Although porous-surfaced orthopedic implants have been designed for fixation by bone ingrowth, there is clinical evidence that this does not always occur. Initial implant movement relative to host bone can result in attachment by a nonmineralized fibrous connective tissue layer. The ranges of movement that result in either bone or fibrous connective tissue fixation are observed in dogs in two independent studies. Experimentally, bone ingrowth can occur in the presence of some movement, albeit very small (up to 28 mu), while excess movement (150 mu or more) can result in attachment by mature connective tissue ingrowth.