| BackgroundIn recent years, metallic biomaterials and absorbable polymeric materials arewidely used in clinical application of bone fracture fixation. However, a limitation ofmetallic biomaterials including stainless steel and titanium is that toxic metallic ionsand/or particles due to the corrosion or wear processes can lead to inflammatorycascades which reduce the biocompatibility. Moreover, the elastic moduli of thesebiomaterials are higher than that of natural bone tissue, resulting in stress shieldingeffects that can lead to reduced stimulation of new bone growth and remodeling.Thirdly, a second surgical intervention for the implant removal is needed, whichbrings extra burdens to the patients. Absorbable polymeric materials includingPolylactic acid (PLA) are also used in clinic, but they are unsatisfying because of thepoor mechanical strength, unpredictable degradation rates and causation of asepticinflammation. Autogenous and allogeneic bone are used for filling bone defects, whilethere is a limited supply of autogenous bone, and graft-versus-host disease limits thetherapeutic approach of allogeneic bone marrow transplantation. As a degradablebiomaterial, magnesium alloys have been revealed as promising candidates fororthopedic implants due to their advantages over the commonly used implants,including biodegradability, biocompatibility, and excellent mechanical properties. In aprevious study, an as-extruded ZK60showed good corrosion properties andbiocompatibility in vitro, and was considered as a potential material for biomedicalapplications. Micro-arc oxidation (MAO) treatment, a common technique forprotecting magnesium alloys against corrosion, showed an improvement in thecorrosion resistance and blood compatibility in vitro. However, in vivo research on the corrosion behavior of ZK60as well as the performance of MAO treatment is yetto be established. Nowadays, the in vivo studies of biodegradable magnesium alloyare mainly focusing on the degradation behavior and biocompatibility of magnesiumcortical bone screws. However, these results are incomparable due to the unifieddegradation environments. With this consideration, in the present study, we tried toestablish mouse femur cancellous bone defect model for in vivo research ofbiodegradable magnesium alloy, evaluate the in vivo degradability of MAO-treatedZK60and ZK60, and compared with PLLA, study the biocompatibility of the ZK60magnesium alloy and MAO treatment. There are three parts in this study:Part One: Establishment of mouse femur cancellous bone defect model for invivo investigation of biodegradable magnesium alloyObjectiveThe aim of this part was to set up mouse femur bone defect model for the studyof in vivo degradation behavior of magnesium alloy.MethodsThe anatomy parameters of mouse distal femur were analyzed usingmicro-computerized tomography (Micro-CT), and we set a cylinder region ofinterest (ROI) positioned within the cancellous bone area with long axis along thetransepicondylar line. The maximum length and diameter of the cylindrical ROIobtained were recorded and the size of bone defect was defined. The experimentalmaterials MAO-ZK60, ZK60and PLLA were prepared according to the defect sizeand were placed into the defect region.ResultsThe maximum length and diameter of the cylindrical ROI obtained were6.32±0.32mm and2.42±0.19mm, respectively. For the convenience of operation, weestablished the animal model with cylindrical cancellous bone defect sized Φ2×6mmwith long axis along the transepicondylar line. The Micro-CT scan of the laboratoryanimals2weeks after implantation showed that only one case was failure, the other implants were fixed well and were in correct position. No fracture occurred aroundthe implants was observed. The success rate was94.44%.ConclusionsThe mouse femur cancellous bone defect model was easy to establish, and wasstable during post-operative observation. This model is suitable for the in vivoinvestigation of biodegradable magnesium alloy.Part Two: Comparison of degradation behavior of MAO-ZK60and untreatedZK60in vivo.ObjectiveThe aim of this part was to investigate how micro-arc oxidation (MAO)treatment influences the in vivo corrosion behavior of ZK60.MethodsTwelve cylinders were machined from as-extruded ZK60, with six cylinderstreated with MAO and six left untreated. They were implanted into the defect regionof animal models as established in Part One. Micro-computerized tomography(micro-CT) was used to quantitatively analyze corrosion in a non-destructive mannerin vivo and the corrosion rate was calculated based on the volume measurements ofthe residual implants.ResultsThe residual implant volume, calculated according to direct3D measurements,showed that the untreated ZK60cylinders lost approximately72.3%and85.0%of itsinitial volume after12weeks and26weeks post-implantation, respectively. Bycomparison, the degradation volume of the MAO-treated ZK60cylinders wasapproximately60.8%and73.3%, respectively. The in vivo corrosion rates calculated from the volume reduction of the implant, indicated that the corrosion rates decreasedin all groups during implantation, and the MAO-treated ZK60cylinders had asignificantly slower corrosion rate than the untreated ZK60cylinders. By using theMAO treatment, the general corrosion rate of ZK60alloy after26weeks implantationwas reduced by13.8%.ConclusionsMAO treatment had a significant effect on protecting the implant from furthercorrosion.Part Three: The associated bone response and biocompatibility of MAO-ZK60alloyObjectiveThe aim of this part was to investigate whether MAO-ZK60alloy reacts in vivowith an appropriate host response.MethodsTwelve cylinders were machined from as-extruded ZK60, with six cylinderstreated with MAO and six left untreated; poly-L-lactic acid (PLLA) pins were used asa control to compare biocompatibility. They were implanted into the defect region ofanimal models as established in Part One. Micro-computerized tomography was usedto analyze surrounding bone response. Histological analyses of the bone tissuesaround the implants were used to assess bone response in relation to the implants. Thephysiological response of the rats post-implantation was obtained by clinicalobservation, blood biochemical analysis and histology analysis of heart, liver, spleen,lung and kidney. ResultsAll three implant materials were well-tolerated. All the animals showed minorswelling and mild wound reactions surrounding the incision, without the appearanceof subcutaneous gas cavities. By week2, cavities were observed surrounding both theMAO and untreated magnesium alloy implants by Micro-CT scans. By week26newbony sheaths were found circling the implant. In the control group, no cavity wasdetected and smooth outlines of hard callus was observed at the surface of PLLAimplant at week2,12,26. The histological analysis on the distal femur at26weekspostimplantation showed new bone sheath in place of cancellous bone surroundingthe two magnesium alloy implants. A fibrous encapsulation around the PLLAimplant was observed and reactive ossification had formed beneath the fibrous tissuelayer. The element mapping analysis showed a high density of Ca and P around theMAO-ZK60implant, indicating the new bone formation, while little concentration ofCa and P appeared within the bone tunnel of PLLA. The blood biochemicalexamination showed that some values of GPT and GOT were out of the recommendedrange, but no difference was observed among the time points, respectively. For serumK, all tested values were out of the recommended value, however, no difference wasobserved among the implantation points. All other values were within therecommended level. No abnormal or biocompatibility issues were found in the heart,kidney, lung, spleen or liver.ConclusionsCompared with PLLA, the ZK60alloy showed good osteoconductivity andosteoinductivity, and had good biocompatibility in vivo. |
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