The effect of cross-linker on endogenous enzymatic activity within radicular dentin and bond strength between intraradicular post and radicular dentin

The primary aim was to evaluate the effect of 1-ethyl-3-(3-dimethylamino-propyl) carbodiimide (EDC) on endogenous enzymatic activity within radicular dentin and push-out bond strength of adhesively luted fiber posts, at baseline and after artificial aging. Additionally, the effect of different cementation strategies on endogenous enzymatic activity and fiber post retention was evaluated. The experiment was carried out on extracted human teeth, following endodontic treatment and fiber post cementation. Three cementation strategies were performed: resin cement in combination with etch-and-rinse (EAR) adhesive system, with self-etch (SE) system and self-adhesive (SE) cement. Each of the mentioned strategies had a control and experimental (EDC) group in which root canal was irrigated with 0.3M EDC for 1 minute. The push-out bond strength test was performed 24h after cementation and after 40.000 thermocycles. In order to investigate the effect of EDC and different cementation strategies, in situ zymography analyses of the resin-dentin interfaces were conducted. Statistical analyses were conducted with the software Stata 12.0 (Stata Corp, College Station, Texas, USA) and the significance was set for p<0.05. The results of statistical analysis (ANOVA) showed that the variables “EDC”, “root region” and “artificial aging” significantly influenced fiber posts’ retention to root canal (p<0.05). The highest values were observed in coronal third. The mean values observed after artificial aging were lower when compared to baseline, however EDC was effective in preserving bond strength. The level of enzymatic activity varied between the groups and EDC had a beneficial effect on silencing the enzymatic activity. Within the limitations of the study, it was concluded that the choice of cementation strategy did not influence posts’ retention, while EDC contributed to the preservation of bond strength after artificial aging and reduced enzymatic activity within radicular dentin. In vivo trials are necessary to confirm the results of this in vitro study.

Influence of application mode of universal adhesives on bond strength performances and enzymatic activity of coronal and radicular dentin

Objective: The aims of this thesis were to analyze the application mode of the universal adhesives (UA) and to give instructions for clinical procedures. The etching mode of UA on the bond strength to dentin and on the risk of retention, marginal discoloration, marginal adaptation and post-operative sensitivity (POS) was analyzed by two systematic reviews. Three in vitro studies were conducted: 1) evaporation mode of a UA on coronal dentin; 2) cementation approach on radicular dentin; 3) adhesion of metal brackets to enamel. Materials and methods: Two systematic review were conducted firstly, then in vitro study to investigate the evaporation mode in presence or not of pulpal pressure by means of μTBS, and the enzymatic activity using in situ zymography, at T0 and T6. The cementation of a fiber into radicular dentin with different resin-cements was studied, by push-out bond strength evaluation. Orthodontic brackets were cemented according to 4 adhesive protocols and shear bond strength test was conducted. Two adhesive removal techniques were evaluated, and spectrophotometry was used. Results: The probability of POS occurrence was less in SE. SEE approach seems to perform better than SE. Air-drying resulted in higher μTBS. Suction-evaporation, aging and ER mode increased MMPs activity. Differences in NL expression were present at T0 for fiber post study, and the aging produced an increase in marginal infiltration. Brackets cemented with new universal cement with previous etchant application showed good μTBS values. Conclusion: SEE performed better than SE and TE with UA in terms of uTBS. Evaporating with air-drying is better for UA in terms of uTBS and enzymatic activity. Aging and choice of resin cement for cementation of fiber posts influenced the PBS. Brackets cementation with a new resin- cement seems to offer the highest bond strength and leaves more cement remnants after the bracket removal.

Sistemi adesivi con applicazioni in campo biomedicale

Polymeric adhesives have been used for many applications like suture and embolization, instead of classic surgical methods or as for dental uses. In this work both subjects have been investigated and the results separated in two parts. In the first, new dentinal adhesives with different polymerizable groups (methacrylic or vinyl-ethereal) were synthesized. A low sensitivity to hydrolysis and equal or enhanced properties, compared to existing commercial products, were considered essentials. Moreover, these monomers need to polymerize by radical photopolymerization and functional groups of different characteristics were tested. All these products were characterized by microtensile bond strength test to determine the bonding strength between the adhesive and tooth. Concerning embolization, cyanoacrylates are nowadays the most-used adhesives in surgery. Thus, they must respond to several requirements. For instance, polymerization time and adhesive strength need to be low, to avoid diffusion of the products in the body and adhesion to the catheter. In order to overcome these problems we developed new cyanoacrylates, which practically instantly polymerize upon contact with blood but do not demonstrate strong adhesion to the catheter, thank to the presence of fluorine atoms, linked to the ester chain. The synthesis of these products was carried out in several steps, such as the depolymerization of the corresponding oligomers at high temperature in acid conditions. Two types of adhesion strengths were determined. Bonding strength between human veins and a microcatheter was determined in vitro by using organic materials as the most realistic model. Another test, on two layers of skin, was conducted to verify the possible use of these new cyanoacrylates as a glue for sutures. As a conclusion, we were able to demonstrate that some of the prepared monomers posses adhesive strength and polymerization time lower than the commercial product Glubran2.

Experimental and numerical investigation on short and long term performance of macro-synthetic fibre reinforced concrete materials

Fibre Reinforced Concretes are innovative composite materials whose applications are growing considerably nowadays. Being composite materials, their performance depends on the mechanical properties of both components, fibre and matrix and, above all, on the interface. The variables to account for the mechanical characterization of the material, could be proper of the material itself, i.e. fibre and concrete type, or external factors, i.e. environmental conditions. The first part of the research presented is focused on the experimental and numerical characterization of the interface properties and short term response of fibre reinforced concretes with macro-synthetic fibers. The experimental database produced represents the starting point for numerical models calibration and validation with two principal purposes: the calibration of a local constitutive law and calibration and validation of a model predictive of the whole material response. In the perspective of the design of sustainable admixtures, the optimization of the matrix of cement-based fibre reinforced composites is realized with partial substitution of the cement amount. In the second part of the research, the effect of time dependent phenomena on MSFRCs response is studied. An extended experimental campaign of creep tests is performed analysing the effect of time and temperature variations in different loading conditions. On the results achieved, a numerical model able to account for the viscoelastic nature of both concrete and reinforcement, together with the environmental conditions, is calibrated with the LDPM theory. Different type of regression models are also elaborated correlating the mechanical properties investigated, bond strength and residual flexural behaviour, regarding the short term analysis and creep coefficient on time, for the time dependent behaviour, with the variable investigated. The experimental studies carried out emphasize the several aspects influencing the material mechanical performance allowing also the identification of those properties that the numerical approach should consider in order to be reliable.

Zirconia in dentistry

The above factors emphasize the scope of this thesis for further investigations on zirconia, the improvement of all-ceramic zirconia restorations, and especially the interaction of zirconia and veneering and its influence on the performance of the whole restoration. The introduction, chapter 1, gave a literature overview on zirconia ceramics. In chapter 2, the objective of the study was to evaluate the effect of abrading before and after sintering using alumina-based abrasives on the surface of yttria-tetragonal zirconia polycrystals. Particular attention was paid to the amount of surface stress–assisted phase transformation (tetragonal→monoclinic) and the presence of microcracks. Chapter 3 is based on the idea that the conventional sintering techniques for zirconia based materials, which are commonly used in dental reconstruction, may not provide a uniform heating, with consequent generation of microstructural flaws in the final component. As a consequence of the sintering system, using microwave heating, may represent a viable alternative. The purpose of the study was to compare the dimensional variations and physical and microstructural characteristics of commercial zirconia (Y-TZP), used as a dental restoration material, sintered in conventional and microwave furnaces. Chapter 4 described the effect of sandblasting before and after sintering on the surface roughness of zirconia and the microtensile bond strength of a pressable veneering ceramic to zirconia.

Fiber-reinforced ceramics for thermostructural applications, produced by polymer impregnation pyrolysis

Several CFCC (Continuous Fiber Composite Ceramics) production processes were tested, concluding that PIP (Polymer Impregnation, or Infiltration, Pyrolysis) and CBC (Chemically Bonded Ceramics) based procedures have interesting potential applications in the construction and transportation fields, thanks to low costs to get potentially useful thermomechanical performances. Among the different processes considered during the Doctorate (from the synthesis of new preceramic polymers, to the PIP production of SiC / SiC composites) the more promising results came from the PIP process with poly-siloxanes on basalt fabrics preforms. Low processing time and costs, together with fairly good thermomechanical properties were demonstrated, even after only one or two PIP steps in nitrogen flow. In alternative, pyrolysis in vacuum was also tested, a procedure still not discussed in literature, but which could originate an interesting reduction of production costs, with only a moderate detrimental effect on the mechanical properties. The resulting CFCC is a basalt / SiCO composite that can be applied for continuous operation up to 600°C, also in oxidant environment, as TG and XRD demonstrated. The failure upon loading is generally pseudo-plastic, being interlaminar delamination the most probable rupture mechanism. . The strength depends on several different factors (microstructure, polymer curing and subsequent ceramic phase evolution, fiber pull-out, fiber strength, fiber percentage) and can only be optimized empirically. In order to be open minded in selecting the best technology, also CBC (Chemically Bonded Ceramics) matrixes were considered during this Doctorate, making some preliminary investigations on fire-resistant phosphate cements. Our results on a commercial product evidenced some interesting thermomechanical capabilities, even after thermal treatments. However the experiments showed also phase change and possible cracking and deformations even on slow drying (at 130°C) and easy rehydration upon exposure to environmental humidity.

Studio degli effetti della mordenzatura con acido fluoridrico sul disilicato di litio utilizzato in odontoiatria

Scopo dello studio: valutare i cambiamenti indotti da diversi trattamenti di mordenzatura sulla morfologia superficiale e sulla microstruttura di due vetro-ceramiche a base disilicato di litio (IPS e.max® Press e IPS e.max® CAD) ed esaminarne gli effetti sia sull’adesione con un cemento resinoso che sulla resistenza alla flessione. Materiali e metodi: Settanta dischetti (12 mm di diametro, 2 mm di spessore) di ogni ceramica sono stati preparati e divisi in 5 gruppi: nessun trattamento (G1), HF 5% 20s (G2), HF 5% 60s (G3), HF 9.6% 20s (G4), HF 9.6% 60s (G5). Un campione per ogni gruppo è stato analizzato mediante profilometro ottico e osservato al SEM. Per gli altri campioni è stato determinato lo shear bond strength (SBS) con un cemento resinoso. Dopo l’SBS test, i campioni sono stati caricati fino a frattura utilizzando il piston-on-three-ball test per determinarne la resistenza biassiale alla flessione. Risultati: L’analisi morfologica e microstrutturale dei campioni ha rivelato come diversi trattamenti di mordenzatura producano delle modifiche nella rugosità superficiale che non sono direttamente collegate ad un aumento dei valori di adesione e dei cambiamenti microstrutturali che sono più rilevanti con l’aumento del tempo di mordenzatura e di concentrazione dell’acido. I valori medi di adesione (MPa) per IPS e.max® CAD sono significativamente più alti in G2 e G3 (21,28 +/- 4,9 e 19,55 +/- 5,41 rispettivamente); per IPS e.max® Press, i valori più elevati sono in G3 (16,80 +/- 3,96). La resistenza biassiale alla flessione media (MPa) è più alta in IPS e.max® CAD (695 +/- 161) che in IPS e.max® Press (588 +/- 117), ma non è non influenzata dalla mordenzatura con HF. Conclusioni: il disilicato di litio va mordenzato preferibilmente con HF al 5%. La mordenzatura produce alcuni cambiamenti superficiali e microstrutturali nel materiale, ma tali cambiamenti non ne influenzano la resistenza in flessione.

Development of new collagen cross-linkers for adhesive bonding stabilization

Aims: This thesis aimed to investigate the influence of different collagen cross-linkers, as separate primers or contained within desensitizing agents, on the longevity of dental restorations and on the dentinal enzymatic activity immediately, or after aging in vitro. Methods: A series of studies was conducted using several different cross-linking molecules and several adhesive systems. Four studies investigated the longevity of the hybrid layer by means of microtensile bond strength test, and the enzymatic activity using gelatin and in situ zymography, immediately or after 1 year of aging in the artificial saliva. The first study tested samples bonded with or without a cross-linking agent, that were previously aged for 5 years. The degradation of the hybrid layer was observed using transmission electron microscopy, the enzymatic activity in the hybrid layer using in situ zymography. Raman spectroscopy was used to investigate whether the active substance was still within the hybrid layer after 5 years. Results: The results of the studies showed that collagen cross-linkers were efficient in preserving bond strength after aging in vitro when used as separate primers on demineralized or partially demineralized dentin. In the cases when the cross-linker was utilized on mineralized dentin, bond strength results were higher than in the control groups immediately and after aging, however, no difference in enzymatic activity was detected after aging. Conclusions: The tested cross-linker molecules used as separate primers in etch-and-rinse and self-etch adhesives seem to be clinically applicable, since the procedure is not overly time-consuming and seems to preserve the hybrid layer over time. As for the cross-linkers contained in the desensitizing agent, when utilized before the adhesive procedures, it has shown to increase the bond strength of self-etch adhesives, but further studies are needed to better understand its effect on the enzymatic activity and crosslinking effects on mineralized dentin.

Considerations on indirect adhesive restorations at the material and tooth interfaces

Objective: Lithium-silicate (LiSi) ceramic is nowadays widely used in dentistry. However, for the longevity of LiSi indirect restorations, it is important to pretreat the material and the dental substrate adequately. However, is not certain how the simplification of the manufacturing and conditioning procedures influences the bonding performances of LiSi ceramic restorations. Accordingly, the aims of this thesis were to investigate the effect of: 1) different LiSi ceramic surface decontamination procedures on the shear bond strength (SBS) to resin composite; 2) different types of lithium-disilicate (LiDi) (pressed vs CAD-CAM) on SBS to resin composite; 3) an experimental metal salt-based zirconium oxynitrate etchant [ZrO(NO3)2] on bonding performances to dentin. Materials and Methods: SBS test was used to investigate the influence of different cleaning protocols applied, or different processing techniques (CAD or PRESS) on the bond strength to composite resin. The third study tackled the interface between restorative materials and dentin, and investigated the microtensile bond strength test (µTBS), nanoleakage expression analysis (NL), gelatin zymography and in situ zymography of dentin conditioned with an experimental metal salt-based zirconium oxynitrate etchant [ZrO(NO3)2]. Results: MEP showed comparable bond strength to the double HP etching and higher compared to other groups. BS of press LiSi to composite was higher than that of CAD/CAM LiSi. ZON pretreatment increased bond strength to dentin when used with a universal adhesive, and inhibited dentinal endogenous enzymes. Conclusions: While simplification of the LiSi conditioning and cleaning procedures seems to yield bond strength comparable to the traditional procedures, it could be recommended in the clinical practice. However, pressed LiSi still seems to perform better in terms of bond strength compared to the CAD/CAM LiSi. Further, the novel ZON etchant seems to perform better compared to the traditional phosphoric dentin etching.

Evaluation of bone strength: microtomographic tecniques

This thesis is a part of a larger study about the characterization of mechanical and histomorphometrical properties of bone. The main objects of this study were the bone tissue properties and its resistance to mechanical loads. Moreover, the knowledge about the equipment selected to carry out the analyses, the micro-computed tomography (micro-CT), was improved. Particular attention was given to the reliability over time of the measuring instrument. In order to understand the main characteristics of bone mechanical properties a study of the skeletal, the bones of which it is composed and biological principles that drive their formation and remodelling, was necessary. This study has led to the definition of two macro-classes describing the main components responsible for the resistance to fracture of bone: quantity and quality of bone. The study of bone quantity is the current clinical standard measure for so-called bone densitometry, and research studies have amply demonstrated that the amount of tissue is correlated with its mechanical properties of elasticity and fracture. However, the models presented in the literature, including information on the mere quantity of tissue, have often been limited in describing the mechanical behaviour. Recent investigations have underlined that also the bone-structure and the tissue-mineralization play an important role in the mechanical characterization of bone tissue. For this reason in this thesis the class defined as bone quality was mainly studied, splitting it into two sub-classes of bone structure and tissue quality. A study on bone structure was designed to identify which structural parameters, among the several presented in the literature, could be integrated with the information about quantity, in order to better describe the mechanical properties of bone. In this way, it was also possible to analyse the iteration between structure and function. It has been known for long that bone tissue is capable of remodeling and changing its internal structure according to loads, but the dynamics of these changes are still being analysed. This part of the study was aimed to identify the parameters that could quantify the structural changes of bone tissue during the development of a given disease: osteoarthritis. A study on tissue quality would have to be divided into different classes, which would require a scale of analysis not suitable for the micro-CT. For this reason the study was focused only on the mineralization of the tissue, highlighting the difference between bone density and tissue density, working in a context where there is still an ongoing scientific debate.