The reversal of tissue differentiation around screws.

Mechanical factors influence the morphology and function of differentiated as well as undifferentiated cells. Observations of tissues surrounding screws, during a 4-week period of stability followed by a 4-week period of instability, demonstrate resorption of the newly formed callus and subsequently to screw lossening. Four weeks of instability followed by 4 weeks of stability produced not only a cessation of the osteoclastic activity, but also the appearance of an osteoclastic activity. Thus, the holding power of screws is not finally determined at the moment of their insertion nor after cellular differentiation has occurred around the screws. Local mechanical conditions throughout the period of fracture consolidation are of eqaul importance.