Assessing the microstructure and in vitro degradation behavior of Mg-xGd screw implants using μCT

Biodegradable implants are taking an increasingly important role in the area of orthopedic implants with the aim to replace permanent implants for temporary bone healing applications.During the implant preparation process,the material’s surface and microstructure are being changed by stresses induced by machining.Hence degradable metal implants need to be fully characterized in terms of the influence of machining on the resulting microstructure and corrosion performance.In this study,micro-computed tomography(μCT)is used for the quantification of the degradation rate of biodegradable implants.To our best knowledge,for the first time quantitative measures are introduced to describe the degradation homogeneity in 3D.This information enables a prediction in terms of implant stability during the degradation in the body.Two magnesium gadolinium alloys,Mg-5Gd and Mg-10 Gd(all alloy compositions are given in weight%unless otherwise stated),in the shape of M2 headless screws have been investigated for their microstructure and their degradation performance up to 56 days.During the microstructure investigations particular attention was paid to the localized deformation of the alloys,due to the machining process.In vitro immersion testing was performed to assess the degradation performance quantified by subsequent weight loss and volume loss(usingμCT)measurements.Although differences were observed in the degree of screw’s near surface microstructure being influenced from machining,the degradation rates of both materials appeared to be suitable for application in orthopedic implants.From the degradation homogeneity point of view no obvious contrast was detected between both alloys.However,the higher degradation depth ratios between the crests and roots of Mg-5Gd ratios may indicated a less homogeneous degradation of the screws of these alloys on contract to the ones made of Mg-10Gd alloys.Due to its lower degradation rates,its more homogeneous microstructure,its weaker texture and better degradation performance extruded Mg-10Gd emerged more suitable as implant material than Mg-5Gd.

Pore characterization of PM Mg–0.6Ca alloy and its degradation behavior under physiological conditions

Several material parameters affect degradation characteristics of Mg and its alloys under physiological conditions.Porous Mg materials are interesting for their simultaneous degradation and drug delivery capabilities.However,an increase in pore surface area is detrimental to both degradation resistance and subsequent mechanical properties.The present work aims at determining the threshold porosity value in Mg–0.6 Ca specimens produced by powder metallurgy(PM)below which low degradation rates persist with acceptable mechanical properties.Seven different porous Mg–0.6 Ca specimens containing both closed and open pore structures were fabricated with porosities ranging from 3%to 21%.Degradation profiles were obtained via a semi static immersion test over 16 days under physiological conditions using Dulbecco’s modified Eagle’s medium with Glutamax and 10%fetal bovine serum as supplements.The results are related to morphological pore parameters like pore size distribution,pore interconnectivity and pore curvatures that were quantified using an ex situμCT analysis.In general,with decreasing porosity a decrease in pore interconnectivity is seen followed by rounding of the pores.Low degradation rates(MDR<0.3 mm/year)are observed in specimens until 10%porosity,however,the upper bound for reproducible degradation is observed to be in specimens until 12%porosity.This porosity level also marks the transition from closed to open pore nature with a simultaneous change in pore interconnectivity from less than 10%to greater than 95%,below and above this porosity level,respectively.The tensile strength and elongation to failure recorded for specimens with 10%porosity were 70 MPa and 2%,respectively displaying positive traits of both homogenous degradation and mechanical properties.The results suggest that high pore interconnectivity is the dominant factor controlling degradation and mechanical properties in porous Mg-0.6 Ca specimens.The results also indicate a good sintering response of Mg-0.6 Ca specimens providing further material development towards biomaterial applications.

On the material dependency of peri-implant morphology and stability in healing bone

The microstructural architecture of remodeled bone in the peri-implant region of screw implants plays a vital role in the distribution of strain energy and implant stability.We present a study in which screw implants made from titanium,polyetheretherketone and biodegradable magnesium-gadolinium alloys were implanted into rat tibia and subjected to a push-out test four,eight and twelve weeks after implantation.Screws were 4 mm in length and with an M2 thread.The loading experiment was accompanied by simultaneous three-dimensional imaging using synchrotron-radiation microcomputed tomography at 5μm resolution.Bone deformation and strains were tracked by applying optical flow-based digital volume correlation to the recorded image sequences.Implant stabilities measured for screws of biodegradable alloys were comparable to pins whereas non-degradable biomaterials experienced additional mechanical stabilization.Peri-implant bone morphology and strain transfer from the loaded implant site depended heavily on the biomaterial utilized.Titanium implants stimulated rapid callus formation displaying a consistent monomodal strain profile whereas the bone volume fraction in the vicinity of magnesium-gadolinium alloys exhibited a minimum close to the interface of the implant and less ordered strain transfer.Correlations in our data suggest that implant stability benefits from disparate bone morphological properties depending on the biomaterial utilized.This leaves the choice of biomaterial as situational depending on local tissue properties.

High-resolution ex vivo analysis of the degradation and osseointegration of Mg-xGd implant screws in 3D

Biodegradable magnesium(Mg)alloys can revolutionize osteosynthesis,because they have mechanical properties similar to those of the bone,and degrade over time,avoiding the need of removal surgery.However,they are not yet routinely applied because their degradation behavior is not fully understood.In this study we have investigated and quantified the degradation and osseointegration behavior of two biodegradable Mg alloys based on gadolinium(Gd)at high resolution.Mg-5Gd and Mg-10Gd screws were inserted in rat tibia for 4,8 and 12 weeks.Afterward,the degradation rate and degradation homogeneity,as well as bone-to-implant interface,were studied with synchrotron radiation micro computed tomography and histology.Titanium(Ti)and polyether ether ketone(PEEK)were used as controls material to evaluate osseointegration.Our results showed that Mg-5Gd degraded faster and less homogeneously than Mg-10Gd.Both alloys gradually form a stable degradation layer at the interface and were surrounded by new bone tissue.The results were correlated to in vitro data obtained from the same material and shape.The average bone-to-implant contact of the Mg-xGd implants was comparable to that of Ti and higher than for PEEK.The results suggest that both Mg-xGd alloys are suitable as materials for bone implants.