Comparative Degradation Behavior of Carbonate Apatite-Coated and Hydroxyapatite-Coated Mg-Ca Alloy Plates and Screws in Rabbit Femurs

Carbonate apatite (CAp) and hydroxyapatite (HAp) coatings were formed on Mg-0.8 mass% Ca (MgCa) alloy plates and screws as bioabsorbable and non-absorbable corrosion suppression coatings, respectively, and implanted in rabbit femurs alongside uncoated devices for 8 and 24 weeks. The degradation behavior of CAp-coated and HAp-coated MgCa, as well as the surrounding bone formation were investigated using scanning electron microscopy and histological observations. The CAp and HAp coatings effectively suppressed the initial rapid corrosion of MgCa devices. Subsequently, while the CAp-coated devices showed no apparent corrosion pits, the HAp-coated devices showed millimeter-sized pits. The 24-week mean corrosion rate of CAp-coated devices was about half that of HAp-coated devices and about one-fifth of uncoated devices. Initially, both CAp and HAp coatings equivalently enhanced new bone formation, promoting screw-bone bonding. However, over time, the screw-bone bonding remained unchanged for CAp-coated screws, whereas it decreased for HAp-coated screws due to substrate corrosion and dissolution of corrosion products. Notably, the CAp coating began to dissolve between 8 and 24 weeks without harming the surrounding tissue, particularly in areas where new bone had adhered. In contrast, the HAp coating almost remained its original shape. It was demonstrated that the CAp coating can effectively suppress MgCa device corrosion and gradually dissolve over time.