Tissue adaptation as a dynamical process far from equilibrium.

It is well established that tissue growth, maintenance, and degeneration are biochemically regulated processes influenced by mechanical function. Biomechanical models have been developed to predict adaptive processes; for example, computer simulation of bone remodeling around orthopaedic implants can accurately predict the effect of certain implant design variables. However, the same success remains to be achieved with other adaptive processes such as joint morphogenesis or osteoporosis. We propose that, to become capable of stimulating such adaptive processes, biomechanical models should capture the inherently irreversible nature of tissue adaptation and therefore should not rely on the assumption of a "homeostatic" equilibrium. In this article, it is proposed that tissue adaptation is an unstable process of moving between tissue states that are far from the equilibrium state--and that to simulate it, independent sensors and positive feedback stimuli should be employed.