Conclusion: The use of biodegradable Mg interference screws is safe and its degradation accelerates graft healing to bone tunnels after ACL reconstruction, implying a potential alternative to current permanent Ti interference screw for clinical applications. http://dx.Introduction: Our recent R&D in medical biodegradable magnesium (Mg) for orthopaedic application may overcome above limitations of the rigid and permanent fixation attributed to similar mechanical properties of Mg compared to the

more »

... one. Mg also promotes bone formation and enhances fracture healing by stimulating more callus formation reported in our previous studies. From our previous studies, fracture fixation with Mg implants alone does not provide sufficient mechanical support over the period of time required for fracture fixation. Therefore, we coated the Mg implant with a thin layer of polymer film and combined it with the common fracture repair locked plate which is made of titanium (Ti) together to develop a new kind of locking plate for fracture fixation. In this new hybrid construct, Ti implants provide enough mechanical strength while Mg screw provides initial stability at the fracture line. Degradation of the Mg screw with healing over time can also promote the fracture healing. Furthermore, the coating film can protect the Mg screw(s) from direct metal-to-metal contact with Ti-plate, which would prevent the electrochemical corrosion of Mg screw head and also reduce its degradation. Methods: The Mg screw in this study was made of a high purity (4N, 99.99%) Mg substrate. A thin layer of PLA coating film was also applied on the surface of the Mg screw using dip coating method. In our in vivo study, New Zealand White Rabbits are used as the sizable experimental animals. This study has got the animal operating license from Department of Health in Hong Kong Special Administrative Region and animal experimentation ethics approval from Animal Experimentation Ethics Committee in The Chinese University of Hong Kong. An innovative "Z" shape osteotomy was conducted at the right tibia shaft of the rabbit, which is more closely to the clinical indications. A specifically designed dynamic compression plate (DCP) made of titanium and common Ti screws were used to fix the fractured bones. Mg screw with the coating was put in the middle part of the plate where just through the fracture site. The control group used fixation method with Ti screws only to fix the fractured bones. Radiography (including both X-ray and micro-CT), four-point bending mechanical test, histological and histomorphometric analysis were used to evaluate the fracture healing of tibia. Results: Radiography data showed that the fixation with Mg screw stimulated significantly more callus formation than the fixation with all Ti screws. In the 3rd week, callus formation was firstly observed around the fracture sites. Fluorescence labeling data showed the mineral deposition rate in the Mg group was also higher than that in the Ti group in the 4th and 5th weeks. The largest callus formation area differences were observed in the 6th week. However, radiography data at the 12th week showed slight difference due to the remodeling effect. The mechanical test data in the 6th and 12th weeks both showed that fixation with Mg screw had comparable ultimate strength and failure energy compared to fixation with all Ti screws (n Z 10). Histological and histomorphometric results showed that the degradation of Mg screw stimulated more callus formation and more bone tissue around the fracture sites were generated. Discussion: The mechanical test data indicated that the replacement of Mg screw would not affect the stability of fixation. Mg screw with Ti implants provides enough mechanical support at the early-stage of fracture healing and continue to provide Mg ions to promote the fracture healing. This composite fracture healing system stimulated more callus formation than using current Ti implants from the radiography data and histological data. So the Mg screws with Ti implants together provides both enough support and healing promotion effect, which can be further developed and widely used in future clinical applications. http://dx.