Biodegradable Orthopedic Implants Research Articles

Preparation and characterization of strontium hydroxyapatite Sr10(PO4)6(OH)2·10H2O suitable for orthopedic applications

A hydrated form of strontium hydroxyapatite Sr10(PO4)6(OH)2·10H2O was prepared. The synthesis was carried out at room temperature using a stoichiometric mixture of di-ammoniumhydrogenphosphate and strontium hydroxide octahydrate. The product was characterized by XRD, FTIR, SEM and mass spectroscopy. The X-ray pattern is that of low crystalline strontium hydroxyapatite Sr10(PO4)6(OH)2. It is suggested that 10molH2O which cannot be removed without heating do not belong to the coordinated shell of the compound, but are retained in the structure channels. The presence of this loosely bonded water, together with its low crystalline structural organization make the compound suitable for the preparation of biodegradable orthopedic implants.

Effects of zirconium and strontium on the biocorrosion of Mg-Zr-Sr alloys for biodegradable implant applications.

The successful applications of magnesium (Mg) alloys as biodegradable orthopedic implants are mainly restricted due to their rapid degradation rate in the physiological environment, leading to a loss of mechanical integrity. This study systematically investigated the degradation behaviors of novel Mg-Zr-Sr alloys using electrochemical techniques, hydrogen evolution, and weight loss in simulated body fluid (SBF). The microstructure and degradation behaviors of the alloys were characterized using optical microscopy, XRD, SEM, and EDX. The results indicate that Zr and Sr concentrations in Mg alloys strongly affected the degradation rate of the alloys in SBF. A high concentration of 5 wt% Zr led to acceleration of anodic dissolution, which significantly decreased the biocorrosion resistance of the alloys and their biocompatibility. A high volume fraction of Mg17Sr2 phases due to the addition of excessive Sr (over 5 wt%) resulted in enhanced galvanic effects between the Mg matrix and Mg17Sr2 phases, which reduced the biocorrosion resistance. The average Sr release rate is approximately 0.15 mg L-1 day-1, which is much lower than the body burden and proves its good biocompatibility. A new biocorrosion model has been established to illustrate the degradation of alloys and the formation of degradation products on the surface of the alloys. It can be concluded that the optimal concentration of Zr and Sr is less than 2 wt% for as-cast Mg-Zr-Sr alloys used as biodegradable orthopedic implants.

Preparation and characterization of as-extruded Mg–Sn alloys for orthopedic applications

Abstract In this study, as-extruded Mg–Sn alloys with various Sn content were prepared and characterized for orthopedic applications. The results of microstructure observations and X-ray diffraction analysis showed that as-extruded Mg–Sn alloys were composed of α-Mg and Mg 2 Sn phases, and the content of Mg 2 Sn phase increased with increasing Sn content. The microstructure of as-extruded Mg–Sn alloy with 1 wt.% Sn was equiaxed grain, while the one with a higher Sn content was inhomogeneous microstructure and the grain size of the long elongated grains decreased with increasing Sn content. Tensile test revealed that the yield strength and ultimate tensile strength of as-extruded Mg–Sn alloys increased while the elongation decreased with increasing Sn content. Immersion and electrochemical tests indicated that the microstructure of as-extruded Mg–Sn alloys affected their corrosion properties, and the increase of Mg 2 Sn phase resulted from the increase of the Sn content led to a higher corrosion rate. The cytotoxicity test showed that as-extruded Mg–1Sn and Mg–3Sn alloys met the requirement of cell toxicity for orthopedic applications. Our analyses showed that as-extruded Mg–1Sn and Mg–3Sn alloys were promising to be used as biodegradable orthopedic implants.

The Study of Microstructure of the MgCa Alloys in their Use in Biodegradable Orthopedic Implants

The need for structural materials in temporary implant applications has grown in the recent years; materials that provide short – term structural support and which can be reabsorbed into the body after healing are being sought. These are materials that are biocompatible and biodegradable. These constitute a novel class of bioactive biomaterials which are expected to support the healing process of a diseased tissue and to degrade thereafter. Magnesium alloys attracted great attention as a new kind of degradable biomaterial. Mg shows great promise as a potential biocompat ible and biodegradable material in biomedical applications where it has gained the interest of researchers in the field. Biodegradable and bioabsorbable magnesium – based alloys provide a number of benefits over traditional permanent implants. There are however some disadvantages to the use of Mg alloys, one of the most critical being the release of hydrogen and alkalinization resulted from the corrosion of Mg. In connection to these drawbacks, a possible solution could be finding alloying elements which would contribute to the reduction of the corrosion rate in the human body. Studies show that a promising alloy for Mg, could be Calcium - a major component of the human bone and also an essential element in the chemical composition of cells. The present paper shall focus on the elaboration of Mg-Ca alloys, respectively Mg0,63Ca to Mg0,8 5 Ca, in the form of bars. These bars were obtained by cast in an inert atmosphere in the presence of argon, in order to be analyzed as biodegradable orthopedic implants. The structure of the alloy has been studied through SEM analyses, X-Ray diffraction, and EDAX to determine the chemical composition, as well as the distribution of elements in the structure. The main desiderate is finding an alloy which would have a minimum healing period postsurgery, pathophysiology and toxicology and a promising degradation behavior.

The effects of pulse electrodeposition parameters on morphology and formation of dual-layer Si-doped calcium phosphate coating on AZ31 alloy

Controlling the corrosion rate of magnesium alloys in vivo to match the bone repair is crucially important for application as biodegradable orthopedic implants. To solve this challenge, a Si-doped Ca–P coating with a novel dual-layer structure was deposited on AZ31 alloy substrate by pulse electrodeposition. The morphology, composition and formation of dual-layer coating were believed to be determined by pulse electrodeposition parameters, thus the deposition time, temperature, duty cycle, pH value of the electrolyte and the standing duration of mixed electrolyte were regulated to seek possible influences and mechanism on the coating formation. Weight gain and pH monitoring tests were also used to evaluate the performance of coating under the varied parameters. The results indicated that deposition temperature and time remarkably influenced the morphology, homogeneity and crystallinity of Ca–P coating. A uniform and compact coating with better degradation performance was obtained at 60°C with deposition time of 40min. The shorter standing duration of the electrolyte lead to an incomplete coating and uneven distribution of Si contents. The two coupled deposition models contributed to this dual-layer structure of Si-doped Ca–P coating and possible formation mechanism was proposed.

In vivo study of nanostructured diopside (CaMgSi2O6) coating on magnesium alloy as biodegradable orthopedic implants

Abstract In order to improve the corrosion resistance and bioactivity of a biodegradable magnesium alloy, we have recently prepared a nanostructured diopside (CaMgSi2O6) coating on AZ91 magnesium alloy through a combined micro-arc oxidation (MAO) and electrophoretic deposition (EPD) method (reported elsewhere). In this work, we performed a detailed biocompatibility analysis of the implants made by this material and compared their performance with those of the uncoated and micro arc oxidized magnesium implants. The biocompatibility evaluation of samples was performed by culturing L-929 cells and in vivo animal study, including implantation of samples in greater trochanter of rabbits, radiography and histological examinations. The results from both the in vitro and in vivo studies indicated that the diopside/MAO coated magnesium implant significantly enhanced cell viability, biodegradation resistance and new bone formation compared with both the uncoated and the micro-arc oxidized magnesium implants. Our data provides an example of how the proper surface treatment of magnesium implants can overcome their drawbacks in terms of high degradation rate and gas bubble formation under physiological conditions.

Surface microstructure and in vitro analysis of nanostructured akermanite (Ca2MgSi2O7) coating on biodegradable magnesium alloy for biomedical applications

Magnesium (Mg) alloys, owing to their biodegradability and good mechanical properties, have potential applications as biodegradable orthopedic implants. However, several poor properties including low corrosion resistance, mechanical stability and cytocompatibility have prevented their clinical application, as these properties may result in the sudden failure of the implants during the bone healing. In this research, nanostructured akermanite (Ca2MgSi2O7) powder was coated on the AZ91 Mg alloy through electrophoretic deposition (EPD) assisted micro arc oxidation (MAO) method to modify the properties of the alloy. The surface microstructure of coating, corrosion resistance, mechanical stability and cytocompatibility of the samples were characterized with different techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electrochemical corrosion test, immersion test, compression test and cell culture test. The results showed that the nanostructured akermanite coating can improve the corrosion resistance, mechanical stability and cytocompatibility of the biodegradable Mg alloy making it a promising material to be used as biodegradable bone implants for orthopedic applications.

Synthesis and evaluation of MgF2 coatings by chemical conversion on magnesium alloys for producing biodegradable orthopedic implants of temporary use

The aim of the present work was the synthesis of biodegradable MgF2 coatings by chemical conversion on the commercial Elektron 21 and AZ91D magnesium alloys, in aqueous HF solutions for different concentrations and temperatures. The chemical composition and morphology of the coatings were analyzed by scanning electron microscopy (SEM-EDX) and X-ray diffraction (XRD). On the other hand, their corrosion behavior was evaluated by gravimetric and electrochemical measurements in Hank's solution at 37°C for different immersion times. The experimental results revealed that chemical conversion in HF produced MgF2 coatings which corrosion resistance was enhanced by increasing the HF concentration. Further, the microstructure and composition of the base alloy played a key role on the growth and degradation mechanisms of the MgF2 coatings.

Zirconium, calcium, and strontium contents in magnesium based biodegradable alloys modulate the efficiency of implant-induced osseointegration

Development of new biodegradable implants and devices is necessary to meet the increasing needs of regenerative orthopedic procedures. An important consideration while formulating new implant materials is that they should physicochemically and biologically mimic bone-like properties. In earlier studies, we have developed and characterized magnesium based biodegradable alloys, in particular magnesium-zirconium (Mg-Zr) alloys. Here we have reported the biological properties of four Mg-Zr alloys containing different quantities of strontium or calcium. The alloys were implanted in small cavities made in femur bones of New Zealand White rabbits, and the quantitative and qualitative assessments of newly induced bone tissue were carried out. A total of 30 experimental animals, three for each implant type, were studied, and bone induction was assessed by histological, immunohistochemical and radiological methods; cavities in the femurs with no implants and observed for the same period of time were kept as controls. Our results showed that Mg-Zr alloys containing appropriate quantities of strontium were more efficient in inducing good quality mineralized bone than other alloys. Our results have been discussed in the context of physicochemical and biological properties of the alloys, and they could be very useful in determining the nature of future generations of biodegradable orthopedic implants.

Biodegradable Orthopedic Magnesium-Calcium (MgCa) Alloys, Processing, and Corrosion Performance.

Magnesium-Calcium (Mg-Ca) alloy has received considerable attention as an emerging biodegradable implant material in orthopedic fixation applications. The biodegradable Mg-Ca alloys avoid stress shielding and secondary surgery inherent with permanent metallic implant materials. They also provide sufficient mechanical strength in load carrying applications as opposed to biopolymers. However, the key issue facing a biodegradable Mg-Ca implant is the fast corrosion in the human body environment. The ability to adjust degradation rate of Mg-Ca alloys is critical for the successful development of biodegradable orthopedic implants. This paper focuses on the functions and requirements of bone implants and critical issues of current implant biomaterials. Microstructures and mechanical properties of Mg-Ca alloys, and the unique properties of novel magnesium-calcium implant materials have been reviewed. Various manufacturing techniques to process Mg-Ca based alloys have been analyzed regarding their impacts on implant performance. Corrosion performance of Mg-Ca alloys processed by different manufacturing techniques was compared. In addition, the societal and economical impacts of developing biodegradable orthopedic implants have been emphasized.

Effect of Casting Method of AZ91 D Magnesium Alloy on the Degradation Behaviour in Simulated Body Fluid

Abstract Magnesium-based alloys as biodegradable orthopedic implants have been successful for their high degradation rates in the physiological environment and the consequent loss in the mechanical integrity. Casting method used in production of magnesium-based alloys influences the microstructure of implants due to cooling conditions. In this study, the effects of casting method (permanent mold casting and sand mold casting) on the degradation of AZ91D magnesium alloy in Hank's simulated body fluid at 36.5 ± 0.5°C have been investigated using immersion technique of cylindrical specimens. The specimens were subjected to simulated body fluid for 8 h and 24 h. The results have shown that the cooling rate from melt and grain size have great influence on the degradation rate of magnesium alloy.

The past, present and near future of materials for use in biodegradable orthopaedic implants

The aim of bone replacement or fracture treatment methodologies is to induce tissue regeneration respecting anatomy and try to recover functionality. This goal was initially achieved in the 17th century by using animal or human grafts and several medical devices made of natural and synthetic materials are currently used having a whole range of chemical and physical properties. Research in this field continues to seek a solution to the disadvantages usually found when using grafts: immunological reactions, the risk of microbiological contamination, the absence of donors, the need for several surgical interventions and the risk of disease transmission. Basic and applied research must thus be carried out not only in the development of biology and studies about embryonic stem cells but also in the field of new material development. This tendency may be clearly detected by looking at the vast numbers of studies related to using metallic, polymer and ceramic materials and, at present, compound or hybrid materials having potential use in orthopaedic implants. Most of them fulfil conditions regarding biocompatibility and non-toxicity and could be considered when designing biodegradable materials thereby making it feasible to identify a range of research subjects on biomaterials. This paper starts by identifying material development periods and then establishes the advantages and disadvantages of groups which have been considered for bone regeneration and identifies some guidelines which should be taken into account in the field of biodegradable materials in the near future. There is still a long way to go in this subject, especially regarding the field of materials science and technology

Short-Term and Long-Term Effects of Orthopedic Biodegradable Implants

Presently, orthopedic and oral/maxillofacial implants represent a combined $2.8 billion market, a figure expected to experience significant and continued growth. Although traditional permanent implants have been proved clinically efficacious, they are also associated with several drawbacks, including secondary revision and removal surgeries. Non-permanent, biodegradable implants offer a promising alternative for patients, as they provide temporary support and degrade at a rate matching tissue formation, and thus, eliminate the need for secondary surgeries. These implants have been in clinical use for nearly 25 years, competing directly with, or maybe even exceeding, the performance of permanent implants. The initial implantation of biodegradable materials, as with permanent materials, mounts an acute host inflammatory response. Over time, the implant degradation profile and possible degradation product toxicity mediate long-term biodegradable implant-induced inflammation. However, unlike permanent implants, this inflammation is likely to cease once the material disappears. Implant-mediated inflammation is a critical determinant for implant success. Thus, for the development of a proactive biodegradable implant that has the ability to promote optimal bone regeneration and minimal detrimental inflammation, a thorough understanding of short- and long-term inflammatory events is required. Here, we discuss an array of biodegradable orthopedic implants, their associated short- and long- term inflammatory effects, and methods to mediate these inflammatory events.

EDM Machining Capabilities of Magnesium (Mg) Alloy WE43 for Medical Applications

Besides high geometrical accuracy medical devices strongly require a good surface integrity of the produced parts. Surfaces should be optimized to reduce friction or to increase cell adhesion for optimal fitting, mechanical stability and biocompatibility to reduce the risk of inflammatory reactions. Therefore surfaces should posses tailored roughness, a closed structure without pores and cracks and should not contain any toxic substances as a result of the machining process. New biomaterials like magnesium alloys for biodegradable orthopaedic implants are very difficult to machine with conventional processes especially for complex and filigree 3D-structures. Therefore, alternative manufacturing technologies are desired and need to be developed. This paper analyzes the capabilities of state-of-the-art EDM process technologies regarding achievable surface integrity.

Influence of circumferential notch and fatigue crack on the mechanical integrity of biodegradable magnesium‐based alloy in simulated body fluid

Applications of magnesium alloys as biodegradable orthopaedic implants are critically dependent on the mechanical integrity of the implant during service. In this study, the mechanical integrity of an AZ91 magnesium alloy was studied using a constant extension rate tensile (CERT) method. The samples in two different geometries that is, circumferentially notched (CN), and circumferentially notched and fatigue cracked (CNFC), were tested in air and in simulated body fluid (SBF). The test results show that the mechanical integrity of the AZ91 magnesium alloy decreased substantially (∼50%) in both the CN and CNFC samples exposed to SBF. Fracture surface analysis revealed secondary cracks suggesting stress corrosion cracking susceptibility of the alloy in SBF.

In vitro degradation behavior and cytocompatibility of Mg-Zn-Zr alloys.

Zinc and zirconium were selected as the alloying elements in biodegradable magnesium alloys, considering their strengthening effect and good biocompatibility. The degradation rate, hydrogen evolution, ion release, surface layer and in vitro cytotoxicity of two Mg–Zn–Zr alloys, i.e. ZK30 and ZK60, and a WE-type alloy (Mg–Y–RE–Zr) were investigated by means of long-term static immersion testing in Hank’s solution, non-static immersion testing in Hank’s solution and cell-material interaction analysis. It was found that, among these three magnesium alloys, ZK30 had the lowest degradation rate and the least hydrogen evolution. A magnesium calcium phosphate layer was formed on the surface of ZK30 sample during non-static immersion and its degradation caused minute changes in the ion concentrations and pH value of Hank’s solution. In addition, the ZK30 alloy showed insignificant cytotoxicity against bone marrow stromal cells as compared with biocompatible hydroxyapatite (HA) and the WE-type alloy. After prolonged incubation for 7 days, a stimulatory effect on cell proliferation was observed. The results of the present study suggested that ZK30 could be a promising material for biodegradable orthopedic implants and worth further investigation to evaluate its in vitro and in vivo degradation behavior.

Bio-corrosion characterization of Mg–Zn–X (X = Ca, Mn, Si) alloys for biomedical applications

The successful applications of magnesium-based alloys as biodegradable orthopedic implants are mainly inhibited due to their high degradation rates in physiological environment. This study examines the bio-corrosion behaviour of Mg-2Zn-0.2X (X = Ca, Mn, Si) alloys in Ringer's physiological solution that simulates bodily fluids, and compares it with that of AZ91 magnesium alloy. Potentiodynamic polarization and electrochemical impedance spectroscopy results showed a better corrosion behaviour of AZ91 alloy with respect to Mg-2Zn-0.2Ca and Mg-2Zn-0.2Si alloys. On the contrary, enhanced corrosion resistance was observed for Mg-2Zn-0.2Mn alloy compared to the AZ91 one: Mg-2Zn-0.2Mn alloy exhibited a four-fold increase in the polarization resistance than AZ91 alloy after 168 h exposure to the Ringer's physiological solution. The improved corrosion behaviour of the Mg-2Zn-0.2Mn alloy with respect to the AZ91 one can be ascribed to enhanced protective properties of the Mg(OH)(2) surface layer. The present study suggests the Mg-2Zn-0.2Mn alloy as a promising candidate for its applications in degradable orthopedic implants, and is worthwhile to further investigate the in vivo corrosion behaviour as well as assessed the mechanical properties of this alloy.

In vitro degradation and mechanical integrity of Mg–Zn–Ca alloy coated with Ca-deficient hydroxyapatite by the pulse electrodeposition process

The key to manufacturing magnesium-based alloys that are suitable as biodegradable orthopaedic implants is how to adjust their degradation rates and mechanical integrity in the physiological environment. In this study, to solve this challenge, a soluble Ca-deficient hydroxyapatite (Ca-def HA) coating was deposited on an Mg–Zn–Ca alloy substrate by pulse eletrodeposition. This deposition can be demonstrated by X-ray diffractometry and energy dispersion spectroscopy analyses, and the Ca/P atomic ratio of as-deposited coating is about 1.33 (within the range from 1.33 to 1.65). By regulating the appropriate pulse amplitude and width, the Ca-def HA coating shows better adhesion to Mg–Zn–Ca alloy, whose lap shear strength is increased to 41.8 ± 2.7 MPa. Potentiodynamic polarization results in Kokubo’s simulated body fluid (SBF) indicate that the corrosion potential of Mg alloy increases from −1645 to −1414 mV, while the corrosion current density decreases from 110 to 25 μA/cm 2, which illustrates that the Ca-def HA coating improves the substrate corrosion resistance significantly. Since orthopaedic implants generally serve under conditions of stress corrosion, the mechanical integrity of the Mg–Zn–Ca alloy was measured using the slow strain rate tensile (SSRT) testing technique in SBF. The SSRT results show that the ultimate tensile strength and time of fracture for the coated Mg–Zn–Ca alloy are higher than those of the uncoated one, which is beneficial in supporting fractured bone for a longer time. Thus Mg–Zn–Ca alloy coated with Ca-def HA is be a promising candidate for biodegradable orthopaedic implants, and is worthwhile to further investigate the in vivo degradation behavior.

Effect of laser surface melting on corrosion behaviour of AZ91D Mg alloy in simulated-modified body fluid

High corrosion rate in physiological environment of the body is the major drawback of magnesium alloys for their successful applications as biodegradable orthopaedic implants. In the present study, corrosion behaviour of AZ91D magnesium alloy after laser surface melting (LSM) was studied in modified-simulated body fluid at 37 °C. The improved corrosion resistance of AZ91D alloy using LSM was found to depend on the solidification microstructure in the laser-melted zone. The general and pitting corrosion resistance of laser-treated surface was significantly enhanced due to the refined continuous network of β-Mg17Al12 phases and the increased Al concentration in the laser-melted zone.

Evaluating the stress corrosion cracking susceptibility of Mg–Al–Zn alloy in modified-simulated body fluid for orthopaedic implant application

Applications of magnesium alloys as biodegradable orthopaedic implants are critically dependent on the mechanical integrity of the implant during service. In this study, the stress corrosion cracking susceptibility of sand-cast Mg–Al–Zn alloy in modified-simulated body fluid was evaluated using the slow strain rate test method. The study suggests that the stress corrosion cracking susceptibility of the sand-cast magnesium alloy is not substantial and this aspect should not be a concern for its implant applications.