Analysis of Magnesium Screw Degradation in a Rabbit Using Micro-CT

Objectives: Recently, magnesium (Mg) alloys have been more closely studied as a potential biomaterial for degradable implants.  Results to date are demonstrating great promise to regenerate both hard and soft musculoskeletal tissues, which is valuable due to the necessity for engineering a bone/cartilage interface for the temporomandibular joint (TMJ).  As a first step towards improving TMJ-implants, the objective of this study was to implant commercially available Mg-alloys as screws in-vivo in rabbit mandibles, and use micro-CT (computed tomography) analysis for bone formation and Mg degradation.

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.