Experimental Studies On In Vivo Implantation Of Biodegradable AZ31B Magnesium Alloy In Rabbit Mandibular

At present,the stainless steel,titanium and absorbable polymeric materials are widely used in clinical application of maxillofacial fracture,orthognathic surgery and bone graft surgery.A limitation of these current materials is the possible release of toxic metallic ions and/or particles through corrosion or wear processes that lead to inflammatory cascades which reduce biocompatibility and cause tissue loss.Moreover, the elastic moduli of the stainless steel and titanium are not well matched with that of natural bone tissue,resulting in stress shielding effects.Current metallic materials are essentially neutral in vivo,remaining as permanent fixtures,which in the case of plates, screws and pins used to secure serious fractures,must be removed by a second surgical procedure after the tissue has healed sufficiently.Polylactic acid(PLA),polyglycolic acid(PGA) and other absorbable polymer materials have also been used in clinic,but there are still inadequate:poor hydrophilicity;cells weak absorption;causation of aseptic inflammation;poor mechanical strength.As a degradable biomaterial for osteosynthesis,magnesium alloys provide potential advantages for patients with bone fractures or defects due to a good biocompatibility and a high primary stability.The elastic modulus and compressive yield strength of magnesium are closer to those of natural bone than is the case for other commonly used metallic implants.There are lots of experimental methods in the study of corrosion resistance of magnesium alloy and biocompatibility,which can be broadly divided into in vitro and in vivo studies.The corrosion of the magnesium alloy can be observed from time to time directly in vitro studies.As a means of analysis of the bio-safety and biocompatibility of the magnesium alloy,the biological response can be observed at the degradation process of magnesium alloy with the in vivo studies.In order to evaluate the feasibility of the clinical application of magnesium alloy in maxillofacial bone injury reparation,AZ31B magnesium alloy samples with two dimensions(stabilization splint and dictyo-plate) were implanted on the submaxilla surface of New-Zealand rabbits,with comparison of Ti-6A1-4V titanium alloy as a control.Biochemical analysis of the experimental animals blood and urine,tissue and implant macro-observation,scanning electron microscope,energy spectrum analysis,immunohistochemistry,RT-PCR and other experimental means were used to investigate the biodegradation mechanism of AZ31B magnesium alloy and the response of the submaxilla.There are three parts in this study:To study the biological response of the experimental animals mandibular and body to the degradation process of magnesium alloy and the degradation characteristics of the magnesium alloy at the location.AZ31B magnesium alloy samples with two dimensions(group A and B) were implanted on the submaxilla surface of New-Zealand rabbits,with comparison of Ti-6A1-4V titanium alloy as a control(group C).The animals in each of the groups are divided into two groups equally,the experimental operation time points are two weeks and eight weeks.The animals in group Al were sacrificed after two weeks,the soft tissue around the implants,heart,liver,kidney and spleen were made into sections for histomorphology analysis.The sample of the blood and urine were collected on each week from the implantation to the eight weeks.The concentration of magnesium ions of every sample was biochemical detected.After eight weeks,the bone tissue around the implants,heart,liver,kidney and spleen were made into sections for histomorphology analysis.The methods of group B and C are the same to group A. After douching with alcohol to remove excess blood and animal tissue,confined preservation,the magnesium alloy implants in group A and B were observed by SEM and analyzed by EDS. After two weeks of implantation,magnesium alloy implant fixation system was wrapped with a layer of fibrous connective tissue.Micrangium and bone trabecula were found in the fiber composition by optical microscope.Calcium salt deposition, mononuclear cells,eosinophile granulocyte and a small amount of osteoblasts distribution were also observed.There is a layer of fibrous connective tissue around the titanium implants.A large number of fibroblasts can be observed without obvious bone trabecula,new capillaries or calcium salt deposition.After eight weeks of implantation, newly formed hard tissue was observed both on the edge and in the hole of magnesium alloy implants,with the combination of magnesium alloy plate closely.The new bone tissue had differentiated maturity,with the surface of the bone plate and cancellous bone inside.There was still connective tissue fibers around the titanium implants.No obvious abnormalities were observed in histopathological sections of the internal organs of animals in group A and B.The concentration of magnesium an blood on different time points in group A is undulate in a small range(0.79-1.05mmol/L). Compared with the group C,the concentration of magnesium ions in urine was increased and fluctuated intensity after the implantation of magnesium alloy.After two and eight weeks of implantation,the magnesium alloy implants in group A and B has lost metallic luster and clear edge.White loose material can be observed on the surface after exsiccation.The product of the degradation on the surface of magnesium alloy implants is rough and curmbly,and shows irregular-shaped cracks.The main component of the material layer is Ca,Mg,P,Al,Zn,Mn and so on.AZ31B magnesium alloy implant can be beneficial to the new bone formation. The implantation of the magnesium alloy did not show negative influence on the recirculating,immune and urinary systems of the rabbits.Magnesium ions concentration in the blood was in the range of normal values. To investigate the biodegradation mechanism of AZ31B magnesium alloy and the response of the surrounding bone as it was used in repairing different types of bone injury in rabbit mandibular.AZ31B alloy samples with two dimensions(stabilization splint and dictyo-plate) were implanted on the submaxilla surface of New-Zealand rabbits,with comparison of Ti-6A1-4V titanium alloy as a control.Two kinds of bone coloboma were made on the submaxilla surface in order to model the different areas of bone defect.After three and six months,newly formed bone tissues around the implants were made into sections for histomorphology analysis.The BMP-2 was detected by SABC immunohistochemistry. SEM and EDS analysis were used to investigate the layer of degradation product.After the implantation,all the implants were covered by a fibrous capsule. Hydrogen bubbles appeared frequently in the fibrous capsule of dictyo-plate implants. On the country,hydrogen bubble was not found around stabilization splint implants at all.After three months,over 50%of the stabilization splint body was embedded by the newly formed bone.After six months,only one head of a screw could be seen.The small bone defect(10mmx2mm) under the stabilization splint implant healed very well. While the big bone defect(15mmxl5mm) model showed another scene after 24 weeks, both newly formed bone and osteolysis were found under the dictyo-plate implants. Histomorphology analysis and immunohistochemistry showed higher expression of BMP-2 in newly formed bone,which displayed the osteoblast activity.The degradation product was the magnesium calcium phosphates.Magnesium alloy implants with small outsize in repairing of small bone defects demonstrated good bone induction,while the bigger implant surface area contacted with body fluid produced more magnesium ion that will lead local osteolytic reaction. Magnesium ions from biodegradation of magnesium alloy and the alkaline environment cause bone morphogenetic protein(BMP-2) increase,thus causing a partial new bone formation.Excessive amount of magnesium ions concentration and the too high pH value will cause excessive levels of BMP-2 secretion,which will activate osteoclasts, resulting in osteolytic situation.To investigate the effects of AZ31B magnesium alloy with B-TCP coating on rabbit mandibular ossify.To explore the biodegradation mechanism of AZ31B magnesium alloy with 8-TCP coating and the response of the surrounding bone.AZ31B magnesium alloy implants with and without B-TCP coating(group A and B ) were implanted on the submaxilla surface of New-Zealand rabbits,with comparison of Ti-6A1-4V titanium alloy as a control(group C ).Four weeks after the implantation, the soft tissue and the bone around the implants were made into sections for histomorphology analysis.After douching with alcohol to remove excess blood and animal tissue,confined preservation,the magnesium alloy implants in group A and B were observed by SEM and analyzed by EDS.After four weeks of implantation,magnesium alloy implants with B-TCP coating (group A) were wrapped with a layer of fibrous connective tissue.There were not much newly formed bone around the implants.Only a small amount of new bone tissue and island of new bone with single-layer distribution of osteoblasts can be observed in sections.Few new bone tissue was found around titanium implants.In group C,newly formed hard tissue was observed around the magnesium alloy without B-TCP coating. The new bone tissue had differentiated maturity,and the new bone islands connected to each other.After four weeks of implantation,the product of the degradation on the surface of magnesium alloy with B-TCP coating implants shows irregular-shaped cracks.Bad connectivity between cracks and no obvious layered collapse were observed.More obvious layered collapse and irregular-shaped cracks were found on the surface of the magnesium alloy without B-TCP coating.The main component of the material layer is Ca,Mg,P,A1,Zn,C,0 and so on.Some residual B-TCP composition is still on the surface of magnesium alloy after four weeks of degradation in vivo (group A).B-TCP coating,as a biodegradable ceramic materials,can be coated on the surface of AZ31B magnesium alloy by chemical methods.In the early stage of in vivo implantation,B-TCP can be used as a barrier structure to control the biodegradation rate of AZ31B magnesium alloy.