The Scissors Model of Microcrack Detection in Bone: Work in Progress
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The Scissors Model of Microcrack Detection in Bone: Work in Progress David Taylor1, Lauren Mulcahy1,2, Gerardo Presbitero1, Pietro Tisbo1, Clodagh Dooley1, Garry Duffy2 and T.Clive Lee2 1 Trinity Centre for Bioengineering, Trinity College, Dublin 2, Ireland. 2 Department of Anatomy, Royal College of Surgeons in Ireland, Dublin, Ireland. ABSTRACT We have proposed a new model for microcrack detection by osteocytes in bone. According to this model, cell signalling is initiated by the cutting of cellular processes which span the crack. We show that shear displacements of the crack faces are needed to rupture these processes, in an action similar to that of a pair of scissors. Current work involves a combination of cell biology experiments, theoretical and experimental fracture mechanics and system modelling using control theory approaches. The approach will be useful for understanding effects of extreme loading, aging, disease states and drug treatments on bone damage and repair; the present paper presents recent results from experiments and simulations as part of current, ongoing research.
INTRODUCTION The idea for the so-called “scissors model” first came about in 2001: the concept has now appeared in a number of publications [1-5] and currently is being actively researched by a team which includes materials scientists, cell biologists, microscopists and experts in engineering control theory. In this paper we take the opportunity to present for the first time some of our most recent results, from experimental activities which are still ongoing. Because this paper was written to form part of a special symposium devoted to body tissues under extreme loading and disease, we have pointed out those areas where our approach has contributed, or may contribute in the future, to the understanding of bone mechanics under these particular conditions. However it should be emphasised that we have still a lot to do in the future in considering these particular aspects in more detail. REPAIR OF BONE MICRODAMAGE: DOES IT MATTER? Before considering the detection and repair of small cracks in bone – which is the subject of this paper – it is worth asking: “Does it matter?”. Do these cracks threaten the structural integrity of bone in normal use? This is a question which we can answer using a different model which we have developed over the last 12 years, a phenomenological model which describes fatigue damage and repair in bone using the available data, which are now quite extensive, and including statistical scatter via the Weibull approach. This model has been described in a number of previous publications [6-9]; it does not consider at all the underlying physical mechanisms, but it is able to predict experimental data from different animals and different test protocols
rather well, it gives reasonable predictions of phenomena such as the effect of exercise on stress fracture risk and as a result has recently has been employed in the field of sports science [10]. Here we use the model to consider the risk of fatigue
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