Methods: Pure Mg (99.9%) and Mg-alloy AZ31 (3%wt-aluminum 1%wt-zinc) screws were implanted in rabbit mandibles (skeletally mature, female, New Zealand White) for 4 and 12 weeks (n=3). Screws were machined, heat treated (removes machining stress), cleaned (isopropanol), individually cataloged, and sterilized (UV). Screw dimensions were 1mm diameter, 2mm shaft, M1x0.25mm threads, and weighed approximately 8mg. Micro-CT analyses were performed to measure bone formation and Mg degradation.
Results: At 4 weeks, Mg screws appeared to have degraded more than the AZ31. The diameter of AZ31 screws ranged from 0.82-0.96mm, and the Mg ranged from 0.66-0.83mm. At 12 weeks more degradation occurred. The diameter of AZ31 screws were approximately 0.8mm and the Mg ranged from 0-0.8mm; in some locations the Mg completely degraded. Bone overgrowth was more pronounced at 12 weeks compared to 4 weeks and occurred approximately 50% of the time.
Conclusion: Understanding the behavior of Mg in-vivo in the craniofacial region is an important step towards improving the lifespan of TMJ-implants. Bone overgrowth was an important finding. Results indicate that in the future, Mg-alloys could be used as a coating on current implants to promote bone overgrowth. Pure Mg degraded faster than AZ31 when implanted as a screw. Degradation properties will be assessed again once Mg-alloys are used as coatings.
Acknowledgement: Andrew Holmes: screw fabrication.
Keywords: Implants, Magnesium, Regeneration and TMJ and masticatory muscles