IN-VIVO SKELETAL RESPONSES TO POROUS-SURFACED IMPLANTS SUBJECTED TO SMALL INDUCED MOTIONS

Cylindrical porous-coated implants were placed in the distal femoral m etaphyses of twenty dogs and were subjected to zero, twenty, forty, or 150 micrometers of oscillatory motion for eight hours each day for si x weeks with use of a specially designed loading apparatus, The in viv o skeletal responses to the different magnitudes of relative motion we re evaluated, Histological analysis demonstrated growth of bone into t he porous coatings of all of the implants, including those that had be en subjected to 150 micrometers of motion, However, the ingrown bone w as in continuity with the surrounding bone only in the groups of impla nts that had not been subjected to motion or that had been subjected t o twenty micrometers of motion; in contrast, the implants that had bee n subjected to forty micrometers of motion were surrounded in part by trabecular bone but also in part by fibrocartilage and fibrous tissue, and those that had been subjected to 150 micrometers of motion were s urrounded by dense fibrous tissue, Trabecular microfractures were iden tified around three of the five implants that had been subjected to fo rty micrometers of motion and around four of the five that had been su bjected to 150 micrometers of motion, suggesting that the ingrown bone had failed at the interface because of the large movements. The archi tecture of the surrounding trabecular bone also was altered by the mic romotion of the implant. The implants that had stable ingrowth of bone were surrounded by a zone of trabecular atrophy, whereas those that h ad unstable ingrowth of bone were surrounded by a zone of trabecular h ypertrophy, The trabeculae surrounding the fibrocartilage or fibrous t issue that had formed around the implants that had been subjected to f orty or 150 micrometers of motion had been organized into a shell of d ense bone tangen-tial to the implant (that is, a neocortex outside the non-osseous tissue). CLINICAL RELEVANCE: The findings of the present study quantitate the in vivo patterns of bone in-growth and remodeling that occur in association with different magnitudes of micromovement of porous-coated implants. Small movements (zero and twenty micrometer s) are compatible with stable ingrowth of bone and atrophy of the surr ounding trabecular bone, whereas larger movements (forty and 150 micro meters) result in less stable or unstable ingrowth of bone, the format ion of fibrocartilage or fibrous tissue around the implant, and hypert rophy of the surrounding trabecular bone, This study not only quantifi ed the magnitudes of relative micromotion that cause these different s keletal responses but also may help in the interpretation of radiograp hs of patients who have a porous-coated prosthesis.