Multi-modal investigation of the bone micro- and ultrastructure, and elemental distribution in the presence of Mg-xGd screws at mid-term healing stages

Magnesium(Mg)–based alloys are becoming attractive materials for medical applications as temporary bone implants for support of fracture healing,e.g.as a suture anchor.Due to their mechanical properties and biocompatibility,they may replace titanium or stainless-steel implants,commonly used in orthopedic field.Nevertheless,patient safety has to be assured by finding a long-term balance between metal degradation,osseointegration,bone ultrastructure adaptation and element distribution in organs.In order to determine the implant behavior and its influence on bone and tissues,we investigated two Mg alloys with gadolinium contents of 5 and 10 wt percent in comparison to permanent materials titanium and polyether ether ketone.The implants were present in rat tibia for 10,20 and 32 weeks before sacrifice of the animal.Synchrotron radiation-based micro computed tomography enables the distinction of features like residual metal,degradation layer and bone structure.Additionally,X-ray diffraction and X-ray fluorescence yield information on parameters describing the bone ultrastructure and elemental composition at the bone-to-implant interface.Finally,with element specific mass spectrometry,the elements and their accumulation in the main organs and tissues are traced.The results show that Mg-xGd implants degrade in vivo under the formation of a stable degradation layer with bone remodeling similar to that of Ti after 10 weeks.No accumulation of Mg and Gd was observed in selected organs,except for the interfacial bone after 8 months of healing.Thus,we confirm that Mg-5Gd and Mg-10Gd are suitable material choices for bone implants.

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.

Radiofrequency induced heating of biodegradable orthopaedic screw implants during magnetic resonance imaging

Magnesium(Mg)-based implants have re-emerged in orthopaedic surgery as an alternative to permanent implants.Literature reveals little information on how the degradation of biodegradable implants may introduce safety implications for patient follow-up using medical imaging.Magnetic resonance imaging(MRI)benefits post-surgery monitoring of bone healing and implantation sites.Previous studies demonstrated radiofrequency(RF)heating of permanent implants caused by electromagnetic fields used in MRI.Our investigation is the first to report the effect of the degradation layer on RF-induced heating of biodegradable orthopaedic implants.WE43 orthopaedic compression screws underwent in vitro degradation.Imaging techniques were applied to assess the corrosion process and the material composition of the degraded screws.Temperature measurements were performed to quantify implant heating with respect to the degradation layer.For comparison,a commercial titanium implant screw was used.Strongest RF induced heating was observed for non-degraded WE43 screw samples.Implant heating had shown to decrease with the formation of the degradation layer.No statistical differences were observed for heating of the non-degraded WE43 material and the titanium equivalent.The highest risk of implant RF heating is most pronounced for Mg-based screws prior to degradation.Amendment to industry standards for MRI safety assessment is warranted to include biodegradable materials.

Evaluating the morphology of the degradation layer of pure magnesium via 3D imaging at resolutions below 40 nm

Magnesium is attractive for the application as a temporary bone implant due to its inherent biodegradability,non-toxicity and suitable mechanical properties.The degradation process of magnesium in physiological environments is complex and is thought to be a diffusion-limited transport problem.We use a multi-scale imaging approach using micro computed tomography and transmission X-ray microscopy(TXM)at resolutions below 40 nm.Thus,we are able to evaluate the nanoporosity of the degradation layer and infer its impact on the degradation process of pure magnesium in two physiological solutions.Magnesium samples were degraded in simulated body fluid(SBF)or Dulbecco’s modified Eagle’s medium(DMEM)with 10%fetal bovine serum(FBS)for one to four weeks.TXM reveals the three-dimensional interconnected pore network within the degradation layer for both solutions.The pore network morphology and degradation layer composition are similar for all samples.By contrast,the degradation layer thickness in samples degraded in SBF was significantly higher and more inhomogeneous than in DMEM+10%FBS.Distinct features could be observed within the degradation layer of samples degraded in SBF,suggesting the formation of microgalvanic cells,which are not present in samples degraded in DMEM+10%FBS.The results suggest that the nanoporosity of the degradation layer and the resulting ion diffusion processes therein have a limited influence on the overall degradation process.This indicates that the influence of organic components on the dampening of the degradation rate by the suppression of microgalvanic degradation is much greater in the present study.