Optimal mechanical design of anatomical post-systems for endodontic restoration

This paper analyses the mechanical behaviour of a new reinforced anatomical post-systems (RAPS) for endodontic restoration. The composite restorative material (CRM) completely fills the root canal (as do the commonly used cast metal posts) and multiple prefabricated composite posts (PCPs) are employed as reinforcements. Numerical simulations based on 3D linearly elastic finite element models under parafunctional loads were performed in order to investigate the influence of the stiffness of the CRM and of the number of PCPs. Periodontal ligament effects were taken into account using a discretised anisotropic nonlinearly elastic spring system, and the full discrete model was validated by comparing the resulting stress fields with those obtained with conventional restorations (cast gold-alloy post, homogeneous anatomical post and cemented single PCP) and with the natural tooth. Analysis of the results shows that stresses at the cervical/middle region decrease as CRM stiffness increases and, for large and irregular root cavities that apical stress peaks disappear when multiple PCPs are used. Accordingly, from a mechanical point of view, an optimal RAPS will use multiple PCPs when CRM stiffness is equal to or at most twice that of the dentin. This restorative solution minimises stress differences with respect to the natural tooth, mechanical inhomogeneities, stress concentrations on healthy tissues, volumes subject to shrinkage phenomena, fatigue effects and risks of both root fracture and adhesive/cohesive interfacial failure.     

Finite element analysis of a new customized composite post system for endodontically treated teeth

This paper investigated the mechanical behavior of a new customized post system built up with a composite framework presently utilized for crowns, bridges, veneers and inlay/onlay dental restorations. The material has been shaped so to follow perfectly the profile of the root canal in order to take advantage of the better mechanical properties of composites with respect to metallic alloys commonly used for cast posts.

The analysis has been carried out with 3D finite element models previously validated on the basis of experimental work. The new post system has been compared to a variety of restorations using either prefabricated or cast posts. The structural efficiency of the new restoration has been evaluated for an upper incisor under different loading conditions (mastication, bruxism, impact).

Results prove that maximum stress values in restored teeth are rather insensitive to post types and materials. However, the new customized composite restoration allows to reduce significantly the stresses inside the dentinal regions where conservative clinical interventions are not possible.     

Effects of Mechanical and Chemical Pretreatments of Zirconia or Fiber Posts on Resin Cement Bonding

The bonding strength between resin cement and posts is important for post and core restorations. An important method of improving the bonding strength is the use of various surface pretreatments of the post. In this study, the surfaces of zirconia (fiber) posts were treated by mechanical and/or chemical methods such as sandblasting and silanization. The bonding strength between the zirconia (fiber) post and the resin cement was measured by a push-out method after thermocycling based on the adhesion to Panavia F 2.0 resin cement. The zirconia and fiber posts exhibited different bonding strengths after sandblasting and/or silanization because of the different strengths and chemical structures. The zirconia post showed a high bonding strength of up to 17.1 MPa after a combined treatment of sandblasting and silanization because of the rough surface and covalent bonds at the interface. This effect was also enhanced by using 1,2-bis(trimethoxysilyl)ethane for the formation of a flexible layer at the interface. In contrast, a high bonding strength of 13.9 MPa was obtained for the fiber post treated by silane agents because the sandblasting treatment resulted in damage to the fiber post, as observed by scanning electron microscopy. The results indicated that the improvement in the bonding strength between the post and the resin cement could be controlled by different chemical and/or mechanical treatments. Enhanced bonding strength depended on covalent bonding and the surface roughness. A zirconia post with high bonding strength could potentially be used for the restoration of teeth in the future.     

The optimal density of cellular solids in axial tension

For cellular bodies with uniform cell size, wall thickness, and shape, an important question is whether the same volume of material has the same effect when arranged as many small cells or as fewer large cells. To answer this question, for finite element models of periodic structures of Mooney-type material with different structural geometry and subject to large strain deformations, we identify a nonlinear elastic modulus as the ratio between the mean effective stress and the mean effective strain in the solid cell walls, and show that this modulus increases when the thickness of the walls increases, as well as when the number of cells increases while the volume of solid material remains fixed. Since, under the specified conditions, this nonlinear elastic modulus increases also as the corresponding mean stress increases, either the mean modulus or the mean stress can be employed as indicator when the optimum wall thickness or number of cells is sought.     

An elastic rod model to evaluate effects of ionic concentration on equilibrium configuration of DNA in salt solution

As a coarse-gained model, a super-thin elastic rod subjected to interfacial interactions is used to investigate the condensation of DNA in a multivalent salt solution. The interfacial traction between the rod and the solution environment is determined in terms of the Young–Laplace equation. Kirchhoff’s theory of elastic rod is used to analyze the equilibrium configuration of a DNA chain under the action of the interfacial traction. Two models are established to characterize the change of the interfacial traction and elastic modulus of DNA with the ionic concentration of the salt solution, respectively. The influences of the ionic concentration on the equilibrium configuration of DNA are discussed. The results show that the condensation of DNA is mainly determined by competition between the interfacial energy and elastic strain energy of the DNA itself, and the interfacial traction is one of forces that drive DNA condensation. With the change of concentration, the DNA segments will undergo a series of alteration from the original configuration to the condensed configuration, and the spiral-shape appearing in the condensed configuration of DNA is independent of the original configuration.     

Modulating DNA configuration by interfacial traction: an elastic rod model to characterize DNA folding and unfolding

As a continuum model of DNA, a thin elastic rod subjected to interfacial interactions is used to investigate the equilibrium configuration of DNA in intracellular solution. The interfacial traction between the rod and the solution environment is derived in detail. Kirchhoff’s theory of elastic rods is used to analyze the equilibrium configuration of a DNA segment under the action of the interfacial traction. The influences of the interfacial energy factor and bending stiffness on the toroidal spool formation of the DNA segment are discussed. The results show that the equilibrium configuration of DNA is mainly determined by competition between the interfacial energy and elastic strain energy of the DNA itself, and the interfacial traction is one of the forces that drives DNA folding and unfolding.     

Song post exposure, song features, and predation risk

Male birds use song to attract mates and deter other males,but in doing so, they also attract the attention of predatorsand parasites. Such viability costs are inherent in reliablesignals, potentially causing females to prefer mates that displayfrom the most exposed sites. However, viability costs of sexualsignals may be ameliorated by affecting the choice of microhabitat,which in turn may affect the design of song features that aremost efficiently transmitted in this microhabitat. We estimatedthe exposure of song posts (microsites used by males when singing)used by passerine birds in relation to prey selection by thesparrowhawk Accipiter nisus, by calculating the proportion ofmales that sang from song posts that were at the maximum levelof the vegetation, in an attempt to quantify the costs of sexualselection. We quantified prey susceptibility to predation asthe difference between the log-transformed observed number ofprey minus the log-transformed expected number of prey in theenvironment. This prey susceptibility index increased with increasingsong post exposure similarly in sexually dichromatic and monochromaticspecies, although the prey susceptibility index was relatedto sexual dichromatism. Song post exposure was dependent onhabitat, but comparative models controlling for the potentiallyconfounding effects of habitat, sexual dichromatism, hole nesting,coloniality, body mass, cognitive capacities, and flying abilitiesindicated that the relationship between the prey susceptibilityindex and song post exposure is strong. Path analyses of therelationship between song post exposure, sexual dichromatism,and prey susceptibility index revealed that selection actingon sexual dichromatism and song post exposure has secondaryimpact on prey susceptibility index. The opposite causal mechanismsby which predation affects sexual traits are less likely. Thesemodels suggest that female preference for high song posts ordichromatic plumage increases predation risk on an evolutionarytime scale.     

Epiphyseal-based designs for tibial plateau components--I. Stress analysis in the frontal plane

Two-dimensional, finite element studies were conducted of the proximal tibia before and after joint arthroplasty. Equivalent-thickness models projected onto the mid-frontal plane were created for the natural, proximal tibia and for the proximal tibia with four different types of tibial plateau components. All components simulated bony ingrowth fixation, i.e. no cement layer existed between component and bone. In addition, the interface between component and bone was assumed to be intimately connected, representing complete bony ingrowth and a rigid state of fixation. Loads consisted of bi-condylar and uni-condylar forces. Results indicated that conventional plateau designs with central posts or multiple pegs led to higher stress magnitudes in the trabecular bone near the distal ends of the post/pegs and stress shielding at more proximal locations. A design without posts or pegs whose interface geometry mimics the epiphyseal plate minimizes bone stress shielding. An implant consisting of separate components covering each condyle was found effective in limiting component tilting and the consequent tensile stresses caused by non-symmetrical, uni-condylar loading.     

An Analysis and Comparison of Convergence and Uniqueness of Time-Independent Bone Adaptation Models

Several stimuli are proposed in the bone remodeling theory. It is not clear, if a unique solution exists and if the result is convergent using a certain stimulus. In this study, the strain stimulus, strain energy stimulus and the von Mises stress stimulus for bone remodeling are compared and applied to a square plate model using the finite element method. In the plane stress state, the remodeling equilibrium equations are transformed into functions of only the principal strains and the graphs of these functions are drawn in a diagram using the principal strains as the variables of two coordinate axes. The equation of the sum of principal strain squared equal to a constant is a circle in the diagram. The remodeling equilibrium equation of the strain stimulus is a quadrangle fitting into the circle, the remodeling equilibrium equation of the strain energy stimulus is an ellipse and the remodeling equilibrium equation of the von Mises stress stimulus is also an ellipse close to the principal strains circle when we take the same constants in the above equations. Using the finite element method, two models are performed with the uniform initial elastic properties and with the semi-random initial distribution of the elastic properties. The principal strains as the final finite element results converge within 2% of the objective constant for all the different stimuli. The obtained Young's moduli of two models as the adaptation object are different but in equilibrium, i.e. the equilibrium solution of adaptation model is not unique. The principal strains can not be used to examine the uniqueness of solution, since two different solutions can have the same results of principal strains. Using a certain stimulus, certain initial properties and a certain iterative equation, the solution is unique in equilibrium. The results using the model in this study show also that the same results can be obtained using any of the three stimuli when a proper constant in each remodeling equilibrium equation is chosen.     

Radial growth rate increases in naturally occurring ponderosa pine trees: a late-20th century CO2 fertilization effect?

The primary objective of this study was to determine if gradually increasing levels of atmospheric CO2, as opposed to 'step' increases commonly employed in controlled studies, have a positive impact on radial growth rates of ponderosa pine (Pinus ponderosa) in natural environments, and to determine the spatial extent and variability of this growth enhancement. We developed a series of tree-ring chronologies from minimally disturbed sites across a spectrum of environmental conditions. A series of difference of means tests were used to compare radial growth post-1950, when the impacts of rising atmospheric CO2 are best expressed, with that pre-1950. Spearman's correlation was used to relate site stress to growth-rate changes. Significant increases in radial growth rates occurred post-1950, especially during drought years, with the greatest increases generally found at the most water-limited sites. Site harshness is positively related to enhanced radial growth rates. Atmospheric CO2 fertilization is probably operative, having a positive effect on radial growth rates of ponderosa pine through increasing water-use efficiency. A CO2-driven growth enhancement may affect ponderosa pine growing under both natural and controlled conditions.     

预成纤维桩在口腔修复中的临床效果评价

目的：探讨预成纤维桩在口腔修复中的临床效果。方法：将2012年8月至2012年11月来我科就诊的100例口腔修复患者按照抽签法随机地均分为对照组与观察组,均为50颗患牙。对照组给予铸造金属桩口腔修复,观察组给予预成纤维桩口腔修复,比较两组疗效、满意度、平均每颗纤维桩所需时间及牙周指数变化。结果：经半年的随访,对照组修复成功率为80．00％（40／50）,明显小于观察组（100．00％,50／50）,两组相比,差异具有统计学意义（P〈0．05）;对照组满意度为60．00％,明显小于观察组（92．00％）,两组相比,差异具有显著的统计学意义（P〈0．01）;对照组平均每颗纤维桩所需时间为（102．22±22．09）min,观察组为（66．37±11．78）min,两组相比,差异具有统计学意义（P〈0．05）;两组术前牙周指数（BI、PLI、PD及松动度）差异均无统计学意义（P〉0．05）,但术后2周、3个月、6个月两组牙周指数差异均具有统计学意义（P〈0．05,P〈0．01）。结论：顸成纤维桩应用于口腔修复中,效果尤佳,患者满意度高,美观效果好,值得在临床上加以推广并应用。     

An analytical model of joint contact

The stress distribution in the region of contact between a layered elastic sphere and a layered elastic cavity is determined using an analytical model to stimulate contact of articulating joints. The purpose is to use the solution to analyze the effects of cartilage thickness and stiffness, bone stiffness and joint curvature on the resulting stress field, and investigate the possibility of cracking of the material due to tensile and shear stresses. Vertical cracking of cartilage as well as horizontal splitting at the cartilage-calcified cartilage interface has been observed in osteoarthritic joints. The current results indicate that for a given system (material properties mu and nu constant), the stress distribution is a function of the ratio of contact radius to layer thickness (a/h), and while tensile stresses are seen to occur only when a/h is small, tensile strain is observed for all a/h values. Significant shear stresses are observed at the cartilage-bone interface. Softening of cartilage results in an increase in a/h, and a decrease in maximum normal stress. Cartilage thinning increases a/h and the maximum contact stress, while thickening has the opposite effect. A reduction in the indenting radius reduces a/h and increases the maximum normal stress. Bone softening is seen to have negligible effect on the resulting contact parameters and stress distribution.     

Flow-tissue interaction with compliance mismatch in a model stented artery

The insertion of an endovascular prosthesis is known to have a thrombogenic effect that is also a consequence of the interaction between the flowing blood and the stented arterial segment; in fact the prosthesis induces a compliance mismatch and a possible small expansion along the vessel that eventually gives rise to an anomalous distribution of wall shear stresses. The fluid dynamics inside a rectilinear elastic vessel with compliance and section variation is studied here numerically. A recently introduced perturbative approach is employed to model the interaction between the fluid and the elastic tissue; this approximate technique is first validated by comparison with a complete solution within a simple one-dimensional model of the same system. Then it is applied to an axisymmetric model in order to evaluate the flow dynamics and the distribution of wall shear stress in the stented vessel. Compliance mismatch is shown to produce more intense negative wall shear stresses in the stented segment while rapid variations of wall shear stress are found at the stent ends. These effects are enhanced when the prosthesis is accompanied by a small increase of the vessel lumen.     

Recruitment of doctors to non-standard grades in the NHS: analysis of job advertisements and survey of advertisers

Objectives To estimate the proportion of advertised non-consultant hospital posts that do not conform to nationally recognised terms and conditions of service and to investigate why these posts exist, who fills them, and what the doctors in such jobs do.Design Analysis of job advertisements and a cross sectional survey of advertisers.Setting Job advertisements in one of the leading UK publications listing hospital doctor vacancies (BMJ Careers).Results Nearly a quarter of non-consultant posts advertised in the two study periods (23% and 21%) were for non-standard grade posts. A questionnaire was sent to the medical staffing officer for each post. Of 430 questionnaires sent out 192 (45%) were returned. 98 trusts said they advertised non-standard grades because there was no more funding from the deanery for approved posts and 75 because service needs could not be met by doctors in training grades. In 132 posts (69%) the post holder would be required to do on-call work, and 50 advertisers (26%) required on-call duty for 1 in 5 or more frequently, which would conflict with the European Working Time Directive. 131 advertisers (68%) expected the posts to be filled by doctors from outside the European Economic Area.Conclusions Non-standard grade posts are mostly being created to meet service requirements when there is no more funding for standard training posts and are expected to be filled by doctors from overseas. Doctors in such posts can be more easily exploited and their careers hindered. The Department of Health''s annual census should include non-standard grade doctors.     

Stress transfer in collagen fibrils reinforcing connective tissues: effects of collagen fibril slenderness and relative stiffness

Unlike engineering fibre composite materials which comprise of fibres that are uniform cylindrical in shape, collagen fibrils reinforcing the proteoglycan-rich (PG) gel in the extra-cellular matrices (ECMs) of connective tissues are taper-ended (paraboloidal in shape). In an earlier paper we have discussed how taper of a fibril leads to an axial stress up-take which differs from that of a uniform cylindrical fibre and implications for fibril fracture. The present paper focuses on the influence of fibre aspect ratio, q (slenderness), and Young's modulus (stiffness), relative to that of the gel phase, E(R), on the magnitude of the axial tensile stresses generated within a fibril and wider implications on failure at tissue level. Fibre composite models were evaluated using finite element (FE) and mathematical analyses. When the applied force is low, there is elastic stress transfer between the PG gel and a fibril. FE modelling shows that the stress in a fibril increases with E(R) and q. At higher applied forces, there is plastic stress transfer. Mathematical modelling predicts that the stress in a fibril increases linearly with q. For small q values, fibrils may be regarded as fillers with little ability to provide tensile reinforcement. Large q values lead to high stress in a fibril. Such high stresses are beneficial provided they do not exceed the fracture stress of collagen. Modulus difference regulates the strain energy release density, u, for interfacial rupture; large E(R) not only leads to high stress in a fibril but also insures against interfacial rupture by raising the value of u.     

Methods for the analysis of triacylglycerols

This article discusses the methods most commonly employed in the analysis of the triacylglycerols (TAGs) in natural fats and considers the main advantages and disadvantages of each and the techniques for optimising analytical conditions. Complete analysis of the composition of a natural fat calls for a method of extracting and purifying the triglyceride fraction, normally by preparatory thin-layer and column chromatography. Determination of the individual components of triglyceride mixtures still entails certain difficulties, namely, the separation and identification of the TAGs in natural fats. High-performance liquid chromatography (HPLC) offers significant advantages over gas and thin-layer chromatography. Many workers have developed non-aqueous, reversed-phase HPLC systems capable of successfully resolving complex mixtures of TAGs, and combining reversed-phase (RP) HPLC and argentation chromatography may improve the results. Identification of the TAGs separated by HPLC becomes an extremely complex task if many different fatty acids are involved and if the sn-stereoscopic positions on the glycerol are to be determined. Enzymatic analysis and chiral-phase chromatography are capable of localising fatty acids on the TAG molecule. In closing, some of the most interesting biomedical applications of TAG analysis are summarised.     

Spontaneous Succession versus Technical Reclamation in the Restoration of Disturbed Sites

We address the question: under which circumstances can we rely upon spontaneous succession and when are technical measures more effective in restoration programs? To answer this question, the position of a disturbed site along the productivity–stress gradient was considered. The probability of attaining a target stage by spontaneous succession decreases toward both ends of the productivity–stress gradient, whereas the acceptance of technical measures generally increases. In correspondence with that, the monetary cost of restoration increases toward the ends of the gradient. Therefore, spontaneous succession is advocated especially if environmental site conditions are not very extreme.     

The hospital component of vocational training for general practice.

Trainees and educationalists in general practice have some grounds for suggesting that the hospital component of vocational training should be restructured and teaching improved. However, the implications for other trainees and secondary care have to be considered. Changes that are needed include a curriculum for senior house officers in each specialty; appointment of training consultants with the necessary skills; and a different attitude by everyone towards study leave, including arrangements for funding. The optimum duration of hospital posts for trainees in general practice might be shorter than now, but the effects on others must be considered and competencies guaranteed in a briefer training period. Changes in the regulations for vocational training could help to improve specialist experience if trainees in general practice were allowed to be supernumerary. Alternatively, senior house officer posts for trainees in general practice could be split between secondary and primary care, thus encouraging a broader perspective.     

Selection of microbial mutants tolerant to extreme environmental stress using continuous culture-control design

The design of controllers for a continuous selection technique (BOICS; Brown and Oliver, 1982) is considered. This technique is used to obtain microbial mutants that are tolerant to extreme environmental stress. Applications of BOICS have been hampered by the problem of controller design. In this paper, a modified implementation of BOICS is considered which has a number of practical advantages. A model-based approach to controller design is taken. The case in which the stress is due to an inhibitory substance in the growth environment is considered. The analysis is intended to be applicable to any reasonable combination of organism and inhibitor. Conventional linear and time-invariant controllers are considered. Guidelines for the selection of controller parameters' values are suggested. The application of these guidelines requires that certain process parameters' values be identified. Methods by which these parameters' values can be identified are suggested. Simulation results indicate that the resulting controllers perform satisfactorily. This is confirmed by experimental data from a model selection experiment. A recipe for the design of controllers is a necessary part of a protocol for BOICS. It is hoped that the solution to the controller design problem that is offered in this paper will encourage further applications for the technique.     

Probabilistic fatigue analysis of all-ceramic crowns based on the finite element method

The reliability evaluation and lifetime prediction of oral restorations for dentists have always been the major concern in the field of prosthodontics. This study introduced a new methodology for dealing with stress–strength interference and an analysis to quantify the reliability of all-ceramic crowns. The response stress was based on finite element models of all-ceramic crowns, which were constructed by reverse techniques. The method utilized in this study was a probabilistic analysis that considered the all-ceramic crown as a structural component associated with multiple factors involved in the all-ceramic system including occlusal loads and mechanical properties of porcelain. The results suggested the survival probability of all-ceramic crowns after 1, 5, 10, 15, and 20 years could be computed to be 98.89%, 98.03%, 97.50%, 97.13%, and 96.93%, respectively. Probabilistic analysis methods offer a systematic technique to incorporate and account for the nature of uncertainty in ceramic restorations and allow prediction of the reliability of all-ceramic crowns under cyclic occlusal loads with accuracy.