BIOPHYSICAL STIMULI ON CELLS DURING TISSUE DIFFERENTIATION AT IMPLANTINTERFACES

If musculoskeletal tissues are indeed efficient for their mechanical f unction, it is most reasonable to assume that this is achieved because the mechanical environment in the tissue influences cell differentiat ion and expression. Although mechanical stimuli can influence the tran sport of bioactive factors, cell deformation and cytoskeletal strain, the question of whether or not they have the potential to regulate tis sue differentiation sequences (for example, during fracture healing or embryogenesis) has not been answered. To assess the feasibility of bi ophysical stimuli as mediators of tissue differentiation, we analysed interfacial tissue formation adjacent to a micromotion device implante d into the condyles of dogs. A biphasic finite element model was used and the mechanical environment in the tissue was characterised in term s of (i) forces opposing implant motion, (ii) relative velocity betwee n constituents, (iii) fluid pressure, (iv) deformation of the tissue a nd (v) strain in the tissue. It was predicted that, as tissue differen tiation progressed, subtle but systematic mechanical changes occur on cells in the interfacial tissue. Specifically, as the forces opposing motion increase, the implant changes from being controlled by the maxi mum-allowable displacement (motion-control) to being controlled by the maximum-available load (force-control). This causes a decrease in the velocity of the fluid phase relative to the solid phase and a drop in interstitial fluid pressure accompanied by a reduction in peri-prosth etic tissue strains. The variation of biophysical stimuli within the t issue can be plotted as 'mechano-regulatory pathway', which identifies the transition from motion-control to force-control as a branching ev ent in the tissue differentiation sequence. (C) 1997 Elsevier Science Ltd.