Department of Biology|
University of York|
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Professor John D Currey
My scientific work
I remember, more than 25 years ago at a party, saying to a girl that my real ambition was to find out what happens when a bone breaks. That is still what I am after. (Her ambition was to get off with the millionaire partygiver). We know a great deal about the stresses and strains that you have to impose on bones in order to make them break, but we still have very little idea of what goes on in a nitty-gritty way as cracks start to form, coalesce, and become dangerous. Bone has a quite extraordinarily intimate relationship between its mineral (hydroxyapatite) and its main protein (collagen). The apatite crystals are plate-like and very thin (3º nm or so), and insert themselves between the collagen molecules to produce an architecture that is still unclear. When bone starts to break thousands of little microcracks form. These microcracks are positioned sensibly in relation to the histological structure of the bone, but we don't know where they form in relation to the ultrastructure. Typically when microcracks form they only reach a few microns in length before they come to a halt. The big question is, what brings them to a halt? (Microcracks in themselves can be a good thing, because they absorb energy as they form, and the ability to form microcracks makes the bone tough. There is a corresponding disadvantage that they make the bone less stiff.)
My major interest recently is in this microcracking. The literature is full of reference to 'microcracks', but what people have meant by this is cracks 100º µm or so long. Such long cracks are already on their way to being dangerous. We found multitudes of cracks that were 5 ºµm or so long, and which are much more interesting because they absorb huge amounts of energy as they form. For several years I have been going to conferences saying 'Look folks, these are the important cracks, get a confocal and look!' Solid resistance at first, for some reason, but people are at last coming round to the idea, and the literature is growing. I have been pursuing these microcracks, seeing, for instance, how there are less of them in brittle bone, less of them in bone that has been irradiated, and we are getting some idea about how they grow and multiply.
I also work on other areas of bone mechanics.
1) What is the effect of ageing, disease, and treatment on the mechanical properties of bone? I am interested in how bone gets less tough with age, and what causes this - is it a change in the stability of the collagen (answer probably yes) or is it a change in the mineral, or is it that the bone becomes deranged or changed at a somewhat higher level (the histological).
The detailed way this bone breaks up in old versus young, osteoporotic and osteoarthritic pathologies is giving us a clearer understanding of the cause of the incidence of fractures in various pathological states.
2) What is the range of the mechanical properties of bone and how are they adapted to the functional requirements of the animal? I have produced, by hard experimentation over the years, and am adding to, a large database of the mechanical properties of various bones. We are finding some very extreme properties, some of which can be clearly seen to be adaptive, others less so. I am also concerned with whether the more usual differences one finds in 'ordinary' bones are adaptive. I have also compared fatigue and other properties of highly mineralised bone with that of similarly mineralised mother of pearl, and found that mother of pearl is far superior (presumably because mother of pearl is designed to be highly mineralised, bone isn't).
3) I have had a blinding glimpse of the obvious and, using my large data set, looked at the relationship between the stiffness of the bone and its bending strength. There is an extremely tight proportional relationship, which no-one has really commented on before. This sounds like a piece of Ho Hum, but is actually very important, because it shows that the bending strength of bone is determined by its yield stress (still with me?). The extent to which it is not the yield stress, tells us other related things about the fracture process in bending. This kind of thing is rather well known to mechanical engineers in other materials, but its presence and significance in bone has not been described before.
I am looking at similar effects in tension, and find very interesting differences.
4) Nanoindentation is a technique that allows one to examine the stiffness of very small areas of bone. We are using this technique to look at how the stiffness changes between the inside of Haversian systems and outside, and with orientation, and with age.
5) People who have been moaning at me occasionally about not producing a second edition of my book may or may not be pleased to know that it now published:
I have various collaborations, of varying intensity.
Peter Zioupos. Royal Military College Shrivenham (Cranfield University). Peter is my ex post-doc, and we are still writing papers together and collaborating on further work.
Professor Steve Weiner. In February 2003 I worked with Steve Weiner and Paul Zaslanski at the Weizmann Institute in Rehovot, Israel, on the development of a speckle interferometer. This can measure minute displacements (and therefore strains) over reasonably large areas. We intend to use it to answer some new questions that we can now ask about stress/strain/damage relationships in bone.'
Dr Jae-Young Rho. University of Memphis (Tennessee). This is a tri-partite collaboration between myself, Jae and Peter Zioupos. Jae has a nanoindentor, capable of determining the hardness and Young's modulus of very small areas of bone. We have been doing work together on the properties of different layers in bone. Tragically, Jae-Young died at the very end of 2002, of a heart attack, at the unbearably early age of 43, leaving a wife and young family. So that collaboration has ended.