Influence of Stress on the Degradation Behavior of Mg LAE442 Implant Systems

In vivo investigation of open-pored magnesium scaffolds LAE442 with different coatings in an open wedge defect

The magnesium alloy LAE442 showed promising results as a bone substitute in numerous studies in non-weight bearing bone defects. This study aimed to investigate the in vivo behavior of wedge-shaped open-pored LAE442 scaffolds modified with two different coatings (magnesium fluoride (MgF₂, group 1)) or magnesium fluoride/calcium phosphate (MgF₂/CaP, group 2)) in a partial weight-bearing rabbit tibia defect model. The implantation of the scaffolds was performed as an open wedge corrective osteotomy in the tibia of 40 rabbits and followed for observation periods of 6, 12, 24, and 36 weeks. Radiological and microcomputed tomographic examinations were performed in vivo. X-ray microscopic, histological, histomorphometric, and SEM/EDS analyses were performed at the end of each time period. µCT measurements and X-ray microscopy showed a slight decrease in volume and density of the scaffolds of both coatings. Histologically, endosteal and periosteal callus formation with good bridging and stabilization of the osteotomy gap and ingrowth of bone into the scaffold was seen. The MgF₂ coating favored better bridging of the osteotomy gap and more bone-scaffold contacts, especially at later examination time points. Overall, the scaffolds of both coatings met the requirement to withstand the loads after an open wedge corrective osteotomy of the proximal rabbit tibia. However, in addition to the inhomogeneous degradation behavior of individual scaffolds, an accumulation of gas appeared, so the scaffold material should be revised again regarding size dimension and composition.

Novel zinc alloys for biodegradable surgical staples

BACKGROUND

The development of biodegradable surgical staples is desirable as non-biodegradable Ti alloy staples reside in the human body long after wound healing, which can cause allergic/foreign-body reactions, adhesion, or other adverse effects. In order to develop a biodegradable alloy suitable for the fabrication of surgical staples, we hypothesized that Zn, a known biodegradable metal, could be alloyed with various elements to improve the mechanical properties while retaining biodegradability and biocompatibility. Considering their biocompatibility, Mg, Ca, Mn, and Cu were selected as candidate alloying elements, alongside Ti, the main material of clinically available surgical staples.

AIM

To investigate the in vitro mechanical properties and degradation behavior and in vivo safety and feasibility of biodegradable Zn alloy staples.

METHODS

Tensile and bending tests were conducted to evaluate the mechanical properties of binary Zn alloys with 0.1–6 wt.% Mg, Ca, Mn, Cu, or Ti. Based on the results, three promising Zn alloy compositions were devised for staple applications (wt.%): Zn-1.0Cu-0.2Mn-0.1Ti (Zn alloy 1), Zn-1.0Mn-0.1Ti (Zn alloy 2), and Zn-1.0Cu-0.1Ti (Zn alloy 3). Immersion tests were performed at 37 °C for 4 wk using fed-state simulated intestinal fluid (FeSSIF) and Hank's balanced salt solution (HBSS). The corrosion rate was estimated from the weight loss of staples during immersion. Nine rabbits were subjected to gastric resection using each Zn alloy staple, and a clinically available Ti staple was used for another group of nine rabbits. Three in each group were sacrificed at 1, 4, and 12 wk post-operation.

RESULTS

Additions of ≤1 wt.% Mn or Cu and 0.1 wt.% Ti improved the yield strength without excessive deterioration of elongation or bendability. Immersion tests revealed no gas evolution or staple fracture in any of the Zn alloy staples. The corrosion rates of Zn alloy staples 1, 2, and 3 were 0.02 mm/year in HBSS and 0.12, 0.11, and 0.13 mm/year, respectively, in FeSSIF. These degradation times are sufficient for wound healing. The degradation rate is notably increased under low pH conditions. Scanning electron microscopy and energy dispersive spectrometry surface analyses of the staples after immersion indicated that the component elements eluted as ions in FeSSIF, whereas corrosion products were produced in HBSS, inhibiting Zn dissolution. In the animal study, none of the Zn alloy staples caused technical failure, and all rabbits survived without complications. Histopathological analysis revealed no severe inflammatory reaction around the Zn alloy staples.

CONCLUSION

Staples made of Zn-1.0Cu-0.2Mn-0.1Ti, Zn-1.0Mn-0.1Ti, and Zn-1.0Cu-0.1Ti exhibit acceptable in vitro mechanical properties, proper degradation behavior, and in vivo safety and feasibility. They are promising candidates for biodegradable staples.

Translational status of biomedical Mg devices in China

Magnesium (Mg) and its alloys as temporary medical implants with biodegradable and properly mechanical properties have been investigated for a long time. There are already three kinds of biodegradable Mg implants which are approved by Conformite Europeene (CE) or Korea Food and Drug Administration (KFDA), but not China Food and Drug Administration (CFDA, now it is National Medical Products Administration, NMPA). As we know, Chinese researchers, surgeons, and entrepreneurs have tried a lot to research and develop biodegradable Mg implants which might become other new approved implants for clinical applications. So in this review, we present the representative Mg implants of three categories, orthopedic implants, surgical implants, and intervention implants and provide an overview of current achievement in China from academic publications and Chinese patents. We would like to provide a systematic way to translate Mg and its alloy implants from experiment designs to clinical products.

Crevice corrosion - A newly observed mechanism of degradation in biomedical magnesium

Crevice-induced corrosion is not desirable to occur in metallic magnesium (Mg) during many industrial applications. However, orthopedic implants made of Mg alloys have been demonstrated to degrade faster between the joining surface of bone plates and screws after implantation, suggesting the crevice corrosion may occur in the physiological environment. In this paper, a resin device is designed to parallel high purity magnesium (HP-Mg) plates with closely spaced slits. After a standard corrosion test in the phosphate-buffered saline (PBS) solution, the paralleled HP-Mg samples embedded in the custom-made resin device corrode faster than those without the resin device. The corrosion morphology of Mg with the resin device exhibits features of crevice corrosion with many deep holes and river-like texture. Moreover, implantation of the bone plate and screws in vivo demonstrates similar corrosion morphology as that of the in vitro test, suggesting the occurrence of crevice-enhanced corrosion in the bone-bone plate interface, as well as the contact area between the bone plate and the screws. STATEMENT OF SIGNIFICANCE: Understanding the corrosion behavior of Mg and Mg alloys after implantation is one of the main challenges for developing desirable biodegradable Mg alloys or effective methods to adjust the corrosion rate of Mg-based implants. In this paper, we attempted to understand the corrosion behaviors of HP-Mg at the joining surface between HP-Mg plates or HP-Mg screws and bone tissues after implantation. We designed an in vitro setup to mimic the crevice environment of the in vivo joining surface and found that the crevices existing on the HP-Mg would significantly accelerate the corrosion rate and change the corrosion morphology of HP-Mg plates. The in vivo implantation also showed similar corrosion morphology caused by crevice corrosion, which appeared at the joining surface between HP-Mg plates or HP-Mg screws and bone tissues. Then, we proposed a new corrosion mechanism of Mg-based alloys inside the crevice. The findings of this study can help us broaden our cognition on the corrosion behavior of Mg and Mg alloy-based orthopedic implants.

Biodegradable Surgical Staple Composed of Magnesium Alloy

Currently, surgical staples are composed of non-biodegradable titanium (Ti) that can cause allergic reactions and interfere with imaging. This paper proposes a novel biodegradable magnesium (Mg) alloy staple and discusses analyses conducted to evaluate its safety and feasibility. Specifically, finite element analysis revealed that the proposed staple has a suitable stress distribution while stapling and maintaining closure. Further, an immersion test using artificial intestinal juice produced satisfactory biodegradable behavior, mechanical durability, and biocompatibility in vitro. Hydrogen resulting from rapid corrosion of Mg was observed in small quantities only in the first week of immersion, and most staples maintained their shapes until at least the fourth week. Further, the tensile force was maintained for more than a week and was reduced to approximately one-half by the fourth week. In addition, the Mg concentration of the intestinal artificial juice was at a low cytotoxic level. In porcine intestinal anastomoses, the Mg alloy staples caused neither technical failure nor such complications as anastomotic leakage, hematoma, or adhesion. No necrosis or serious inflammation reaction was histopathologically recognized. Thus, the proposed Mg alloy staple offers a promising alternative to Ti alloy staples.

Magnesium matrix nanocomposites for orthopedic applications: A review from mechanical, corrosion, and biological perspectives

Magnesium (Mg) and some of its alloys have attracted extensive interests for biomedical applications as they exhibit biodegradability and low elastic modulus that is closer to natural bones than the currently used metallic implant materials such as titanium (Ti) and its alloys, stainless steels, and cobalt-chromium (Co-Cr) alloys. However, the rapid degradation of Mg alloys and loss of their mechanical integrity before sufficient bone healing impede their clinical application. Our literature review shows that magnesium matrix nanocomposites (MMNCs) reinforced with nanoparticles possess enhanced strength, high corrosion resistance, and good biocompatibility. This article provides a detailed analysis of the effects of nanoparticle reinforcements on the mechanical properties, corrosion behavior, and biocompatibility of MMNCs as promising biodegradable implant materials. The governing equations to quantitatively predict the mechanical properties and underlying synergistic strengthening mechanisms in MMNCs are elucidated. The potential, recent advances, challenges and future research directions in relation to nanoparticles reinforced MMNCs are highlighted. STATEMENT OF SIGNIFICANCE: Critically reviewing magnesium metal matrix nanocomposites (MMNCs) for the biomedical application. Clear definitions of strengthening mechanisms using reinforcement particle in the magnesium matrix, as there were controversial in governing equations of strengthening parameters. Providing better understanding of the effect of particle size, volume fraction, interfacial bonding, and uniform dispersion of reinforcement particles on MMNCs.

The effect of tensile and fluid shear stress on the in vitro degradation of magnesium alloy for stent applications

Magnesium alloys have gained great attention as biodegradable materials for stent applications. Cardiovascular stents are continuously exposed to different types of mechanical loadings simultaneously during service, including tensile, compressive and fluid shear stress. In this study, the in vitro degradation of WE43 wires was investigated under combined effect of tensile loading and fluid shear stress and compared with that experienced an individual loading condition. For the individual mechanical loading treatment, the degradation of magnesium wires was more severely affected by tensile loading than fluid shear stress. Under tensile loading, magnesium wires showed faster increment of corrosion rates, loss of mechanical properties and localized corrosion morphology with the increasing tensile loadings. With the combined stress, smaller variation of the corrosion rates as well as the slower strength degeneration was shown with increasing stress levels, in comparison with the individual treatment of tensile loading. This study could help to understand the effect of complex stress condition on the corrosion of magnesium for the optimization of biodegradable magnesium stents.

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Tensile loading induced the localized corrosion and fast degeneration of mechanical properties of magnesium wires.

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Relatively homogeneous corrosion of magnesium wires was observed under fluid shear stress.

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Under combined stresses, smaller variation of the corrosion rates and the slower strength decay was shown with increasing stresses.

Research of a novel biodegradable surgical staple made of high purity magnesium

Surgical staples made of pure titanium and titanium alloys are widely used in gastrointestinal anastomosis. However the Ti staple cannot be absorbed in human body and produce artifacts on computed tomography (CT) and other imaging examination, and cause the risk of incorrect diagnosis. The bioabsorbable staple made from polymers that can degrade in human body environment, is an alternative. In the present study, biodegradable high purity magnesium staples were developed for gastric anastomosis. U-shape staples with two different interior angles, namely original 90° and modified 100°, were designed. Finite element analysis (FEA) showed that the residual stress concentrated on the arc part when the original staple was closed to B-shape, while it concentrated on the feet for the modified staple after closure. The in vitro tests indicated that the arc part of the original staple ruptured firstly after 7 days immersion, whereas the modified one kept intact, demonstrating residual stress greatly affected the corrosion behavior of the HP-Mg staples. The in vivo implantation showed good biocompatibility of the modified Mg staples, without inflammatory reaction 9 weeks post-operation. The Mg staples kept good closure to the Anastomosis, no leaking and bleeding were found, and the staples exhibited no fracture or severe corrosion cracks during the degradation.

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A modified structure with about 100° interior angle of U-shape was selected by using FEA.

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In vitro immersion experiment showed homogeneous corrosion behavior of the modified HP-Mg surgical staple.

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In vivo implantation suggested that the modified HP-Mg surgical staple had enough closure strength and good biocompatibility.

In vivo study of magnesium plate and screw degradation and bone fracture healing

Each year, millions of Americans suffer bone fractures, often requiring internal fixation. Current devices, like plates and screws, are made with permanent metals or resorbable polymers. Permanent metals provide strength and biocompatibility, but cause long-term complications and may require removal. Resorbable polymers reduce long-term complications, but are unsuitable for many load-bearing applications. To mitigate complications, degradable magnesium (Mg) alloys are being developed for craniofacial and orthopedic applications. Their combination of strength and degradation make them ideal for bone fixation. Previously, we conducted a pilot study comparing Mg and titanium devices with a rabbit ulna fracture model. We observed Mg device degradation, with uninhibited healing. Interestingly, we observed bone formation around degrading Mg, but not titanium, devices. These results highlighted the potential for these fixation devices. To better assess their efficacy, we conducted a more thorough study assessing 99.9% Mg devices in a similar rabbit ulna fracture model. Device degradation, fracture healing, and bone formation were evaluated using microcomputed tomography, histology and biomechanical tests. We observed device degradation throughout, and calculated a corrosion rate of 0.40±0.04mm/year after 8 weeks. In addition, we observed fracture healing by 8 weeks, and maturation after 16 weeks. In accordance with our pilot study, we observed bone formation surrounding Mg devices, with complete overgrowth by 16 weeks. Bend tests revealed no difference in flexural load of healed ulnae with Mg devices compared to intact ulnae. These data suggest that Mg devices provide stabilization to facilitate healing, while degrading and stimulating new bone formation.