Research progress on biodegradable magnesium phosphate ceramics in orthopaedic applications

Kaushik Sarkar
Department of Ceramic Engineering, Indian Institute of Technology (BHU), Varanasi 221005, India. E-mail: kaushik.cer@iitbhu.ac.in

First published on 30th July 2024

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1. Bioceramics for bone replacement

To treat bone disorders and maintain bone health, there is a growing demand for synthetic bone substitute resources with appropriate physicomechanical and biological characteristics. Synthetic bone substitute materials include metals, polymers, and ceramics. Among the wide range of synthetic bone substitutes, ceramic materials are thoroughly investigated for their usage in orthopaedic applications. These bioceramics can be bioinert (alumina, zirconia, or a composite), bioactive (hydroxyapatite, bioactive glasses, and glass ceramics) and bioresorbable (tricalcium phosphate, bioglass). For bioactive ceramics, an interfacial bond will form between the implant and host tissue, whereas no interfacial bond will form between the host tissue and bioinert ceramics. In the case of bioresorbable ceramics, the implant will degrade inside the body and help in bone tissue formation. Because bone tissue is a combination of ceramic-based inorganic (hydroxyapatite) and organic (collagen type I) phases, significant investigation on bioresorbable and bioactive ceramics was performed for the purpose of engineering bone tissue.^1,2 The synthetic calcium phosphate (CaP) group of ceramics (hydroxyapatite; HA, tricalcium phosphate; TCP) have been widely used as bone substitute materials due to their ability to form a bone-like chemical bond with the living tissue, nonimmunogenic biocompatibility and chemical similarity to bone.^2–5 In the present scenario, CaP ceramics are used in various forms, such as powder, granules, coatings, cement, and dense or porous blocks, to treat bone defects in craniofacial and maxillofacial reconstructive bone surgery.⁶ Several studies have been conducted to prove the in vitro and in vivo biocompatibility and bone forming ability of TCP and HA ceramics.^7–9 However, a longer time frame for the degradation of HA and TCP ceramics raises the question of the natural character of the resorbable class of bioceramics. So, in view of degradation kinetics, new kinds of bioceramics have been evaluated for faster degradation in vivo with comparable biocompatibility and bone forming ability.^4,7–9

Magnesium phosphate (MgP) ceramics have recently received considerable interest from researchers due to their similar biocompatibility to calcium phosphate ceramics and bone-regeneration capability. The rationale for developing MgP ceramics is their suitable degradation behaviour and the importance of Mg²⁺ ions in bone remodeling. Mg, like Ca, is a bone component ion and serves essential roles in bone metabolism, such as stabilizing DNA and cell membranes, regulating bone density, influencing Ca²⁺ and Na⁺ ion channels, and increasing cell proliferation and differentiation by controlling cellular-level signalling pathways.^10–12 The deficiency of Mg retards cartilage and bone calcification processes and develops impaired bone with low toughness.¹¹ A healthy adult human needs an average of 24 grams of magnesium, where almost 60% of it is found in bone and teeth. The daily intake of Mg must be 420 mg per day for an adult male and 320 mg per day for an adult female. According to experiments, MgP ceramics are more biodegradable than CaP ceramics and have no deleterious effects on surrounding tissues.^13–16 Several in vitro investigations have shown that MgP bioceramics, both crystalline and amorphous, can promote cell adhesion, growth, and proliferation.^14,17–23 The tuneable degradation properties of magnesium phosphate make it an appropriate scaffold material for supporting cell adhesion and osseointegration.^21–26 Some recent publications reviewed magnesium phosphates for their potential application in bone cement, cement properties, and biological applications.^27–29 In another review, the microwave processing of MgP bioceramics was discussed thoroughly.³⁰ The available review articles were primarily focused on the formulation of bone cement, in-situ setting behaviour and their biocompatible nature. In this present article, we tried to provide a more comprehensive review of magnesium phosphate bioceramics, mainly different synthesis processes of MgP, the effect of porosity, the role of doping elements, the degradation ratio and the in vivo bone regeneration capacity. We believe that this review article will significantly influence further research on degradable magnesium phosphate ceramics for bone tissue engineering.

2. Role of magnesium in bone

Magnesium is one of the important elements found in mammals. It is the fourth most rich cation in the human body after Na, K, and Ca. Table 1 indicates the presence of Mg in different body parts, and out of that, 60% of Mg is found in bone and teeth. As Ca and Mg belong to a similar group in the periodic table, Mg shows analogous chemical characteristics and can replace Ca in bone minerals. An insufficiency of Mg²⁺ ions in human physiology can cause a low bone growth rate, osteoporosis, and bone fragility.³¹ The presence of adequate magnesium reduces the risk of bone fracture and increases bone ductility and density. Mg²⁺ ions play a crucial role in bone tissue development by increasing the attachment and proliferation of osteoblast cells by controlling cellular level signalling mechanisms.^11,32 Mg²⁺ acts as a co-factor for various enzymes, allowing it to bind to protein molecules and induce osteoblastic cell proliferation and differentiation.³³ This ion also plays a critical part in the maintenance of DNA and RNA structure and bone remodelling.^10,31 Mg²⁺ can help to maintain the cell's structural integrity and activate platelet-derived growth factor (PDGF). This PDGF may boost DNA and collagen production and promote osteoblast cell attachment, proliferation, and migration.³⁴ Xue et al. showed that the presence of Mg²⁺ ions (1 wt%) in tricalcium phosphate can effectively increase osteoblast cell proliferation and differentiation.³⁵ The presence of Mg²⁺ ions has also been shown to inhibit pre-osteoclast cell growth and differentiation, slowing the bone resorption process.³⁶ Thus, Mg-based ceramics can show a significant role in orthopaedic applications for bone defect repair.

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3. CaO–P₂O₅vs. MgO–P₂O₅ systems

The CaO–P₂O₅(CaP) based systems have been well explored and commonly used synthetic bioceramics in orthopaedic applications. Table 2 shows the different forms of CaPs and their important properties. On the other hand, the MgO–P₂O₅ (MgP) system has been explored in the last few years as a potential bioceramic. The research on MgP bioceramics is still in its early stages, and more thorough research is necessary for their wide application as an orthopaedic biomaterial. Depending on the Mg/P ratios, the MgP ceramics have different phases comparable to the phases of CaP bioceramics. Table 3 shows the various phases of MgP ceramics as differentiated by their Mg/P ratios. Most of the MgP phases contain crystallization water, whereas only limited CaP phases have water molecules in their structure. The Mg/P molar ratio for the MgP system is within the range of 0.5 to 1.5, which nearly resembles the CaP system. Bobierrite [Mg₃(PO₄)₂·8H₂O], Catiite [Mg₃(PO₄)₂·22H₂O], and Farringtonite [Mg₃(PO₄)₂] all have a Mg/P ratio of 1.5 and a similar Ca/P ratio can be found in tricalcium phosphate [Ca₃(PO₄)₂, TCP] phases for the CaP system. Both the α and β phases of TCP ceramics are extensively used as a bone substitute material. So their counterpart in the MgP system, the farringtonite [Mg₃(PO₄)₂] phase, has gained research interest for application as an orthopaedic biomaterial.^27,28 Monocalcium phosphate anhydrous [monetite, MCP, Ca(H₂PO₄)2] and dicalcium phosphate dehydrate [brushite, DCP, CaHPO₄·2H₂O] are two important phases in CaP having a Ca/P ratio of 1.0 which are mainly used as bone cement. Similarly, the struvite and newberyite phases of the MgP system have an Mg/P molar ratio of 1.0 and are highly reactive, like brushite and monetite. The high solubility of different MgP phases, comparable to CaP phases, makes them a suitable choice for degradable bioceramics.^38–40 The presence of NH⁴⁺ and K⁺ ions in MgP compounds such as struvite can effectively speed up the solubility. The compounds of the MgP system are metastable in nature, as they have structural water molecules at ambient temperature. Upon heating at a sufficient temperature, almost all the phases are converted to magnesium pyrophosphate (Mg₂P₂O₇) or farringtonite [Mg₃(PO₄)₂].²⁷ Similar to TCP ceramics of CaP, the farringtonite [Mg₃(PO₄)₂] has high solubility and biological properties. In contrast, the less soluble HA in the CaP group has a Ca/P ratio of 1.67, and their counterpart Mg/P molar ratio cannot be found in any of the MgP compounds. From Table 3, the solubility of MgP phases can be compared to that of CaP ceramics in Table 2, and it can be seen that all the MgP phases are chemically resorbable. However, it must be noted that the resorption properties are also dependent on the various physical properties, such as density, entrapped pores, microstructure, grain size, purity of precursor powder and their ratios, etc.

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4. Synthesis of magnesium phosphate bioceramics

After implantation in the defect region, the ceramics start to degrade with time and provide a suitable micro environment for bone regeneration and osseointegration. The degradation rate depends on the phase purity and physical properties, which are strongly dependent on different processing routes. Various process methods have been adopted to prepare the MgP ceramics, such as the solution precipitation method, acid–base reaction, microwave assisted process, and solid state synthesis process. Table 4 represents the different raw materials and their processing route for the synthesis of different forms of magnesium phosphate bioceramics. Because struvite and farringtonite are the most reported ceramics in biomedical applications, the main focus of this section was given to the synthesis of these two phases. Different forms of MgP such as MgHPO₄·3H₂O, Mg₃PO₄·5H₂O, Mg₃PO₄·8H₂O, Mg₃PO₄·22H₂O and Mg₂PO₄OH·4H₂O may be precipitated from different aqueous conditions of magnesium and phosphate containing solution.¹⁷ The MgP ceramics, in amorphous, semi-crystalline and crystalline form, were synthesised by the aqueous precipitation reaction method using magnesium chloride and trisodium phosphate as precursor materials.^42,43 The thermogravimetric analysis and the differential scanning calorimetry results [Fig. 1(a)] of the as synthesized MgP powder indicated that the crystallization temperature lies around 700 °C and both the amorphous and crystalline phase showed excellent cytocompatibility for MC3T3-E1 cells.⁴²

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Wang et al. prepared MgP ceramics utilizing the chemical precipitation technique and the XRD results [Fig. 1(b)] of 400 °C, 600 °C, and 800 °C sintered samples showed that all the peaks belonged to Mg₃(PO₄)₂ phase.⁴⁶ Synthesis of MgP ceramics by the acid–base reaction is an exothermic process where the typical precursor material is dead burnt magnesium oxide (MgO) or tri magnesium phosphate [Mg₃(PO₄)₂].^18,55 These precursor materials are then allowed to react with water soluble monatomic or diatomic phosphate salts of potassium, ammonium, and sodium. Struvite, the most frequent product of ammonium magnesium phosphate, is formed when ammonium phosphate salt is used. Depending upon the precursor material [MgO or Mg₃(PO₄)₂], the by-product and possible reaction routes are proposed.²⁸

MgO + (NH4)2HPO4 + 5H2O → NH4MgPO4·6H2O + NH3↑	(1)

2Mg3(PO4)2 + 3(NH4)2HPO4 + 36H2O → 6NH4MgPO4·6H2O + H3PO4	(2)

When MgO was used (eqn (1)) along with struvite, ammonia vapour was also released into the air. In the case of the farringtonite precursor, phosphoric acid (eqn (2)) was the by product which decreased the pH of MgP-based cement.⁵⁶ The release of ammonia may cause an unpleasant environmental odour, which can be overcome by using potassium or sodium phosphate salt in place of ammonium phosphate. The formation of K-struvite (potassium magnesium phosphate hexahydrate, KMgPO₄·6H₂O) and Na-struvite (sodium magnesium phosphate heptahydrate, NaMgPO₄·7H₂O) took place when ammonium phosphate was replaced by the potassium phosphate salt and sodium phosphate salt, respectively. Wang et al. synthesised MgP powder by using magnesium nitrate [Mg(NO₃)₂·6H₂O] and ammonium dihydrogen phosphate [NH₄H₂PO₄].⁴⁶ The precipitation product, after dropwise addition of Mg(NO₃)₂·6H₂O into NH₄H₂PO₄ solution was sintered at 400 °C, 600 °C and 800 °C.

For the acid–base reaction method, the reaction kinetics and remnant precursor in the final product strongly depend on the physical properties and the mixing ratio of the precursor materials.⁵⁵ Another influential parameter is the solid to liquid ratio for the MgP cement system in terms of setting time, injectability and mechanical strength.^51,56 In general, the major drawback of this process is the in situ exothermic reaction at the time of phase conversion, which may cause local tissue damage.⁵⁵ To overcome this problem, a microwave assisted synthesis method was employed by the researchers. MgP cement was prepared by microwave heating for 5 min of the mixture of Mg(OH)₂ and water based solution of H₃PO₄. It was found that the microwave-treated precursor material had not shown an exothermic reaction and possessed better mechanical properties than the material which experienced the exothermic reaction.²¹ An amorphous magnesium phosphate nanosphere precursor was prepared using a microwave-assisted synthesis method in a household micro oven.²² By mixing MgCl₂·6H₂O, KH₂PO₄ and NaHCO₃ in distilled water the reaction solution was prepared for microwave heat treatment. Farringtonite powder can also be prepared by using a high temperature treatment of acid–base reaction product. Mg(OH)₂ and H₃PO₄ in a molar ratio of 3:2 were mixed and allowed to react for 24 h. The dried precipitate product was then heat treated at 850 °C for 6 h to form farringtonite.^44,45

Another important method of synthesis of MgP ceramics is the solid state synthesis process. In this process, MgHPO₄·3H₂O and Mg(OH)₂ powder were mixed in a 2:1 molar ratio and calcined at a high temperature to form the Farrington [Mg₃(PO₄)₂] phase.^20,57–59 The farringtonite phase formation temperature was 1000 °C,⁵⁸ 1050 °C^57,59 and 1100 °C.²⁰ The authors have not reported the mixing time and precursor particle size, which might be a possible reason for different calcination temperatures. In another study, Mg₃(PO₄)₂ was prepared using MgHPO₄·3H₂O and Mg(OH)₂ powders by a two step sintering process.⁵² In the first step, the powder mixture was calcined at 1050 °C for 5 h, and the crushed powder was used to prepare the sample, which was sintered at 1175 °C for 4 h. He et al. used NH₄H₂PO₄ and 4MgCO₃·Mg(OH)₂·5H₂O for the synthesis of Mg₃(PO₄)₂ powder.⁵³ The raw powders were mixed in a ball mill, followed by calcination at 1000 °C for 2 h. This calcined powder was then crushed and compacted by cold isostatic pressing at 200 MPa for 2 min and sintered at 1200 °C.

5. Porous MgP scaffolds

Bone is a natural composite that contains a hierarchical 3D porous structure. This porous structure provides sufficient space for the nerves, blood vessels, nutrients and cell transport. Also, the porous structure acts as a bone cell attachment site, which can help in bone remodelling. Keeping these things in mind, researchers have tried to prepare porous ceramics as potentially viable bone substitute materials. The primary considerations were pore size (macro, micro), pore distribution, pore connectivity, pore morphology and sufficient mechanical strength. The macropores (>100 μm) allow cell ingrowth, and the micropores (<20 μm) provide a high specific area for cell attachment. A sufficient number of articles have been published, which indicates that porous ceramics are very helpful for bone regeneration.

Porous MgP-based ceramics were also prepared to check the enhanced biocompatibility both in vitro and in vivo. A MgP scaffold with controlled porosity was produced by a paste extruding deposition (PED) system, which showed high drug release efficiency and cytocompatibility.^25,26,45 Porous three dimensional MgP was prepared along with the incorporation of HA containing polymeric microspheres, which showed controlled biodegradation and mechanical strength loss.⁴⁵ Porous MgP was prepared at room temperature using Mg as a porogen, where macro-pores were formed due to hydrogen gas bubble evolution.⁶⁰ Meininger et al. used 3D printing to prepare porous Sr-incorporated MgP bioceramics, which have compressive strength within the range of cancellous bone.⁵² Kim et al. prepared micro porous MgP bioceramics using NaCl as a pore former.⁶¹ These porous MgP ceramics showed better in vitro bioactivity and higher in vivo bone forming ability, compared to the non porous MgP ceramics in a rabbit model. In another study, Kim et al. successfully incorporated a novel indene compound in the 3D printed porous MgP scaffold. This MgP scaffold showed a stimulatory effect on osteoblast cell differentiation and in vivo bone regeneration in rat models.⁴⁴ However, the porous scaffold was not correlated with the other physico-mechanical properties. Sarkar et al. prepared pure, 0.5 wt% Zn, 0.5 wt% Si and 2 wt% Sr-doped MgP scaffolds by utilizing naphthalene as a pore forming agent and reported that the porosity was the main parameter for the improved cytocompatibility.⁶²

6. Doped magnesium phosphate ceramics

When a degradable class of material is implanted inside the body, it is expected that the implant gradually degrades with the same rate of formation of new bone tissue with no toxic effects. For magnesium phosphate bioceramics, the primary advantages are its suitable degradation behaviour, mechanical strength, and enhanced biological properties. These properties further can be tailored by the incorporation of some trace elements (Na, Li, Ag, Zn, Sr, Si), which are generally found in bone minerals. Several studies have already shown that doping with these metallic ions (Na⁺, Li⁺, Ag⁺, Zn²⁺, Sr²⁺, Si⁴⁺) could be an effective way to tailor the degradation behaviour and enhanced biological response of ceramic materials.^63–66 As the research on MgP bioceramics is at a preliminary stage, the dopant induced biocompatibility has not been well explored, unlike the CaP group of ceramics. Meininger et al. prepared pure, 0.5 and 1.0 at% Sr-doped MgP ceramics via a solid state sintering method.⁵² At first, the precursor powders were ball milled and then calcined at 1050 °C for 5 h and then 3D printed to prepare MgP scaffolds. The pure and Sr-doped scaffolds were sintered at 1175 °C and 1100 °C for 4 h, respectively and evaluated in terms of in vitro degradation behaviour and mechanical properties. It was found that the compressive strength increased from 22 MPa of MgP to 36 MPa of 0.5% Sr-doped MgP ceramics after the hardening reaction. In another study, Meininger et al. prepared 0, 8.2, 16.4, and 24.6 wt% Sr incorporated MgP cement powder by a solid phase sintering process at 1050 °C for 5 h.⁵⁴ He et al. synthesized undoped and Sr doped (1, 1.5, 2 and 3 at%) MgP ceramics by a solid phase reaction method where the powders were calcined at 1000 °C for 2 h.⁵³ The calcined powders were then uniaxially compacted at 5 MPa, followed by cold isostatic pressing at 200 MPa for 2 minutes. The undoped and Sr doped samples were sintered at 1200 °C and 1100 °C for 2 h. The results showed that the 1.5% Sr-doped MgP ceramics have the highest compressive strength, excellent degradation rate and suitable cytocompatibility. Similarly, Sarkar et al. prepared pure Zn (0.25, 0.5 wt %), and Sr (1, 2, 3 wt %)-doped MgP ceramics. The precursor powders were mixed using a planetary ball mill and then calcined at 1150 °C for 5 h. Samples were prepared from calcined powder and sintered at 1200 °C for 2 h.^67–69 They have reported that the addition of dopants increases the sintering kinetics and thereby enhanced densification was achieved. 0.5 wt% Zn doping increases the relative density from 90.15% to 96.80% of MgP ceramics.⁶⁷ Similarly, the density was increased for 3 wt% Sr-doped MgP ceramics from 90.15% to 95.84% after sintering.⁶⁸ Kumar et al. synthesised Si (0.5, 1.25, 2.0 wt%)-doped MgP ceramics and found that the sintered density was improved from 90.15% to 93.10%. Therefore, it can be summarized that the doping of metallic ions in MgP ceramics can change the degradation behaviour and cytocompatibility, which can enhance the in vivo biocompatibility.

7. Biological properties

7.1. Degradation studies

Degradation kinetics is one of the primary considerations for the development of any bioceramic. The local ionic concentration changes as the material starts to degrade and these changes in ionic activity can strongly influence various biochemical processes. Two types of mechanisms cause the degradation of bioceramics: (i) active degradation and (ii) passive degradation. Active degradation is the process where the material starts to dissolve by the osteoclast cell-mediated resorption process. The passive degradation of the material is the simple chemical dissolution of the surfaces when the material's solubility product exceeds the ionic concentration of the surrounding liquid. For solubility-mediated degradation, researchers have used different types of aqueous solutions, such as simulated body fluid (SBF), and phosphate buffered saline (PBS). The degradation kinetics is an important study in terms of weight loss, pH change of the media, and the released ion concentration. This degradation study is helpful for the comparative rate of degradation of different compositions and ionic concentrations, which can correlate with the cell response. In ideal conditions, the dissolution rate of the material should be the same as the rate of formation of new bone tissue and the material will gradually be replaced by the newly grown bone. MgP ceramics have a higher dissolution rate compared to CaP ceramics.^49,70 Studies have shown that MgP ceramics could be dissolved by both passive dissolution and osteoclastic cell-mediated resorption, whereas HA ceramics were dissolved by a cell-mediated resorption process.⁴⁹ Wu et al. reported that the ratio of MgP to CaP controlled the in vitro degradation rate of the combination of MgP and CaP ceramics (Table 5), and the degradation rate increased with the amount of MgP.^13,71Fig. 2 represents the higher weight loss ratio of the MgP system compared to that of calcium phosphate ceramics. Similarly, Liu et al. showed that the composite of magnesium phosphate and tricalcium phosphate showed a lower in vitro degradation rate compared to pure magnesium phosphate.⁴⁷

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The in vivo degradation study has also proven that the MgP ceramics are more degradable than the CaP ceramic groups.^56,70 Porosity is also an influencing factor for the degradation rate of bioceramics. With increasing the sintering temperature the porosity decreases, which causes a lower dissolution rate.⁴⁶ The rate of degradation can be controlled by changing the porosity within the structure by acquiring different processing technologies.^44,61

7.2. Cytocompatibility

Traditionally MgP ceramics have been used in refractories, rapid construction repair, and waste remediation.^72–76 No particular attention was given to MgP ceramics in the biomedical field because of extensive research on CaP-based ceramics groups. CaP-based ceramics mimic the chemical composition of bone, which makes them suitable for synthetic bone material. It was in 1995 that Driessens et al. first implanted MgP ceramics subcutaneously in rats.⁷⁷ After this experiment, numerous research results have been published which showed that MgP ceramics are inherently nontoxic in nature. In general, osteoblast cell lines such as MC3T3-E1 and MG63 were used to study the positive effect of MgP ceramics on cell adhesion and proliferation. In a comparison manner, MgP ceramics showed higher cell adhesion and proliferation than CaP-based ceramics.^16–18 Ewald et al. reported that MG-63 cells have higher cellular level activity and growth rate on MgP ceramics compared to their CaP ceramic counterparts.¹⁸ In other studies, Mg containing phosphate bonded ceramics showed that MG-63 cells are normally attached to the ceramic surface along with a stimulated proliferation rate.^13,46,48,78 Similarly, MC3T3-E1 preosteoblast cells showed a significant proliferation rate and attachment on the Mg containing phosphate bonded bioceramics.^21,26,42,61 Kim et al. tested the cytocompatibility of MgP ceramics using Human bone marrow mesenchymal stem cells (BMSCs) and MC3T3-E1 preosteoblast cell lines.⁴⁴ They reported that both the cell lines can proliferate and differentiate on the MgP ceramics system. Mouse bone mesenchymal stem cells (mBMSCs) were seeded on Mg₃(PO₄)₂ samples and found significantly higher cell proliferation compared to TCP ceramics after 5 days of culture.⁵³ Amorphous magnesium phosphate nanospheres prepared by microwave irradiation showed cytocompatibility for mouse 7F2 osteoblast cells.²⁴

The osteogenic response of MgP ceramics in comparison to CaP ceramics and polystyrene controls was also studied and the results were not consistent. Ewald et al. studied osteogenic protein expression of bone specific proteins including osteopontin, bone sialoprotein, alkaline phosphatase, and osteocalcin on MgP and CaP systems.¹⁸ The MgP system showed higher expression of osteogenic protein production compared to the polystyrene controls but significantly lower than the CaP system. Ostrowski et al. assessed the osteogenic differentiation of MC3T3-E1 cells for the MgP ceramics.⁴² The authors have reported that the ALP activity of amorphous MgP was higher for 7 days and then decreased at 14 days, in comparison to TCP ceramics. The osteogenesis related relative gene expression, such as collagen type I (Col I), runt-related transcription factor 2 (Runx2), osteocalcin (OCN), bone sialoprotein (BSP) and alkaline phosphatase (ALP) for mBMSCs, was higher for Mg₃(PO₄)₂ compared to the TCP sample after culturing for 7 days, in the absence of osteogenesis induction supplements. In contrast, with an osteogenesis induction supplement the Mg₃(PO₄)₂ showed lower expression of osteogenesis related genes.⁵³

7.3. Antibacterial properties

Surgical intervention is required for the repair of critical bone fractures and non union bone defects where biomaterials are used. Postoperative bacterial infection is one of the main reasons for the failure of implantation and causes serious morbidity. About 2–5% of the total orthopaedic surgeries involve complicated bacterial infections.⁷⁹ The serious outcome often leads to the removal of the implant, surgical damage of tissue, antibiotic therapy and also prolonged hospitalization of the patient.⁸⁰ Generally, any biomaterial is vulnerable to bacterial infection at any time in its lifetime. So the antimicrobial activity of any biomaterial also needs to be evaluated for successful implantation. For bioceramics, CaP ceramics have been developed with the incorporation of different antimicrobial agents such as antibodies, and inorganic ions (Ag).^80–82 As the MgP ceramics are still under evaluation for application in the biomedical field, the antibacterial properties of these ceramics need to be assessed. Mestres et al. reported that MgP cement had antibacterial activity on S. sanguinis bacteria.⁵⁵ The growth rate inhibition of bacteria was attributed to the reactive oxygen species and the bactericidal effect was due to the excessive increase of pH. In another study, Mestres et al. observed the antibacterial property of MgP cements against Escherichia coli, Pseudomonas aeruginosa, and Aggregatibacter actinomycetemcomitans.⁸³ The authors claimed that the bacteriostatic effect and reduced biofilm formation was because of a change in osmolarity and the pH of the solution. On the contrary, the newberyite single phase of the MgP system has not shown any inhibition effect on bacterial growth.⁸⁴ So the intrinsic antimicrobial properties of this new class of bioceramics should be enhanced to combat surgical site infection.

7.4. In vivo biocompatibility

CaP ceramics are commercially available bone substitute materials that are being currently used by medical practitioners due to their excellent biocompatibility. MgP ceramics can be thought of as an alternative to CaP bone substitute material for bone repair because of their excellent cytocompatibility. The research on MgP ceramics for the biomedical field is still in the primary stage and very few research articles are available on in vivo biocompatibility studies. The in vivo biocompatibility of MgP was first checked by Driessens et al. after implanting subcutaneously in rats.⁷⁷ They found that the mechanical strength of the retrieved MgP samples decreased with time. Wu et al. implanted calcium magnesium phosphate (CMPC) cement in the femur bone of a rabbit.¹³ After 6 months, histological analysis indicated that the presence of MgP ceramics in CaP composition enhanced the bone formation at the defect region. Yu et al. have extensively checked the inherent nontoxic and biocompatible nature of MgP ceramics.¹⁶ After the implantation of MgP in the rabbit femur for 6 months, they found that the implant was completely replaced by newly generated bone. In vivo degradation behaviour of MgP ceramics, in comparison with CaP ceramics, was studied after implanting at femoral extensor muscles of Sprague–Dawley rats.⁴⁰ Micro-CT results showed that MgP samples had a considerably fast degradation rate and CaP samples were comparatively less soluble after 15 months of implantation. Similar results were found in another study by Kanter and co workers.⁵⁶ They implanted MgP and CaP ceramics in the medial femoral condyle portion of merino sheep and found that after 10 months, the CaP sample showed very poor degradation. The brushite sample showed a small reduction in volume and the HA was unchanged during the implantation timeframe. In contrast, the MgP samples showed fast resorption kinetics and bone forming ability. From μ-CT analysis (Fig. 3) it was observed that the struvite prepared with PLR of 2.0 gm/ml completely resorbed and the defect was filled with mineralized tissue, whereas the diameter of struvite prepared with PLR of 3.0 gm mL⁻¹ reduced to ∼3–4 mm after 10 months of implantation. Therefore, these results clearly indicated that the MgP ceramics are more degradable than the traditional CaP ceramics under in vivo conditions.

Detailed in vivo studies in terms of biodegradation, and new bone formation of the MgP bioceramics are rarely available in the published literature. Kim et al. prepared micro porous MgP scaffolds using 3D printing technology and studied bone regeneration after implanting in rabbit calvarial defects (with 4 and 6 mm diameters). Micropores lower than 25 μm (MgP25), between 25 and 53 μm (MgP53) and non porous (MgP0) MgP scaffolds using NaCl pore former were prepared and implanted for 4 and 8 weeks in the defect site. For the 4 mm defect site, the porous MgP25 and MgP53 were dissolved completely after 4 weeks and an amorphous natural trabecular bone-like structure was found in the micro-CT analysis. In contrast, the MgP0 micro-CT analysis showed that the scaffolds retained their strut-like morphological shape. On the other hand, for the defect size of 6 mm, relatively thick bone formations were found for the MgP scaffolds. After 4 weeks, it was observed that the porous MgP scaffold-induced newly formed bone reached the center of the defect area. At 8 weeks, the micro-CT analysis showed that the volume of newly formed trabecular bone was significantly increased for the porous MgP scaffolds. Histological analysis showed that bone and blood vessels formed into the center of the calvarial defect and around the scaffold rods after 4 weeks of implantation for the 4 mm defect site. A higher number of blood vessels were found for MgP53 compared to the MgP25 and MgP0 scaffolds. After 8 weeks, the presence of bone-lining cells on the surface of trabecular-like bone indicates that bone regeneration and remodelling are in progress. Fig. 4 shows that both the MgP53 and MgP25 defect areas contained well-organized bone marrow like tissue and plenty of blood cells, which is an indication of the synthesis of new bone. The quantitative analysis of 4 mm defect samples showed that the defect area was covered up 42.5% for MgP53 and 35.6% for MgP25 by the new bone tissue compared to the area of 21.62% for non-porous MgP0 at 4 weeks. After 8 weeks, comparatively more mature bone tissue was found with a filling area of 60.8% for MgP53, 50.3% for MgP25, and 28.5% for MgP0 at the 4 mm defect sites. However, a slower bone regeneration rate was observed for the 6-mm defect model both after 4 and 8 weeks of implantation. The bone formation was found to be 17.93%, 15.49% and 7.21% for MgP53, MgP25 and MgP0, respectively, at the implanted area after 4 weeks of implantation. At 8 weeks, the regeneration of bone tissue was found to be 34.4%, 35.4% and 30.4% for MgP53, MgP25 and non-porous MgP0, respectively, which is significantly lower than that of the 4 mm defect model.

A large animal model such as merino sheep (4–6 years, 94 ± 8 kg) was also used to check the bone forming ability of MgP ceramics.⁸⁶ After 10 months of implantation, the μ-CT analysis showed that the ceramic material degraded with time and was replaced by newly generated trabecular bone. Bioactivity and bone forming ability can be increased by incorporating different bioactive substances in degradable biomaterials. Kim et al. loaded osteogenic substances in MgP ceramics and implanted them in the calvarial defect of a rat to examine the bone regeneration capacity. After 8 weeks, the osteogenic molecule-loaded MgP ceramics showed enhanced bone regeneration at the defect site compared to the pure MgP ceramics. Histological analysis indicated that the bone maturation was higher for the osteogenic substance-loaded MgP ceramics.⁴⁴

8. Conclusion and future remarks

The present comprehensive review provides basic insights into magnesium phosphate ceramics in terms of their synthesis routes and changes in physicochemical properties by incorporating metallic ion doping, as well as their in vitro and in vivo biocompatibility. In recent years, MgP-based bioceramics have been explored as potential degradable bioceramics. These degradable class ceramics can be used to treat bone defects, and for craniofacial and maxillofacial reconstructions. In this report, we have covered almost all types of synthesis methods for the preparation of MgP bioceramics. Also, the effect of different doping elements in terms of their role in degradation kinetics and in vitro and in vivo bioactivity is discussed. The application of doping elements is a unique process for changing the degradation kinetics of bioceramics and therefore the nature and rate of the defect healing process. However, very limited information is available in terms of the in vivo study for MgP ceramics compared to the in vitro study. The in vivo studies have mainly recognized the bone forming ability after implantation. Research on MgP-based ceramics still remains in the primary stage from the perspective of biomedical applications and systematic thorough investigation is required to make commercially viable bioceramics. For that, detailed in vivo studies need to be carried out keeping in mind the faster bone regeneration and osseointegration capacity. The MgP ceramics could be a good choice for localized delivery applications of drugs and biomolecules for treating bone cancer and osteoporosis or for faster bone defect healing. Incorporation of biomolecules such as BMP-2, VEGF, and TGF can effectively increase the healing rate by speeding up the osteogenesis and angiogenesis process in the near vicinity of their application site. The release kinetics of the drugs and biomolecules should be modulated in a patient-specific manner. The release kinetics of drugs and biomolecules can be tuned by the incorporation of doping elements and the introduction of porosity into MgP ceramics.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The author would like to acknowledge the Director of IIT (BHU) Varanasi and the Department of Ceramic Engineering of IIT (BHU) Varanasi for their support.

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