Histologic and mechanical evaluation for bone bonding of polymer surfaces grafted with a phosphate-containing polymer

A phosphate-containing polymer was covalently immobilized onto a polyethylene (PE) rod, a poly(ethylene terephthalate) (PET) thread, and a PET film by a surface graft polymerization technique, followed by immersion in calcium phosphate solution to deposit a thin hydroxyapatite (HA) layer on the modified polymer surfaces. The PE rod had a tapered shape, while the PET thread was fixed with clips after implantation, both to minimize the micromovement which may occur in bone. The PE rod was implanted through press-fitting in the femur of rat. Significant enhancement was observed for direct contact of the implant surface with a newly formed bone for both the grafted only and the further HA-deposited PE rods in comparison with the untreated PE at 4, 5, and 6 weeks after implantation. The rats implanted with the modified PET thread in the femur were sacrificed 3 and 6 weeks after implantation. Statistically significant differences were observed for the untreated versus the grafted plus HA-deposited rods at 6 weeks after implantation. To study the resorption of the deposited HA on the methacryloyloxyethylene phosphate-grafted surface, HA-deposited PET films were subcutaneously implanted in the back of rats. The deposited HA was rapidly resorbed within 3 weeks of implantation. These results suggest that the phosphate polymer chains grafted on the PE and PET surfaces effectively induced nucleation and growth of HA crystals. It seems likely that the thin HA layer additionally deposited in vitro onto the grafted PE and PET surfaces was resorbed rapidly and then promoted the growth of HA crystals in vivo.