AIP Conference Proceedings 2092, 020021 (2019); https://doi.org/10.1063/1.5096689 2092, 020021
© 2019 Author(s).
Magnesium-carbonate apatite metal
composite: Potential biodegradable material for orthopaedic implant
Cite as: AIP Conference Proceedings 2092, 020021 (2019); https://doi.org/10.1063/1.5096689 Published Online: 09 April 2019
Ahmad Jabir Rahyussalim, Sugeng Supriadi, Achmad Fauzi Kamal, Aldo Fransiskus Marsetio, and Pancar Muhammad Pribadi
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Magnesium-Carbonate Apatite Metal Composite: Potential Biodegradable Material for Orthopaedic Implant
Ahmad Jabir Rahyussalim
1,a), Sugeng Supriadi
2,3,b), Achmad Fauzi Kamal
1, Aldo Fransiskus Marsetio
1, Pancar Muhammad Pribadi
21Department of Orthopaedic and Traumatology, Faculty of Medicine Universitas Indonesia – Cipto Mangunkusumo Hospital, Jakarta, 10320, Indonesia
2Department of Mechanical Engineering, Faculty of Engineering Universitas Indonesia, Kampus Baru UI, Depok, 16424, West-Java Indonesia
3Research Center for Biomedical Engineering, Universitas Indonesia, Depok 16424 West-Java Indonesia Corresponding author: a)[email protected], b)[email protected]
Abstract. Magnesium and carbonate are important elements and available in the human body as biological apatites. Good absorbable biomaterial for implants is on the search. Biomaterial used in orthopaedic implants must fulfill some criteria, including the degradation time must be in parallel with the physiologic timeline of bone healing. Magnesium is also a biodegradable metal with characteristic similar to the bone. However, the degradation time is quite short. Incorporating magnesium with carbonate apatite involve a complex process. Studies found structural changes upon synthesis of the two elements. This review of the literature would analyze the potential of two elements, magnesium and carbonate apatite, as bioabsorbable orthopaedic implants. Modification to increase the corrosion resistance of the composite is also needed, as the material must be able to comply with the bone healing process before degrading or lose its mechanical properties.
Keywords: apatite, biodegradable implant, carbonate apatite, magnesium, metal composite
INTRODUCTION
Osteosynthesis is a common procedure performed in orthopaedics field. The procedure involves fixation of fractured bone, expecting able to facilitate bone healing process, either by primary or secondary process. Joint arthroplasty is another common procedure in the field, in which degenerated or damaged joint is replaced by prosthesis. These kinds of procedure require the use of implant(s). Implants that are chosen must be composed of biocompatible materials. The term of biocompatible materials suggests that the materials must produce side effect as minimal as possible after implantation inside the body. Minimal body host cells and tissues reaction towards the implant material is expected to provide proper biological safety of the implants [1].
The ability of biomaterials to produce adverse effect on body cells and tissues can be categorized as cytotoxic, carcinogenic, mutagenic, pyrogenic, and allergenic or thrombogenic. The adverse effect may appear in the body as hypersensitivity (allergic) reaction. It may also be induced by the complication of long-term material implantation, such as debris from implant wear and corrosion process.
Additionally, these implants are also expected able to promote bone healing process, either by chemically or mechanically. By chemically means that the implant supports the healing system by providing a favorable environment for bone healing, either by increasing bioactivity of the bone, increasing osteoconduction, good osseointegration, resistant to wear and corrosion process. By mechanically means the stability of the implant that able to prevent mechanical failure. Mechanical stability is determined by many properties, such as tensile strength, yield strength, elastic modulus, corrosion and fatigue resistance, surface finish, creep, and hardness [1].
Although minor, magnesium (Mg2+) and carbonate (CO32-) are important elements and available in the human body as biological apatites. Bone apatite has an average concentration of 0.6 wt. (%) Mg2+ and 7.5 wt. (%) CO32- [2].
Besides, magnesium is a natural micronutrient in the human body. Around 250-300 mg magnesium is acquired from daily food intake. As micronutrient, the element plays important roles in energy metabolism and regulating organ function such as heart, muscle, nerve, bone, and kidney. As a metal, the element is a potential biodegradable material with mechanical properties similar to bone [3,4].
Incorporating magnesium with carbonate apatite involve a complicated process. Studies found structural changes upon synthesis of the two elements, such as destabilization of apatite lattice, contraction on the A-axis dimension, expansion on the C-axis dimension, and increase in solubility [2,5-6]. Although, some modification can stabilize the construction [7].
This review of the literature would analyze the potential of two elements, magnesium and carbonate apatite, as bioabsorbable orthopaedic implants.
MAGNESIUM AS ORTHOPAEDIC IMPLANT MATERIAL
Magnesium is a natural element that circulating inside the human body. Magnesium macronutrient is metabolite product and acquired from daily food intake. WHO recommends daily consumption of magnesium as much as 280- 300 mg/day for adults, 250 mg/day for children, and 80 mg/day for infants. The element has essential roles in energy metabolism and regulating organ function such as heart, muscle, nerve, bone, and kidney [3].
On the other hand, magnesium is also a biodegradable metal with characteristic similar to the bone. The Young modulus elasticity is 41-45 GPa, compared to bone’s 15-25 GPa. The density is 1.74-2.1 g/cm3, compared to bone’s 1.8-2.1g/cm3 [3,4]. These characteristics are comparable to titanium, the most commonly used base material for orthopaedic implants nowadays, which has young modulus of 106-115 GPa and density of 4.43-4.52 g/cm3 [4].
Biodegradability of Magnesium
Every metal would undergo a corrosion process. The rate of corrosion is determined by several factors, for example the environment acidity, concentration, ion type, protein absorption ability, and biochemical activity towards surrounding tissue [8,9]. Electrochemical reaction from the corrosion process will have a byproduct in the form of oxide, hydroxide, and H2 gas species. Physiological body fluid environment would accelerate the corrosion process even further due to high electrochemical potentials. Ion migrates from the metallic surface to the surrounding fluid.
These electrochemical process would induce hydroxide layers on the Mg surface [10].
M M
n ne
Anodic reaction (1)2 2
2 H O 2 e
H 2 OH
Cathodic reaction (2)( )
n
M
nOH
M OH
n Final reaction (3)The oxide layers act as a passive layer or barrier to prevent ion migration to the metallic surface. However, chloride ion from human tissue environment would able to break down, corrode, and dissolve the metal.
2 2
( ) 2 2
Mg OH Cl
MgCl OH
(4) The corrosion process also produce a H2 byproduct, which would ceased after 1 week [11]. These H2 gas is released by the metal on the rate of 0,01mL/cm2.day. This dosage is found to be safe and tolerable for human body [12].There are three types of corrosion that may occur on Magnesium [10]:
Galvanic corrosion occurs when two different metal with different electrochemical potential stick to each other, compounded with electrolyte. Magnesium is an active metal anode, thus, easily to be corroded upon adherence with other metal.
Intergranular corrosion occurs when the edge of crystallite material (secondary phase) is more corrosive than the core. Commonly found on aluminum and stainless steel. However, it also observed on Mg-Al composite.
Aluminum is found able to increase magnesium’s intergranular corrosion rate.
Pitting corrosion occurs as localized corrosion due to aggressive environment causing passive layer damage.
The formed pit is very corrosive and able to extend up to deep matrix layer. This phenomenon may be induced by microstructure impurity of the alloy, added with chloride environment. After the pit is formed, the rest of the metal would easily corrode within short time, and localized stress would ensue. Later on, crack will be formed, loss of weight bearing follows, and ultimately implant failure would happen.
Corrosion of magnesium and its alloys may be affected by two factors, namely buffer system and inorganic ion, as well as mechanical stress. Buffer system on human body tends to maintain the acidity in normal, neutral pH. The chloride ion is aggressive in eroding passive layer and triggering pitting corrosion. On the other hand, anion HPO42-
/PO43-/HCO3-/CO32- and Ca2+ ion protects the passive layer erosion by forming phosphate calcium and sodium carbonate precipitates [10].
Mechanical stress may accelerate the corrosion process of magnesium alloy, although high-stress force is needed.
One study found that found that the rate of corrosion of the alloy could increase more than 10 times after repeated compression force on magnesium alloy [13]. Ion chloride flow that passes the metal would give shear stress. A study by Leversque et al. found that low shear stress would decrease corrosion rate, while high shear stress would increase it [14].
Some strategy could be done to improve the corrosion rate and biocompatibility of magnesium. Development of processing and manufacturing methods may be able to optimize the composition and microstructure (granule size, crystalline phase structure, texture). Reduction in granule size would improve the corrosion resistance. This is due to the increased density of inter-granule bonding, which leads to the formation and adherence of passive layers. Other strategies are by coating the magnesium with polymer coat or synthesize an alloy form [12].
Effect of alloying towards the mechanical properties and corrosion rate of magnesium
The objective of alloying metal is usually to optimize the grain size, improve the corrosion resistance, increase mechanical strength, as well as facilitating the manufacturing process. There are four types of magnesium processing, which are:
Pure magnesium: in order to acquire pure magnesium, the content of copper (Cu) cannot be more than 100- 300 ppm, nickel (Ni) cannot be more than 20-50 ppm, iron (Fe) cannot be more than 35-50 ppm, and beryllium (Be) cannot be more than 5 ppm [15].
Aluminum alloy (AZ91, AZ31, LAE422, AM60, etc): the formed composite can decrease the grain size (less than 5% content). Moreover, Mg17Al12 is found to have a protective effect towards corrosion (AZ91D) [16].
o Manganese (Mn): Mg-Al-Mn alloy is found able to reduce the grain size (less than 0.4% content). This composite is also able to decrease the corrosion rate by altering the iron (Fe), copper (Cu), and Nickel (Ni) components into the intermetallic compound. Composite of Z31, AZ61, and AZ21 was found to have increased strength and hardness. On the other hand, excessive Mn addition would decrease the corrosion resistance, due to increased vulnerability towards galvanic effect [17].
Rare earth elements alloy (AE21, WE43, etc): rare earth elements are group of 17 elements, which can be grouped into two classes: high solubility (yttrium (Y), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium(Er), thulium (Tm), ytterbium (Yb), and lutetium(Lu)), and limited solubility (neodymium (Nd), lanthanum (La), cerium (Ce), praseodymium (Pr), samarium (Sm), europium (Eu)). These elements are sometimes added to increase the strength and corrosion resistance by solidification and precipitation hardening [15].
Aluminum-free alloy (WE43, MgCa 0.8, MgZn6, etc)
o Calcium (Ca): the element can decrease the grain size (with less than 0.5% content). However, the higher the amount of Ca content, the lower the corrosion resistance. The optimum concentration of Ca must ≤1%
[17].
o Zinc (Zn): the element can decrease the corrosion rate by altering the iron (Fe), copper (Cu), and Nickel (Ni) components into intermetallic compound. The composition also decreases the grain size, as well as increasing the strength (3% Zn in Mg-Zn-Mn composite). The optimal concentration of Zn is <5%. The higher the concentration, the higher the corrosion rate [18].
o Lithium (Li): the element makes the formed alloy to be flexible and easily configured. By possessing an optimal concentration of <9%, the element can increase corrosion resistance [19].
Toxic effects of magnesium and its alloy
Level of element toxicity may be grouped into 3, namely mild (Mg, Ca, Li, Al, Zr), moderate (Y), and severe (Zn, Ni, Cu, Mn) toxic reaction. The normal amount of magnesium inside human body is 25 gram. Normal value on the blood serum is 0.3-1.06 mmol/L. Magnesium toxicity may give symptoms that are related to homeostasis, such as nausea, kidney failure, even disorder of breathing pattern [10].
CHARACTERISTIC OF CARBONATE APATITE
Carbonate Apatite (CO3Ap; Ca10-a(PO4)6-b(CO3)c(OH)2-d) is a natural inorganic component of the bone tissue.
There are 2 kinds of carbonate apatite that are known based on the substitution of the CO32- component. The general formula for apatite is M10(ZO4)6X2. Substitution of CO32- on the X position (hydroxyl ion (OH–)) is known as type A carbonate apatite, while CO32- substitution on the ZO4 position (phosphate ion (PO43–)) as type B. Type A carbonate apatite is synthesized at high temperature, while the B type is synthesized at low temperature. Type B carbonate apatite is the most commonly found biological apatite. Carbonate content is usually as much as 4-8 wt.% [20,21].
Apatite carbonite can be regarded as bone apatite. However, the powder form cannot be used directly as a bone substitute, because it triggers the formation of crystals from the inflammatory response. Suitable shapes need to be fabricated into granular blocks or granular ceramics by a sintering process. Unfortunately, the sintering process requires high temperatures which induce decomposition of the carbonate apatite [20]. Previous study showed that sintering techniques could be done by treating specimens that contain 12 wt.% of carbonate, sintering at 400°C [22].
Alternative method in making the block shape is by compositional transformation reaction, which is based on dissipation-precipitation reaction. This reaction is similar to what is observed at gypsum formation, in which some part of the precursor phase is dissolved in the liquid, and the precipitates on the liquid would also form the final product. In a physiological environment (weak acid; pH 7.4), carbonate apatite is thermodynamically more stable compared to hydroxyapatite. Therefore, the selected precursor phase must have a moderate level of solubility, for example calcium carbonate, calcium phosphate, and phosphate compounds such as zinc phosphate [20,23].
Body Cell and Tissue Response toward Carbonite Apatite
Under acidic environmental condition (pH 5), sintered hydroxyapatite takes up to 3.8 days to dissolve. Meanwhile, the sintered carbonite apatite takes only 30 seconds [23]. Similar circumstances are found in the physiological environment (pH 7.4), carbonate apatite is found to be more resorbable by osteoclast compared with sintered hydroxyapatite [20].
The osteoblast response to carbonate apatite determines the turnover of the material with bone tissue. Thus, it can be an indicator of osteoconductivity [20]. A study by Ayukawa et al. [23] found that carbonate apatite can highly bound with bone without fibrotic tissue formation. This is different from the hydroxyapatite that forms fibrotic tissue upon bonding.
On the other hand, Nagai et al. [24] also examined and compared carbonate apatite with hydroxyapatite. They found no difference in cell count until the seventh day. However, significant differences were found on differentiation markers of type I collagen, alkaline phosphatase, osteopontin, and osteocalcin in the carbonate apatite group.
Carbonate apatite is found able to improve the regulation of the osteoblast cells differentiation. Thus, carbonite apatite has higher osteoconductivity, similar characteristic with bone tissue at the cellular level, and able to trigger remodeling process that resembles a natural bone remodeling process.
A recent study evaluated bone defects in rabbits at 18 months after reconstruction with carbonate apatite granules made from dicalcium phosphate dihydrate blocks using a micro CT scan. The study found that after 24 weeks of implantation, the hydroxyapatite block was not replaced by bone tissue. On the other hand, carbonate apatite is resorbed and replaced by new bone tissue. Carbonite apatite that is implanted at the metaphysical of proximal tibia is resorbed twice faster than the one that is implanted at the epiphysis of distal femur. The resorption rate of carbonate apatite is found to be linear. Therefore, it can be estimated that complete resorption of this material would occur after 1-1.5 years [25].
The potential of Carbonate Apatite – Magnesium Composite as orthopaedic implant
As magnesium has been extensively studied as biodegradable material, while carbonate apatite has been studied as bone substitute, they are potential to be used as basic material for orthopaedic implant. The natural mixture of magnesium and carbonate can be found in natural bone apatite, in which the crystalline structure is composed of 4–
7.5 wt.% carbonate and 0.5-0.6 wt.% magnesium, with plate-like morphology [26,27]. However, the synthesizing process of those two materials would not be a simple process.
One study that tried to combine the magnesium powder and carbonate apatite found that the incorporation destabilized the apatite lattice. This combination caused a significant decrease in crystal size, contraction on the dimension of A-axis, expansion on the dimension of C-axis, and increased the solubility [2, 5-6]. Another study succeeded in making three-dimensional composite scaffolds by synthesizing both components of this material on anionic collagen-containing medium, followed by cross-linking using a solution of 0.25% glutaraldehyde dan lyophilization process [7]. This cross-link process makes this construction stable, without changing the morphology or pore structure.
Additionally, the mechanical properties and degradation rate of the material must be aptly studied. The cross- linked construction is also found to be able to survive intact for up to 14 days, does not provide cytotoxic effects, and supports cell attachment. Up to date, there is still no studies regarding the mechanical properties of magnesium- carbonate apatite composite. Additionally, modification to increase the corrosion resistance of the composite is also needed, as the material must be able to comply with the bone healing process before degrading or lose its mechanical properties.
PRELIMINARY RESULTS OF OUR STUDY
We are currently conducting a study in fabrication of magnesium and carbonate apatite. The flowchart of our study is depicted in the Figure 1. In order to decide the fabrication parameters, we determined the compaction pressure and sintering parameter by literature study, while the compaction temperature was determined by experimental study.
FIGURE 1. Research methods flowchart of our study in fabrication of magnesium and carbonate apatite
Based on the literature, we decided to use 265 MPa compaction pressure [28,29]. As for the sintering parameter, 550°C of sintering temperature, 1 hour of holding time, and 5°C/min heat rate were used [30]. Based on experimental study, we fabricated specimens of pure magnesium with the dimensions of Ø10,5 x 10 mm through powder metallurgy with the different compaction temperature on each specimen (200°C, 250°C, 300°C), then sintered under the same sintering parameter. By using microstructure analysis through metallography examination, the porosity of the microstructure was evaluated. The examination revealed that the specimen with 300°C of compaction temperature showed the least porosity. Therefore, we decided to use 300°C as the compaction temperature in our study.
After deciding all of the fabrication parameters, we fabricated magnesium with variables amount of carbonate apatite in each specimen, which differs from 5, 10, and 15wt.%. During fabrication, we found that the specimens were burnt and major mass loss was observed after sintering process (Figure 2). Based on that finding, we tried to analyze the process by x-ray diffraction testing and reviewing the literature.
FIGURE 2. Specimens before sintering (left) and after sintering (right)
XRD examination found that there is an alteration of the specimens’ composition due to sintering process.
However, there was no major alteration occurred and the Mg/xCA still present as the main compositions. Upon reviewing the literature, we also found that elevation of temperature on carbonate apatite would lead to decarbonation process [31]. This flammable CO2 gas would need to be eradicated during sintering process.
Following the evaluation process, we decided to proceed only with the Mg/5CA and molded it into miniplate form.
We tried to reduce the CO2 gas by vacuuming the sintering chamber at the temperature of 300°C – 550°C. The results indicate that there was less burnt material after the sintering processes were done.
FIGURE 3. MG/5CA miniplate specimens before sintering (left) and after sintering (right)
Bending test was done to the specimen, with the pure Mg specimen and Mg ECAP used as comparison. Three kinds of flexural properties were tested, namely the flexural stress, flexural strain, and the modulus of elasticity. From the results, it was shown that the specimens that made through the powder metallurgy methods (both pure magnesium and Mg/5CA) were inferior regarding the flexural properties when it is being compared with the Mg ECAP specimens.
The main reason could have been the sintering process used during the fabrication. The low mechanical properties of the pure magnesium could be attributed to the presence of the stable oxide layer around each powder particles, hindering the consolidation process done during the sintering process. On the other hand, the low mechanical properties of the Mg/5CA specimens are worsened by the inability for the carbonate apatite particles to consolidate due to the different sintering temperature of the carbonate apatite and the magnesium powder particle. Sintering
pressure could suitably assist the densification process magnesium and bioceramic particle with higher sintering temperature.
FIGURE 4. Graph comparison of the mechanical properties of Mg/5CA miniplate specimen
CONCLUSION AND FUTURE IMPROVEMENT
Magnesium is a biodegradable metal with high potential as a base material for orthopaedic implant. In combination with carbonate apatite, it is expected that the degradation time would be longer. Additionally, augmentation of osteoconductive properties is also expected. However, the best fabrication method for this composite has not been discovered.
We initiated the research for fabrication of these two materials. Initial findings showed that the process of sintering induces the release of CO2 from the carbonate apatite. Further investigation regarding suitable consolidation process that would reduce the CO2 byproduct is needed. Alternatively, the coating process is maybe another option for fabrication method. After the composite is considered stable, mechanical properties, and biotoxicity tests would be conducted.
ACKNOWLEDGEMENT
The authors declare that there is no conflict of interest regarding the publication of this paper. This literature review is part of our study, which is funded by International Indexed Publications for Student Final Project (PITTA) grant by Universitas Indonesia and Decentralization and Competitive grant by Indonesian Ministry of Research Technology and Higher Education.
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