The cementum-dentin junction also contains glycosaminoglycans and collagen fibrils

Matrix Metalloproteinases in the Periodontium-Vital in Tissue Turnover and Unfortunate in Periodontitis

The extracellular matrix (ECM) is a complex non-cellular three-dimensional macromolecular network present within all tissues and organs, forming the foundation on which cells sit, and composed of proteins (such as collagen), glycosaminoglycans, proteoglycans, minerals, and water. The ECM provides a fundamental framework for the cellular constituents of tissue and biochemical support to surrounding cells. The ECM is a highly dynamic structure that is constantly being remodeled. Matrix metalloproteinases (MMPs) are among the most important proteolytic enzymes of the ECM and are capable of degrading all ECM molecules. MMPs play a relevant role in physiological as well as pathological processes; MMPs participate in embryogenesis, morphogenesis, wound healing, and tissue remodeling, and therefore, their impaired activity may result in several problems. MMP activity is also associated with chronic inflammation, tissue breakdown, fibrosis, and cancer invasion and metastasis. The periodontium is a unique anatomical site, composed of a variety of connective tissues, created by the ECM. During periodontitis, a chronic inflammation affecting the periodontium, increased presence and activity of MMPs is observed, resulting in irreversible losses of periodontal tissues. MMP expression and activity may be controlled in various ways, one of which is the inhibition of their activity by an endogenous group of tissue inhibitors of metalloproteinases (TIMPs), as well as reversion-inducing cysteine-rich protein with Kazal motifs (RECK).

Rapid Intrafibrillar Mineralization Strategy Enhances Adhesive-Dentin Interface

Biomimetic mineralization of dentin collagen appears to be a promising strategy to optimize dentin bonding durability. However, traditional postbonding mineralization strategies based on Ca/P ion release still have some drawbacks, such as being time-consuming, having a spatiotemporal mismatch, and having limited intrafibrillar minerals. To tackle these problems, a prebonding rapid intrafibrillar mineralization strategy was developed in the present study. Specifically, polyacrylic acid-stabilized amorphous calcium fluoride (PAA-ACF) was found to induce rapid intrafibrillar mineralization of the single-layer collagen model and dentin collagen at just 1 min and 10 min, as identified by transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. This strategy has also been identified to strengthen the mechanical properties of demineralized dentin within a clinically acceptable timeframe. Significantly, the bonding strength of the PAA-ACF-treated groups outperformed the control group irrespective of aging modes. In addition, the endogenous matrix metalloproteinases as well as exogenous bacterial erosion were inhibited, thus reducing the degradation of dentin collagen. High-quality integration of the hybrid layer and the underlying dentin was also demonstrated. On the basis of the present results, the concept of "prebonding rapid intrafibrillar mineralization" was proposed. This user-friendly scheme introduced PAA-ACF-based intrafibrillar mineralization into dentin bonding for the first time. As multifunctional primers, PAA-ACF precursors have the potential to shed new light on prolonging the service life of adhesive restorations, with promising significance.

Evaluation of resveratrol-doped adhesive with advanced dentin bond durability

OBJECTIVES

This paper aimed to evaluate the influence of resveratrol-doped adhesive on the durability and antibiofilm capability of dentin bonding.

METHODS

Experimental adhesives were prepared by incorporating resveratrol into a universal adhesive at concentrations of 0 (control), 0.1, 1, and 10 mg/mL. The microtensile bond strength, fracture modes, and adhesive-dentin interface nanoleakage were assessed after 24 h of water storage, 10,000 times of thermocycling or 1-month of collagenase ageing. Relevant antibiofilm capability on Streptococcus mutans (S. mutans), in situ zymography, degree of conversion, and cytotoxicity of resveratrol-doped adhesives were also determined.

RESULTS

Irrespective of thermocycled or collagenase ageing, the resveratrol-doped adhesive (1 mg/mL) maintained the bond strength and reduced the nanoleakage expression. Meanwhile, the inhibitory ability on endogenous protease activity and S. mutans biofilm formation with acceptable biocompatibility were obtained.

CONCLUSIONS

This study suggested that the resveratrol-doped adhesive achieved effective improvement on dentin bond durability and secondary caries management.

CLINICAL SIGNIFICANCE

The application of the resveratrol-doped adhesive indicates promising benefits to increase the lifetime of composite restorations.

Small leucine-rich proteoglycans in physiological and biomechanical function of bone

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Proteoglycans (PGs) and glycosaminoglycans (GAGs) play vital roles in key signaling pathways to regulate bone homeostasis.

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The highly negatively charged GAGs are crucial in retaining bound water and modulating mechanical properties of bone.

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Age-related changes of PGs, GAGs, and bound water contribute to deterioration of bone quality during aging.

Proteoglycans (PGs) contain long unbranched glycosaminoglycan (GAG) chains attached to core proteins. In the bone extracellular matrix, PGs represent a class of non-collagenous proteins, and have high affinity to minerals and collagen. Considering the highly negatively charged character of GAGs and their interfibrillar positioning interconnecting with collagen fibrils, PGs and GAGs play pivotal roles in maintaining hydrostatic and osmotic pressure in the matrix. In this review, we will discuss the role of PGs, especially the small leucine-rich proteoglycans, in regulating the bioactivity of multiple cytokines and growth factors, and the bone turnover process. In addition, we focus on the coupling effects of PGs and GAGs in the hydration status of bone extracellular matrix, thus modulating bone biomechanical properties under physiological and pathological conditions.

Residual stress and osmotic swelling of the periodontal ligament

Osmotic swelling and residual stress are increasingly recognized as important factors in soft tissue biomechanics. Little attention has been given to residual stress in periodontal ligament (PDL) biomechanics despite its rapid growth and remodeling potential. Those tissues that bear compressive loads, e.g., articular cartilage, intervertebral disk, have received much attention related to their capacities for osmotic swelling. To understand residual stress and osmotic swelling in the PDL, it must be asked (1) to what extent, if any, does the PDL exhibit residual stress and osmotic swelling, and (2) if so, whether residual stress and osmotic swelling are mechanically significant to the PDL's stress/strain behavior under external loading. Here, we incrementally built a series of computer models that were fit to uniaxial loading, osmotic swelling and residual stretch data. The models were validated with in vitro shear tests and in vivo tooth-tipping data. Residual stress and osmotic swelling models were used to analyze tension and compression stress (principal stress) effects in PDL specimens under external loads. Shear-to-failure experiments under osmotic conditions were performed and modeled to determine differences in mechanics and failure of swollen periodontal ligament. Significantly higher failure shear stresses in swollen PDL suggested that osmotic swelling reduced tension and thus had a strengthening effect. The in vivo model's first and third principal stresses were both higher with residual stress and osmotic swelling, but smooth stress gradients prevailed throughout the three-dimensional PDL anatomy. The addition of PDL stresses from residual stress and osmotic swelling represents a unique concept in dental biomechanics.

Microstructural heterogeneity of the collagenous network in the loaded and unloaded periodontal ligament and its biomechanical implications

The periodontal ligament (PDL) is a highly heterogeneous fibrous connective tissue and plays a critical role in distributing occlusal forces and regulating tissue remodeling. Its mechanical properties are largely determined by the extracellular matrix, comprising a collagenous fiber network interacting with the capillary system as well as interstitial fluid containing proteoglycans. While the phase-contrast micro-CT technique has portrayed the 3D microscopic heterogeneity of PDL, the topological parameters of its network, which is crucial to understanding the multiscale constitutive behavior of this tissue, has not been characterized quantitatively. This study aimed to provide new understanding of such microscopic heterogeneity of the PDL with quantifications at both tissue and collagen network levels in a spatial manner, by combining phase-contrast micro-CT imaging and a purpose-built image processing algorithm for fiber analysis. Both variations within a PDL and among the PDL with different shapes, i.e. round-shaped and kidney-shaped PDLs, are described in terms of tissue thickness, fiber distribution, local fiber densities, and fiber orientation (namely azimuthal and elevation angles). Furthermore, the tissue and collagen fiber network responses to mechanical loading were evaluated in a similar manner. A 3D helical alignment pattern was observed in the fiber network, which appears to regulate and adapt a screw-like tooth motion under occlusion. The microstructural heterogeneity quantified here allows development of sample-specific constitutive models to characterize the PDL's functional and pathological loading responses, thereby providing a new multiscale framework for advancing our knowledge of this complex limited mobility soft-hard tissue interface.

Biglycan and chondroitin sulfate play pivotal roles in bone toughness via retaining bound water in bone mineral matrix

Recent in vitro evidence shows that glycosaminoglycans (GAGs) and proteoglycans (PGs) in bone matrix may functionally be involved in the tissue-level toughness of bone. In this study, we showed the effect of biglycan (Bgn), a small leucine-rich proteoglycan enriched in extracellular matrix of bone and the associated GAG subtype, chondroitin sulfate (CS), on the toughness of bone in vivo, using wild-type (WT) and Bgn deficient mice. The amount of total GAGs and CS in the mineralized compartment of Bgn KO mouse bone matrix decreased significantly, associated with the reduction of the toughness of bone, in comparison with those of WT mice. However, such differences between WT and Bgn KO mice diminished once the bound water was removed from bone matrix. In addition, CS was identified as the major subtype in bone matrix. We then supplemented CS to both WT and Bgn KO mice to test whether supplemental GAGs could improve the tissue-level toughness of bone. After intradermal administration of CS, the toughness of WT bone was greatly improved, with the GAGs and bound water amount in the bone matrix increased, while such improvement was not observed in Bgn KO mice or with supplementation of dermatan sulfate (DS). Moreover, CS supplemented WT mice exhibited higher bone mineral density and reduced osteoclastogenesis. Interestingly, Bgn KO bone did not show such differences irrespective of the intradermal administration of CS. In summary, the results of this study suggest that Bgn and CS in bone matrix play a pivotal role in imparting the toughness to bone most likely via retaining bound water in bone matrix. Moreover, supplementation of CS improves the toughness of bone in mouse models.

A Bottom-Up Approach Grafts Collagen Fibrils Perpendicularly to Titanium Surfaces

Currently, titanium dental implant apposition to bone is achieved via osseointegration leading to ankylosis. A biomimetic Sharpey's fiber-type interface could be constructed around collagen fibrils robustly attached and projecting perpendicularly from the titanium surface. We present a proof-of-concept for a method to create upright-standing collagen nanofibrils covalently bonded to a titanium surface. The method involves activation of the titanium surface using a plasma discharge treatment followed by functionalization with an oxyamine-terminated silane coupling molecule. Using Rapoport's salt, the N-termini of individual type I collagen monomers are converted to ketones. When presented to the functionalized titanium surface, these ketones form oxime linkages with the silanes thus immobilizing the collagen. In a two-step process, these covalently bonded monomers act as sites for the formation of fibrils. Many fibril-surface junctions were observed by scanning electron microscopy on three different surfaces. These findings set the stage for working toward a high surface density of such features which might act as a platform from which to build a synthetic ligament.

New perspective to improve dentin-adhesive interface stability by using dimethyl sulfoxide wet-bonding and epigallocatechin-3-gallate

OBJECTIVES

To determine whether dentin-adhesive interface stability would be improved by dimethyl sulfoxide (DMSO) wet-bonding and epigallocatechin-3-gallate (EGCG).

METHODS

Etched dentin surfaces from sound third molars were randomly assigned to five groups according to different pretreatments: group 1, water wet-bonding (WWB); group 2, 50% (v/v) DMSO wet-bonding (DWB); groups 3-5, 0.01, 0.1, and 1 wt% EGCG-incorporated 50% (v/v) DMSO wet-bonding (0.01%, 0.1%, and 1%EGCG/DWB). Singlebond universal adhesive was applied to the pretreated dentin surfaces, and composite buildups were constructed. Microtensile bond strength (μTBS) and interfacial nanoleakage were respectively examined after 24 h water storage or 1-month collagenase ageing. In situ zymography andStreptococcus mutans (S. mutans) biofilm formation were also investigated.

RESULTS

After collagenase ageing, μTBS of groups 4 (0.1%EGCG/DWB) and 5 (1%EGCG/DWB) did not decrease (p > 0.05) and was higher than that of the other three groups (p < 0.05). Nanoleakage expression of groups 4 and 5 was less than that of the other three groups (p < 0.05), regardless of collagenase ageing. Metalloproteinase activities within the hybrid layer in groups 4 and 5 were suppressed. Furthermore, pretreatment with 1%EGCG/DWB (group 5) efficiently inhibited S. mutans biofilm formation along the dentin-adhesive interface.

SIGNIFICANCE

This study suggested that the synergistic action of DMSO wet-bonding and EGCG can effectively improve dentin-adhesive interface stability. This strategy provides clinicians with promising benefits to achieve desirable dentin bonding performance and to prevent secondary caries, thereby extending the longevity of adhesive restorations.

Removal of water binding proteins from dentin increases the adhesion strength of low-hydrophilicity dental resins

OBJECTIVES

To investigate the role of proteoglycans (PGs) on the physical properties of the dentin matrix and the bond strength of methacrylate resins with varying hydrophilicities.

METHODS

Dentin were obtained from crowns of human molars. Enzymatic removal of PGs followed a standard protocol using 1 mg/mL trypsin (Try) for 24 h. Controls were incubated in ammonium bicarbonate buffer. Removal of PGs was assessed by visualization of glycosaminoglycan chains (GAGs) in dentin under transmission electron microscopy (TEM). The dentin matrix swelling ratio was estimated using fully demineralized dentin. Dentin wettability was assessed on wet, dry and re-wetted dentin surfaces through water contact angle measurements. Microtensile bond strength test (TBS) was performed with experimental adhesives containing 6% HEMA (H₆) and 18% HEMA (H₁₈) and a commercial dental adhesive. Data were statistically analyzed using ANOVA and post-hoc tests (α = 0.05).

RESULTS

The enzymatic removal of PGs was confirmed by the absence and fragmentation of GAGs. There was statistically significant difference between the swelling ratio of Try-treated and control dentin (p < 0.001). Significantly lower contact angle was found for Try-treated on wet and dry dentin (p < 0.002). The contact angle on re-wet dentin was not recovered in Try-treated group (p = 0.9). Removal of PGs significantly improved the TBS of H₆ (109% higher, p < 0.001) and H₁₈ (29% higher, p = 0.002) when compared to control. The TBS of commercial adhesive was not affected by trypsin treatment (p = 0.9).

SIGNIFICANCE

Changing the surface energy of dentin by PGs removal improved resin adhesion, likely due to more efficient water displacement, aiding to improved resin infiltration and polymerization.

Effects of resveratrol/ethanol pretreatment on dentin bonding durability

To determine the effects of resveratrol/ethanol solution on the durability of resin-dentin bonding interfaces. Sixty-four non-caries third molars were randomly divided into four groups (n = 16) after sectioning, and then pretreated with one of the following concentrations of resveratrol/ethanol solutions: 0 (control group), 1, 10 and 20 mg/mL, followed by a universal adhesive and resin composites. All microtensile samples were divided into three subgroups: immediate group, collagenase ageing group and thermocycled group. The microtensile bond strength (MTBS), failure modes, interfacial nanoleakage and in situ zymography were measured, whereas the inhibitory effects of pretreated dentin slices on S. mutans biofilms were determined by confocal laser scanning microscopy and MTT assay. The results indicated that bonding strength was not only influenced by pretreatment factors (P < 0.05) but also ageing factors (P < 0.05). Regardless of collagenase ageing or thermocycling, the 10 mg/mL resveratrol/ethanol pretreatment group presented significantly higher (P < 0.05) MTBS and lower (P < 0.05) expression of nanoleakage than the control group, showed better inhibitory effect of matrix metalloproteinases and S. mutans activity with acceptable cytotoxicity. Meanwhile, cohesive failure in dentin decreased gradually with increasing resveratrol concentration. Therefore, the resveratrol/ethanol solution had the potential to serve as a versatile dentin primer, which can effectively improve dentin bonding durability and prevent secondary caries.

Biomechanical pathways of dentoalveolar fibrous joints in health and disease

Spatial and temporal adaptations within periodontal tissues and their interfaces result from functional loads. Functional loads can be physiologic and/or pathologic in nature. The prolonged effect of these loads can alter the overall biomechanics of a dentoalveolar fibrous joint (dentoalveolar joint) by changing the form of the tooth root and its socket. This "sculpting" of the tooth root and alveolar bony socket is a consequence of several mechano-biological changes that occur within the periodontal complex of a load-bearing dentoalveolar joint. These include changes in biochemical expressions, structure, elemental composition, and mechanical properties of alveolar bone, the underlying tissues of the roots of teeth, and their interfaces. These physicochemical changes in tissues continue to prompt mechano-responsive biochemical activities at the attachment sites of periodontal ligament (soft) with bone (hard), and ligament with cementum (hard), which are the entheses of a load-bearing dentoalveolar joint. Forces at soft-hard tissue attachment sites between disparate materials with different stiffness values theoretically generate strain singularities or discontinuities. These discontinuities under prolonged functional loading increase the probability for failure to occur specifically at the enthesial zones. However, in a normal dentoalveolar joint, gradual stiffness gradients exist from ligament to bone, and from ligament to cementum. The gradual transitions in stiffness from softer ligament (lower stiffness) to harder bone or cementum (higher stiffness) or vice versa optimize tissue and interfacial strains. Optimization of tissue and ligament-enthesial physical and chemical properties facilitates transmission of cyclic forces of varying magnitudes and frequencies that collectively maintain the overall biomechanics of a dentoalveolar joint. The objectives of this review are 3-fold: (i) to illustrate physicochemical adaptations at the periodontal ligament entheses of a human periodontal complex affected by subgingival calculus; (ii) to demonstrate how to "program" the hallmarks of periodontitis in small-scale vertebrates in vivo to generate spatiotemporal maps of physicochemical adaptations in a diseased dentoalveolar joint; and (iii) to correlate dentoalveolar joint biomechanics in healthy and diseased states to spatiotemporal maps of physicochemical adaptations within respective periodontal tissues. This interdisciplinary approach demonstrates that physicochemical adaptations within periodontal tissues using the mechanics of materials (tissue mechanics), materials science (tissue composition), and mechano-biology (matrix molecules) can help explain the mechano-adaptation of dentoalveolar joints in normal and diseased functional states. Multiscale biomechanics and mechano-biology approaches can provide insights into the functional competence of a diseased relative to a normal dentoalveolar joint. Insights gathered from interdisciplinary and multiscale biomechanics approaches include the following: (i) physiologic loads related to chewing maintain a balance between mineral-forming and-resorbing biochemical cellular events, resulting in gradual stiffness gradients at the periodontal ligament entheses, and, in turn, sustain the overall biomechanics of a normal "healthy" dentoalveolar joint; (ii) pathologic loads resulting from tissue degradation and physical changes to the periodontal complex promote an abrupt stiffness gradient at the periodontal ligament entheses. The shift from gradual to an abrupt stiffness gradient could prompt a shift in the biochemical cascades, exacerbate mechano-responsive biochemical expressions at periodontal ligament entheses farther away from the site of insult, and culminate in joint degradation; (iii) sustained pathologic function on periodontally diseased joints exacerbates degradation of periodontal ligament entheses providing insights into "rescue therapy", such as the use of an adequate "mechanocal dose" to regain joint function; and (iv) spatiotemporal maps of changes in biochemical expressions, and physicochemical properties of strain-dominated affected sites, including the periodontal ligament entheses, can guide anatomy-specific therapeutics for tissue regeneration and/or disease control with the purpose of regaining dentoalveolar joint function. Modulation of occlusal loads could minimize disease progression and potentially assist in regaining functional attachment of ligament to bone and/or ligament to cementum of the dentoalveolar joint. Elucidating mechanisms that drive the breakdown of the functionally active periodontal complex burdened with microbes will provide the required critical insights into regenerative medicine and/or biomimetic approaches that would facilitate rescue/regain of dentoalveolar joint function.

Regional contribution of proteoglycans to the fracture toughness of the dentin extracellular matrix

This study investigated the contribution of small leucine rich proteoglycans (SLRPs) to the fracture toughness of the dentin extracellular matrix (ECM) by enzymatically-assisted selective removal of glycosaminoglycan chains (GAGs) and proteoglycans (PGs) core protein. We adapted the Mode III trouser tear test to evaluate the energy required to tear the dentin ECM. Trouser-shaped dentin specimens from crown and root were demineralized. Depletion of GAGs and PGs followed enzymatic digestion using chondroitinase ABC (c-ABC) and matrix metalloproteinase 3 (MMP-3), respectively. The legs from specimen were stretched under tensile force and the load at tear propagation was determined to calculate the tear energy (T, kJ/m²). SLRPs decorin and biglycan were visualized by immunohistochemistry and ECM tear pattern was analyzed in SEM. Results showed T of crown ECM was not affected by PGs/GAGs depletion (p = 0.799), whereas the removal of PGs significantly reduced T in root dentin ECM (p = 0.001). Root dentin ECM exhibited higher T than crown (p < 0.03), however no regional difference are present after PG depletion (p = 0.480). Immunohistochemistry confirmed removal of GAGs and PGs. SEM images showed structural modifications after PGs/GAGs removal such as enlargement of dentinal tubules, increased interfibrillar spaces and presence of untwisted fibrils with increased diameter. Findings indicate that the capacity of the PGs to unfold and untwist contribute to the dentin ECM resistance to tear, possibly influencing crack growth propagation. The regional differences are likely an evolutionary design to increase tooth survival, that undergoes repetitive mechanical loading and load stress dissipation over a lifetime of an individual.

From Translation to Protein Degradation as Mechanisms for Regulating Biological Functions: A Review on the SLRP Family in Skeletal Tissues

The extracellular matrix can trigger cellular responses through its composition and structure. Major extracellular matrix components are the proteoglycans, which are composed of a core protein associated with glycosaminoglycans, among which the small leucine-rich proteoglycans (SLRPs) are the largest family. This review highlights how the codon usage pattern can be used to modulate cellular response and discusses the biological impact of post-translational events on SLRPs, including the substitution of glycosaminoglycan moieties, glycosylation, and degradation. These modifications are listed, and their impacts on the biological activities and structural properties of SLRPs are described. We narrowed the topic to skeletal tissues undergoing dynamic remodeling.

Structural and biomechanical changes to dentin extracellular matrix following chemical removal of proteoglycans

Proteoglycans are biomacromolecules with significant biomineralization and structural roles in the dentin extracellular matrix. This study comprehensively assessed the mechanical properties and morphology of the dentin extracellular matrix following chemical removal of proteoglycans to elucidate the structural roles of proteoglycans in dentin. Dentin extracellular matrix was prepared from extracted teeth after complete tissue demineralization. Chemical removal of proteoglycans was carried-out using guanidine hydrochloride for up to 10 days. The removal of proteoglycans was determined by dimethylmethylene blue colorimetric assay and histological staining analyses using transmission electron microscopy and optical microscopy. The modulus of elasticity of dentin matrix was determined by a 3-point bending test method. Partial removal of proteoglycans induced significant modifications to the dentin matrix, particularly to type I collagen. Removal of proteoglycans significantly decreased the modulus of elasticity of dentin extracellular matrix (p < 0.0001). In conclusion, the subtle disruption of proteoglycans induces pronounced changes to the collagen network packing and the bulk modulus of elasticity of dentin matrix.

Chlorhexidine-encapsulated mesoporous silica-modified dentin adhesive

OBJECTIVES

This work aims to explore the feasibility of chlorhexidine-encapsulated mesoporous silica (CHX@pMSN) as a modifier of a commercial dental adhesive via the evaluation of physicochemical properties and antibacterial capabilities of adhesive-dentin interface.

METHODS

Therapeutic adhesives were developed in the present study by incorporating CHX@pMSN into a commercial adhesive at four mass fractions (0, 1, 5 and 10 wt.%). The antibacterial capability on Streptococcus mutans (S. mutans) biofilm, conversion degree, adhesive morphology, microtensile bond strength (MTBS) and nanoleakage expression were evaluated comprehensively.

RESULTS

MTT and CLSM evaluation showed that CHX@pMSN-doped adhesive inhibits S. mutans biofilm growth, while CHX is released from the modified adhesive continuously. The incorporation of CHX@pMSN did not affect immediate bond strength at the concentration of 1% and 5% (P >  0.05). Moreover, these bonds were mainly preserved in 5% CHX@pMSN group after one month of collagenase ageing. Meanwhile, CHX@pMSN-doped adhesive groups exhibited similar nanoleakage distribution compared with the control.

CONCLUSION

This study showed that the 5% CHX@pMSN-modified adhesive achieved balance amongst unaffected immediate bonding strength, well-preserved bonds against collagenase ageing and effective inhibition of S. mutans biofilm growth.

CLINICAL SIGNIFICANCE

CHX@pMSN-modified dentin adhesive can potentially extend the service life of adhesive restoration in clinic.

High-performance therapeutic quercetin-doped adhesive for adhesive-dentin interfaces

Almost half of dental restorations have failed in less than 10 years, and approximately 60% of practice time has been consumed to replace these dental restorations. As such, contemporary dentin adhesives should be modified to treat secondary caries and prevent the degradation of adhesive–dentin interfaces. To achieve this goal, we developed a versatile therapeutic adhesive in the present study by incorporating quercetin, which is a naturally derived plant extract, into a commercial adhesive at three concentrations (100, 500 and 1000 µg/mL). An unmodified adhesive served as a control. The antibacterial ability on Streptococcus mutans biofilm, conversion degree, microtensile bond strength, failure modes, in situ zymography, nanoleakage expression and cytotoxicity of quercetin-doped adhesive were comprehensively evaluated. Results showed that the quercetin-doped adhesive (500 µg/mL) preserved its bonding properties against collagenase ageing and inhibited the growth of S. mutans biofilm. Efficient bonding interface sealing ability, matrix metalloproteinase inhibition and acceptable biocompatibility were also achieved. Thus, a simple, safe and workable strategy was successfully developed to produce therapeutic adhesives for the extension of the service life of adhesive restorations.

Dentin on the nanoscale: Hierarchical organization, mechanical behavior and bioinspired engineering

OBJECTIVE

Knowledge of the structural organization and mechanical properties of dentin has expanded considerably during the past two decades, especially on a nanometer scale. In this paper, we review the recent literature on the nanostructural and nanomechanical properties of dentin, with special emphasis in its hierarchical organization.

METHODS

We give particular attention to the recent literature concerning the structural and mechanical influence of collagen intrafibrillar and extrafibrillar mineral in healthy and remineralized tissues. The multilevel hierarchical structure of collagen, and the participation of non-collagenous proteins and proteoglycans in healthy and diseased dentin are also discussed. Furthermore, we provide a forward-looking perspective of emerging topics in biomaterials sciences, such as bioinspired materials design and fabrication, 3D bioprinting and microfabrication, and briefly discuss recent developments on the emerging field of organs-on-a-chip.

RESULTS

The existing literature suggests that both the inorganic and organic nanostructural components of the dentin matrix play a critical role in various mechanisms that influence tissue properties.

SIGNIFICANCE

An in-depth understanding of such nanostructural and nanomechanical mechanisms can have a direct impact in our ability to evaluate and predict the efficacy of dental materials. This knowledge will pave the way for the development of improved dental materials and treatment strategies.

CONCLUSIONS

Development of future dental materials should take into consideration the intricate hierarchical organization of dentin, and pay particular attention to their complex interaction with the dentin matrix on a nanometer scale.

Periodontal ligament entheses and their adaptive role in the context of dentoalveolar joint function

OBJECTIVE

The dynamic bone-periodontal ligament (PDL)-tooth fibrous joint consists of two adaptive functionally graded interfaces (FGI), the PDL-bone and PDL-cementum that respond to mechanical strain transmitted during mastication. In general, from a materials and mechanics perspective, FGI prevent catastrophic failure during prolonged cyclic loading. This review is a discourse of results gathered from literature to illustrate the dynamic adaptive nature of the fibrous joint in response to physiologic and pathologic simulated functions, and experimental tooth movement.

METHODS

Historically, studies have investigated soft to hard tissue transitions through analytical techniques that provided insights into structural, biochemical, and mechanical characterization methods. Experimental approaches included two dimensional to three dimensional advanced in situ imaging and analytical techniques. These techniques allowed mapping and correlation of deformations to physicochemical and mechanobiological changes within volumes of the complex subjected to concentric and eccentric loading regimes respectively.

RESULTS

Tooth movement is facilitated by mechanobiological activity at the interfaces of the fibrous joint and generates elastic discontinuities at these interfaces in response to eccentric loading. Both concentric and eccentric loads mediated cellular responses to strains, and prompted self-regulating mineral forming and resorbing zones that in turn altered the functional space of the joint.

SIGNIFICANCE

A multiscale biomechanics and mechanobiology approach is important for correlating joint function to tissue-level strain-adaptive properties with overall effects on joint form as related to physiologic and pathologic functions. Elucidating the shift in localization of biomolecules specifically at interfaces during development, function, and therapeutic loading of the joint is critical for developing "functional regeneration and adaptation" strategies with an emphasis on restoring physiologic joint function.

Quercetin as a simple but versatile primer in dentin bonding

A quercetin/ethanol solution may serve as a simple but versatile primer to obtain desirable bonding stability and prevent secondary caries.

Management of Root Resorption Using Chemical Agents: A Review

Root resorption (RR) is defined as the loss of dental hard tissues because of clastic activity inside or outside of tooth the root. In the permanent dentition, RR is a pathologic event; if untreated, it might result in the premature loss of the affected tooth. Several hypotheses have been suggested as the mechanisms of root resorption such as absence of the remnants of Hertwig's epithelial root sheath (HERS) and the absence of some intrinsic factors in cementum and predentin such as amelogenin or osteoprotegerin (OPG). It seems that a barrier is formed by the less-calcified intermediate cementum or the cementodentin junction that prevents external RR. There are several chemical strategies to manage root resorption. The purpose of this paper was to review several chemical agents to manage RR such as tetracycline, sodium hypochlorite, acids (citric acid, phosphoric acid, ascorbic acid and hydrochloric acid), acetazolamide, calcitonin, alendronate, fluoride, Ledermix and Emdogain.

Effect of proteoglycans at interfaces as related to location, architecture, and mechanical cues

INTRODUCTION

Covalently bound functional GAGs orchestrate tissue mechanics through time-dependent characteristics.

OBJECTIVE

The role of specific glycosaminoglycans (GAGs) at the ligament-cementum and cementum-dentin interfaces within a human periodontal complex were examined. Matrix swelling and resistance to compression under health and modeled diseased states was investigated.

MATERIALS AND METHODS

The presence of keratin sulfate (KS) and chondroitin sulfate (CS) GAGs at the ligament-cementum and cementum-dentin interfaces in human molars (N=5) was illustrated by using enzymes, atomic force microscopy (AFM), and AFM-based nanoindentation. The change in physical characteristics of modeled diseased states through sequential digestion of keratin sulfate (KS) and chondroitin sulfate (CS) GAGs was investigated. One-way ANOVA tests with P<0.05 were performed to determine significant differences between groups. Additionally, the presence of mineral within the seemingly hygroscopic interfaces was investigated using transmission electron microscopy.

RESULTS

Immunohistochemistry (N=3) indicated presence of biglycan and fibromodulin small leucine rich proteoglycans at the interfaces. Digestion of matrices with enzymes confirmed the presence of KS and CS GAGs at the interfaces by illustrating a change in tissue architecture and mechanics. A significant increase in height (nm), decrease in elastic modulus (GPa), and tissue deformation rate (nm/s) of the PDL-C attachment site (215±63-424±94nm; 1.5±0.7-0.4±0.2GPa; 21±7-48±22nm/s), and cementum-dentin interface (122±69-360±159nm; 2.9±1.3-0.7±0.3GPa; 18±4-30±6nm/s) was observed.

CONCLUSIONS

The sequential removal of GAGs indicated loss in intricate structural hierarchy of hygroscopic interfaces. From a mechanics perspective, GAGs provide tissue recovery/resilience. The results of this study provide insights into the role of GAGs toward conserved tooth movement in the socket in response to mechanical loads, and modulation of potentially deleterious strain at tissue interfaces.

Multiscale biomechanical responses of adapted bone-periodontal ligament-tooth fibrous joints

Reduced functional loads cause adaptations in organs. In this study, temporal adaptations of bone-ligament-tooth fibrous joints to reduced functional loads were mapped using a holistic approach. Systematic studies were performed to evaluate organ-level and tissue-level adaptations in specimens harvested periodically from rats (N=60) given powder food for 6 months over 8,12,16,20, and 24 weeks. Bone-periodontal ligament (PDL)-tooth fibrous joint adaptation was evaluated by comparing changes in joint stiffness with changes in functional space between the tooth and alveolar bony socket. Adaptations in tissues included mapping changes in the PDL and bone architecture as observed from collagen birefringence, bone hardness and volume fraction in rats fed soft foods (soft diet, SD) compared to those fed hard pellets as a routine diet (hard diet, HD). In situ biomechanical testing on harvested fibrous joints revealed increased stiffness in SD groups (SD:239-605 N/mm) (p<0.05) at 8 and 12 weeks. Increased joint stiffness in early development phase was due to decreased functional space (at 8 weeks change in functional space was -33 μm, at 12 weeks change in functional space was -30 μm) and shifts in tissue quality as highlighted by birefringence, architecture and hardness. These physical changes were not observed in joints that were well into function, that is, in rodents older than 12 weeks of age. Significant adaptations in older groups were highlighted by shifts in bone growth (bone volume fraction 24 weeks: Δ-0.06) and bone hardness (8 weeks: Δ-0.04 GPa, 16 weeks: Δ-0.07 GPa, 24 weeks: Δ-0.06 GPa). The response rate (N/s) of joints to mechanical loads decreased in SD groups. Results from the study showed that joint adaptation depended on age. The initial form-related adaptation (observed change in functional space) can challenge strain-adaptive nature of tissues to meet functional demands with increasing age into adulthood. The coupled effect between functional space in the bone-PDL-tooth complex and strain-adaptive nature of tissues is necessary to accommodate functional demands, and is temporally sensitive despite joint malfunction. From an applied science perspective, we propose that adaptations are registered as functional history in tissues and joints.

Calcium orthophosphates (CaPO4): occurrence and properties

The present overview is intended to point the readers' attention to the important subject of calcium orthophosphates (CaPO4). This type of materials is of the special significance for the human beings because they represent the inorganic part of major normal (bones, teeth and antlers) and pathological (i.e., those appearing due to various diseases) calcified tissues of mammals. For example, atherosclerosis results in blood vessel blockage caused by a solid composite of cholesterol with CaPO4, while dental caries and osteoporosis mean a partial decalcification of teeth and bones, respectively, that results in replacement of a less soluble and harder biological apatite by more soluble and softer calcium hydrogenorthophosphates. Therefore, the processes of both normal and pathological calcifications are just an in vivo crystallization of CaPO4. Similarly, dental caries and osteoporosis might be considered as in vivo dissolution of CaPO4. In addition, natural CaPO4 are the major source of phosphorus, which is used to produce agricultural fertilizers, detergents and various phosphorus-containing chemicals. Thus, there is a great significance of CaPO4 for the humankind and, in this paper, an overview on the current knowledge on this subject is provided.

The role of proteoglycans in the nanoindentation creep behavior of human dentin

Attempts to understand the mechanical behavior of dentin and other mineralized tissues have been primarily focused on the role of their more abundant matrix components, such as collagen and hydroxyapatite. The structural mechanisms endowing these biological materials with outstanding load bearing properties, however, remain elusive to date. Furthermore, while their response to deformation has been extensively studied, mechanisms contributing to their recovery from induced deformation remain poorly described in the literature. Here, we offer novel insights into the participation of proteoglycans (PG) and glycosaminoglycans (GAG) in regulating the nanoindentation creep deformation and recovery of mineralized and demineralized dentin. Accordingly, after the enzymatic digestion of either PGs and associated GAGs or only GAGs, the nanoindentation creep deformation of dentin increased significantly, while the relative recovery of both the mineralized and demineralized dentin dropped by 40-70%. In summary, our results suggest that PGs and GAGs may participate in a nanoscale mechanism that contributes significantly to the outstanding durability of dentin and possibly other mineralized tissues of similar composition.

The narwhal (Monodon monoceros) cementum-dentin junction: a functionally graded biointerphase

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.

Adaptive properties of human cementum and cementum dentin junction with age

OBJECTIVES

The objective of this study was to evaluate age related changes in physical (structure/mechanical properties) and chemical (elemental/inorganic mineral content) properties of cementum layers interfacing dentin.

METHODS

Human mandibular molars (N=43) were collected and sorted by age (younger=19-39, middle=40-60, older=61-81 years). The structures of primary and secondary cementum (PC, SC) types were evaluated using light and atomic force microscopy (AFM) techniques. Chemical composition of cementum layers were characterized through gravimetric analysis by estimating ash weight and concentrations of Ca, Mn, and Zn trace elements in the analytes through inductively coupled plasma mass spectroscopy. The hardness of PC and SC was determined using microindentation and site-specific reduced elastic modulus properties were determined using nanoindentation techniques.

RESULTS

PC contained fibrous 1-3 µm wide hygroscopic radial PDL-inserts. SC illustrated PC-like structure adjacent to a multilayered architecture composing of regions that contained mineral dominant lamellae. The width of the cementum dentin junction (CDJ) decreased as measured from the cementum enamel junction (CEJ) to the tooth apex (49-21 µm), and significantly decreased with age (44-23 µm; p<0.05). The inorganic ratio defined as the ratio of post-burn to pre-burn weight increased with age within primary cementum (PC) and secondary cementum (SC). Cementum showed an increase in hardness with age (PC (0.40-0.46 GPa), SC (0.37-0.43 GPa)), while dentin showed a decreasing trend (coronal dentin (0.70-0.72 GPa); apical dentin (0.63-0.73 GPa)).

SIGNIFICANCE

The observed physicochemical changes are indicative of increased mineralization of cementum and CDJ over time. Changes in tissue properties of teeth can alter overall tooth biomechanics and in turn the entire bone-tooth complex including the periodontal ligament. This study provides baseline information about the changes in physicochemical properties of cementum with age, which can be identified as adaptive in nature.

Elastic discontinuity due to ectopic calcification in a human fibrous joint

Disease can alter natural ramp-like elastic gradients to steeper step-like profiles at soft-hard tissue interfaces. Prolonged function can further mediate mechanochemical events that alter biomechanical response within diseased organs. In this study, a human bone-tooth fibrous joint was chosen as a model system, in which the effects of bacterial-induced disease, i.e. periodontitis, on natural elastic gradients were investigated. Specifically, the effects of ectopic biomineral, i.e. calculus, on innate chemical and elastic gradients within the cementum-dentin complex, both of which are fundamental parameters to load-bearing tissues, are investigated through comparisons with a healthy complex. Complementary techniques for mapping changes in physicochemical properties as a result of disease included micro X-ray computed tomography, microprobe micro X-ray fluorescence imaging, transmission electron and atomic force microscopy (AFM) techniques, and AFM-based nanoindentation. Results demonstrated primary effects as derivatives of ectopic mineralization within the diseased fibrous joint. Ectopic mineralization with no cementum resorption, but altered cementum physicochemical properties with increasing X-ray attenuation, exhibited stratified concretion with increasing X-ray fluorescence counts of calcium and phosphorus elements in the extracellular matrix in correlation with decreased hygroscopicity, indenter displacement, and apparent strain-relieving characteristics. Disease progression, identified as concretion through the periodontal ligament (PDL)-cementum enthesis, and sometimes the originally hygroscopic cementum-dentin junction, resulted in a significantly increased indentation elastic modulus (3.16±1.19 GPa) and a shift towards a discontinuous interface compared with healthy conditions (1.54±0.83 GPa) (Student's t-test, P<0.05). The observed primary effects could result in secondary downstream effects, such as compromised mechanobiology at the mechanically active PDL-cementum enthesis that can catalyze progression of disease.

Biomechanics of a bone-periodontal ligament-tooth fibrous joint

This study investigates bone-tooth association under compression to identify strain amplified sites within the bone-periodontal ligament (PDL)-tooth fibrous joint. Our results indicate that the biomechanical response of the joint is due to a combinatorial response of the constitutive properties of organic, inorganic, and fluid components. Second maxillary molars within intact maxillae (N=8) of 5-month-old rats were loaded with a μ-XCT-compatible in situ loading device at various permutations of displacement rates (0.2, 0.5, 1.0, 1.5, 2.0 mm/min) and peak reactionary load responses (5, 10, 15, 20 N). Results indicated a nonlinear biomechanical response of the joint, in which the observed reactionary load rates were directly proportional to displacement rates (velocities). No significant differences in peak reactionary load rates at a displacement rate of 0.2mm/min were observed. However, for displacement rates greater than 0.2mm/min, an increasing trend in reactionary rate was observed for every peak reactionary load with significant increases at 2.0mm/min. Regardless of displacement rates, two distinct behaviors were identified with stiffness (S) and reactionary load rate (LR) values at a peak load of 5 N (S(5 N)=290-523 N/mm) being significantly lower than those at 10 N (LR(5 N)=1-10 N/s) and higher (S(10 N-20 N)=380-684 N/mm; LR(10 N-20 N)=1-19 N/s). Digital image correlation revealed the possibility of a screw-like motion of the tooth into the PDL-space, i.e., predominant vertical displacement of 35 μm at 5 N, followed by a slight increase to 40 μm at 10 N and 50 μm at 20 N of the tooth and potential tooth rotation at loads above 10 N. Narrowed and widened PDL spaces as a result of tooth displacement indicated areas of increased apparent strains within the complex. We propose that such highly strained regions are "hot spots" that can potentiate local tissue adaptation under physiological loading and adverse tissue adaptation under pathological loading conditions.

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.

Influence of hydration on nanoindentation induced energy expenditure of dentin

Improved understanding of the effects of hydration and drying in mineralized tissues is highly desirable, particularly for physiologically hydrated biological materials such as dentin. We investigated the influence of hydration on the nanomechanical properties of healthy dentin and hypothesized that drying leads to an increase in indentation induced energy expenditure and hardness. Hydrated and dry dentin were tested with a UMIS set up with a Berkovich indenter at a maximum load of 50 mN. Values representative of the energy expenditure behavior were presented as dissipated energy, U(d), recovered energy, U(e), normalized energy expenditure index, ψ, and hardness, H. Energy expenditure index results, which normalize the energy expenditure for each test and describe the relative energy dissipation-recovery behavior of a material, suggested that, for the relatively severe contact strains about a sharp Berkovich indenter, dissipation dominates the mechanical response of both the hydrated and dry dentin. In support of our initial hypothesis, dry dentin presented a significantly higher energy expenditure index than hydrated dentin (p<0.0001). These results were primarily associated with a lower U(e) that was found upon drying. Hydration also decreased H significantly (p<0.0001). In summary, this study presents the first direct measurements of the energy expenditure behavior of hydrated and dry dentin using instrumented nanoindentation.

Insights into the structure and composition of the peritubular dentin organic matrix and the lamina limitans

Dentin is a mineralized dental tissue underlying the outer enamel that has a peculiar micro morphology. It is composed of micrometer sized tubules that are surrounded by a highly mineralized structure, called peritubular dentin (PTD), and embedded in a collagen-rich matrix, named intertubular dentin. The PTD has been thought to be composed of a highly mineralized collagen-free organic matrix with unknown composition. Here we tested the hypothesis that proteoglycans and glycosaminoglycans, two important organic structural features found in dentin, are key participants in the microstructure and composition of the PTD. To test this hypothesis dentin blocks were demineralized with 10 vol% citric acid for 2 min and either digested with 1mg/ml TPCK-treated trypsin with 0.2 ammonium bicarbonate at pH 7.9 (TRY) or 0.1 U/mL C-ABC with 50mM Tris, 60mM sodium acetate and 0.02% bovine serum albumin at pH 8.0 (C-ABC). TRY is known to cleave the protein core of dentin proteoglycans, whereas C-ABC is expected to selectively remove glycosaminoglycans. All specimens were digested for 48 h in 37°C, dehydrated in ascending grades of acetone, immersed in HMDS, platinum coated and imaged using an FE-SEM. Images of demineralized dentin revealed a meshwork of noncollagenous fibrils protruding towards the tubule lumen following removal of the peritubular mineral and confirmed the lack of collagen in the peritubular matrix. Further, images revealed that the peritubular organic network originates from a sheet-like membrane covering the entire visible length of tubule, called lamina limitans. Confirming our initial hypothesis, after the digestion with C-ABC the organic network appeared to vanish, while the lamina limitans was preserved. This suggests that glycosaminoglycans are the main component of the PTD organic network. Following digestion with TRY, both the organic network and the lamina limitans disappeared, thus suggesting that the lamina limitans may be primarily composed of proteoglycan protein cores. In summary, our results provide novel evidence that (1) PTD lacks collagen fibrils, (2) PTD contains an organic scaffold embedded with mineral and (3) the PTD organic matrix is manly composed of glycosaminoglycans, whereas the lamina limitans is primarily made of proteoglycans protein cores.

Discontinuities in the human bone-PDL-cementum complex

A naturally graded interface due to functional demands can deviate toward a discontinuous interface, eventually decreasing the functional efficiency of a dynamic joint. It is this characteristic feature in a human bone-tooth fibrous joint bone-PDL-tooth complex that will be discussed through histochemistry, and site-specific high resolution microscopy, micro tomography(Micro XCT™), X-ray fluorescence imaging and wet nanoindentation techniques. Results demonstrated two causes for the occurrence of 5-50 μm narrowed PDL-space: 1) microscopic scalloped regions at the PDL-insertion sites and macro-scale stratified layers of bone with rich basophilic lines, and 2) macroscopic bony protrusions. Narrowed PDL-complexes illustrated patchy appearance of asporin, and when imaged under wet conditions using an atomic force microscope (AFM), demonstrated structural reorganization of the PDL, collagen periodicity, organic-dominant areas at the PDL-cementum and PDL-bone entheses and within cementum and bone. Scanning electron microscopy (SEM) results confirmed AFM results. Despite the narrowed PDL, continuity between PDL and vasculature in endosteal spaces of bone was demonstrated using a Micro XCT™. The higher levels of Ca and P X-ray fluorescence using a microprobe were correlated with higher elastic modulus values of 0.1-1.4 and 0.1-1.2 GPa for PDL-bone and PDL-cementum using wet nanoindentation. The ranges in elastic modulus values for PDL-bone and PDL-cementum entheses in 150-380 μm wide PDL-complex were 0.1-1.0 and 0.1-0.6 GPa. Based on these results we propose that strain amplification at the entheses could be minimized with a gradual change in modulus profile, a characteristic of 150-380 μm wide functional PDL-space. However, a discontinuity in modulus profile, a characteristic of 5-50 μm wide narrowed PDL-space would cause compromised mechanotransduction. The constrictions or narrowed sites within the bone-tooth fibrous joint will become the new "load bearing sites" that eventually could cause direct local fusion of bone with cementum.

Monomer elution from nanohybrid and ormocer-based composites cured with different light sources

OBJECTIVES

To study monomer elution from four resin-based composites (RBCs) cured with different light sources.

METHODS

Twenty-eight premolars were randomly allocated to four groups. Standardized cavities were prepared and restored with a nanohybrid (Filtek Supreme XT or Tetric EvoCeram), an ormocer (Admira) or a microhybrid RBC (Filtek Z250) which served as control. Buccal restorations were cured with a halogen and oral restorations with an LED light-curing unit. Elution of diurethane dimethacrylate (UDMA), Bisphenol A diglycidylether methacrylate (BisGMA), triethylene glycol dimethacrylate (TEGDMA) and 2-hydroxyethyl methacrylate (HEMA) was analyzed using high-performance liquid chromatography (HPLC) 1h to 28 days post-immersion in 75% ethanol. Data were analyzed using multivariate and repeated measures analysis of variance (α = 0.05).

RESULTS

The greatest elution of UDMA and BisGMA occurred from Tetric EvoCeram and the least from Filtek Z250 (p < 0.05). LED and halogen light-curing units gave similar results for all RBCs (p > 0.05) except Tetric EvoCeram which showed greater elution for the LED unit (p < 0.05). TEGDMA was below the limit of quantification. HEMA eluted in similar concentrations from Filtek Supreme and Tetric EvoCeram (p > 0.05).

SIGNIFICANCE

The two nanohybrid RBCs eluted more cross-linking monomers than the ormocer and the control microhybrid RBC. Continuous elution over 28 days indicates that RBCs act as a chronic source of monomers in clinical conditions. Light source may affect monomer elution since differences were found for one out of four RBCs. Mathematical models for elution kinetics of UDMA and BisGMA indicated two elution mechanisms.

A crystallinity study of dental tissues and tartar by infrared spectroscopy

In this paper we report a study of an important property of biomineralized phases, crystallinity, on the basis of previous results for synthetic apatite. Crystallinity is not only important for understanding biomineralization, it is also related to the maturation and mechanisms of growth of calcium phosphates in biological surroundings. We studied two kinds of sample, teeth as an example of biomineralized tissues and dental calculi (adhering) as an example of mineralization without participation of biological agents, except possibly bacteria. The investigation focused on study of ν(1)-ν(3) infrared absorption bands of PO(4)(3-) phosphates. We used ATR (attenuated total reflection) analysis to examine human dental tissues and tartar on several samples. The results confirm for the first time previous assumptions about the growth and maturation of dental calculi, i.e., crystallinity progresses from regions of high crystallinity to regions of lower crystallinity, and, in addition, its quantification with spatial resolution in the sample. A gradual pattern was observed in dental calculus. Another result from this study was that cementum and dentine had similar crystallinity, despite their different biological and mechanical functions.

Papain-gel degrades intact nonmineralized type I collagen fibrils

Papain-gel has been utilized as a chemo-mechanical material for caries removal due to its ability to preserve underlying sound dentin. However, little is known about the effect of the papain enzyme on intact type I collagen fibrils that compose the dentin matrix. Here we sought to define structural changes that occur in intact type I collagen fibrils after an enzymatic treatment with a papain-gel. Intact and nonmineralized type I collagen fibrils from rat tail were obtained and treated with a papain-gel (Papacarie) for 30 s, rinsed with water and imaged using an atomic force microscope (AFM). Additionally, polished healthy dentin specimens were also treated using the same protocol described above and had their elastic modulus (E) and hardness (H) measured by means of AFM-based nanoindentation. AFM images showed that the papain-gel induced partial degradation of the fibrils surface, yet no rupture of fibrils was noticed. The distinction between gap and overlap zones of fibrils vanished in most regions after treatment, and overlap zones appeared to be generally more affected. Mechanical data suggested a gradual decrease in E and H after treatments. A significant two-fold drop from the values of normal dentin (E=20+/-1.9, H=0.8+/-0.08 GPa) was found after four applications (E=9.7+/-3.2, H=0.24+/-0.1 GPa) (P<0.001), which may be attributed to the degradation of proteoglycans of the matrix. In summary, this study provided novel evidence that intact nonmineralized type I collagen fibrils are partially degraded by a papain-gel.

Structure and mechanical properties of Ank/Ank mutant mouse dental tissues--an animal model for studying periodontal regeneration

Enamel, dentine and cementum are dental tissues with distinct functional properties associated with their unique hierarchical structures. Some potential ways to repair or regenerate lost tooth structures have been revealed in our studies focused on examining teeth obtained from mice with mutations at the mouse progressive ankylosis (ank) locus. Previous studies have shown that mice with such mutations have decreased levels of extracellular inorganic pyrophosphate (PP(i)) at local sites resulting in ectopic calcification in joint areas and in formation of a significantly thicker cementum layer when compared with age-matched wild-type (WT) tissue [Ho AM, Johnson MD, Kingsley DM. Role of the mouse ank gene in control of tissue calcification and arthritis. Science 2000;289:265-70; Nociti Jr FH, Berry JE, Foster BL, Gurley KA, Kingsley DM, Takata T, et al. Cementum: a phosphate-sensitive tissue. J Dent Res 2002;81:817-21]. As a next step, to determine the quality of the cementum tissue formed in mice with a mutation in the ank gene (ank/ank), we compared the microstructure and mechanical properties of cementum and other dental tissues in mature ank/ank vs. age-matched WT mice. Backscattered scanning electron microscopy (SEM) imaging and transmission electron microscopy (TEM) analyses on mineralized tissues revealed no decrease in the extent of mineralization between ank/ank cementum vs. WT controls. Atomic-force-microscopy-based nanoindentation performed on enamel, dentine or cementum of ank/ank vs. age-matched WT molars revealed no significant difference in any of the tested tissues in terms of hardness and elastic modulus. These results indicate that the tissue quality was not compromised in ank/ank mice despite faster rate of formation and more abundant cementum when compared with age-matched WT mice. In conclusion, these data suggest that this animal model can be utilized for studies focused on defining mechanisms to promote cementum formation without loss of mechanical integrity.

Structure, chemical composition and mechanical properties of human and rat cementum and its interface with root dentin

This work seeks to establish comparisons of the physical properties of rat and human cementum, root dentin and their interface, including the cementum-dentin junction (CDJ), as a basis for future studies of the entire periodontal complex using rats as animal models. In this study the structure, site-specific chemical composition and mechanical properties of cementum and its interface with root dentin taken from 9- to 12-month-old rats were compared to the physiologically equivalent 40- to 55-year-old human age group using qualitative and quantitative characterization techniques, including histology, atomic force microscopy (AFM), micro-X-ray computed tomography, Raman microspectroscopy and AFM-based nanoindentation. Based on results from this study, cementum taken from the apical third of the respective species can be represented as a woven fabric with radially and circumferentially oriented collagen fibers. In both species the attachment of cementum to root dentin is defined by a stiffness-graded interface (CDJ/cementum-dentin interface). However, it was concluded that cementum and the cementum-dentin interface from a 9- to 12-month-old rat could be more mineralized, resulting in noticeably decreased collagen fiber hydration and significantly higher modulus values under wet conditions for cementum and CDJ (E(rat-cementum)=12.7+/-2.6 GPa; E(rat-CDJ)=11.6+/-3.2 GPa) compared to a 40- to 55-year-old human (E(human-cementum)=3.73+/-1.8 GPa; E(human-CDJ)=1.5+/-0.7 GPa). The resulting data illustrated that the extensions of observations made from animal models to humans should be justified with substantial and equivalent comparison of data across age ranges (life spans) of mammalian species.

Effect of bacterial collagenase on resin-dentin bonds degradation

The objective of this study is to evaluate the effect of a bacterial collagenase on the degradation of resin-dentin bonds. Human dentin surfaces were bonded with: an etch-&-rinse self-priming adhesive (SB), a two-step self-etching primer/adhesive (SEB), and a 1-step self-etching adhesive (OUB). Composite build-ups were constructed. The bonded teeth were stored (24 h, 3 months, 1 year) in distilled water or in a buffered bacterial collagenase solution. Half of the specimens were stored as intact bonded teeth (Indirect Exposure/IE). The other half were sectioned into beams prior to storage (Direct Exposure/DE). After storage the intact teeth were sectioned into beams and all specimens were tested for microtensile bond strengths (MTBS). ANOVA and multiple comparisons tests were performed. Fractographic analysis was performed by scanning electron microscopy. The inclusion of bacterial collagenase in the storing solution did not lower the MTBS values over those seen in specimens stored in water. SB and SEB bonds strength were equal, and were superior to OUB. After 3 months of DE, SB and OUB bonded specimens showed decreases in MTBS; similar reductions required 1 year for SEB/DE. MTBS did not decrease in IE specimens except for OUB. Resin and collagen dissolution were evident in DE groups after storing.

The tooth attachment mechanism defined by structure, chemical composition and mechanical properties of collagen fibers in the periodontium

In this study, a comparison between structure, chemical composition and mechanical properties of collagen fibers at three regions within a human periodontium, has enabled us to define a novel tooth attachment mechanism. The three regions include, (1) the enthesis region: insertion site of periodontal ligament (PDL) fibers (collagen fibers) into cementum at the root surface, (2) bulk cementum, and (3) the cementum-dentin junction (CDJ). Structurally, continuity in collagen fibers was observed from the enthesis, through bulk cementum and CDJ. At the CDJ the collagen fibers split into individual collagen fibrils and intermingled with the extracellular matrix of mantle dentin. Under wet conditions, the collagen fibers at the three regions exhibited significant swelling suggesting a composition rich in polyanionic molecules such as glycosaminoglycans. Additionally, site-specific indentation illustrated a comparable elastic modulus between collagen fibers at the enthesis (1-3 GPa) and the CDJ (2-4 GPa). However, the elastic modulus of collagen fibers within bulk cementum was higher (4-7 GPa) suggesting presence of extrafibrillar mineral. It is known that the tooth forms a fibrous joint with the alveolar bone, which is termed a gomphosis. Although narrower in width than the PDL space, the hygroscopic CDJ can also be termed as a gomphosis; a fibrous joint between cementum and root dentin capable of accommodating functional loads similar to that between cementum and alveolar bone. From an engineering perspective, it is proposed that a tooth contains two fibrous joints that accommodate the masticatory cyclic loads. These joints are defined by the attachment of dissimilar materials via graded stiffness interfaces, such as: (1) alveolar bone attached to cementum with the PDL; and (2) cementum to root dentin with the CDJ. Thus, through variations in concentrations of basic constituents, distinct regions with characteristic structures and graded properties allow for attachment and the load bearing characteristics of a tooth.

Observing growth steps of collagen self-assembly by time-lapse high-resolution atomic force microscopy

Insights into molecular mechanisms of collagen assembly are important for understanding countless biological processes and at the same time a prerequisite for many biotechnological and medical applications. In this work, the self-assembly of collagen type I molecules into fibrils could be directly observed using time-lapse atomic force microscopy (AFM). The smallest isolated fibrillar structures initiating fibril growth showed a thickness of approximately 1.5 nm corresponding to that of a single collagen molecule. Fibrils assembled in vitro established an axial D-periodicity of approximately 67 nm such as typically observed for in vivo assembled collagen fibrils from tendon. At given collagen concentrations of the buffer solution the fibrils showed constant lateral and longitudinal growth rates. Single fibrils continuously grew and fused with each other until the supporting surface was completely covered by a nanoscopically well-defined collagen matrix. Their thickness of approximately 3 nm suggests that the fibrils were build from laterally assembled collagen microfibrils. Laterally the fibrils grew in steps of approximately 4 nm, indicating microfibril formation and incorporation. Thus, we suggest collagen fibrils assembling in a two-step process. In a first step, collagen molecules assemble with each other. In the second step, these molecules then rearrange into microfibrils which form the building blocks of collagen fibrils. High-resolution AFM topographs revealed substructural details of the D-band architecture of the fibrils forming the collagen matrix. These substructures correlated well with those revealed from positively stained collagen fibers imaged by transmission electron microscopy.