How everyday stress shapes us

Skeletal Function and Form
April 26, 2002

The skeleton is highly adaptive - its structure, and the gross form that arises, changes dynamically throughout its lifetime. There are two factors at play: intrinsic genetic regulation and extrinsic interactions. Bone resists stresses and adapts to these physical factors in real time by constant micro-fracture, repair and remodelling. Julius Wolff first established the causal link between physical forces and morphology of the skeleton and the idea that skeletal tissues adapt to function in the prevailing environment. Carl Hueter and Richard Volkmann extended this concept to later skeleto-genesis in their law, which states that mechanical forces affect growth and ossification directly. These ideas are elaborated and extended by Dennis Carter and Gary Beaupre in their textbook on skeletal form and function.

This excellent book is a comprehensive introduction to the study of the modulation of bone form and function by mechanical stimuli. It is an up-to-date reappraisal of how growth and form in biology are intimately bound up in physical and mechanical factors. Mechano-biological principles also extend to other tissues, but the principles and effects are heightened in tissues that perform mechanical functions such as the skeleton. The authors explain how mechano-biological factors regulate skeletal development, which begins in the embryo, and adaptation, repair and regeneration in later life. Pattern formation and constraints in embryonic development are not exclusive to regulatory genes and morphogens. Rather, genes encode mechano-biological rules in early development.

The authors "aspire to bridge important gaps between disciplines in a manner that will provide common ground for understanding and future investigation", and I think they achieve this. The text is concise, direct and erudite. It is of interest to biologists, bioengineers, biomedics and clinical orthopaedics. In scientific fields such as bone-tissue engineering, aspects of mechano-biology have been neglected at the point when cells and accompanying factors are loaded into synthetic scaffolds. This work has shown the need to incorporate this knowledge into tissue-engineering strategies. Furthermore, the quantitative aspects of skeletal development and growth in health and disease are of direct interest in clinical orthopaedics.

The differentiation of mesenchymal stem cells is in part directed by mechano-biological factors. A work such as this focuses attention on the importance of mechanical cues in development, differentiation and growth of tissues. Written by well-established academics in biomechanics, the book's strength lies in the many examples of mathematical modelling it gives to promote understanding of the inter-relationship between the shape and structure of musco-skeletal elements and their function. Two principle techniques are used throughout: finite element analysis and simulations of stress-loading histories that can be calculated using engineering mechanics. One is immediately aware of how well computer analyses can generate realistic representations of forces acting on these elements and predict biological responses.

The models are deliberately straightforward and understandable by the non-specialist. However there are some confusing engineering terms that seem abstract to the non-engineer. Since the transduction mechanisms that sense and signal biological responses are still not completely understood, there remains a degree of phenomenological hand-waving in these computer-generated models. An integration of findings between chapters seems somewhat lacking, though perhaps the authors found it necessary to compartmentalise the subject matter finely so as to avoid too much complexity. In addition, it would have been useful if more was explained of the practical implications and practical uses of the models to reinforce the importance of a mechano-biological approach.

There is a complex web of multiple interactions at play in determining biological responses to physical factors. The appropriate level of mechanical stimulation for bone development and maintenance is generated by genotype, systemic factors and physico-chemistry. What is understood is that fluid pressure and shear stresses form the basis of the mechano-biology of skeletal tissues and many other connective tissues. What emerges from cartilage, perichondral, periosteal and endochondral growth and ossification is the key role that mechanical loading plays in patterning, geometry and histomorphology.

Noteworthy are the sections on skeletal evolution. "Nothing makes sense in biology except in the light of evolutionary biology" (Theodosius Dobzhansky) and it is important to know for what aspects of structure and form are being selected. So often this is crucial to understanding optimisation and constraints on skeletal design: why living things take the form they do and function the way they do. Mechano-biological rules play a large part in constraining evolution and development.

This is an informative and enjoyable book, but I would like to have seen more illustrative histology and diagrams to explain the growth and development of each skeletal tissue, which would have made the detailed descriptions easier to grasp. But this is a quibble. Overall, the book is a comprehensive synopsis of the mechano-biology of the skeleton, extended and updated with many rigorous analytical models generated by the authors. A recommended read for graduate studies in biology and orthopaedics as well as for practising orthopaedic surgeons.

David Green is a research fellow, University of Southampton.

Skeletal Function and Form: Mechanobiology of Skeletal Development, Aging and Regeneration

Author - Dennis R. Carter and Gary S. Beaupré
ISBN - 0 521 79000 X
Publisher - Cambridge University Press
Price - £45.00
Pages - 318

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