Degradation behavior and osseointegration of Mg-Zn-Ca screws in different bone regions of growing sheep:a pilot study

Magnesium(Mg)-based implants are highly attractive for the orthopedic field and may replace titanium(Ti)as support for fracture healing.To determine the implant-bone interaction in different bony regions,we implanted Mg-based alloy ZX00(Mg<0.5 Zn<0.5 Ca,in wt%)and Ti-screws into the distal epiphysis and distal metaphysis of sheep tibiae.The implant degradation and osseointegration were assessed in vivo and ex vivo after 4,6 and 12weeks,using a combination of clinical computed tomography,medium-resolution micro computed tomography(mCT)and high-resolution synchrotron radiation mCT(SRmCT).Implant volume loss,gas formation and bone growth were evaluated for both implantation sites and each bone region independently.Additionally,histological analysis of bone growth was performed on embedded hard-tissue samples.We demonstrate that in all cases,the degradation rate of ZX00-implants ranges between 0.23 and 0.75mm/year.The highest degradation rates were found in the epiphysis.Bone-to-implant contact varied between the time points and bone types for both materials.Mostly,bone-volume-to-total-volume was higher around Ti-implants.However,we found an increased cortical thickness around the ZX00-screws when compared with the Tiscrews.Our results showed the suitability of ZX00-screws for implantation into the distalmeta-and epiphysis.