John D. Currey

The Mechanical Adaptations of Bones

This book relates the mechanical and structural properties of bone to its function in man and other vertebrates. John Currey, one of the pioneers of modern bone research, reviews existing information in the field and particularly emphasizes the correlation of the structure of bone with its various uses. Originally published in 1984. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These paperback editions preserve the original texts of these important books while presenting them in durable paperback editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

The effect of porosity and mineral content on the Young's modulus of elasticity of compact bone

The Youngs modulus of elasticity, the calcium content and the volume fraction (1-porosity) of 23 tension specimens and 80 bending specimens, taken from compact bone of 18 species of mammal, bird and reptile, were determined. There was a strong positive relationship between Youngs modulus and both calcium content and volume fraction. A power law model fits the data better than a linear model. Youngs modulus has a roughly cubic relationship with both calcium content and volume fraction. Over 80% of the total variation in Youngs modulus in this data set is explained by these two variables.

Mechanical properties of mother of pearl in tension

Mother of pearl, or nacre, is one of a number of characteristic skeletal structures of molluscs, occurring in cephalopods, gastropods and bivalves. It consists of plates of aragonite, about 0.3 μm thick, arranged in sheets, with a tenuous protein matrix. Mechanical tests of nacre from all three classes show that it has a tensile strength of between 35 and 110 MN m-2. It is slightly viscoelastic, and shows marked, though not extensive, plastic deformation. The maximum measured strain was 0.018. While undergoing plastic deformation the material shows considerable optical changes. The regions where plastic flow is occurring show ‘tension lines’, probably equivalent to similar lines in bone. These are probably caused by voids forming in the protein matrix. The work of fracture is very different in different loading directions, being about 1.65 x 103 J m-2 when fractured across the grain, and 1.5 x 102 J m-2 when fractured along it. Nacre shows considerable ability to stop cracks. An attempt is made to explain qualitatively the mechanical behaviour of nacre in terms of its submicroscopic structure. It is concluded that the precise geometric arrangement of the plates is most important, and that this constraint may make nacre less suitable for shells that must be built quickly.

Changes in the stiffness, strength, and toughness of human cortical bone with age.

Aging adversely affects the elastic and ultimate properties of human cortical bone as seen in uniaxial tests in quasi static loading, high strain rate impact or fatigue. Little is known about the full effects of aging on toughness and its relationship with strength. In the present article the elastic modulus (E), strength (sigma f), fracture toughness (KC and J-integral), and work of fracture (Wf) were determined in specimens of male human femoral bone aged between 35-92 years. In this way we investigated whether fracture of bone in three situations, allowing various amounts of damage prior to fracture, can provide a better insight into the fracture process and also the relative importance of these experimental methods for assessing the soundness of bone material. We found a steady and significant decrease with age for all these mechanical measures. E fell by 2.3%, from its value of 15.2 GPa at 35 years of age, per decade of later life; sigma f fell similarly from 170 MPa by 3.7%; KC from 6.4 MPa m1/2 by 4.1%; J-integral from 1.2 kJ m-2 by 3%, and the Wf from 3.4 kJ m-2 by 8.7%. In aging bone there was a deterioration in the elastic properties of the material. This reduced the (elastically calculated) critical stress intensity level (KC) required to initiate a macrocrack, or the nonlinear energy associated with the onset of fracture (J). The macrocrack was preceded by less damage, and once created needed less energy to drive through the tissue (Wf).

The mechanical properties of bone tissue in children

Specimens of femoral cortical bone from eighteen subjects between two and forty-eight years old were loaded in bending. Compared with the bone of adults, that of children had a lower modulus of elasticity, a lower bending strength, and a lower ash content. However, the childrens bone deflected more and absorbed more energy before breaking. It also tended to absorb more energy after fracture had started. The typical greenstick fracture surface of many specimens of childrens bone requires more energy for its production than the relatively smooth surface of adult specimens.

The role of collagen in the declining mechanical properties of aging human cortical bone.

The importance of the mechanical role of collagen in bone is becoming increasingly more clear as evidence mounts on the detrimental effects of altered collagen on the mechanical properties of bone. We previously examined a set of mechanical properties (material stiffness, strength, and toughness) of human femoral bone (ages 35-92) and found that a gradual deterioration in these properties occurs with age. The present study examines the collagen of the same specimens and relates the collagen properties to the mechanical ones. In the collagen we measured the concentration of stable mature crosslinks, the shrinkage temperature, and the rate of contraction during isometric heating. The changes in the concentration of mature (pyridinium and deoxypyridinium) crosslinks showed no clear relationship to age nor did they correlate with the mechanical properties. The shrinkage temperature declined with age and correlated with a bones toughness. The maximum rate of contraction was strongly correlated with three different measures of tissue toughness, but much less to stiffness and strength. Our results reinforce speculation regarding the toughening role of collagen in bone mechanics and suggest that the fragility of aging bone may be related to collagen changes.

The mechanical properties of bone

2 Department of Anatomy and Highway Safety Research Institute, The University of Michigan, Ann Arbor, Mich. 48104. BONE is the material with which the orthopaedic surgeon deals. Consequently, some knowledge of its mechanical properties is of importance for an understanding of the mechanism and management of fractures, as well as the design of prosthetic or orthotic appliances and protective gear, e.g., crash helmets. The behavior of a body under a load or force is a function not only of the form and structure of the body, but also of the mechanical properties of the material composing the body. For example, a steel beam will support a higher load before breaking and will behave differently under loading than will an oak beam of exactly the same shape and dimensions because of differences in the mechanical properties and structure of steel and of wood.

Area Effects in Cepaea

The snails Cepaea nemoralis and C. hortensis are remarkable for an extensive and stable polymorphism involving the colour and banding of the shell. It was formerly thought that the variation in frequency of the different morphs between populations was random. Cain & Sheppard, for nemoralis and Clarke, for C. hortensis, have shown that in many English colonies visual selection by thrushes, and no doubt other predators, strongly influences the frequencies of the morphs, the more conspicuous on a given background being more heavily predated. In consequence populations tend to match their backgrounds, but remain polymorphic. In some districts of high chalk downland, this correspondence with background does not occur. The predominance of a few morphs irrespective of habitat and background characterizes areas vastly larger than that of a panmictic population. Such a constancy of morph frequencies over a large and diverse area in spite of visual selection we call an area effect. The principal district we have studied is the Marlborough Downs, where in an area of several square kilometres there are no five-banded C. nemoralis although in a contiguous area they predominate. Part of the non-five-banded area has a vast excess of browns, and another part of yellows. The form spread-banded and the cross-product ratio of pink and yellow to unbanded and banded also show such effects. In some places the morph frequencies change with extraordinary abruptness over 100 to 300 m. The area effects are not due to differential incidence of visual predation, nor, since they bear no relation to variation of habitat, to differences in its direction. In only two subareas do we think that visual selection is affecting morph frequencies. The observed frequency distributions cannot be accounted for by sampling drift (‘genetic drift’) at the present day since the numbers involved are far too large and the frequencies too constant over large areas. In the few populations that have been observed for up to 10 years, no major changes of frequency have been found. The probability of a reduction to a few isolated populations because of ploughing up or drought in the last 200 years and subsequent drift and expansion is shown by the known agricultural history of the district to be slight. Restriction by spread of C. hortensis is also unlikely. A few colonies with restricted variation which might seem to show the action of drift or the founder effect are only extreme examples of local tendencies. Moreover, subfossil material from just off the south-western corner of the district strongly suggests that the area effects seen there have been in existence since Neolithic times. A survey of another district of high downland (Lambourn Downs) has shown a similar state of affairs to that on the Marlborough Downs, with a large area characterized by excess of yellow and mid-banded, and adjacent to it localities in which visual selection is effective. Observations from various other places on and off the Chalk also indicate that area effects are frequent on the Chalk, but that away from it visual selection is the principal agent determining local variation in gene frequencies. There is good evidence that the pigmentation of the body, which is apparently multifactorially controlled, also shows area effects; and part of the correspondence between body colour and background shade reported by Cain & Sheppard may be due to them. The evidence available for C. hortensis suggests that this species also shows area effects in shell characters. Yellow, pink or brown may predominate in C. nemoralis, but area effects in banding seem due mostly to the excess or defect of the modifier M3 which reduces the five-banded phenotype to the form with only the middle band. It seems clear that the area effects are caused by some form of selection, but the topography, geology and vegetation of the Marlborough Downs gives no clue to what this could be for banding. Brown is known to be common only in the northern half of the range of nemoralis, and hortensis to extend much farther north than does nemoralis. A study of the distributions of the two species and of the brown morph on the Marlborough Downs suggests that local features of topography of open downland may produce localized climatic conditions influencing the relative distributions of the species and the abundance of brown. The abruptness of change of gene frequencies in both colour and banding might be caused by the change-over from one balanced gene complex to another requiring very different frequencies. Examination of Fisher’s equation for stability of a polymorphism maintained by heterosis (the most likely condition in these species) shows that in the districts where visual selection is effective in altering gene frequencies in nemoralis, the heterozygote advantage can only be of the order of a few percent, and that local differences of a few percent in the selective disadvantages of the homozygotes concerned could well produce area effects as marked as those we have observed. For biological purposes it is essential to recognize the difference between changes in gene frequency caused by selection and those produced by the effects of sampling error. ‘Genetic drift’ has been generally used to refer to the latter, but Sewall Wright uses it for all apparently random changes, whatever their cause, and perhaps for all changes in gene frequency; we therefore use sampling drift for the effects of sampling error. Surveys based only on the observations of frequencies and population size in widely scattered populations do not allow one to distinguish between the effects of selection that varies in direction and intensity from place to place (although more or less constant in time) and those of sampling drift. In general it is exceedingly difficult to identify the result of sampling drift in the wild except in certain situations. Casual collecting over such a district as the Marlborough Downs might well give the impression that sampling drift was effective there, but a more intensive survey shows the contrary.

Mechanical properties of bone tissues with greatly differing functions

Abstract The mechanical properties of three types of bone: deers antler, cows femur and fin whales tympanic bulla, are compared. The antler has a very high work of fracture, the bulla a very low one. The bulla has a very high modulus of elasticity, the antler a low one. The cows femur is the strongest in bending, the bulla is very weak. It is shown how these mechanical properties, which are associated with markedly different amounts of mineralization, nicely suit the bones for their differeing functions.

The extent of microcracking and the morphology of microcracks in damaged bone

Strain-induced damage in bovine laminar bone has been examined using laser scanning confocal microscopy (LSCM). The specimens were loaded in a fluorescein solution, which penetrated the newly formed cracks in the specimen. The microcracking, and the larger cracking, induced by strain were very clearly visible. The microcracking occurred diffusely in regions of high strain (stress), but was particularly obvious in the vicinity of large machined stressconcentrators. The microcracking could be shown not to be artefactual, that is, it was produced by strain, and not by specimen preparation. The microcracking interacted with the structure of the bone, often having a wavy appearance related to the histology. Microcracks seemed to be particularly associated with the most highly mineralized parts of the bone. LSCM is a technique holding great promise for the investigation of the initiation and development of damage in mineralized hard tissues, and other translucent materials.