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In a beagle dog study, the peri-implant bone changes around flexible (Polyactive^(R)) and rigid hydroxyapatite (HA) implants were investigated radiographically by quantitative digital subtraction analysis and by assessment of marginal bone height, with the aid of a computerized method. A loss of approximately 1 mm of marginal bone height was observed for both the dense Polyactive and the HA implants, after 6 months of loading. This value appeared to be stable from 12 weeks of loading onward. Along the total length of the implant during the first 6 weeks of loading, both the flexible (dense Polyactive) and the rigid (HA) implants showed a decrease in density. However, after this 6-week period, the bone density around the implants increased, and after 18 weeks the original bone density was reached. The flexible Polyactive implants provoked less decrease in density than the rigid HA implants, although not to a statistically significant level. This finding sustains the hypothesis that flexible implant materials may transfer stresses to the surrounding bone more favorably.

Calcification is a crucial step in the bone-bonding mechanism of PEO/PBT hydrogel copolymers (Polyactive^(R)), a new generation of bone-fillers. A beagle dog study was conducted to determine whether the preoperative presence of a calcium phosphate layer (precalcification) on a PEO/PBT 80/20 copolymer would further increase the bone-bonding rate. Standard bone cavities were filled with either precalcified or nonprecalcified porous cylindric PEO/PBT 80/20 implants, or hydroxyapatite granules held together with PEO/PBT 70/30, or were left unfilled. A significantly higher percentage of mineralized component was present in the cavities filled with the precalcified PEO/PBT 80/20 copolymer than in the control defects. As a result of swelling by fluid-uptake, the press-fit inserted copolymer implants showed a significant reduction in pore size, thus preventing optimal bone ingrowth. Both precalcification of the copolymer and underfilling of the defect, to create space for the copolymer to increase in diameter, stimulate postoperative calcification and bone in-growth in PEO/PBT 80/20 copolymers.