Li-Hong He

Mechanical behaviour of porous hydroxyapatite.

The aim of the study was to investigate the role of microstructure and porosity on the mechanical behaviour of sintered hydroxyapatite. Hydroxyapatite disks with four different porosities were used in this investigation. With a nanoindentation system, elastic modulus, hardness, contact stress-strain relationship, energy absorption and indentation creep behaviour were investigated. The elastic modulus and hardness of hydroxyapatite exhibited an exponential relationship (e(-bP)) with the porosity P, which is similar to Rices finding with the minimum solid area model. High porosity samples showed more substantial inelastic behaviour, including higher energy absorption, no linear elastic region in the contact stress-strain curve and some indentation creep behaviour. We conclude that porous microstructure endows hydroxyapatite with inelastic deformation properties, which are important in a material for bone substitution usage.

A novel polymer infiltrated ceramic dental material

OBJECTIVES To evaluate the mechanical behavior of a prototype porous ceramic interpenetrating polymer-ceramic material containing 15-20% polymer. METHODS After sample preparation, elastic modulus, hardness, stress-strain relationship and indentation creep response were measured by a nanoindentation system. Fracture toughness was measured by the single-edge-notched beam (SENB) method. SEM was employed to observe the fractured surface and analyze the fracture mechanisms. RESULTS The polymer infiltrated ceramic material has elastic modulus, hardness, and fracture toughness values of 30.14GPa, 2.59GPa, and 1.72 MPam(1/2), respectively. The material illustrates a significant indentation size effect for elastic modulus and hardness, and has similar indentation creep behavior to human enamel. Manufacturing procedures such as the density of pre-infiltrated porous ceramic and processing pressure influence the final properties of the material. SIGNIFICANCE This polymer infiltrated ceramic material is anticipated to become a new member of the dental CAD/CAM family.

Enamel--a functionally graded natural coating.

OBJECTIVE The aim of this study is to illustrate the graded proper properties of enamel from the outer (near occlusal surface) to the inner region (near enamel-dentine junction) in a cross-sectioned surface and discuss how natural design achieve the graded functions. METHODS Nanoindentation, Raman spectroscopes, and SEM were employed to compare the inner and outer regions of the cross-sectioned enamel from different angles, namely mechanical properties such as elastic modulus and hardness, indentation energy absorption ability, indentation creep ability, indentation residual stress distribution pattern, compositional differences, and microstructural differences. RESULTS Inner enamel has lower elastic modulus and hardness but higher creep and stress redistribution abilities than outer counterpart, which is related to the gradual compositional change through the enamel. SIGNIFICANCE Enamel can be regarded as a functionally graded natural biocomposite, which will require special attention using numerical analysis to fully appreciate the consequences of such a structure for the mechanical behaviour of teeth and restorations placed therein. Moreover, the smart design of nature can be a good model for us in functional graded materials/coatings design and development.

Nanoindentation creep behavior of human enamel

In this study, the indentation creep behavior of human enamel was investigated with a nanoindentation system and a Berkovich indenter at a force of 250 mN with one-step loading and unloading method. A constant hold period of 900 s was incorporated into each test at the maximum load as well at 5 mN minimum load during unloading. The indentation creep at the maximum load and creep recovery at the minimum load was described with a double exponential function and compared with other classic viscoelastic models (Debye/Maxwell and Kohlrausch-Williams-Watts). Indentation creep rate sensitivity, m, of human enamel was measured for the first time with a value of approximately 0.012. Enamel displayed both viscoelastic and viscoplastic behavior similar to that of bone. These results indicate that, associated with entrapment of particulates between teeth under functional loading and sliding wear conditions, the enamel may inelastically deform but recover upon its release. This behavior may be important in explaining the excellent wear resistance, antifatigue, and crack resistant abilities of natural tooth structure.

Tooth wear and wear investigations in dentistry

Tooth wear has been recognised as a major problem in dentistry. Epidemiological studies have reported an increasing prevalence of tooth wear and general dental practitioners see a greater number of patients seeking treatment with worn dentition. Although the dental literature contains numerous publications related to management and rehabilitation of tooth wear of varying aetiologies, our understanding of the aetiology and pathogenesis of tooth wear is still limited. The wear behaviour of dental biomaterials has also been extensively researched to improve our understanding of the underlying mechanisms and for the development of restorative materials with good wear resistance. The complex nature of tooth wear indicates challenges for conducting in vitro and in vivo wear investigations and a clear correlation between in vitro and in vivo data has not been established. The objective was to critically review the peer reviewed English-language literature pertaining to prevalence and aetiology of tooth wear and wear investigations in dentistry identified through a Medline search engine combined with hand-searching of the relevant literature, covering the period between 1960 and 2011.

A natural functionally graded biocomposite coating – Human enamel

Human enamel has been found to be a coating with excellent mechanical performance, and has undergone extensive investigation and discussion. However, most of the reported studies consider the enamel as a homogeneous anisotropic biocomposite. The current study illustrated the graded properties of the biocomposite from its functional load-bearing direction. Within the thickness of the enamel, from the outer surface towards the enamel-dentin junction (EDJ), the elastic modulus (E(x)) and hardness (H(x)) of enamel exist in an exponential relationship with normalized thickness (x) as E(x)=111.64x(0.18) (R(2)=0.94) and H(x)=4.41x(0.16) (R(2)=0.87) GPa, respectively. Moreover, the creep ability of enamel increases towards the EDJ. The graded properties of the biocomposite can be explained by both microstructural and compositional changes along the thickness of the material towards the EDJ. Finite element analysis indicates that the graded properties of enamel have important roles in reducing the enamel-dentin interface stresses and maintaining the integrity of the multilayer tooth structure. The results provide a new angle to understand the excellent mechanical behaviour of the multilayer tooth structure and may inspire the development of new functionally graded materials and coating structures.

A novel polymer infiltrated ceramic for dental simulation.

Simulation of tooth preparation using rotary cutting instruments is viewed as beneficial and essential in dental training. Various types of materials have been used for simulation systems in dental preclinical training. However, the phantom tooth materials used for simulation have not changed significantly for decades and they are acknowledged to be different from natural teeth. This study investigated the mechanical properties and microstructure of a widely used phantom tooth material and compared them with a novel, polymer infiltrated, ceramic. It was concluded that the polymer infiltrated ceramic has mechanical properties more similar to natural teeth than current phantom tooth materials, suggesting that it might be a good candidate material for phantom teeth for trainees to acquire initial tactile sense for tooth preparation.

Correlation of mineral density and elastic modulus of natural enamel white spot lesions using X-ray microtomography and nanoindentation.

Our objectives were to correlate the mineral density (MD) and elastic modulus (E) of natural white spot lesions (WSLs) and compare them with analytical and numerical models. Five natural WSLs from four extracted sound premolar teeth were scanned at a voxel size of 7.6μm using a desktop X-ray microtomography (XRMT) system. Five hydroxyapatite phantoms with densities ranging from 1.52 to 3.14gcm⁻³ were used as calibration standards for each scan. MD throughout the WSLs was quantified using an MD calibration equation derived from hydroxyapatite phantoms. Subsequently, teeth were cross-sectioned and the E modulus was measured systematically across the WSLs at intervals of 25 and 50μm using nanoindentation. The MD and E modulus of WSLs correlated well. The relationship may be expressed as E=E⁰exp(-bP) (R²=0.952) with E⁰ the elastic modulus of the fully dense material, P the porosity and b a constant. The results for sound enamel were compared with Spears model. The limitation of Spears model to the WSLs is discussed and an alternative model developed by Rice for porous materials is proposed. Clinical implications of this work for quantifying de-/remineralization of teeth are pointed out. We conclude that XRMT can be utilized to extrapolate the E modulus of WSLs. This provides a basis for non-destructive, longitudinal analysis of WSLs in de-/remineralization studies of enamel.

Influence of the indenter tip geometry and environment on the indentation modulus of enamel

The aim of the investigation was to study the influence of indenter tip geometry on the conventionally obtained indentation modulus of enamel by nanoindentation. Indentation tests on bovine enamel using three different diamond pyramidal indenters with half face angles 65.27°, 45°, and 35.26° were conducted to evaluate the indentation modulus using the Oliver–Pharr method [W.C. Oliver and G.M. Pharr, J. Mater. Res. 7, 1564 (1992)]. In addition, three different dehydration conditions were studied: wet under Hanks balanced salt solution, laboratory dried, and vacuum dehydrated. For the Berkovich indenter (65.27°) and 45° pyramidal indenters, there was only a small difference between indentation modulus values, whereas for the cube-corner indenter (35.26°) a ratio of 2.4 between laboratory dry and wet samples was found. A detailed evaluation, including indentation creep and recovery as well as pileup, resulted in a reduction of this latter ratio to 1.7. This still large difference was rationalized on the basis of the different deformation mechanisms generated by indenters of different face angles.

A suitable base material for composite resin restorations: zinc oxide eugenol.

OBJECTIVE This in vitro study evaluated the effects of a zinc oxide eugenol (ZOE) base on the mechanical properties of a composite resin restoration. METHODS Class I cavities were prepared on plastic teeth and filled with ZOE plus composite resin, following standard clinical procedures. The samples were sectioned sagittally and the ZOE-resin interface was exposed. After polishing, nanoindentation was performed on the region near the interface, and elastic modulus and hardness were plotted in the form of a color contour map. SEM was employed to observe the interface between composite resin and ZOE base. RESULTS In the region close to the ZOE base, the elastic modulus and hardness of composite resin reduced to the values of 9.71+/-0.54 and 0.51+/-0.05 GPa, respectively. Eugenol from ZOE had detrimental effects on the composite resin only to a distance of less than 100 microm from the ZOE base. CONCLUSION Although eugenol suppresses polymerization slightly, by considering the biological advantages of ZOE, together with the results of the current investigation, ZOE may still be considered a suitable base material for composite resin. Bonding is essential for composite resin restorations over ZOE bases to avoid shrinkage detachment.