Pre-clinical testing of human size magnesium implants in miniature pigs: Implant degradation and bone fracture healing at multiple implantation sites

Two miniature pig models to assess safety and performance of degradable osteosynthesis implants are presented. Both models provide multiple implantation sites with human size implants. In the first model, different types of magnesium plates and screws for fracture fixation were used to study local and systemic safety aspects in 14 Gottingen minipigs. Implant degradation, gas release and accumulation of alloying elements in organs were assessed for non-coated and plasmaelectrolytic coated magnesium implants and compared to the titanium reference. The observed implant degradation was mostly uniform and did not seem to depend on the implantation site and implant condition. The coating was effective in delaying initial gas release and degradation. No rare earth alloying elements could be detected in local lymph nodes, kidneys, livers or spleens. In the second model with Gottingen and Yucatan minipigs, full osteotomies were inflicted to four different anatomical sites and treated with magnesium plates and screws to assess fracture healing performance. Two Gottingen pilot minipigs showed promising results including a mandible osteosynthesis which healed within 6 weeks. The subsequent study was compromised by the more massive jaws of the used Yucatan minipigs. Three out of seven animals had to be sacrificed within two months as the stability of magnesium and titanium reference implants in the mandible was surpassed. In conclusion, the resorbable magnesium implants showed promising in vivo properties. For the analysis of human standard sized implants under full chewing load conditions, lighter Gottingen minipigs were more suitable than heavier Yucatan minipigs.