Nano finite element modeling of the mechanical behavior of biocomposites using multi-scale (virtual internal bond) material models.

It is evident that biocomposites, specifically mineralized Type-I collagen fibrils, have strong mechanical properties, such as a desirable combination of elastic modulus, fracture toughness, and fracture strength. The mineral Hydroxyapatite [Hap] by itself is stiffer, and it is not clear whether a collagen fiber by itself has a lower breaking strength than the mineralized fiber. Hence, the objective of this paper is to develop, outline, apply, and demonstrate issues involving a new nano explicit finite element based framework, by which the mechanical behavior of mineralized collagen fibrils and their constituents can be studied. A multi-scale virtual internal bond model is used to model the material behavior and failure of such biocomposites. In this research two models have been studied. The first model attempts to illustrate the hypothesis that materials are less sensitive to flaws at nanoscale and the second model studies the mechanical behavior of a nano sized dahlite mineral crystal commonly found in collagen fibril. Two important implementation characteristics have been introduced and illustrated, namely that scaled properties can be used at the micro and nano length scales along with scaled dimensions and secondly the loading time can be appropriately scaled without the loading becoming a dynamic loading.

Finite element analysis of a transmandibular implant.

Finite element analysis (FEA) was used to study the influence of bony height and baseplate effect on the transmandibular implant (TMI). FEA was used to assess the stress distribution in the mandible, with and without a baseplate. Forces were applied to the center of a bar space. The von Mises stress (SEQV) was displayed in bone, and shear stress also was measured. Tensile stress was always seen at the so-called saddle area; compressive stress was seen at the inferior border. Tensile strain at the saddle area of a 6-mm height mandible is always 9.1 times greater (corresponding to more SEQV) than that of an 18-mm mandible. At same bone heights, saddle area stresses are similar with or without a baseplate. However, stress at the lateral transossseous post interface with bone was significantly different between different bone height groups. Shear stress around the lateral transossseous post was smaller than does the SEQV of the same site. Decreased bone height dramatically increases stress at the saddle area independent of the baseplate presence. These data suggest that bone apposition is found in the severely atrophic mandibles. Thus, there is no need for a baseplate in a slight to mild atrophic mandible.

Multiscale mechanics of hierarchical structure/property relationships in calcified tissues and tissue/material interfaces.

This paper presents a review plus new data that describes the role hierarchical nanostructural properties play in developing an understanding of the effect of scale on the material properties (chemical, elastic and electrical) of calcified tissues as well as the interfaces that form between such tissues and biomaterials. Both nanostructural and microstructural properties will be considered starting with the size and shape of the apatitic mineralites in both young and mature bovine bone. Microstructural properties for human dentin and cortical and trabecular bone will be considered. These separate sets of data will be combined mathematically to advance the effects of scale on the modeling of these tissues and the tissue/biomaterial interfaces as hierarchical material/structural composites. Interfacial structure and properties to be considered in greatest detail will be that of the dentin/adhesive (d/a) interface, which presents a clear example of examining all three material properties, (chemical, elastic and electrical). In this case, finite element modeling (FEA) was based on the actual measured values of the structure and elastic properties of the materials comprising the d/a interface; this combination provides insight into factors and mechanisms that contribute to premature failure of dental composite fillings. At present, there are more elastic property data obtained by microstructural measurements, especially high frequency ultrasonic wave propagation (UWP) and scanning acoustic microscopy (SAM) techniques. However, atomic force microscopy (AFM) and nanoindentation (NI) of cortical and trabecular bone and the dentin-enamel junction (DEJ) among others have become available allowing correlation of the nanostructural level measurements with those made on the microstructural level.

A clinical case report: interface analysis of a successful well-functioning transmandibular implant from a cadaver mandible.

Transmandibular implants (TMI) are indicated both for functional reconstruction of the severely atrophic mandible and when routine augmentation is unpredictable. This study investigates the interface of bone around a TMI, retrieved from the cadaver. The TMI had successfully functioned for 7 years. The mandible was immersed in 10% formaldehyde and sectioned into nine appropriate pieces. Samples were embedded in polymethylmethacrylate, and cut around the transosseous posts and cortical screws in both vertical and horizontal sections. Samples were analyzed at 400 MHz (nominal lateral resolution, 2.5 microm) using a UH3 Scanning Acoustic Microscope (Olympus, Tokyo, Japan). The middle of implant specimens 1-4 were cut to 50 microm, and stained by toluidine blue for light microscopy. Dental X-rays showed no bone resorption around any implant. On a 2-mm lateral scan, almost uniform interface space was seen between bone and implant surface in cortical screws. There are wider spaces around the transmandibular posts in the superior area. Histology revealed the small area of direct contact. There is bone marrow space in the interface, with no significant fibrous tissue. We interpret these results at the interface to be because of adaptation for stress distribution.

Histomorphometric Comparison of Transmandibular Implant and Titanium Implant.

This study was a histomorphometric comparison of the transmandibular implant (TMI) system and titanium implant using a confocal laser scanning microscope (CLSM). Two dogs were used. In each dog, 16 implant holes were made in the bilateral mandibular angle. Four cortical screws in the TMI, four titanium plasma spray (TPS) implants, and four hydroxyapatite (HA) coating implants were inserted and four holes were left as a control. Two dogs were sacrificed at 3 and 6 months. The samples were cut longitudinally and examined under CLSM. The contact rate of trabecular bone was smaller than that of cortical bone in all implants. The contact rates in TMI at 3 and 6 months was significantly smaller than those of TPS and HA and there was no difference between TPS and HA. Although the contact rate of TMI was low, only a small contact area may be enough for TMI because of its box form structure.

Physicochemical interactions at the dentin/adhesive interface using FTIR chemical imaging.

To date, much of our understanding of dentin bonding has been based on investigations performed on sound, healthy dentin. This is not the substrate generally encountered in clinical practice, rather dentists must frequently bond to caries-affected dentin. Because of the extreme complexity and variability of the caries-affected dentin substrate, conventional characterization techniques do not provide adequate information for defining those factors that impact bond formation. Using Fourier-transform infrared imaging, we characterized the inhomogeneities and compositional differences across the length and breadth of the caries-affected dentin/adhesive interface. Differences in mineral/matrix ratio, crystallinity, and collagen organization were noted in the comparison of caries-affected and healthy dentin. As compared to healthy dentin, there were striking differences in depth of demineralization, adhesive infiltration, and degree of conversion at the interface with caries-affected dentin.

Parametric study of the effect of phase anisotropy on the micromechanical behaviour of dentin-adhesive interfaces.

A finite element (FE) model has been developed based upon the recently measured micro-scale morphological, chemical and mechanical properties of dentin-adhesive (d-a) interfaces using confocal Raman microspectroscopy and scanning acoustic microscopy (SAM). The results computed from this FE model indicated that the stress distributions and concentrations are affected by the micro-scale elastic properties of various phases composing the d-a interface. However, these computations were performed assuming isotropic material properties for the d-a interface. The d-a interface components, such as the peritubular and intertubular dentin, the partially demineralized dentin and the so-called "hybrid layer" adhesive-collagen composite, are probably anisotropic. In this paper, the FE model is extended to account for the probable anisotropic properties of these d-a interface phases. A parametric study is performed to study the effect of anisotropy on the micromechanical stress distributions in the hybrid layer and the peritubular dentin phases of the d-a interface. It is found that the anisotropy of the phases affects the region and extent of stress concentration as well as the location of the maximum stress concentrations. Thus, the anisotropy of the phases could effect the probable location of failure initiation, whether in the peritubular region or in the hybrid layer.

Evaluation of the micromechanical elastic properties of potential bone-grafting materials.

The micromechanical elastic properties of potential bone-graft materials were compared with that of the human mandible. Six different potential bone-replacement materials were used: Bio-Oss (Osteohealth), OsteoGraf/N-700 (Ceramed), Pepgen P15 (Ceramed), Interpore200 (Interpore Cross International), Allogro (Ceramed), and Dynagraft (GenSci Dental). As a control, mandibular cortical bone was obtained from a 17-year-old woman. Micromechanical elastic property analysis was obtained with the use of a UH3 scanning acoustic microscope (Olympus Co., Tokyo, Japan) (SAM) at 400 MHz in the burst mode. Each sample was measured at three areas. The data were analyzed statistically by SPSS (SPSS, Inc.) with the use of the Student t test. In human bone, the reflection coefficients r of the x dimension (r = 0.75 +/- 0.01) was statistically higher than those of the y (0.72 +/- 0.05) and the z (0.72 +/- 0.01) directions. The order of stiffness magnitude was found to be Pepgen (r = 0.73 +/- 0.05) >/= OsteoGraf (0.72 +/- 0.03) > Bio-Oss (0.71 +/- 0.02) > Interpore (0.69 +/- 0.10) > Dynagraft (0.43 +/- 0.05) > Allogro (0.36 +/- 0.04). For these samples, Interpore alone showed a large deviation in properties in the same specimen. With regard to the elastic properties solely, bone-grafting materials made from bovine or processed marine coral appear to be reasonable choices as graft materials.

Micromechanical properties of demineralized dentin collagen with and without adhesive infiltration.

In a previous study, we reported the upper limit of Young's modulus of the unprotected protein at the dentin/adhesive interface to be 2 GPa. In this study, to obtain a more exact value of the moduli of the components at the d/a interface, we used demineralized dentin collagen with and without adhesive infiltration. The prepared samples were analyzed using micro-Raman spectroscopy (micro RS) and scanning acoustic microscopy (SAM). Using an Olympus UH3 SAM (Olympus Co., Tokyo), measurements were recorded with a 400 MHz burst mode lens (120 degrees aperture angle; nominal lateral resolution, 2.5 microm). A series of calibration curves were prepared using the relationship between the ultrasonically measured elastic moduli of a set of known materials and their SAM response. Finally, both the bulk and bar wave elastic moduli were computed for a set of 13 materials, including polymers, ceramics, and metals. These provided the rationale for using extensional wave measurements of the elastic moduli as the basis for extrapolation of the 400 MHz SAM data to obtain Young's moduli for the samples: E = 1.76 +/- 0.00 GPa for the collagen alone; E = 1.84 +/- 0.65 GPa for the collagen infiltrated with adhesive; E = 3.4 +/- 1.00 GPa for the adhesive infiltrate.

Micromechanical analysis of dentin/adhesive interface by the finite element method.

The interfacial microstructure and spatial distribution of the modulus of elasticity have a profound effect on load transfer at the dentin/adhesive (d/a) interface. The microstructure is influenced by the varying degree of demineralization of intertubular and peritubular dentin during etching as well as the depth of adhesive penetration into the hybrid layer. These factors lead not only to a unique microstructure in the vicinity of the dentinal tubules, but also to a mechanically graded hybrid layer. This article investigates the micromechanical stress distribution at a d/a interface with the use of finite element analysis (FEA). Such analysis is now feasible given the newly measured moduli of elasticity at micro- and nanoscales. The results indicate that the morphological and micromechanical properties of the d/a interface affects the stress field such that the fracture/failure is likely to initiate in the stress-concentration zone of peritubular dentin next to the hybrid/exposed-collagen layer. The results suggest that devising a full-depth high modulus hybrid layer may considerably reduce the stress concentration zone and the magnitude of stress concentration in the peritubular dentin next to the hybrid/exposed-collagen layer.

Micromechanics/structure relationships in the human mandible.

OBJECTIVES: A clear understanding of the relationship between the micromechanical properties and orientation of the osteons within the mandible is important to understand mandibular function, fracture repair, treatment of temporo-mandibular joint disorders, the materials and organization of dental implants. The objective of this research was to obtain the micromechanical properties of human mandibular cortical bone as a function of orientation from TMJ to TMJ. METHODS: A mandible obtained from a deceased 66 year-old female free of bone disease was used. The mandible was embedded in polymethylmethacrylate. The micromechanical properties analysis was obtained using the UH3 scanning acoustic microscope (SAM; Olympus Co., Tokyo, Japan). The coordinates system is defined such that the inferior border of mandibular is positioned on the x-y plane. x is along the anterior-posterior direction, y is in the horizontal direction and z is in superior-inferior direction. RESULTS: The osteonal orientations were almost parallel to the x axis and eventually branched into two directions towards the coronoid process and condylar head. The SAM revealed that almost the whole area of the mandible body was found to be transversely isotropic in the plane perpendicular to the x axis. In the parallel and oblique directions, all data were transversely isotropic with respect to the x axis. Data of the perpendicular osteons were transversely isotropic with respect to the z axis. SIGNIFICANCE: Having actual micromechanical properties as a function of orientation in the mandible could provide base line data for: fracture repair; choice of bone replacement materials.

Identification of collagen encapsulation at the dentin/adhesive interface.

PURPOSE: The purpose of this study was to investigate the dentin/adhesive interfacial characteristics of three current commercial adhesives with different relative hydrophilic/hydrophobic composition, using a nondestructive staining technique. MATERIALS AND METHODS: Dentin surfaces of 18 unerupted human third molars were randomly selected for treatment with one of three commercial dentin bonding agents according to manufacturers' instructions for the "wet" bonding technique. The adhesives were ranked based on hydrophilic/hydrophobic component ratios (ie, ability to dissolve in water), highest to lowest, as follows: Uno (Pulpdent) > Prime&Bond NT (PBNT, Dentsply Caulk) > Single Bond (SB, 3M ESPE). Dentin/adhesive (d/a) interface sections were stained with Goldner's trichrome, a classical bone stain, and examined using light microscopy. RESULTS: The extent and degree to which the adhesive encapsulates the demineralized dentin matrix is reflected in the color differences in the stained sections. The depth of demineralization appeared comparable among these bonding systems, but adhesive infiltration varied from highest to lowest as follows: Uno > PBNT > SB. CONCLUSIONS: The differential staining technique provided a clear representation of the depth of dentin demineralization and extent/degree of adhesive encapsulation of the exposed collagen at the d/a interface. This technique provides a mechanism for readily identifying vulnerable sites at the d/a interface. The composition of the one-bottle adhesive systems has a substantial effect on the interfacial structure of the d/a bond.

Scanning acoustic microscopy investigation of frequency-dependent reflectance of acid- etched human dentin using homotopic measurements.

Composite restorations in modern restorative dentistry rely on the bond formed in the adhesive-infiltrated acid-etched dentin. The physical characteristics of etched dentin are, therefore, of paramount interest. However, characterization of the acid-etched zone in its natural state is fraught with problems stemming from a variety of sources including its narrow size, the presence of water, heterogeneity, and spatial scale dependency. We have developed a novel homotopic (same location) measurement methodology utilizing scanning acoustic microscopy (SAM). Homotopic measurements with SAM overcome the problems encountered by other characterization/imaging methods. These measurements provide us with acoustic reflectance at the same location of both the pre- and post-etched dentin in its natural state. We have applied this methodology for in vitro measurements on dentin samples. Fourier spectra from acid-etched dentin showed amplitude reduction and shifts of the central frequency that were location dependent. Through calibration, the acoustic reflectance of acid-etched dentin was found to have complex and non-monotonic frequency dependence. These data suggest that acid-etching of dentin results in a near-surface graded layer of varying thickness and property gradations. The measurement methodology described in this paper can be applied to systematically characterize mechanical properties of heterogeneous soft layers and interfaces in biological materials.

Adhesive/Dentin interface: the weak link in the composite restoration.

Results from clinical studies suggest that more than half of the 166 million dental restorations that were placed in the United States in 2005 were replacements for failed restorations. This emphasis on replacement therapy is expected to grow as dentists use composite as opposed to dental amalgam to restore moderate to large posterior lesions. Composite restorations have higher failure rates, more recurrent caries, and increased frequency of replacement as compared to amalgam. Penetration of bacterial enzymes, oral fluids, and bacteria into the crevices between the tooth and composite undermines the restoration and leads to recurrent decay and premature failure. Under in vivo conditions the bond formed at the adhesive/dentin interface can be the first defense against these noxious, damaging substances. The intent of this article is to review structural aspects of the clinical substrate that impact bond formation at the adhesive/dentin interface; to examine physico-chemical factors that affect the integrity and durability of the adhesive/dentin interfacial bond; and to explore how these factors act synergistically with mechanical forces to undermine the composite restoration. The article will examine the various avenues that have been pursued to address these problems and it will explore how alterations in material chemistry could address the detrimental impact of physico-chemical stresses on the bond formed at the adhesive/dentin interface.

Physico-mechanical properties determination using microscale homotopic measurements: application to sound and caries-affected primary tooth dentin.

Microscale elastic moduli, composition and density have rarely been determined at the same location for biological materials. In this paper, we have performed homotopic measurements to determine the physico-mechanical properties of a second primary molar specimen exhibiting sound and caries-affected regions. A microscale acoustic impedance map of a section through this sample was acquired using scanning acoustic microscopy (SAM). Scanning electron microscopy was then used to obtain mineral mass fraction of the same section using backscattered images. Careful calibration of each method was performed to reduce system effects and obtain accurate data. Resorption, demineralization and hypermineralization mechanisms were considered in order to derive relationships between measured mineral mass fraction and material mass density. As a result, microscale mass density was determined at the same lateral resolution and location as the SAM data. The mass density and the acoustic impedance were combined to find the microscale elastic modulus and study the relationship between microscale composition and mechanical properties.

Enzymatic biodegradation of HEMA/bisGMA adhesives formulated with different water content.

Dentin adhesives may undergo phase separation when bonding to wet demineralized dentin. We hypothesized that adhesives exhibiting phase separation will experience enhanced biodegradation of methacrylate ester groups. The objective of this project was to study the effect of enzyme-exposure on the release of methacrylic acid (MAA) and 2-hydroxyethyl methacrylate (HEMA) from adhesives formulated under conditions simulating wet bonding. HEMA/bisGMA(2,2-bis[4(2-hydroxy-3-methacryloyloxy-propyloxy)-phenyl] propane), 45/55 w/w ratio, was formulated with different water content: 0 Wt % (A00), 8 wt % (A08), and 16 wt % (A16). After a three day prewash, adhesive discs were incubated with/without porcine liver esterase (PLE) in phosphate buffer (PB, pH 7.4) at 37 degrees C for 8 days. Supernatants were collected daily and analyzed for MAA and HEMA by HPLC. For all formulations, daily MAA release in the presence of PLE was increased compared to MAA release in PB. HEMA release in the presence of PLE was not detected while HEMA release was consistently measured in PB. A08 and A16 released significantly larger amounts of HEMA compared to A00. Analysis of the cumulative release of analytes showed that the leachables in PLE was significantly increased (p < 0.05) as compared with that released in PB indicating that MAA release was not only formed from unreacted monomers but from pendant groups in the polymer network. However, the levels of analytes HEMA in PB or MAA in PLE were increased in A08 and A16 as compared with A00, which suggests that there could be a greater loss of material in HEMA/bisGMA adhesives that experience phase separation under wet bonding conditions.

A micromechanical elastic property study of trabecular bone in the human mandible.

Micromechanical properties of human mandibular trabecular bone, with particular interest to any site differences were investigated. A mandible was harvested from a 66 year-old female cadaver free from bone disease. It was embedded in PMMA, cut into 2mm sections and polished. Micromechanical property measurements were obtained using the UH3 Scanning Acoustic Microscope (SAM) (Olympus Co., Tokyo, Japan) at 400MHz in the burst mode. 6 vertical slices from the right and 6 horizontal slices from the left were chosen. In each of the 12 samples, 3 points were measured; first in the center, the other 2 from the margins. Data were analyzed statistically by SPSS (SPSS, Inc.) using Student's t-test. The average value of reflection coefficient r is 0.58+/-0.079 with the range from 0.46 to 0.64; E=25.0+/-5.64 GPa. There is no significant difference in properties in the osteonal direction of related cortical bone and those found between the marginal area and center areas. The average value of r from the right side, 0.60+/-0.07, is statistically higher than the average value of from the left side, 0.56+/-0.07. Micromechanical properties of both mandibular trabecular and cortical bone have almost the same values.

Elastic anisotropy of bone and dentitional tissues.

The calculation of the scalar compressive and shear anisotropy factors developed for single crystal refractory compounds has been adapted to the anisotropic elastic stiffness coefficients determined by a number of ultrasonic measurements of bone based on transverse isotropic symmetry. Later, this work was extended to include the ultrasonic measurements of bone based on orthotropic symmetry. Recently, the five transverse isotropic elastic constants for both wet and dry human dentin were determined using resonant ultrasound spectroscopy. The five transverse isotropic elastic constants for wet bovine enamel and dentin had been calculated based on modeling of ultrasonic wave propagation measurements and related data in the literature. The scalar compressive and shear anisotropy factors have been calculated from both these sets of data and are compared with a representative set from those published previously for both human and bovine bone and both fluoro- and hydroxyl-apatites.