Haversian canal structures can be associated with size effects in cortical bone

Prediction of periprosthetic failure may be improved by an improved model of bone elasticity which includes microstructural information. Micropolar theory facilitates such information to be included in a continuum model. We assessed the extent of bone’s micropolar behaviour in bending both numerically and experimentally. The numerical model was consistent with micropolar behaviour, and experimental results exhibited size effects that may have been confounded by surface roughness effects, as predicted numerically

Modelling micropolar behaviour in cortical bone

This presentation looks at modelling micropolar behaviour in cortical bon

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

A computational and experimental investigation into the micropolar elastic behaviour of cortical bone

Cortical bone is a natural composite, heterogeneous material with a complex hierarchical microstructure. The description of this microstructure in terms of the mechanical properties of cortical bone may be important in the understanding of periprosthetic stress concentrations. Micropolar elasticity is a higher order continuum theory which may more effectively describe the influence of the microstructure in cortical bone on its mechanical behaviour. Micropolar elasticity predicts a size effect in three-point bending, which has been investigated computationally and experimentally on bovine mid-diaphyseal cortical bone. Computational models of an idealised heterogeneous material, with vascular canal-like structures running along the length of the beam, demonstrated a size effect in the longitudinal and transverse directions which was dependent on the surface condition of the beam. Idealised models with smooth surface layers increased in stiffness as specimens decreased in size, whilst idealised model beams intersected by the internal microstructure demonstrated an equally strong, yet opposite, effect. These FE size effects were further corroborated by analytical studies which demonstrated similar size effects. Experimental three-point bending studies of bovine cortical bone specimens orientated both longitudinally and transversely were consistent with the equivalent numerical models where the internal microstructure intersected the surface. These results suggest the micropolar characteristic length in bending is of the order of the size of the Haversian canal system in secondary osteons and the vascular channels in plexiform cortical bone. The ramifications of this are that the microstructure of cortical bone is of fundamental importance in understanding size effects and stress concentrations in the material. This finding is important in understanding and developing the design and longevity of prosthetic devices and in being able to improve the interaction between an implant and the surrounding cortical bone.Cortical bone is a natural composite, heterogeneous material with a complex hierarchical microstructure. The description of this microstructure in terms of the mechanical properties of cortical bone may be important in the understanding of periprosthetic stress concentrations. Micropolar elasticity is a higher order continuum theory which may more effectively describe the influence of the microstructure in cortical bone on its mechanical behaviour. Micropolar elasticity predicts a size effect in three-point bending, which has been investigated computationally and experimentally on bovine mid-diaphyseal cortical bone. Computational models of an idealised heterogeneous material, with vascular canal-like structures running along the length of the beam, demonstrated a size effect in the longitudinal and transverse directions which was dependent on the surface condition of the beam. Idealised models with smooth surface layers increased in stiffness as specimens decreased in size, whilst idealised model beams intersected by the internal microstructure demonstrated an equally strong, yet opposite, effect. These FE size effects were further corroborated by analytical studies which demonstrated similar size effects. Experimental three-point bending studies of bovine cortical bone specimens orientated both longitudinally and transversely were consistent with the equivalent numerical models where the internal microstructure intersected the surface. These results suggest the micropolar characteristic length in bending is of the order of the size of the Haversian canal system in secondary osteons and the vascular channels in plexiform cortical bone. The ramifications of this are that the microstructure of cortical bone is of fundamental importance in understanding size effects and stress concentrations in the material. This finding is important in understanding and developing the design and longevity of prosthetic devices and in being able to improve the interaction between an implant and the surrounding cortical bone

The micropolar behaviour of cortical bone : size and surface effects in 3 point bending

The heterogeneous microstructure of cortical bone may be important in the determination of stress concentrations and shielding in the vicinity of orthopaedic implants. We hypothesise that micropolar elasticity can parameterise the microstructure of cortical bone to better predict local stresses. Threepoint bending tests on bovine mid-diaphyseal bone demonstrated a size effect in which sample stiffness decreased as size reduced. However, computational predictions indicate that the size effect depends entirely on the surface condition: smooth surfaces result in increased stiffness as size decreases, whilst surfaces corrugated by the microstructure demonstrated an identically strong, yet opposite, effect. We have thus established the connection between anti-micropolar behaviour and surface heterogeneity, of significant relevance to all heterogeneous solids. For bone in particular, we have shown that the micropolar characteristic length is consistent with the Haversian canal diameter. Haversian canals are therefore of fundamental importance in understanding local stress and strain fields in cortical bone

On surface effects in model heterogeneous materials and the consequences for a real material : cortical bone

Experimental testing and finite element analysis of model heterogeneous materials consisting of regular periodic arrays of circular voids within metallic or polymeric matrices reveals that the materials exhibit mechanical behaviour consistent with the predictions of micropolar or Cosserat elasticity theory: for samples of similar geometry sample stiffness increases as size reduces. However, this behaviour is only observed in cases where the sample surface remains smooth and continuous, the circular voids constituting the heterogeneity do not intersect the surface. In this paper the results of finite element analyses of model heterogeneous materials in which the surfaces are corrugated because the voids intersect them are presented. The results indicate that rather than exhibiting micropolar behaviour in which sample stiffness increases with reducing size the opposite is observed; the samples become more compliant as size decreases. Interestingly, the rate of decrease in the latter case where the voids intersect the surface is exactly equal to the rate of increase in the former case where the surfaces are smooth. Mechanical testing of a real material, bovine cortical bone, reveals a stiffness variation consistent with that of a heterogeneous material with corrugated surfaces, the smaller bone samples are more compliant than their larger counterparts. The observed rate of change of stiffness is then used to determine the value of an additional elastic constant present within micropolar elasticity theory, the characteristic length. The value obtained for this parameter is shown to be consistent with the length scales associated with the largest scale heterogeneity present within the bone, the Herversian canal system

The micropolar properties of bone

Paper explains the micropolar properties of bone