.

The soft zone in dentine beneath the dentino-enamel junction is thought to play an important role in tooth function, strain distribution and fracture resistance during mastication. Recently reported asymmetry in mechanical properties with tooth side may point at a basic property of tooth function. The aim of our study was to test if this asymmetry was reflected in the nano- and micromechanical properties of dentine. We investigated the mechanical properties of dentine on the buccal and lingual side of nine extracted human teeth using nano- and microindentation. Properties were analysed on the natural log scale, using maximum likelihood to estimate the parameters. Two-sided 0.05-level likelihood ratio tests were used to assess the influences of surface (buccal versus lingual) and dentine depth, measured from the DEJ in crown dentine and from the CDJ in root dentine. Results showed the well known gradual increase in mechanical properties with increasing distance from the DEJ. Coronal dentine showed higher elastic modulus and hardness on the lingual side of teeth for all measurements, while root dentine was harder on the buccal side. Due to the subtlety of these effects and the small number of teeth studied, results failed to reach statistical significance. Results suggest that dentine nano- and micromechanical properties vary with tooth side in agreement with recent literature using macroscopic methods. They also reveal that buccal-lingual ratios of hardness are in opposite directions in crown and root dentine, suggesting compensatory functions.

Mechanical recovery of dentin following remineralization in vitro--an indentation study.

This study sought to gain insights into the steps leading to remineralization and mechanical recovery of hydrated dentin. Mechanical recovery in water was hypothesized to result from effective mineral matrix binding and to occur from the innermost regions outwards due to an increase in the number of nucleation sites. Partially demineralized (0.05 M acetate, pH=5.0, 8h) dentin was remineralized using calcium and phosphate solutions of 10.1 or 9.8 degree of saturation (DS) for hydroxyapatite (pH=7.4) for 4, 8 or 24h. Remineralization used a constant solution composition approach, which allowed for a continuous mineral growth with relatively constant thermodynamic driving forces. Crystal growth rates (R) were calculated using concentrations of calcium and phosphate. Before and after de- and re-mineralization, specimens had their surface and cross-section elastic moduli measured using AFM-nanoindentation in water. DS=10.1 provided higher R and higher mechanical recovery at the surface (p<0.0001). Cross-sectional measurements showed that subsurface mechanical recovery occurred from the innermost demineralized areas gradually outwards for both groups with no statistical differences at different DS, thus suggesting that remineralization is driven by mineral growth within nucleation sites with preserved collagen fibrils. Further, mechanical recovery appeared to initially obey a heterogeneous pattern, which vanished with time. This study provides evidence of mechanical recovery of hydrated dentin after remineralization and novel insights into the steps leading to mechanical recovery of carious dentin.

Tissue-specific calibration of extracellular matrix material properties by transforming growth factor-ß and Runx2 in bone is required for hearing.

Physical cues, such as extracellular matrix stiffness, direct cell differentiation and support tissue-specific function. Perturbation of these cues underlies diverse pathologies, including osteoarthritis, cardiovascular disease and cancer. However, the molecular mechanisms that establish tissue-specific material properties and link them to healthy tissue function are unknown. We show that Runx2, a key lineage-specific transcription factor, regulates the material properties of bone matrix through the same transforming growth factor-ß (TGFß)-responsive pathway that controls osteoblast differentiation. Deregulated TGFß or Runx2 function compromises the distinctly hard cochlear bone matrix and causes hearing loss, as seen in human cleidocranial dysplasia. In Runx2+/⁻ mice, inhibition of TGFß signalling rescues both the material properties of the defective matrix, and hearing. This study elucidates the unknown cause of hearing loss in cleidocranial dysplasia, and demonstrates that a molecular pathway controlling cell differentiation also defines material properties of extracellular matrix. Furthermore, our results suggest that the careful regulation of these properties is essential for healthy tissue function.

Does mentoring matter: results from a survey of faculty mentees at a large health sciences university.

BACKGROUND: To determine the characteristics associated with having a mentor, the association of mentoring with self-efficacy, and the content of mentor-mentee interactions at the University of California, San Francisco (UCSF), we conducted a baseline assessment prior to implementing a comprehensive faculty mentoring program. METHOD: We surveyed all prospective junior faculty mentees at UCSF. Mentees completed a web-based, 38-item survey including an assessment of self-efficacy and a needs assessment. We used descriptive and inferential statistics to determine the association between having a mentor and gender, ethnicity, faculty series, and self-efficacy. RESULTS: Our respondents (n=464, 56%) were 53% female, 62% white, and 7% from underrepresented minority groups. More than half of respondents (n=319) reported having a mentor. There were no differences in having a mentor based on gender or ethnicity (p>or=0.05). Clinician educator faculty with more teaching and patient care responsibilities were statistically significantly less likely to have a mentor compared with faculty in research intensive series (p<0.001). Having a mentor was associated with greater satisfaction with time allocation at work (p<0.05) and with higher academic self-efficacy scores, 6.07 (sd = 1.36) compared with those without a mentor, 5.33 (sd = 1.35, p<0.001). Mentees reported that they most often discussed funding with the mentors, but rated highest requiring mentoring assistance with issues of promotion and tenure. CONCLUSION: Findings from the UCSF faculty mentoring program may assist other health science institutions plan similar programs. Mentoring needs for junior faculty with greater teaching and patient care responsibilities must be addressed.

Variations in human DEJ scallop size with tooth type.

OBJECTIVE: Recent literature suggests that the scalloped structure of the dentino-enamel junction (DEJ) is critical for DEJ stability. Aim of our study was to see if there are differences in scallop size and shape with tooth type. METHODS: Enamel of extracted permanent human teeth was demineralised using EDTA. After fixation and dehydration the scallops of the DEJ were investigated in a scanning electron microscope. Scallop area and shape (circularity) were measured for molars, premolars, canines and incisors. RESULTS: Scallop area showed main effects for tooth type and specimen, while, due to high variability in third molars, there was also an interaction effect (repeated measures two-way ANOVA, p<0.05). Differences between tooth types were statistically significant, suggesting that posterior teeth showed larger scallops compared to anterior teeth. Differences in shape (circularity) were not statistically significant. CONCLUSION: Our results suggest that teeth which are subject to higher masticatory loads (posterior teeth) show larger and more pronounced scallops. These findings might be of interest for improving other interfaces joining dissimilar materials.

A review of adhesion science.

OBJECTIVE: Adhesion or cohesion includes an adherend, adhesive, and intervening interface. Adhesive joints may include one or more interfaces. Adhesion science focuses on understanding the materials properties associated with formation of the interfaces, changes in the interfaces with time, and events associated with failure of the interfaces. METHODS: The key principles for good interface formation are creation of a clean surface, generation of a rough surface for interfacial interlocking, good wetting of the substratum by the adhesive/cohesive materials, adequate flow and adaptation for intimate interaction, and acceptable curing when phase changes are required for final joint formation. RESULTS: Much more effort is needed in the future to carefully assess each of these using available testing methods that attempt to characterize the energetics of the interfaces. Bonding involves potential contributions from physical, chemical, and mechanical sources but primarily relies on micro-mechanical interaction for success. Characterization of the interface before adhesion, during service, and after failure would be much more useful for future investigations and remains as a great challenge. SIGNIFICANCE: Scientists should more rigorously apply techniques such as comprehensive contact angle analysis (rather than simple water wettability) for surface energy determination, and AFM in addition to SEM for surface texture analysis.

Structure, chemical composition and mechanical properties of coronal cementum in human deciduous molars.

OBJECTIVES: It was hypothesized that the coronal cementum containing collagen forms a weak junction with enamel unlike the well integrated DEJ and CDJ. METHODS: The hypothesis was investigated in two parts: (1) evaluate the structure, chemical composition and mechanical properties of coronal cementum and its junction with enamel using scanning electron microscopy, micro-X-ray computed tomography, and atomic force microscopy. The chemical composition and mechanical properties were determined by evaluating the spatial variations of inorganic (PO(4)(3-)nu(1) mode at 960 cm(-1)) and organic (C-H deformation at 1452 cm(-1); C-H stretch at 2940 cm(-1)) contents using Raman microspectroscopy and elastic modulus and hardness values using nanoindentation. (2) Estimate the strength and evaluate the microstructure of coronal cementum interface with enamel using SEM and MicroXCT. RESULTS AND CONCLUSIONS: Coronal cementum is heterogeneous because it is a combination of laminar acellular afibrillar cementum and acellular extrinsic fiber cementum with relatively higher organic content. It integrates micromechanically via a scallop-like weak interface with enamel unlike the biomechanically efficient DEJ and CDJ and is continuous with primary root cementum. A single tooth could exhibit all three types of cementum enamel junctions; an overlap, butt and a gap depending on the sectioning plane. The elastic modulus of coronal cementum (11.0+/-5.8 GPa) is significantly lower (p<0.05; Student's t-test with 95% confidence interval) than primary cementum (15.8+/-5.3 GPa).