In vivo biocompatibility and mechanical study of novel bone-bioactive materials for prosthetic implantation

Two epoxy materials with or without adhesively bonded hydroxyapatite (HA) coatings were studied for their biocompatibility and mechanical pushout strength using in vivo implantation in the rabbit lower femur for a duration of 10 days to 6 months. Both were two-part epoxies cured at room temperature for 24 h, with material 1 (Ampreg 26; SP Systems Limited, Cowes, UK) postcured at 110 degrees C (Tg approximately 80 degrees C) and Material 2 (CG5052; Ciba Geigy Limited, Cambridge, UK) at 125 degrees C (Tg approximately 120 degrees C). Implantation in dead rabbit bone was performed to provide mechanical baseline levels. Polymethylmethacrylate (PMMA) and conventionally HA-coated titanium alloy (Ti-6Al-4V) were used as control materials. In the biological study, different fluorescent dyes were used to label newly formed bone. After 6 weeks of implantation, results from mechanical pushout tests showed that the interfacial shear strength (ISS) values were significantly higher than for dead bones with each of the different implants (p < .01-.001). HA-coated material 2 showed a significantly higher ISS value than the uncoated material (p < .05) after 6 weeks' implantation. However, the ISS value for the uncoated material 2 was significantly higher than for PMMA controls (p < .05). No significant differences in the ISS values were shown between HA-coated materials 1 and 2 and Ti-6Al-4V on in vivo implantation for 6 weeks. Failure points of the pushout test from the three HA-coated materials were defined by scanning electron microscopy. Specimens implanted with both HA-coated epoxies were fractured within the HA-coatings or the bone, while with HA-coated Ti-6Al-4V cracked between the coating and metal implant. The percentage of bone in contact with the implant surface was obtained by image analysis which showed that there were no significant differences between different materials after short time implantation (up to 6 week). Long-term implantation of the HA-coated material 2 showed that the percentage of bone contact had increased from 52.8+/-1.1% (6 week) to 80.0+/-0.3% (3 months) (p < .01) and remained at 81.0+/-0.8% (6 months). Measurements of bone mineralization rate (BMR) showed that after 3 weeks of implantation, there were no significant differences between PMMA and uncoated materials 1 and 2. After 6 weeks, the BMRs in animals implanted with either HA-coated material 1 or 2 were significantly higher than with HA-coated Ti-6Al-4V (p < .05-.0001 in both cases), but with HA-coated material 2 was lower than with this material uncoated (p < .05-.001). No significant differences were found between the two HA-coated epoxy materials. In addition, there were always lower BMRs during the third week of implantation than other periods regardless of biomaterial implanted. The study indicated that the adhesively bonded HA-coated novel epoxy materials were superior to conventional plasma-sprayed Ti-6Al-4V implants with respect to both BMR and bone integration with the implant surfaces. Adhesively bonded HA-coated epoxy materials had similar ISS values to HA-coated Ti-6Al-4V, but the former failed within the bone and coating, while the latter showed splitting between coating and metal.