Tissue response to porous-coated implants lacking initial bone apposition.

Although initial bone apposition of a porous-surfaced implant is desirable, it is not always achieved surgically. A model to study the effect of a gap on the quantity and quality of bone growth in both the cancellous and cortical regions has been developed. Implants were surgically placed in the intramedullary canals of adult dogs producing uniform gap spaces 0.0-2.0 mm wide. Histologic and microradiographic evaluations were conducted after 3, 6, and 12 weeks in situ. The results demonstrate that the initial apposition of a porous implant to the surrounding bone surface is not necessary for fixation by bone ingrowth. New bone will grow up to and within the porous structure of an implant even when there is a gap as large as 2.0 mm. However, the rate and degree of maturity and mineralization is enhanced when the gap width is 0.5 mm or less. The amount of bone activity in the cortical region was greater than in the cancellous region at 3 and 6 weeks after operation. After 12 weeks in situ bone growth in gap spaces and into the porous coating was approximately equal.