Magnesium and its alloys as orthopedic biomaterials: A review

Effect of surface pre-treatments on biocompatibility of magnesium

This study reports the influence of Mg surface passivation on the survival rate of human HeLa cells and mouse fibroblasts in cell culture experiments. Polished samples of commercially pure Mg show high reactivity in the cell culture medium, leading to a pH shift in the alkaline direction, and therefore cell adhesion and survival is strongly impaired. Passivation of the Mg surface in 1M NaOH can strongly enhance cell survival. The best initial cell adhesion is observed for Mg samples incubated in simulated body fluid (M-SBF), which leads to the formation of a biomimetic, amorphous Ca/Mg-phosphate layer with high surface roughness. This surface layer, however, passivates and seals the Mg surface only partially. Subsequent Mg dissolution leads to a significantly stronger pH increase compared to NaOH-passivated samples, which prevents long-term cell survival. These results demonstrate that surface passivation with NaOH and M-SBF together with the associated changes of surface reactivity, chemistry and roughness provide a viable strategy to facilitate cell survival on otherwise non-biocompatible Mg surfaces.

A study on alkaline heat treated Mg-Ca alloy for the control of the biocorrosion rate

To reduce the biocorrosion rate by surface modification, Mg-Ca alloy (1.4wt.% Ca content) was soaked in three alkaline solutions (Na(2)HPO(4), Na(2)CO(3) and NaHCO(3)) for 24h, respectively, and subsequently heat treated at 773K for 12h. Scanning electron microscopy and energy-dispersive spectroscopy results revealed that magnesium oxide layers with the thickness of about 13, 9 and 26microm were formed on the surfaces of Mg-Ca alloy after the above different alkaline heat treatments. Atomic force microscopy showed that the surfaces of Mg-Ca alloy samples became rough after three alkaline heat treatments. The in vitro corrosion tests in simulated body fluid indicated that the corrosion rates of Mg-Ca alloy were effectively decreased after alkaline heat treatments, with the following sequence: NaHCO(3) heated<Na(2)HPO(4) heated<Na(2)CO(3) heated. The cytotoxicity evaluation revealed that none of the alkaline heat treated Mg-Ca alloy samples induced toxicity to L-929 cells during 7days culture.

Magnetic resonance imaging evaluation of intervertebral test spacers: an experimental comparison of magnesium versus titanium and carbon fiber reinforced polymers as biomaterials

INTRODUCTION

Intervertebral spacers are made of different materials, which can affect the postfusion magnetic resonance imaging (MRI) scans. Susceptibility artifacts, especially for metallic implants, can decrease the image quality. This study aimed to determine whether magnesium as a lightweight and biocompatible metal is suitable as a biomaterial for spinal implants based on its MRI artifacting behavior.

MATERIALS AND METHODS

To compare artifacting behaviors, we implanted into one porcine cadaveric spine different test spacers made of magnesium, titanium, and CFRP. All test spacers were scanned using two T1-TSE MRI sequences. The artifact dimensions were traced on all scans and statistically analyzed.

RESULTS

The total artifact volume and median artifact area of the titanium spacers were statistically significantly larger than magnesium spacers (P < 0.001), while magnesium and CFRP spacers produced almost identical artifacting behaviors (P > 0.05).

CONCLUSION

Our results suggest that spinal implants made with magnesium alloys will behave more like CFRP devices in MRI scans.

Metallic Scaffolds for Bone Regeneration

Bone tissue engineering is an emerging interdisciplinary field in Science, combining expertise in medicine, material science and biomechanics. Hard tissue engineering research is focused mainly in two areas, osteo and dental clinical applications. There is a lot of exciting research being performed worldwide in developing novel scaffolds for tissue engineering. Although, nowadays the majority of the research effort is in the development of scaffolds for non-load bearing applications, primarily using soft natural or synthetic polymers or natural scaffolds for soft tissue engineering; metallic scaffolds aimed for hard tissue engineering have been also the subject of in vitro and in vivo research and industrial development. In this article, descriptions of the different manufacturing technologies available to fabricate metallic scaffolds and a compilation of the reported biocompatibility of the currently developed metallic scaffolds have been performed. Finally, we highlight the positive aspects and the remaining problems that will drive future research in metallic constructs aimed for the reconstruction and repair of bone.

Artifacts in spine magnetic resonance imaging due to different intervertebral test spacers: an in vitro evaluation of magnesium versus titanium and carbon-fiber-reinforced polymers as biomaterials

Introduction

Intervertebral spacers are made of different materials, which can affect the postfusion magnetic imaging (MRI) scans. Susceptibility artifacts especially for metallic implants can decrease the image quality. This study aimed to determine whether magnesium as a lightweight and biocompatible metal is suitable as a biomaterial for spinal implants based on its MRI artifacting behavior.

Materials and methods

To compare artifacting behaviors, we implanted into one porcine cadaveric spine different test spacers made of magnesium, titanium, and carbon-fiber-reinforced polymers (CFRP). All test spacers were scanned using two T1-TSE MRI sequences. The artifact dimensions were traced on all scans and statistically analyzed.

Results

The total artifact volume and median artifact area of the titanium spacers were statistically significantly larger than magnesium spacers (p < 0.001), while magnesium and CFRP spacers produced almost identical artifacting behaviors (p > 0.05).

Conclusion

Our results suggest that spinal implants made with magnesium alloys will behave more like CFRP devices in MRI scans. Given its osseoconductive potential as a metal, implant alloys made with magnesium would combine the advantages to the two principal spacer materials currently used but without their limitations, at least in terms of MRI artifacting.

Phosphating treatment and corrosion properties of Mg-Mn-Zn alloy for biomedical application

A phosphating treatment was applied to Mg-Mn-Zn alloy in order to improve the corrosion resistance. Surface morphology and phase constitute were observed and identified by SEM, EDS, SAXS, XRD and XPS. SEM observation showed that a rough and crystalline reaction layer was formed on the surface of Mg alloy. With the increasing of phosphating time, the layer became thicker and denser. It has been showed that the reaction layer was mainly composed of brushite (CaHPO4 x 2H2O). Small amount of Zn2+ was also detected by XPS and EDS. The corrosion resistance of the phosphated samples was measured by the electrochemical polarization and the immersion test in comparison with the bare alloy. The results manifested that the corrosion resistance of Mg alloy was improved by the phosphating treatment, and the corrosion resistance increased with the increase of the phosphating time within 50 min. Immersion tests showed that the phosphate layer could protect magnesium alloy from fast corrosion. The brushite layer has been transformed into hydroxyapatite (HA) during the immersion in the simulated body fluid (SBF) solution, which suggested the brushite layer could provide good biocompatibility.

Enrichment of anodic MgO layers with Ag nanoparticles for biomedical applications

The growing fight against infections caused by bacteria poses new challenges for development of materials and medical devices with antimicrobial properties. Silver is a well known antimicrobial agent and has recently started to be used in nanoparticulate form, with the advantage of a high specific surface area and a continuous release of enough concentration of silver ions/radicals. The synthesis of MgO-Ag nanocomposite coatings by in situ deposition of silver nanoparticles during plasma electrolytic oxidation of a magnesium substrate is presented in this study. The process was performed in an electrolyte containing Ag nanoparticles under different oxidation conditions (i.e., current density, oxidizing time, silver nanoparticles concentration in the electrolyte). Surface morphology, phase composition and elemental composition (on the surface and across the thickness of MgO-Ag nanocomposite coatings) were assessed by scanning electron microscopy, X-ray diffraction, energy X-ray dispersive spectrometry and radio frequency glow discharge optical emission spectroscopy, respectively. The coatings were found to be porous, around 7 mum thick, consisting of a crystalline oxide matrix embedded with silver nanoparticles. The findings suggest that plasma electrolytic oxidation process has potential for the synthesis of MgO-Ag nanocomposite coatings.

In vitro and in vivo evaluation of the surface bioactivity of a calcium phosphate coated magnesium alloy

Magnesium has shown potential application as a bio-absorbable biomaterial, such as for bone screws and plates. In order to improve the surface bioactivity, a calcium phosphate was coated on a magnesium alloy by a phosphating process (Ca-P coating). The surface characterization showed that a porous and netlike CaHPO(4).2H(2)O layer with small amounts of Mg(2+) and Zn(2+) was formed on the surface of the Mg alloy. Cells L929 showed significantly good adherence and significantly high growth rate and proliferation characteristics on the Ca-P coated magnesium alloy (p<0.05) in in-vitro cell experiments, demonstrating that the surface cytocompatibility of magnesium was significantly improved by the Ca-P coating. In vivo implantations of the Ca-P coated and the naked alloy rods were carried out to investigate the bone response at the early stage. Both routine pathological examination and immunohistochemical analysis demonstrated that the Ca-P coating provided magnesium with a significantly good surface bioactivity (p<0.05) and promoted early bone growth at the implant/bone interface. It was suggested that the Ca-P coating might be an effective method to improve the surface bioactivity of magnesium alloy.

Influence of aggressive ions on the degradation behavior of biomedical magnesium alloy in physiological environment

Various electrochemical approaches, including potentiodynamic polarization, open circuit potential evolution and electrochemical impedance spectroscopy (EIS), are employed to investigate the degradation behavior of biomedical magnesium alloy under the influence of aggressive ions, such as chloride, phosphate, carbonate and sulfate, in a physiological environment. The synergetic effects and mutual influence of these ions on the degradation behavior of Mg are revealed. Our results demonstrate that chloride ions can induce porous pitting corrosion. In the presence of phosphates, the corrosion rate decreases and the formation of pitting corrosion is significantly delayed due to precipitation of magnesium phosphate. Hydrogen carbonate ions are observed to stimulate the corrosion of magnesium alloy during the early immersion stage but they can also induce rapid passivation on the surface. This surface passivation behavior mainly results from the fast precipitation of magnesium carbonate in the corrosion product layer that can subsequently inhibit pitting corrosion completely. Sulfate ions are also found to stimulate magnesium dissolution. These results improve our understanding on the degradation mechanism of surgical magnesium in the physiological environment.

Design of a pseudo-physiological test bench specific to the development of biodegradable metallic biomaterials

Endovascular stents have proven effective in treating coronary and peripheral arterial occlusions. Since the first attempts, metals used to make these devices have been generally selected, and designed to be highly resistant to corrosion. Therefore, as almost the totality of metallic biomaterials, they are implanted on a long-term basis. However, complications associated with permanent stents, such as in-stent restenosis and thrombosis, have often been reported. In order to reduce those complications, it would be clinically useful to develop a new family of degradable stents. An interesting material for fabrication of degradable stents is based on magnesium, an essential element involved in human metabolism. Success in using magnesium alloys for the fabrication of endovascular devices is closely related to the properties of the selected alloy. In this context, a test bench was specifically designed to reproduce the physiological conditions to which stents are submitted when implanted in the coronary arteries. Then the test bench was validated using a magnesium-based alloy. Results showed that the corrosion rate and the corrosion mechanisms vary with the applied shear stress and that corrosion products strongly depend on the composition of the corrosive solution. This test bench will thus be useful in further investigations for the development of metallic alloys as degradable biomaterials.

Corrosion Behavior of TiO2 Coating on Magnesium Alloy AM60 in Hank’s Solution

As magnesium is an essential element for the human body, magnesium alloys are expected to be promising implant materials despite their lower corrosion resistance. The most potential advantages of magnesium lie on both excellent biocompatibility and good mechanical properties. A TiO2 coating had been successfully deposited on the surface of AM60 samples by using a thermal spray technique and was then sealed with sodium silicate. The corrosion behavior was investigated by using the potentiodynamic electrochemical technique. The experimental results demonstrated that the TiO2 coating had a porous ceramic structure. The thickness and micro hardness of the layer reached 40 μm and 886 HV, respectively. After additional sealing with sodium silicate, TiO2 coating on magnesium alloy improved corrosion resistance in Hank’s solution.

The development of binary Mg-Ca alloys for use as biodegradable materials within bone

Binary Mg-Ca alloys with various Ca contents were fabricated under different working conditions. X-ray diffraction (XRD) analysis and optical microscopy observations showed that Mg-xCa (x=1-3 wt%) alloys were composed of two phases, alpha (Mg) and Mg2Ca. The results of tensile tests and in vitro corrosion tests indicated that the mechanical properties could be adjusted by controlling the Ca content and processing treatment. The yield strength (YS), ultimate tensile strength (UTS) and elongation decreased with increasing Ca content. The UTS and elongation of as-cast Mg-1Ca alloy (71.38+/-3.01 MPa and 1.87+/-0.14%) were largely improved after hot rolling (166.7+/-3.01 MPa and 3+/-0.78%) and hot extrusion (239.63+/-7.21 MPa and 10.63+/-0.64%). The in vitro corrosion test in simulated body fluid (SBF) indicated that the microstructure and working history of Mg-xCa alloys strongly affected their corrosion behaviors. An increasing content of Mg2Ca phase led to a higher corrosion rate whereas hot rolling and hot extrusion could reduce it. The cytotoxicity evaluation using L-929 cells revealed that Mg-1Ca alloy did not induce toxicity to cells, and the viability of cells for Mg-1Ca alloy extraction medium was better than that of control. Moreover, Mg-1Ca alloy pins, with commercial pure Ti pins as control, were implanted into the left and right rabbit femoral shafts, respectively, and observed for 1, 2 and 3 months. High activity of osteoblast and osteocytes were observed around the Mg-1Ca alloy pins as shown by hematoxylin and eosin stained tissue sections. Radiographic examination revealed that the Mg-1Ca alloy pins gradually degraded in vivo within 90 days and the newly formed bone was clearly seen at month 3. Both the in vitro and in vivo corrosion suggested that a mixture of Mg(OH)2 and hydroxyapatite formed on the surface of Mg-1Ca alloy with the extension of immersion/implantation time. In addition, no significant difference (p>0.05) of serum magnesium was detected at different degradation stages. All these results revealed that Mg-1Ca alloy had the acceptable biocompatibility as a new kind of biodegradable implant material. Based on the above results, a solid alloy/liquid solution interface model was also proposed to interpret the biocorrosion process and the associated hydroxyapatite mineralization.

Time-dependent electrochemical characterization of the corrosion of a magnesium rare-earth alloy in simulated body fluids

The electrochemistry of the corrosion process of a magnesium rare-earth-alloy is studied in detail in simulated body fluid (m-SBF) over the first 5 days. The aim is to investigate the corrosion mechanism under in vitro conditions. For this purpose we also used electrolytes that contain only some of the components of SBF, they were compared to SBF to investigate the influence of the different ions in SBF. The influence of albumin on the corrosion process was studied with a solution containing m-SBF and albumin in physiological concentration. For this study, impedance spectroscopy series measurements were performed. Additional results were gained from polarization curves. We conclude from the study that the corrosion resistance is significantly lower in m-SBF than in simple isotonic NaCl-solution. Albumin may form a blocking layer on the surface in the first hours of exposure. The formed corrosion layers consisting of amorphous apatite have only a low protective ability. Further results show that the corrosion processes in SBFs follow a linear time-law. The results elucidate critical factors and mechanisms of the electrochemical corrosion process of magnesium rare-earth alloys in SBFs, this understanding is crucial for a successful application of Mg alloys in biomedical applications.

Bone-like matrix formation on magnesium and magnesium alloys

Mg metal and its alloys have promise as a biocompatible, degradable biomaterials. This work evaluates the potential of in vitro cell culture work with osteoblast-like cells on Mg based materials, and investigates cell differentiation and growth on Mg alloyed with various non-toxic or low-toxicity elements. Mg based substrates support the adhesion, differentiation and growth of stromal cells towards an osteoblast-like phenotype with the subsequent production of a bone like matrix under in vitro conditions. No significant difference in the final tissue layer is observed on pure Mg, an AZ21 alloy or a 0.5 wt% Ca alloy. Only a 0.8 wt% Ca alloy which shows complete structural disintegration shows minimal cell growth. Due to association of non-soluble degradation products formed when Mg is incubated in physiological-like fluid, mass changes typically used to report Mg degradation are not viable estimates of degradation. Methods quantifying the time dependent change in the mechanical integrity of samples as a function of incubation time are required for a proper assessment of Mg degradation. We conclude that in vitro cell culture of bone cells on Mg substrates is expected to be a viable screening technique to assess the relative biological activity of Mg-based materials.

In vivo corrosion behavior of Mg-Mn-Zn alloy for bone implant application

Magnesium alloy has been implanted in rats to investigate the in vivo degradation behavior of magnesium for bone implant application. After 9 weeks postoperation, 100% implants were fixed and no inflammation was observed. Histological analysis showed new bone was formed around magnesium implant and no difference was found in the histological microstructure of the new bone and the cortical bone. A degradation or reaction layer, which was mainly composed of Ca, P, O, and Mg, was formed on the surface of magnesium alloy implants. High Ca content in the degradation layer displayed that magnesium could promote the deposition of Ca. Residual area calculation has showed that 10-17% magnesium alloy implant has been degraded in vivo. Compared with that of the controlled rats, no increase in serum magnesium and no disorder of kidney were observed after 15 weeks postoperation. After 18 weeks postoperation, 100% magnesium implants were fixed and no inflammation was observed. About 54% magnesium implant has degraded in vivo. Element analysis showed that Zn and Mn in Mg-Mn-Zn alloy distributed homogeneously in the residual magnesium implant, the degradation layer, and the surrounding bone tissue after 18 weeks implantation, indicating that Zn and Mn elements were easily absorbed by bioenvironment.

Low stiffness porous Ti structures for load-bearing implants

The need for unique mechanical and functional properties coupled with manufacturing flexibility for a wide range of metallic implant materials necessitates the use of novel design and fabrication approaches. In this work, we have demonstrated that application of proposed design concepts in combination with laser-engineered net shaping (LENStrade mark) can significantly increase the processing flexibility of complex-shaped metallic implants with three-dimensionally interconnected, designed and functionally graded porosities down to 70vol.%, to reduce effective stiffness for load-bearing implants. Young's modulus and 0.2% proof strength of these porous Ti samples having 35-42vol.% porosity are found to be similar to those of human cortical bone.

Effects of a magnesium adhesive cement on bone stability and healing following a metatarsal osteotomy in horses

OBJECTIVE

To compare biodegradable magnesium phosphate cement (Mg-cement), calcium phosphate cement (Ca-cement), and no cement on bone repair, biocompatibility, and bone adhesive characteristics in vivo in horses.

ANIMALS

8 clinically normal adult horses.

PROCEDURES

Triangular fragments (1-cm-long arms) were created by Y-shaped osteotomy of the second and fourth metatarsal bones (MTII and MTIV, respectively). Fragments were replaced in pairs to compare Mg-cement (MTII, n = 8; MTIV, 8) with Ca-cement (MTIV, 8) or with no cement (MTII, 8). Clinical and radiographic evaluations were performed for 7 weeks, at which time osteotomy sites were harvested for computed tomographic measurement of bone density and callus amount, 3-point mechanical testing, and histologic evaluation of healing pattern and biodegradation.

RESULTS

All horses tolerated the procedure without clinical problems. Radiographically, Mg-cement secured fragments significantly closer to parent bone, compared with Ca-cement or no treatment. Callus amount and bone remodeling and healing were significantly greater with Mg-cement, compared with Ca-cement or no cement. Biomechanical testing results and callus density among treatments were not significantly different. Significantly greater woven bone was observed adjacent to the Mg-cement without foreign body reaction, compared with Ca-cement or no cement. The Mg-cement was not fully degraded and was still adhered to the fragment.

CONCLUSIONS AND CLINICAL RELEVANCE

Both bone cements were biocompatible in horses, and Mg-cement may assist fracture repair by osteogenesis and fragment stabilization. Further studies are warranted on other applications and to define degradation characteristics.

Biodegradable magnesium scaffolds: Part 1: appropriate inflammatory response

Current tissue engineering strategies focus on the replacement of pathologically altered tissues by the transplantation of cells in combination with supportive biocompatible scaffolds. Scaffolds for tissue engineering strategies in musculoskeletal research require an appropriate mechanical stability. In recent studies, considerable attention has thus been given to magnesium alloys as biodegradable implants. The aim of this study was to characterize the biocompatibility of magnesium scaffolds by the inflammatory host response. Open porous scaffolds made of the magnesium alloy AZ91D were implanted into the distal femur condyle of rabbits and were compared to autologous bone, which was transplanted into the contralateral condyle in a 3 and 6 months follow-up group. After 3 months, magnesium scaffolds were already largely degraded and most of the original magnesium alloy has disappeared. Concomitantly, a fibrous capsule enclosed the operation site. Histological analysis revealed that the magnesium scaffolds caused no significant harm to their neighboring tissues. This study shows that even fast degrading magnesium scaffolds show a good biocompatibility and react in vivo with an appropriate inflammatory host response. Magnesium alloy based implants are therefore a very promising approach in the development of mechanically suitable and open porous scaffolds for the replacement of subchondral bone in cartilage tissue engineering.

Diffusion Bonding of Magnesium, Zirconium and Titanium as Implant Material

Titanium, zirconium and magnesium alloys are considered to be biocompatible, and can be used as implants such as hip ball and sockets and to make medical equipments. Biomaterials with hybrid structures in some applications utilizing the beneficial properties of different metals together are considered potential implant materials. Therefore, in this study, experimental trials were attempted to bond pure magnesium, AM60 (6 wt% Al-0.27 wt% Mn), and AZ31 (3 wt% Al-1 wt% Zn) alloys to pure zirconium and Ti6Al4V (6 wt% Al-4 wt% V) alloy to experimentally evaluate the forming bimetallic structures by diffusion bonding technique by vacuum hot pressing. SEM analysis showed the presence of a significant diffusion zone and a presence of diffusion bonding in some metallic couples. It may be suggested that novel hybrid implant materials, composed of diffusion couples of magnesium, zirconium and titanium alloys may emerge in the future.

Biodegradable magnesium–hydroxyapatite metal matrix composites

Recent studies indicate that there is a high demand to design magnesium alloys with adjustable corrosion rates and suitable mechanical properties. An approach to this challenge might be the application of metal matrix composite (MMC) based on magnesium alloys. In this study, a MMC made of magnesium alloy AZ91D as a matrix and hydroxyapatite (HA) particles as reinforcements have been investigated in vitro for mechanical, corrosive and cytocompatible properties. The mechanical properties of the MMC-HA were adjustable by the choice of HA particle size and distribution. Corrosion tests revealed that HA particles stabilised the corrosion rate and exhibited more uniform corrosion attack in artificial sea water and cell solutions. The phase identification showed that all samples contained hcp-Mg, Mg(17)Al(12), and HA before and after immersion. After immersion in artificial sea water CaCO3 was found on MMC-HA surfaces, while no formation of CaCO3 was found after immersion in cell solutions with and without proteins. Co-cultivation of MMC-HA with human bone derived cells (HBDC), cells of an osteoblasts lineage (MG-63) and cells of a macrophage lineage (RAW264.7) revealed that RAW264.7, MG-63 and HBDC adhere, proliferate and survive on the corroding surfaces of MMC-HA. In summary, biodegradable MMC-HA are cytocompatible biomaterials with adjustable mechanical and corrosive properties.

Investigation on the Potential of Magnesium Alloy AZ31 as a Bone Implant

Degradable implants have been in use for bone surgery for decades. However, degradable metal implants are one of the new research areas of biomaterials science. There is a potential for application of degradable metal implants as screws and plates in bones. Magnesium alloys are one of the new candidate materials for degradable implants. Magnesium has good biocompatibility due to its low toxicity, and it is a corroding, i.e. dissolvable, metal. Furthermore, magnesium is needed in human body, and naturally found in bone tissue. Mechanical properties of magnesium alloys are dimensionally comparable to the cortical bone substance. In this study, corrosion behavior of magnesium metal at the bone interface and the possibility of new bone cell formation have been investigated. Cortical bone screws were machined from magnesium alloy AZ31 extruded rod and implanted to hip-bones of sheep via surgery. Three months after surgery, the bone segments carrying these screws were removed from the sacrificed animals. Samples were sectioned to reveal Mg/bone interfaces and investigated using optical microscope, SEM-EDS and radiography. Optical and SEM images showed that there was a significant amount of corrosion on the magnesium screw. The elemental mapping results indicate, due to the presence of calcium and phosphorus elements, that there exists new bone formation at the interface. From the results of this study, it may be stated that the potential for using magnesium alloys as a bone implant material is expected to be significant.

Corrosion degradation and prevention by surface modification of biometallic materials

Metals, in addition to ceramics and polymers, are important class of materials considered for replacement of non-functional parts in the body. Stainless steel 316, titanium and titanium alloys, Co-Cr, and nitinol shape memory alloys are the most frequently used metallic materials. These alloys are prone to corrosion in various extents. This review briefly discusses the important biomaterials, their properties, and the physiological environment to which these materials are exposed. Corrosion performance of currently used metallic materials has been assessed and threat to the biocompatibility from corrosion products/metal ions is discussed. The possible preventive measures to improve corrosion resistance by surface modification and to increase the bioactivity of the metallic surfaces have also been discussed. Importance of the formation of oxide layers on the metal surface, another aspect of corrosion process, has been correlated with the host response. The gap areas and future direction of research are also outlined in the paper.

The effects of magnesium particles in posterolateral spinal fusion: an experimental in vivo study in a sheep model

Object

Magnesium has recently become a material of interest as a biocompatible and biodegradable implant metal. Authors of several reports have noted the potential bone-cell activating or bone-healing effect of high Mg ion concentrations. The classic method for achieving intertransverse process fusion involves using an autologous iliac crest bone graft. Several studies have been performed to investigate enhancement of this type of autograft fusion. To the authors’ knowledge, no research has been conducted in which the efficacy of pure Mg particles in posterolateral spinal fusion has been investigated. The objective of this study was to determine whether Mg particles enhance the effectiveness of intertransverse process lumbar fusion in a sheep model.

Methods

Sixteen skeletally mature female sheep were subjected to intertransverse process spinal fusions with pedicle screw fixation at L2–3 and L5–6. Each animal was given a 5-cm3 bone autograft at one fusion level, and a combined 5-cm3 bone autograft with the addition of 1 cm3 Mg at the other level. Six months after surgery, bone formation was evaluated by gross inspection and palpation, and by radiological, histological, scanning electron microscopic, and x-ray diffraction analyses. Radiological results were graded from 0 to 4 according to the status of the bridging bone, which was determined by evaluating both x-ray films and computed tomography scans. The quality of the spinal fusion was assigned a histological score of 0 to 7, in which a score of 0 represented an empty cleft and a score of 7 represented complete bridging of bone between the transverse processes. The trabecular bone formation at each fusion level and the Ca hydroxyapatite crystalline structure in core biopsy specimens were evaluated using scanning electron microscopy and x-ray diffraction analyses, respectively.

The rate of rigid bone fusion, according to both palpation and radiological assessment, in the combined Mg and autologous bone treatment group was higher (81.25%) than in the autograft bone treatment group (62.5%), but this difference was not statistically significant. The quality of bone fusion, according to the histological grading system and scanning electron microscopy inspection, was higher in the bone fusion segments of the Mg and autologous graft combined group than in the group with autograft-only arthrodesis, and this difference was statistically significant. The x-ray diffraction analyses further confirmed the effect of Mg in promoting the formation of the crystalline portion of the bone (hydroxyapatite).

Conclusions

Based on the results of this study, adding Mg particles to autologous corticocancellous bone in a posterolateral intertransverse process fusion enhances the quality of bone formation. However, radiological findings did not reveal a statistically significant effect of Mg on the rate of solid bone fusion formation between the two transverse processes.

Coronary stents: a materials perspective

The objective of this review is to describe the suitability of different biomaterials as coronary stents. This review focuses on the following topics: (1) different materials used for stents, (2) surface characteristics that influence stent-biology interactions, (3) the use of polymers in stents, and (4) drug-eluting stents, especially those that are commercially available.