Strain-related control of bone (re)modeling: objectives, mechanisms and failures.

a control process dominated by the "error" signals associated with loading situations to which the bone was not habituated rather than one driven by prolonged exposure to customary loading. Within these short periods of loading high strains 2 , high strain rates 3 and interruption of strain cycles with periods of rest 4 all increase a strain regimen's osteogenic potential. The effects of vibration on (re)modeling 5 where strain frequencies are high but strain magnitudes two orders of magnitude lower than those achieved during locomotion may or may not be working through the same "programmes" as those derived from functional levels of strain change. What are the strain-related objectives of the processes of bone modeling and remodeling? Most experiments using artificial loading are too short for any adaptive (re)modeling response to be completed and thus allow strains before intervention to be compared with those after adaptation. According to the "mechanostat theo-ry" the strains after adaptation should be the same as those beforehand since the presumed objective of the adaptive (re)modeling process, will have been to change the bone's mass and architecture to re-establish them. However, while experiments have given some insight into the waveform of strain change capable of stimulating the adaptive (re)model-ing process practically nothing is known of the subset of the total strain information that constitutes the strain-related objective of adaptation. Thus assuming that bone cells have a "target strain environment" is it fixed genetically, influenced by other features of the individual's physiology, or itself varied according to strain history? Does it relate only to peak strains and if so, peak strains anywhere in the bone or at particular places? Are periods of high strains averaged over time with periods of low strains? Does it relate to actual strain change or to some composite measure of strain change such as strain energy density? What is the effect of strain distribution or strain gradients? Is strain change averaged through-Hylonome The strains engendered in bone tissue by functional loading are now widely accepted as providing the controlling stimulus for functional control of bone architecture. While an individual's nutritional and hormonal status may significantly influence the cells responsible for bone modeling and remodeling it is difficult to see how these systemic influences themselves could provide the local, loading-related control necessary to establish and maintain an appropriate balance between architecture and loading at each location throughout the skeleton. Similarly, while neurologically-derived influences …