Recent advances in the local antibiotics delivery systems for management of osteomyelitis

Abstract

Chronic osteomyelitis is a challenging disease due to its serious rates of mortality and morbidity while the currently available treatment strategies are suboptimal. In contrast to the adopted systemic treatment approaches after surgical debridement in chronic osteomyelitis, local drug delivery systems are receiving great attention in the recent decades. Local drug delivery systems using special carriers have the pros of enhancing the feasibility of penetration of antimicrobial agents to bone tissues, providing sustained release and localized concentrations of the antimicrobial agents in the infected area while avoiding the systemic side effects and toxicity. Most important, the incorporation of osteoinductive and osteoconductive materials in these systems assists bones proliferation and differentiation, hence the generation of new bone materials is enhanced. Some of these systems can also provide mechanical support for the long bones during the healing process. Most important, if the local systems are designed to be injectable to the affected site and biodegradable, they will reduce the level of invasion required for implantation and can win the patients’ compliance and reduce the healing period. They will also allow multiple injections during the course of therapy to guard against the side effect of the long-term systemic therapy. The current review presents different available approaches for delivering antimicrobial agents for the treatment of osteomyelitis focusing on the recent advances in researches for local delivery of antibiotics.

HIGHLIGHTS

1. Chronic osteomyelitis is a challenging disease due to its serious mortality and morbidity rates and limited effective treatment options.

2. Local drug delivery systems are receiving great attention in the recent decades.

3. Osteoinductive and osteoconductive materials in the local systems assists bones proliferation and differentiation

4. Local systems can be designed to provide mechanical support for the long bones during the healing process.

5. Designing the local system to be injectable to the affected site and biodegradable will reduces the level of invasion and win the patients’ compliance.

Keywords:

• Osteomyelitis
• bone scaffolds
• tissue engineering
• nanoparticles
• 3 D printing
• bone grafts
• local drug delivery

1. Introduction

Osteomyelitis, a term indicating infection in the bone, is accompanied by acute or chronic inflammatory events. Bone infection usually arises from pyogenic organisms including bacteria, fungi and mycobacteria. In cases such as ischemia, trauma, surgery or those associated with incorporation of artificial joint devices, susceptibility of the bone and its structures to catch the causative microorganism is high (Momodu & Savaliya, 2020).

The opportunistic gram positive Staphylococcus aureus was found to be the most common causative microorganism in bone infection, which calls for approximately 75% of cases of osteomyelitis (Walter et al., 2012; Kavanagh et al., 2018). Other causative microorganisms include enterococcus species, streptococcus species, pseudomonas aeruginosa and enterobacteriaceae (Fraimow, 2009).

The tendency of bacteria to form biofilm of high density of microbial content made of condensed matrix of DNA, proteins and polysaccharides is considered an obstacle in the treatment of osteomyelitis (Zimmerli & Moser, 2012). Formation of biofilm can hinder the accessibility of the antimicrobial agents into the infected site resulting in resistance of the bacteria to the antimicrobial agents and increasing the rates of failure of treatment (Høiby et al., 2011).

Etiologically, three clinical categories of osteomyelitis are identified; (1) A bone infection that is derived or transported through the systemic circulation called hematogenous osteomyelitis, (2) Osteomyelitis associated with a contiguous source such as surgical procedure, incorporation of artificial joint or trauma and (3) Osteomyelitis due to contiguous source associated with vascular insufficiency which is most commonly seen in patients with peripheral vascular diseases and diabetes mellitus (Ford & Cassat, 2017).

Osteomyelitis has more prevalence in the developing countries than in the United States (U.S) (Uskokovic, 2015). Reports from the U.S hospitals admissions in 1999 showed that the overall incidence of osteomyelitis was found to be about 50,000 cases per year or as high as 1 in 675 hospital admissions annually (Rubin et al., 1999). A comprehensive epidemiological study describing the entire spectrum of osteomyelitis in the U.S over 41-year from 1969 to 2009, showed that there was an increased incidence of osteomyelitis due to the increase in the prevalence of risk factors such as diabetes. The major incidence of osteomyelitis among older individuals was mainly because of the higher frequency of disorders such as diabetes mellitus and orthopedic surgeries that lead to infections. The most common etiology in children was hematogenous infections (Kremers et al., 2015).

The three categories of osteomyelitis can occur in acute or chronic phases. Acute osteomyelitis develops over several days or weeks in contrast to the chronic one, which is known to evolve over a span of months or even years. The mechanism of acute infection involves the entry of the microorganisms, which release proteolytic enzymes and toxic free radicals causing inflammation and destruction of the surrounding tissues. This leads to unreachable blood flow to the area of infection, which causes poor vascularized area. The poor vascularization enhances the chance of forming sequestrum that results from the destruction of bone and hence boosts the impediment of penetration of the antimicrobial agents, which accounts later for the chronicity of the infection. Chronic osteomyelitis has different stages demonstrating the progression of the disease as well as the degree of deterioration of bones. Surgery is usually needed to remove the infection focus for the complete resolution of infection.

As it is very important to identify the clinical stage of patients with chronic osteomyelitis before treatment, many classification systems were described by several authors. The most widely used classification system is the one developed by Cierny and colleagues in 1985 (Cierny et al., 1985). This classification provides guiding for surgical or antibiotic therapy approaches (Calhoun et al., 2009). It was developed based on several parameters namely; the anatomical position of the infection, the extent of bone damage and the patient status. According to this classification, there are four anatomic types of osteomyelitis based on the degree of tissues involvement. These four types are: (1) medullary osteomyelitis when the affected part of the bone is restricted only to the intramedullary cavity of the cortical bone and is usually caused by single pathogen, (2) superficial osteomyelitis when the bone surface is damaged, (3) localized osteomyelitis which involves both the cortical and medullary areas in the damaged bone and is usually caused by multiple pathogens and (4) diffuse osteomyelitis when the entire thickness of the bone is damaged and extend to multiple bones and soft tissues structures that leads to bone destruction. These anatomic types are combined with other physiologic factors to define the clinical stage (stages 1–4) of the patient with their corresponding medical intervention (Cierny et al., 2003) as presented in Table 1.

Table 1. Clinical stages and suggested medical intervention of chronic osteomyelitis according to Cierny –Mader (Cierny et al., 2003) classification system.

Clinical stage	Required medical intervention
Stage I	Systemic antimicrobial therapy at early stage. Debridement may be needed at late stage.
Stage II	Systemic antimicrobial therapy at early stage. Debridement may be needed at late stage.
Stage III	Antimicrobial therapy at early stage in addition to limited surgical procedures.
Stage IV	Requires surgical + antimicrobial therapy in addition to post-surgery stabilization.

2. Background on the current treatment strategies

Due to the increased morbidity rates in patients suffering from osteomyelitis and the challenging treatment plan for complete eradication of the infection, extensive efforts are being spent and researches are being devoted that improved the therapeutic strategies to include:

2.1. Surgical procedures

Surgical procedures that involve complete debridement of the formed biofilm and necrotic tissues is a functional and effective approach in the eradication of the infection in complicated chronic osteomyelitis (Inzana et al., 2016). However, the critical problem of developing what is called dead space necessitates the effective management to reduce the chance of re-infection. This involves the implementation of bone grafts in the debrided tissues or the implementation of biomaterial spacers loaded with antibiotics and located in the bone tissue to deliver them locally in the area (Nandi et al., 2009). In a retrospective review study, Segreto et al. (2018) reported that early surgical intervention showed better outcomes than delayed one.

2.2. Systemic antibiotic therapy

Systemic antibiotic therapy is an effective strategy for the eradication of acute infections, and if combined with surgical treatment, it can be used in the treatment of chronic osteomyelitis for the complete eradication of the infectious microorganisms (Lew & Waldvogel, 2004). Antimicrobial therapy could be initiated according to culture media data of collected specimens. According to causative microorganisms, a set of antimicrobial agents were reported as effective therapy for most cases of osteomyelitis (Howard‐Jones & Isaacs, 2013; Senneville & Nguyen, 2013) either alone or as an adjunctive therapy after surgery. Table 2 presents examples of the officially approved products for systemic treatments with their date of approval. The treatment course usually starts with intravenous (IV) antimicrobials to improve the drug concentration in bone. Following, shifting to oral anti-bacterial agents might be considered in patients with good clinical response (Peters & Lipsky, 2013; Peltola & Pääkkönen, 2014).

Table 2. Examples of the currently available antimicrobial therapy targeting osteomyelitis causative pathogens with the systemic route of administration.

Microorganism	Antimicrobial agent	Route	Products approved by the FDA	Date of approval
Methicillin- resistant staphylococcus aureus (MRSA)	Vancomycin	IV	Firvanq^®	2018
			Vancomycin^®	1999
	Tedizolid	IV/ Oral	Sivextro^®	2014
	Daptomycin	IV	Cubicin^®	2003
	Linezolid	IV/ Oral	Zyvox^®	2000
	Trimethoprim/sulfamethoxazole (+oral Rifampin)	IV	Sulfamethoxazole and trimethoprim^®	1991
		Oral	Sulfamethoxazole and trimethoprim^®	1986
	Clindamycin	IV	Cleocin phosphate^®	1972
		Oral	Cleocin hydrochloride ^®	1970
Methicillin- sensitive staphylococcus aureus (MSSA)	Cefazolin	IV	Cefazolin sodium^®	1988
	Nafcillin/oxacillin	IV	Unipen^®	1970
P. aeruginosa	Levofloxacin	Oral	Levofloxacin^®	2011
		IV	Levofloxacin^®	2005
	Ciprofloxacin	IV	Ciprofloxacin^®	2009
		Oral	Ciprofloxacin^®	2004
	Meropenem	IV	Merrem^®	1996
	Cefepime	IV	Maxipime^®	1996
	Piperacillin/tazobactam	IV	Zosyn^®	1993
	Ceftazidime	IV	Fortaz^®	1985
	Imipenem/cilastin	IV	Primaxin^®	1985
Enterobacteriaceae	Moxifloxacin (+antipseudomonal agents)	IV/oral	Moxifloxacin hydrochloride^®	2015
	Ceftriaxone	IV	Cftriaxone^®	2003
	Cefotaxime	IV	Cefotaxime sodium^®	2002
Enterobacteriaceae (Anaerobes)	Clindamycin	IV/oral	Cleocin phosphate^®	1999
			Cleocin HCL^®	1998
	Metronidazole	Oral	Flagyl^®	1995
		IV	Flagyl^®	1981
Enterococcus spp. (Ampicillin resistant)	Vancomycin	IV	Firvanq^®	2018
			Vancomycin^®	1999
Enterococcus spp. (Ampicillin sensitive)	Ampicillin/ sulbactam	IV	Unasyn^®	1986
Enterococcus spp. (Vancomycin resistant)	Tedizolid	IV/oral	Sivextro^®	2014
	Linezolid	IV/ oral	Zyvox^®	2007
	Daptomycin	IV	Cubcin^®	2003
Streptococcus spp.	Penicillin G	IV	Bicillin L-A^®	1952

Empiric antimicrobial therapy that covers all possible microorganisms is vital when the isolation of the causative pathogens from the infection site is not possible. Empiric therapy should target likely the causative pathogen(s) based on patient-specific risk factors and route of infection. The activity of empiric antimicrobial therapy against S. aureus should be taken into consideration in the treatment strategy in all the classifications of osteomyelitis (Conrad, 2010). Nosocomial infections that got from the hospitals are usually derived from methicillin-resistant S. aureus (MRSA), while infections got from somewhere else are often polymicrobial involving Gram-negative bacteria in some cases (Conrad, 2010). The utilization of anti-MRSA antimicrobials ought to be considered as the first-line therapy for empiric inclusion of suspected staphylococcal osteomyelitis, then antimicrobial therapy ought to be modified depending on culture and sensitivity data of appropriately collected specimens (Howell & Goulston, 2011).

Customarily, anti-infection treatment of osteomyelitis is comprised of 2–6 week course of IV administration of antibiotics, followed by oral antibiotics for 6-months course (Calhoun & Manring, 2005). Oral antimicrobial agents should possess such attributes as high bioavailability, good bone accessibility and long half-life (Senneville & Nguyen, 2013). Oral antibiotics that were concluded as being successful include clindamycin, rifampin, trimethoprim-sulfamethoxazole, linezolid and fluoroquinolones (Chihara & Segreti, 2010; Harik & Smeltzer, 2010; Liu et al., 2011; Senneville & Nguyen, 2013).

For parenteral route, Vancomycin, a member of the glycopeptide family, is a broad-spectrum antibiotic, active against Gram-positive bacteria including MRSA and is considered the first line therapy unless the minimum inhibitory concentration is greater than 2 mcg/mL (Senneville & Nguyen, 2013). Clindamycin is active against most Gram-positive bacteria, including staphylococci. It has excellent bioavailability and is administered by IV route for 1–2 weeks followed by oral administration (Liu et al., 2011). Linezolid is active against MRSA and vancomycin-resistant Enterococcus. It can penetrate the bone very well and can be administered via IV or oral routes (Liu et al., 2011). Tedizolid, a new oxazolidinone compound, has fewer side effects compared to linezolid and is effective against MRSA. It was reported that tedizolid monotherapy as well as tedizolid combination therapy with rifampin are active against MRSA in experimental foreign body-associated osteomyelitis (Park et al., 2017).

Oral quinolones are often prescribed for adults suffering from Gram-negative organisms. Ciprofloxacin, moxifloxacin and levofloxacin are used most commonly among the oral quinolones’ family owing to their kinetics and good distribution in bone tissues (Rimmele et al., 2004; Metallidis et al., 2006). Rifampin is active against MRSA. It can be combined with other antibiotics to have synergistic effect and to avoid rapid occurrence of resistant strains (Liu et al., 2011). Moreover, the utilization of rifampin in combination with other antibiotics has more efficiency than monotherapy in treating infections related to surgical hardware implants (Euba et al., 2009; Vergidis et al., 2011). Trimethoprim-sulfamethoxazole was indicated to be successful in the treatment of osteomyelitis in patients with MRSA infected orthopedic implants either alone or in combination with other antibiotics (Sato et al., 2019).

However, systemically administered antimicrobial agents suffer from several drawbacks in the treatment of chronic osteomyelitis including prolonged duration of treatment, increased incidence of side effects, restriction to penetration due to the sequestrum formed in the infection site and increasing the chance of development of resistance to the antimicrobial agent. Resistance develops when the delivery of antibiotics became obstructed. Some bacteria, such as S. epidermidis in prosthesis contaminations, adhere to the formed biofilm which protects the organism from phagocytosis in addition to the obstruction of the antibiotic penetration caused by the biofilm/ sequestrum, hence treatment fails. Similar conclusions were reported by many clinics (Zimmerli, 2014; Khatoon et al., 2018; Álvarez et al., 2019; Shipitsyna et al., 2020).

Thereby, the utilization of local antimicrobial therapy in the management of chronic osteomyelitis in long bones offers the advantage of localizing the therapeutic agent at the infection site in much higher concentration, lowering the incidence of side effects associated with the systemic route and offering the ability to increase the infection control rate and enhance the regeneration of bone tissues using tissue engendering techniques and biomaterial agents.

2.3. Clinically applied topical cements and scaffolds containing antimicrobial agents for managing osteomyelitis

From the surgeons’ point of view, as drawn from clinical investigations and studies conducted in groups of patients over several years, the use of bone cements mixed with antibiotics provides a promising and very efficient strategy for the management of infectious osteomyelitis. This has been routinely used strategy in orthopedic surgeries either applying a readymade delivery system (available in the market) or in-situ mixed dug and cements during the surgery (at operation sit). Clinical studies on local delivery of antibiotics along with the delivery vehicle that contains bone filling materials for the treatment of long bone infections are also available. They have been proven very effective in: (1) early suppression of infection, (2) showing several-folds reduction in the recurrent infection incidence, (3) minimizing the risk of pathological fracture due to internal reinforcement, (4) providing early recovery of extremity’s function and (5) creation of favorable conditions for bone structures restoration (Dzyuba et al., 2016; Mauffrey et al., 2016; Wang et al., 2021). The highlighted advantages of some cements being biodegradable and their ability to sustain the release of antibiotics over weeks of recovery period have greatly encouraged its widespread.

The oldest most widely used non-biodegradable material is Poly (methyl methacrylate) PMMA beads which have been the gold standard for local delivery of the therapeutic agents since decades (Evans & Nelson, 1993; Mohanty et al., 2003). PMMA beads can also be mixed with the antimicrobial agent just before the surgery (Gogia et al., 2009). PMMA beads offer the advantages of the lack of hypersensitivity reactions in the host in addition to maintaining high concentrations of the drug in-situ due to wide surface area for the release of the therapeutic agents (Lalidou et al., 2014).

In a study conducted by Dzyuba et al. (2016), scaffolds made intraoperatively from PMMA was implanted during operations to 30 patients suffering from chronic osteomyelitis. The data collected were compared with those of 30 patients treated with the conventional systemic therapy. Results showed that suppression of infection was better controlled in the group treated with local implant which also showed significant decrease in the recurrent infection incidence.

In a clinical study published by Wang et al. (2021), 19 patients diagnosed with chronic osteomyelitis in long bones underwent debridement procedure to remove the dead tissues followed by implanting a 3 D printed intramedullary titanium nail coated with PMMA impregnated with antimicrobial agents. Procedure involved formation of the 3 D molds that simulates the defect areas followed by using titanium nails coated by PMMA and vancomycin/imipenem to fit the molds. The titanium nails were inserted into the defect areas of the patients’ bones. The infection control was evaluated both clinically and radiographically, which showed that all cases had good infection control and confirmed the effectiveness of the used strategy for the treatment of defected infected bone along with bone healing.

However, PMMA beads have the shortcomings of requiring secondary surgery for removal, being non-biodegradable. In addition to heat production during the process of polymerization for the preparation of the beads, which makes it unsuitable for most antimicrobial agents as heat generation affects their stability. Finally, the in situ developed system suffers from unpredicted release profiles of the drugs, being prepared and inserted during the operation without pre-evaluation (Nandi et al., 2009).

Therefore, attempts have been made to encourage the use of biodegradable and bioabsorbable materials namely; hydroxyapatite, calcium sulfate, calcium phosphate, collagen and others incorporated into different forms (Maier et al., 2013; Anugraha et al., 2019). A local antibiotic delivery system with biodegradable drug carrier can be considered as therapeutically efficient platform for the treatment of osteomyelitis. Scaffolds made from biodegradable materials greatly affect the release rate of the incorporated drug. Therefore, when reasonably controlled drug release is desired, scaffold degradation rate has to be set properly (Dorati et al., 2017b).

In a retrospective investigation, 51 patients suffering from chronic osteomyelitis of lower extremities were distributed into two gropes either treated with combination therapy of vancomycin-loaded calcium sulfate and vancomycin-loaded PMMA as test group or vancomycin-loaded PMMA as a control grope. The combination therapy of both biomaterials achieved synergistic effect in controlling the infection compared to the group given PMMA loaded with vancomycin only (Luo et al., 2016).

The available commercial products for local antibiotic delivery and their pharmaceutical forms are listed in Table 3.

Table 3. commercially available systems for local antibiotic delivery to the affected sites of bones.

Carrier	Antimicrobial agent	Form	Commercial Products	Official Approval
PMMA	Gentamicin	Beads impregnated with the antibiotic	Palacos G^®	FDA, 2003
	Tobramycin	Bone cement incorporating the antibiotic	Simplex P^®	FDA, 2007
	Gentamicin and clindamycin	Bone cement incorporating the antibiotics	Copal^® G + C	FDA, 2019
	Gentamicin and vancomycin	Bone cement incorporating the antibiotics	Copal^® G + V	FDA, 2019
	Gentamicin	Beads impregnated with the antibiotic	Septopal^®	The Therapeutic Goods Administration, Australia 2020
Hydroxyapatite	—	Xenograft granules that can be preloaded with suitable antibiotic	Endobon^®	FDA, 2011
Hydoxyapatite/ Calcium sulfate	Gentamicin.	Synthetic bone substitute incorporated with the antibiotic	Cerament^® G	Health Canada, 2018
Calcium sulfate	Tobramycin.	Bone graft substitute incorporating the antibiotic	Osteoset^®	FDA, 2004
	—	Synthetic bone substitute that can be preloaded with suitable antibiotic.	Stimulan^®	FDA, 2015
Calcium phosphate	—	Synthetic bone substitute that can be preloaded with suitable antibiotic.	Cerasorb^® M	FDA, 2012
	—	Synthetic bone cement that can be preloaded with suitable antibiotic.	Biopex-R^®	The Pharmaceutical and medical devices agency, Japan 2000
Calcium phosphate/ collagen	…	Synthetic bone substitute that can be preloaded with suitable antibiotic.	Cerasorb^® Ortho Foam	FDA, 2020
Collagen	Gentamicin.	Matrix impregnated with the antibiotic	Collatamp^®	Health Canada, 2008

3. Systems under investigation for the local delivery of antibiotic for osteomyelitis management

Locally administered drug delivery systems to the site of infections are still under extensive investigations to improve the drug delivery to the infected bone. Using appropriate carriers, specific amount of the antimicrobial agents and controlling the released rate of the drug can help in the infection control and limit the recurrence rate. Additionally, if the delivery system made osteogenic in nature, they can exert dual function of eradicating the pathogens and assisting the bone regeneration after surgical debridement. Furthermore, these systems can be designed for in situ gelling after being injected to the close vicinity of the infected bone which allow the administration of a support therapy during the course of treatment to guard against a second surgical procedure for implant application or the drawbacks of the systemic therapy. Some clinical trials proved that in some cases, locally administered systems can minimize, the surgical intervention for removing the affected bone (Dzyuba et al., 2016; Luo et al., 2016).

Biodegradable materials that have been investigated for the local application of the antimicrobial agents include natural (Boles et al., 2018) and synthetic polymers (Tsiolis et al., 2011), ceramics (Cao et al., 2016), bioactive glasses (Jia et al., 2015), allograft bone (Kim et al., 2011) and demineralized bone graft (Turner et al., 2001). These materials are designed in many forms or systems for the local delivery to the affected parts as presented in Figure 1.

Figure 1. Local drug delivery systems under investigation for the management of osteomyelitis.

3.1. Scaffolds

Tissue regeneration is a process that happens following an acute injury. Enhancing the tissue regeneration process can occur in the defective site through rebuilding or repairing the tissue structure by providing a scaffold that simulate the body extracellular matrix (ECM). The use of scaffolds combined with therapeutic agents have been investigated and widely applied for the eradication of infections, revitalizing the damaged bones and restoring the loss of functionality of an organ as a result of tissue damages (Patzakis et al., 1993; Rasyid et al., 2009; Bhattacharya et al., 2013). Some other components are widely incorporated in the fabrication of bone tissue scaffold for the tissue engineering purpose including biomaterials, bioactive factors and cells (Oryan et al., 2018; Elkasabgy & Mahmoud, 2019; Morsi et al., 2019; Abdel-Salam et al., 2020).

Scaffold facilitates the processes of attachment, proliferation and differentiation of cells. Being biocompatible, neighboring cells are likely to be infiltrated into the scaffold and proliferate. Biomaterials, whether biodegradable or non-biodegradable, are better to have some properties including: (1) integrating with the biological molecules and cells, (2) promoting cells migration, proliferation, differentiation and revitalization of tissues, (3) exerting osteoinductive property, where osteogenesis is induced through stimulation of immature cells to develop into pre-osteoblasts as well as osteoconductive property, where bone tissue is allowed to grow on their surfaces and (4) providing mechanical structure to the affected bone (Sarigol-Calamak & Hascicek, 2018).

The bioengineered scaffolds are advantageous when applied in such cases of pathological conditions where, tissues do not have sufficient self-healing power, thus, promoting quicker tissue regeneration and rapid healing become mandatory (Porter et al., 2009). Over time, the implanted scaffold supports and promotes tissue regeneration accompanied with local delivery of certain dose of antibiotic for the desired period with minimization of its release to sites other than the targeted one. From this viewpoint, tissue engineering can be considered as a unique type of controlled drug delivery combined with scaffolding biomaterials (Dorati et al., 2017b).

Bone tissue scaffolds would be ideal if they fulfill the following criteria: (1) degradation in the same rate as the rate of bone regeneration, (2) leaving nontoxic and non-immunological degradation products, (3) having the ability to adhere to bone cells and provide mechanical support, (4) having interconnected porous structure of average diameter of 100 µm or greater where 50% or more are open in nature to facilitate cell infiltration and migration into the scaffold and finally (5) Their porous structure can provide satisfactory nutrients and oxygen diffusion (Sarigol-Calamak & Hascicek, 2018). In order to fulfill the previously-mentioned criteria, scaffolds made from bioceramics, polymers (natural and synthetic) and their composites are the most frequently studied types of scaffolds in the management of osteomyelitis.

Different techniques were employed for scaffold fabrication and application to the affected parts of bones either in animal studies and/or in clinical trials, including topical implantation of the compounded system (Singh et al., 2020; Fang et al., 2021), in situ forming implant of an injectable system (Wasupalli & Verma, 2020; Moeinzadeh et al., 2021) and three-dimensional (3 D) printing of a fitting system (Sun et al., 2021; Wang et al., 2021). Graphical presentation of the application methods of scaffolds loaded with antibiotics into the infected bone tissues are shown in Figure 2.

Figure 2. Representation of the methods used for the application of scaffolds loaded with antibiotics into the infected bone tissues.

The implant-based scaffold requires surgical procedure to be applied to the infected bone. They are prepared by several techniques including lyophilization (Nair et al., 2015; Narayan et al., 2017), solvent casting (Siemann, 2005; Choudhury et al., 2015), particulate leaching (Phull et al., 2013; Sabzi et al., 2021) gas foaming (Teng et al., 2007; Diaz-Gomez et al., 2017), or electrospinning techniques (Frohbergh et al., 2012; Su et al., 2012). In a study by Zhou et al. (2018) a bone tissue scaffold made of gelatin and β tricalcium phosphate was prepared by lyophilization technique and loaded with vancomycin. The surgical procedure for applying scaffolds to the rabbits’ tibias after osteomyelitis induction was accompanied by debridement of the necrotic bone tissues for enhancing the therapeutic efficacy. Results showed that scaffolds were able to sustain vancomycin release for eight weeks with good infection control and ability to repair the bone defects.

Another technique of scaffold formation is the in-situ forming of implants after injecting natural and/or synthetic polymer dispersion to the affected site. The most important parameters to control the formation of in-situ forming implant-scaffold is the ability of the polymers to undergo self-assembly transformation from a liquid monomeric phase into a solid or semi-solid polymeric network when affected by various factors such as temperature and pH Changes at the site of injection (Chaudhari et al., 2016). Injectable systems offer several advantages including patients comfort with a less invasive application technique, shorter recovery period and significant cost reduction with effective treatment of bone infections (Mahmoudian & Ganji, 2017). Hydrogel was found to be an ideal bulk platform scaffold for bone regeneration and for the localized release of antibiotics for the treatment of bone infections (Xing et al., 2013; Wan et al., 2015; Morsi et al., 2019). Furthermore, a novel injectable chitosan-based in-situ forming implant loaded with bioactive glass nanoparticles and raloxifene was prepared for the repair of noninfectious bone defect which can be extended for application in cases of bone infections. The injectable formulation had the ability to transform into hydrogel upon being exposed to body temperature (37 °C) within 20 min and offered sustained release of the drug for 56 days. The in vivo studies showed accelerated bone formation in induced non-critical injuries in rats tibias when compared to the untreated bone defect (Abdel-Salam et al., 2020).

Among the advanced technologies, utilizing computer-aided design (CAD) is the three-dimensional (3 D) printing. 3 D printing technologies has been characterized as an effective fabrication technique for customized scaffolds based on tissue engineering and antibiotic therapy (Mills et al., 2018; Elkasabgy et al., 2020; El-Habashy et al., 2021; Shamma et al., 2021). They showed promising potential in construction scaffolds fitting to the shape and site of the bone defect through controlling the cornerstones of 3 D printing such as accuracy and speed of fabrication (Xu et al., 2010; Seol et al., 2012; Lee et al., 2013). Additionally, the porous interconnected inner structure of 3 D printed scaffolds helps in enhancing the process of regeneration of new tissues. this can be useful in bone defects associated with bone infection (Lee et al., 2010, 2011; Shim et al., 2012; Sa & Kim, 2015).

3 D printed scaffolds loaded with tobramycin have been fabricated for the treatment of osteomyelitis where tobramycin was mixed with molten blend polymers of ploy-caprolactone (PCL) and poly (lactic-co-glycolic) acid (PLGA), dispensed using multi-head disposition system and exposed to 500 kPa pneumatic pressure to distribute the mixture through a nozzle made of steel. The bactericidal activity of the 3 D printed scaffolds against S. aureus and E. coli was successfully maintained after the process of thermal printing in an in-vitro study. The in-vivo study after implanting the scaffold in a rat femur model, confirmed its efficacy in developing new bone at the end of eighth week (Shim et al., 2015).

Additionally, Wu et al. revealed that 3 D printed multilayered concentric cylinder construction implant incorporating levofloxacin and tobramycin was able to enhance the antibiotics pharmacodynamic action through a sustained and programmed release pattern. The implanted scaffolds completely eradicated the infectious microorganisms and treated osteomyelitis in animal model with no recurrences after follow up for 2 months (Wu et al., 2016).

3.1.1. Metal implant

Some approaches are developed to prevent implants from bacterial infections as a result of the tremendous number of patients requiring orthopedic implants. Local delivery of bioactive agents through implant coating is one of the effective approaches to deal with infections. Long-term controlled drug release from the implant can be guaranteed without the existence of systemic side effects or development of bacterial biofilms. Among the metals used as orthopedic implants due to their satisfactory characteristics (excellent mechanical strength, good corrosion resistance and biocompatibility) are titanium (Ti) (Liu et al., 2017), cobalt (Co) (Lin et al., 2019) and stainless steel (Gimeno et al., 2013). However, the use of metallic implants suffers from the disadvantages of non-biodegradability and limited processability in the biological environment (Long & Rack, 1998; Chen et al., 2017). The most commonly used metal in implants for osteomyelitis is titanium.

3.1.1.1. Titanium implants

Ti-based alloys have great widespread to be used as metal implants and they are commercially available as load-bearing components in total bone or joint replacement in the form of nails, pedicle screws and fracture plates. However, Ti-based implants have the drawbacks of incidence of implant loosening, in addition to the probability of inhibition of subsequent osseointegration due to formation of a fibrous layer that is non-adherent to the metal implant (Chan et al., 2017). They can also induce immune reactions due to sensitization and activation of pro-inflammatory and anti-inflammatory cytokines which may lead to allergic reactions (Fage et al., 2016). Although titanium alloys have inherent antimicrobial properties, infection may still arise, which requires the application of antimicrobial coat on the implant’s surface in order to reduce infection and colonization of bacteria (Zhao et al., 2009). Furthermore, It is worthy to mention that metal implants are not osteoinductive nor osteoconductive and they do not enhance bone regeneration. Therefore, modification of the surface of an implant for the optimization of its osteoinductive and osteoconductive properties are critical for the implant to maximize its biological function while interacting with bone tissues (Asri et al., 2017). The following represents the most important surface modifications of Ti alloys:

3.1.1.1.1. Titanium implant doped with heavy metals

For the purpose of improving the antimicrobial effect of Ti alloys implants, they were supplemented (doped) with heavy metals that have antibacterial activity resulting in better outcome of treatment. Norambuena et al. (2017) in 2017 added copper (Cu) to the surface of Ti alloy to form titanium-copper oxide at different strengths of Cu. The in-vitro studies demonstrated that high doses of Cu (reaching 80% of the implant weight) had the most reliable effect in diminishing biofilm and bacterial growth of S. epidermidis. Moreover, in an in-vivo study in mice comparing the antibacterial effect of titanium implant coated with silver to the uncoated one, it was found that the number of S. epidermidis bacteria has been diminished by greater than 500-fold in case of the silver-coated implant compared to the uncoated 1 after 2 weeks of treatment (Tîlmaciu et al., 2015). Doping the titanium implant with metals can be serious if the local release of metal ions in the site of implant is high, which may lead to necrosis of cells. furthermore, the systemic exposure to these metal ions can be fatal if they distribute into brain, spleen, liver and other organs. Therefore, the toxicity of the doped metal ions should be under control (Gulati et al., 2021).

3.1.1.1.2. Titanium implants with antibiotics coated surface

In a study carried out by Sharma et al. in 2016 (Sharma et al., 2016), gentamicin loaded into the natural polymeric material, silk fibroin, nanoparticles were prepared and deposited over the Ti implant surface in order to achieve sustained release of gentamicin. The fibroin nanoparticle offered the advantage of changing the surface topography of the bare metal implant, which became rougher and more hydrophilic. Thus, the nanoparticles coated implants were found to be unrivaled for the osteoinduction and osteogenesis through improving osteoblast adhesion, proliferation and differentiation if compared with the uncoated Ti surface. This approach was considered as one of the cost-effective approaches for the treatment of osteomyelitis.

In another study, Ti alloy underwent surface modification through coating with chitosan/lysine biopolymers as a carrier for gentamicin. This modification did not only eradicate the infection through sustained drug release after the initial burst release from the implant, but also improved the adhesion of cells followed by proliferation when tested in simulated body fluid. Results showed that the coat had the capability of bioactivity through improving bone growth and new bone tissue formation. This reinforced the idea of the favored environment of the modified implant to encourage bone regeneration (Raj et al., 2018).

David and colleague (2018) adopted surface modification of Ti metal using chitosan-gelatin polyelectrolyte complex as a carrier for vancomycin. The complex-loaded with vancomycin was incorporated into scaffold formed of gelatin: stronium: hydroxyapatite (HA) simulating the chemical composition of bone. The fabricated scaffold achieved its antibacterial activity through growth inhibition of MRSA and MSSA strains with sustained and controlled release of vancomycin for an adequate period to resolve the infection.

Additionally, modifying the surface of Ti scaffold for providing antibacterial properties was achieved successfully by introducing a biodegradable system made of gentamicin loaded microparticles linked to the pore walls of the Ti scaffolds by cross-linked sodium alginate. The fabricated system provided the required doses of gentamicin to combat the infection after implementation of the scaffold. The system was fully cytocompatible when tested against osteoblast-like cells (Rumian et al., 2016).

Creating and using an intramedullary nail to be implanted in the intramedullary cavity coated with antibiotic and growth factors can be a successful approach for eradicating the infection and encouraging bone regeneration according to Berebichez-Fridman et al. (2017). The placement of nanoparticles on intramedullary nail at the area of infection can guarantee local application of the antimicrobial agents restricted to the contact sites. After certain time, the nanoparticles were able to kill the pathogens and invigorate the formation of new healthy bone, besides providing constant immobilization and stability of the affected bone by the intramedullary nail, which improved bone healing.

3.1.1.1.3. Titanium implants with surfaces’ nanopatterning

Nanoscale dimensions are substantial in the design of surfaces for tissue engineering, as cells can recognize the topographical changes in the extracellular matrices where they can adhere to and proliferate (Jeong et al., 2012). Moreover, nanopatterned surfaces can facilitate the development of new functional materials with increased resistance to bacterial contamination and infection in addition to offering a new platform for tuning implant properties. Nanopatterned surfaces can be of natural source such as cicada wings and dragonfly wings with nanopillar patterns (Diu et al., 2014) or of an artificial source such as Ti (Sengstock et al., 2014), silicon (Ivanova et al., 2013) and polymers (Dickson et al., 2015).

A suggested mechanism for the bactericidal activity of the nanopatterned surfaces when tested on a cicada wing-like nanopatterned surface is that the nanopatterned surfaces have a bactericidal activity arising from physical interactions between the nano-surfaces structures and the bacterial cells because of the remarkable increase in the contact adhesion area (Li, 2016).

Nanpaterterning of Ti implant is made through direct changing the surface of implant at the nanoscopic level. Nanopatterning technology can provide biologically optimized surfaces for implantable materials where cells can adhere to, since bone tissues are composed of nanostructures such as hydroxyapatite crystals, fibrillar collagen and non-collagenous organic proteins (Tan et al., 2014). An in-vitro study using mesenchymal stem cells revealed that the fabricated three-dimensional (3 D) nanopatterned surfaces of titanium induced the osteogenic differentiation with cells on these surfaces (Sjöström et al., 2013). In another in-vitro study, Variola et al. (2014) proved that the Ti nanopatterned surfaces had resistance against the adhesion of candida albicans yeast, S.aureus and E.coli aggregation which proves their antimicrobial activity. More investigations on metal scaffolds are listed in Table 4.

Table 4. More investigations on the types of scaffolds as delivery systems of antimicrobial agents for the management of osteomyelitis.

Types of Scaffolds	Antimicrobial agents	Carriers	Achievements	Study type/ assayed model	References
Metal implants	Gentamicin	Titanium implant	High prophylaxis against implant-related osteomyelitis	In-vivo/Rats	(Diefenbeck et al., 2016)
	Vancomycin	Zeolitic imidazolate nanocrystals	Sustained release of antibiotic Eradication of infection Promotion of osteogenesis	In-vitro	(Karakeçili et al., 2019)
		3D printed Titanium implant	High loading of vancomycin on the implant Sustained release Promotion of osteogenesis	In-vivo/ Rabbits	(Zhang et al., 2020)
	Clindamycin	3D printed coatings on titanium and stainless steel implant	Sustained release of antibiotic for 3 days	In-vitro	(Maver et al., 2021)
Bioceramics	Vancomycin	Hydroxyapatite/ Calcium phosphate	Successful management of diabetic foot infection.	Clinical/ Patients	(Karr, 2011)
		Hydroxyapatite/ Calcium phosphate	Controlled antibiotic release pattern over a 12-day period.	In-vitro	(Thanyaphoo & Kaewsrichan, 2012)
		Hydroxyapatite/ Poly amino acid	Significant bactericidal activity Sustained duration of action for 42 days in-vivo.	In-vivo/Rabbits	(Cao et al., 2016)
		Hydroxyapatite	Increased drug loading in the freeze-gelated matrix -Sustained release up to 5 days.	In-vitro	(Hess et al., 2016)
		Hydroxyapatite	High antibiotic loading Sustained release from the implant.	In-vitro	(Parent et al., 2016)
		Calcium polyphosphate	Sustained release of the antibiotic.	In-vitro	(Comeau & Filiaggi, 2017)
		Calcium phosphate/ calcium sulfate	New bone formation. Low rates of infection recurrence in 31 patients with chronic osteomyelitis.	Clinical/ Patients	(Zhao et al., 2020)
	Gentamicin	Calcium sulfate	Significant bactericidal activity of the scaffold.	In-vitro	(Thein et al., 2013)
		Hydroxyapatite/ Calcium phosphate	Clinical cure of the heel ulcers in diabetic foot patients after 16 weeks without need of amputation.	Clinical/ Patients	(Drampalos et al., 2018)
	Vancomycin/gentamicin	Calcium sulfate	Full recovery in 4 months in a patient with diabetic foot infection.	Clinical/ Patients	(Morley et al., 2016)
		Calcium sulfate/ hydroxyapatite	Prevention of biofilm formation	In-vitro	(Bidossi et al., 2020)
	Ceftriaxone/sulbactam	Bioactive glass	Sustained antibiotic release for up to 42 days in the treated animals. New bone formation.	In-vivo/Rabbits	(Kundu et al., 2011)
	Gatifloxacin/ fluconazole	Bioactive glass	Sustained antibiotic release for up to 6 weeks.	In-vitro	(Soundrapandian et al., 2011)
	Vancomycin/ rhBMP-2	Calcium sulfate	Sustained release in-vivo for up to 14 to 21 days. -Formation of new bone.	In-vivo/Rabbits	(Wang et al., 2011)
	Ceftriaxone/sulbactam	Hydroxyapatite	New bone formation Sustained drug release for 42 days.	In-vivo/Rabbits Clinical/ Patients	(Bhattacharya et al., 2013)
	Linezolid	Calcium deficient apatite (CDA)	Enhanced efficacy of treatment with IV treatment.	In-vivo/Rabbits	(Gaudin et al., 2013)
	Amphotericin B/ voriconazole	Hydroxyapatite/ Calcium sulfate	Maintained effective antifungal concentrations over 96 hours.	In-vitro	(Karr & Lauretta, 2015)
	Levofloxacin	Bioactive glass/ HA nanoparticles	Sustained with pH-dependent release of drug at the infection site.	In-vitro	(Cicuéndez et al., 2018)
	Sitafloxacin/ rifampin	Calcium phosphate	Decreased bacterial colonization Formation of new bone in osteomyelitis model	In-vivo/ Mice	(Trombetta et al., 2019)
	Rifampicin	Nanohydroxyapatite /calcium sulfate	Sustained release for 28 days Eradication of infection New bone formation	In-vivo/Rats	(Qayoom et al., 2020)
Polymeric	Gentamicin	PMMA	Effective treatment of patients with infected nonunioun of the long bones.	Clinical/ Patients	(Selhi et al., 2012)
	Gentamicin/silver ion	Silk fibrin	Effective inhibition of MRSA growth Stimulation of bone regeneration	In-vivo/Rats	(Zhang et al., 2019b)
	Vancomycin/rifampin	Polydioxanone	Inhibition of biofilm formation.	In-vitro	(Waeiss et al., 2014)
	Vancomycin/cefuroxime	PMMA	Successful treatment of the infection in 7 cases with chronic osteomyelitis.	Clinical/ Patients	(Bharti et al., 2016)
	Gentamicin,vancomycin, amikacin, ceftriaxone	PMMA	Effective inhibition of MRSA growth for 42 days	In-vitro	(Noor et al., 2016)
	Vancomycin	PMMA	Prolonged release of the antibiotic from the bone cement over 6 weeks in femoral osteomyelitis model.	In-vivo/Rats	(Oh et al., 2016)
		Chitosan	Sustained release for 26 days Promotion of osteoblast formation	In-vivo/Rabbits	(Tao et al., 2020)
	Fosfomycin	Chitosan	Sustained release for 5 days Eradication of infection	In-vitro	(Tucker et al., 2021)
	Linezolid/daptomycin /vancomycin	PMMA/ PLGA microparticles	Synergistic effect of the antibiotics Sustained drug release for 2 months.	In-vitro	(Parra-Ruíz et al., 2017)
	Vancomycin/amikacin	Chitosan sponge	Prevention and clearance of polymicrobial implant associated-biofilm.	In-vivo/ Mice	(Boles et al., 2018)
	Ciprofloxacin	Poly (hydroxyethyl methacrylate)	Sustained antibiotic release for 5 days Osteoblast differentiation and osteoinduction	In-vitro	(Sreeja et al., 2020)
	Rifampicin	PCL	Sustained antibiotic release for 14 days	In-vitro	(Lee et al., 2020)
Composite	Vancomycin	PLLA/ β TCP	Achieved controlled release of antibiotic. Formation of new bone.	In-vivo/Rats	(Kankilic et al., 2014)
		Hydroxyapatite/ collagen	Controlled release of antibiotic Eradication of bacteria Induction of new bone formation	In-vitro	(Coelho et al., 2015)
		Calcium sulfate / PMMA	Effective control of infection in patients with chronic osteomyelitis Providing high concentrations at the infection site.	Clinical/ Patients	(Luo et al., 2016)
		Gelatin/ β TCP	Extended release of antibiotic from scaffold for 8 weeks -Healing of the bone defect. Eradication of infection.	In-vivo/Rabbits	(Zhou et al., 2018)
		PLA/ nanohydroxyapatite	Sustained antibiotic release Stimulation of bone regeneration	In-vitro	(Zhao et al., 2019)
		Nano- hydroxyapatite / Gelatin / PLA	Significant bacterial reduction in osteomyelitis rat femur model. Formation of new bone.	In-vivo/Rats	(Krishnan et al., 2020)
		Heparinized nanohydroxyapatite/collagen	Sustained release for 19 days Eradication of infection	In-vitro	(Padrão et al., 2021)
		Polyurethane/ hydroxyapatite	Reduced number of bacteria in the bone Had the ability of bone regeneration	In-vivo/Rabbits	(Beenken et al., 2021)
		Hydroxyapatite/ Sodium alginate/ chitosan	Eradication of infection Had osteogenic properties	In-vitro	(Liu et al., 2021)
	Gentamicin	Hydroxyapatite/ collagen	Sustained release of antibiotic for 3 days Prevention of implant-associated infection	In-vitro	(Oshima et al., 2020)
	Ceftriaxone	Hydroxyapatite/ β TCP /chitosan	Prolonged release pattern for more than 5 weeks.	In-vitro	(Kundu et al., 2010)
	Moxifloxacin	Chitosan/ calcium phosphate	Controlled release for 3 days. Induction of osteogenesis	In-vivo/Rabbits	(Radwan et al., 2020)
		Poly-lactide-co-ε-caprolactone/calcium phosphate	Sustained release for 6 weeks Enhanced cellular proliferation and differentiation Diminished sequestrum formation and inflammation	In-vivo/Rabbits	(Radwan et al., 2021)
	Gatifloxacine	β TCP/ PLGA	Osteoconductive scaffolds with efficacy in local treatment of osteomyelitis.	In-vivo/Rabbits	(Tamazawa et al., 2011)
	Rifampicin/ ciprofloxacin	PCL/ β TCP	Good therapeutic activity against the causative bacteria -Sustained drug release.	In-vitro	(Ahola et al., 2012)
	Gentamicin/ vancomycin	PMMA/ β TCP	Eradication of infection through custom made devices in femoral osteomyelitis model.	In-vivo/Rabbits	(Giavaresi et al., 2012)
	Daptomycin	Calcium phosphate / chitosan	Prolonged the duration of release of the antibiotic Maintaining high concentrations of the antibiotic -Better therapeutic outcome following surgical debridement.	In-vivo/Rabbits	(Beenken et al., 2014)
	Ciprofloxacin	Gelatin/ hydroxyapatite	Sustained release of the antibiotic for 60 days. -enhancing of osteogenic differentiation in cells	In-vitro	(Krishnan et al., 2015)
		Hydroxyapatite/ PCL	Controlled the release of antibiotic in implant.	In-vitro	(Nithya & Sundaram, 2015)
	Rifapentine	Hydroxyapatite/ Poly amino acid	Inhibition of bacterial growth. Sustained antibiotic release and cure of chronic osteomyelitis after 12 weeks.	In-vivo/Rabbits	(Yan et al., 2015)
	Tobramycin	PCL/ PEG/ Calcium phosphate/ Hydroxyapatite	Osteoconductivity and resorbtion with sustained antibiotic release.	In-vivo/Rabbits	(Jones et al., 2016)
	Doxycycline	Bioactive glass/ mesoporous silica	Sustained release for 19 days Eradication of infection	In-vitro	(Szewczyk et al., 2021)
	Silver ion	Nano- hydroxyapatite / polyurethane	Controlled release of silver ion, Eradication of infection. Formation of new bone.	In-vivo/Rabbits	(Zhang et al., 2019a)

3.1.2. Bioceramics scaffolds

Enhancing bone regeneration, while promoting controlled delivery of therapeutic agents is one of the chief goals for the development of bioceramic-based scaffolds. Bioceramic scaffolds are types of inorganic biomaterials such as calcium phosphate derivatives and bioactive glass. Hydroxyapatite (HA) and tricalciumphosphate (TCP) are among the most widely used derivatives of calcium phosphate biomaterials and the oldest applied ones are bone substitute (Lew et al., 2012). Bioceramic scaffolds are chemically and structurally similar to the inorganic component of bone, therefore, they have been widely used for bone regeneration purposes (Stevens, 2008).

3.1.2.1. Calcium phosphate bioceramics

Depending on the ratio of calcium to phosphate, the temperature of synthesis of the ceramics and the existence of impurities and water, calcium phosphate can differ in crystal pattern. They can be classified according to their crystal pattern into mono-, di-, tri- (including both α-tricalcium phosphate (α-TCP) and β-tricalcium phosphate (β-TCP)), tetra- calcium phosphate and HA (Lew et al., 2012). Calcium phosphate bioceramics demonstrate promising biological and functional properties as bone scaffold material due to their biocompatibility, high loading capacities, nontoxicity, non-immunogenicity, bioresorbability and osteoinductive features (Kalita et al., 2007). They are perfect and natural candidates as bone-filling drug carriers as they resemble the mineral phase of the bone tissues. Additionally, they participate in stimulating osteoblastic differentiation and proliferation, so they aid in the formation of new bone when these compounds degrade and release phosphate and calcium ions, which can be used as ionic ingredients for bone regeneration (Uskokovic, 2015).

Hence, calcium phosphate is an excellent platform for the incorporation of antibacterial agents to control the release kinetics and to stimulate osteogenesis. The use of injectable calcium phosphate as an experimental biomaterials systems was evaluated for treating induced S. aureus osteomyelitis in an animal models (Inzana et al., 2016). These cements are generally formed when the soluble calcium phosphate is combined with water or another aqueous solution, where this solution will precipitate calcium phosphate when being exposed to body environment. The release of antibiotic from the scaffold is controlled by diffusion rather than the degradation of the matrix (Inzana et al., 2016). Injectable calcium phosphate cements offers many advantages namely; localizing the therapeutic agent at the site of action, minimizing invasion, providing perfect fit to the defect site as they are moldable materials and offering superior biological reactivity due to their porous structure (Ginebra et al., 2018).

Ghosh et al. (2016) successfully incorporated ciprofloxacin and vancomycin into nanoparticulate hydroxyapatite to act as an injectable cement with tunable drug release kinetics, which was found to be less invasive and more affordable. In a more recent study, the application of low frequency pulsed ultrasound resulted in increasing the extent of vancomycin release from vancomycin-loaded calcium phosphate cement, together with prolonging the duration of vancomycin release and accelerating the degradation rate of the applied cement (Shi et al., 2018). Additionally, Inzana et al. (2015) fabricated rifampin- and vancomycin-laden calcium phosphate scaffolds (CPS) by three-dimensional (3 D) printing to treat an implant-associated Staphylococcus aureus bone infection in a murine model. All vancomycin- and rifampin-laden CPS treatments significantly reduced the bacterial burden compared with vancomycin-laden PMMA.

3.1.2.2. Bioactive glasses

Bioactive glass is a type of bioceramics that has been widely applied in experimental models and clinical trials for the treatment of osteomyelitis along with its applications in other bone tissue engineering (Lindfors et al., 2010; Zhang et al., 2010; Drago et al., 2013; Rahaman et al., 2014; Ryan et al., 2019; Abdel-Salam et al., 2020). Bioactive glass is structurally composed of calcium, silicon and phosphate ions. Its action in bone tissue engineering is related to their biocompatibility, osteoconductivity and its ability to release calcium, sodium, silicon and phosphate ions during its dissolution process. These ions are biologically active when released in a rate favorable for the bone-cells proliferation and differentiation (Hupa, 2018). Bioactive glass binds to the damaged bone tissue through chemical bond as a result of the formation of a hydrocarbonate apatite layer on the glass surface which, has similarity to the bone components, thus enhancing the interaction with the damaged bone and promoting the osteogenesis process (Rahaman et al., 2011).

Bioactive glass materials have shown promise for application as antibiotic carrier in the local treatment of osteomyelitis (Rahaman et al., 2014). In a study carried out on rabbit model, the local teicoplanin delivery from borate bioactive-glass implant was found to be more effective in the treatment of MRSA-induced osteomyelitis compared to teicoplanin intravenous administration (Jia et al., 2015). A clinical study on 27 patients suffering from osteomyelitis in long bones was performed by Drago et al. (Drago et al., 2013) using bioactive glass granules. The granules were implanted at the site of the osteomyelitic lesion after complete debridement of the dead tissues. Resulted showed that the bioactive glass granules had antibacterial activity against the causative bacteria without addition of antibiotics. The main downsides of bioactive glasses are the increased concentrations of ions at the site of application of the implant, the slow rate of degradation of the glass and the difficulties encountered in fabrication processes of the scaffold (Mancuso et al., 2017). More investigations on bioceramics scaffolds are listed in Table 4.

3.1.3. Polymeric scaffolds

In the field of polymeric scaffold fabrication, it is important to use biomaterials adaptable to the bone tissues. The ideal polymeric materials for this purpose are those simulating the natural ECM of bone and possessing the essential biochemical and mechanical characteristics (Bose et al., 2012). Natural polymeric materials or synthetic ones offer great potential in designing the scaffolds due to their biocompatibility, biodegradability and being able to interact with cells. Natural polymers including, collagen chitosan, silk and alginate as well as synthetic polymers including poly-lactic acid (PLA), polycaprolactone (PCL) and poly (lactic-co-glycolic) acid (PLGA) have been investigated as bone scaffolds with different morphologies for bone tissue engineering and as drug carriers. The use of synthetic polymers offered easier modification of the mechanical and physicochemical properties in addition to adjusting the biodegradation rates (Bose et al., 2012; Sarigol-Calamak & Hascicek, 2018; Elkasabgy et al., 2019).

3.1.3.1 Natural polymeric scaffolds

Collagen is a naturally occurring protein in the ECM of bone. The high biocompatibility of collagen makes it ideal biomaterial to be used as scaffolding material (Meyer, 2019). ‏The application of collagen implant loaded with antibacterial agents has been investigated for the treatment of acute and chronic osteomyelitis in clinical studies since decades and the composite system made of the drug and collagen was found to be biodegradable and bioresorbable, hence does not need second surgery for the removal of the implant when compared to PMMA (Feil et al., 1990; Ipsen et al., 1991).

Chitosan is another natural polymer produced by deacetylation of chitin, which is a primary component of cell wall in some living organisms (Di Martino et al., 2005). Being of chemical nature that offer modification besides its biological characteristics including biodegradability, biocompatibility, tissue engineering capability and antibacterial activity, it has been widely investigated both in vitro and in animal models as local drug delivery vehicle for the treatment of osteomyelitis (Noel et al., 2008; Wells et al., 2018; Boles et al., 2018; Sarwar et al., 2020). Chitosan suffers from poor mechanical properties if used as a sole component in the implanted scaffold, so it is usually combined with other synthetic polymer such as PCL and PEG to allow tailoring its properties as well as expanding its applications (Malheiro et al., 2010; Lima et al., 2018; Pawar & Srivastava, 2019).

Another example of the naturally occurring polymers is alginate. It is being widely used in the pharmaceutical industry owing to its gelling and stabilizing properties. It has been used also as drug delivery systems for bone through acting as a biodegradable carrier for antibacterial agents and bone cells (Ueng et al., 2007).

3.1.3.2. Synthetic polymeric scaffolds

PLGA, a group of synthetic, biodegradable and biocompatible polymers. They were extensively investigated as building materials for scaffolds and as drug delivery systems. They show minimal toxicity since they degrade into lactic and glycolic acids in the human body. They also offer great ability to sustain drug release which is an important feature for a bone scaffolds (Danhier et al., 2012; Shamma et al., 2017). According to Ueng et al. (2011) it was possible to sustain the release of vancomycin from the polymeric composite system of PLGA and collagen. The composite system of the two polymers revealed adequate osteogenic differentiation of mesenchymal stem cells in a rabbit model (Ueng et al., 2011). Further in-vitro and in-vivo studies applying the biodegradable PLGA-vancomycin beads confirmed their success in eradicating the induced infection in the damaged bone of rabbits model (Ueng et al., 2016).

PLA is also used as scaffolding material due to its biocompatibility and biodegradability (Sarigol-Calamak & Hascicek, 2018). In a study conducted by Tsiolis et al. (2011) linezolid was mixed with PLA to form an implant for application at the bone site where MRSA infection was induced in experimental rabbit model. They proved the inhibition of the induced infection during the first six weeks after the implant application. The system composed of the drug and the polymer succeeded in reducing the microbial count continuously as well as showing good histopathological results.

Another example of the synthetic polymers mostly used for bone regeneration purposes is PCL. It offers many important features namely; biocompatibility, biodegradability, processability and good mechanical characteristics (Waknis & Jonnalagadda, 2011). PCL can exert their action in tissue engineering through contribution to adhesion and proliferation of cells and its ability to interact with the biological aqueous fluids after modifying its hydrophobic properties by blending with natural polymers (Zhang et al., 2013; Siddiqui et al., 2018). Injectable and cytocompatible implants made from physical blends of poloxamine and PCL by hot melting solvent-free method and incorporating ciprofloxacin hydrochloride showed tunable bioerosion rate, osteoconductive features and sustained drug release (Puga et al., 2012). More investigations on polymeric scaffolds are listed in Table 4.

3.1.4. Composite scaffolds

Composite scaffolds are systems where two or more materials are combined into the same system to combine the advantages of both materials. Composite scaffolds offers good load bearing and strong mechanical properties for bone tissue engineering applications (Wei & Ma, 2004; Abdel-Salam et al., 2020). A good example is the combination of bioceramics with polymers in scaffold fabrication. Although bioceramics can provide high structural support and compressive strength, however, they are very brittle. Therefore, when polymers are integrated with bioceramic materials, the composite system acquire mechanical strength and processability in addition to controlling the drug release from the scaffold (Perez et al., 2012; Abdel-Salam et al., 2020).

In a study by Mostafa et al. (2017) ciprofloxacin was incorporated into a scaffold composed of chitosan and bioactive glasses in a ratio of 1–2. The composite carrier was biocompatible and offered optimal adhesion, cell migration and growth as well as sustained drug release profile when compared to the sole chitosan scaffolds (Mostafa et al., 2017). More investigations on composite scaffolds are listed in Table 4.

3.2. Particulate systems

Particulate systems have been investigated as suitable systems for the local delivery of antibacterial agents in the treatment of bone infections (Elkheshen et al., 2013; Mobarak et al., 2014; Hassani Besheli et al., 2017; Cobb et al., 2020). Injectable particulate systems exhibit the advantage of being easier to be delivered to the affected part when compared with the implanted tissue scaffolds. However, they do not usually fully fit in the dead space due to their relative fluidity in contrast with the tissue scaffolds, which fills the dead space ensuring the adhesion and proliferation of the newly formed cells. Various particulate systems have been identified in the local antibiotics treatment of osteomyelitis namely; nanoparticles/nanospheres and microparticles/microspheres.

3.2.1. Nanoparticles/nanospheres

The delivery of drugs in the nano-form have been developed and gained much interest since decades with the aim of improving the bioavailability and therapeutic efficacy of medicinal agents. These nano-formulated drug-carriers have high surface area relative to their mass, which enable them to solve the problems of drugs suffering from delivery obstacles as low solubility. They also minimize the drugs systemic side effects or even help in targeting them to certain organ in the body through adding a tag molecule to the surface of the particles which recognize the required site (Nikezić et al., 2020). Controlled release polymeric nanoparticles (NPs) exhibit biodegradability, which allow the gradual degradation of the polymer with controlling the release of drugs. The rate of drug release from polymeric NPs can be controlled by controlling the particle size as well as the stability and chemical structure of their components (Cong et al., 2015). NPs also offer high protection of drugs against degradation. The therapeutic agents are loaded to these systems through wrapping with polymeric or non-polymeric materials, dissolving in the bulk of the particles or adsorption to the their surface (Han et al., 2018).

Polymeric NPs as delivery systems have been widely investigated in the field of osteomyelitis local treatment aiming to deliver the antibiotics to the infected site with controlled release manner. This could strongly aid the efficient eradication of infection while avoiding the systemic therapy (Posadowska et al., 2015; Hassani Besheli et al., 2017; Zhang et al., 2017, 2018). Gentamicin sulfate was encapsulated in PLGA NPs using double emulsification (water/oil/water) technique. Agar-diffusion tests using S. aureus and S. epidermidis confirmed the antibacterial activity of gentamicin after loading to nanoparticles and the in-vitro release studies revealed sustained gentamicin release for 35 days (Posadowska et al., 2015).

Aragonite is a biocompatible form of calcium carbonate, which proved its potential application in the evolution of advanced and novel drug delivery platforms for bone tissue engineering (Islam et al., 2013). Utilizing cost effective and easy approach to design biodegradable antibiotic system in the nano-form, Saidykhan et al., loaded vancomycin into aragonite NPs. The Nano-system showed biocompatability, good bone resorbability and optimal antibacterial efficacy. The prepared NPs loaded with vancomycin displayed high antibacterial activity against MRSA, confirming that vancomycin retained its potency after loading into aragonite NPs (Saidykhan et al., 2016).

In a study conducted by Zhang et al. (2017) a cationic derivative of chitosan namely; N,N,N-trimethyl chitosan (TMC) formed a complex, in a nano-form, with the anionic vancomycin through ionotropic gelation technique. TMC-NPs exhibited initial burst release along with an ideal sustained release kinetics of vancomycin besides enhancing the product biocompatibility. The beads also promoted new bone formation in rabbit model as a result of the unique degradation behavior.

Gelatin nanospheres acquire inherent advantages including biocompatibility, biodegradability and cost effectiveness, which encouraged their use as efficient carriers for the local delivery of antibiotics. Moreover, gelatin nanospheres, being positively charged, can bind to oppositely charged molecules upon cross-linking, which can be adjusted to control the drug release through controlling their degradation rate (Santoro et al., 2014; Song & Leeuwenburgh, 2014). In-vitro and in-vivo studies by Zhang et al. (2018) showed that gelatin nanospheres loaded with vancomycin had good internalization of vancomycin by macrophages compared to vancomycin delivered systemically which had poor internalization. The in-vivo model using zebrafish larvae confirmed that the local application of vancomycin-loaded gelatin nanospheres into the macrophages had significantly higher survival rate of zebrafish larvae compared to the survival rate after systemic administration of the drug over time.

3.2.2. Microparticles/microspheres

Microparticles having size range in the micrometer scale have been utilized as drug carriers to enhance the delivery of drugs since decades (Ogay et al., 2020). Differences in the size range is the principal differentiating the nanoparticle carriers and the microparticle ones. Microspheres are type of microparticles composed of a polymer shell that surrounds the drug. Microspheres have several advantages such as enhancing the solubility of poorly soluble drugs in aqueous environment or controlling the drug’s half-life (Freiberg & Zhu, 2004). A biodegradable drug delivery system of (PLGA-PEG) microspheres containing gentamicin was successfully prepared using two different techniques; single emulsion and double emulsion solvent evaporation (Dorati et al., 2016). A composite in-situ forming gel using chitosan solution mixed with glycerol phosphate cross-linked with a natural bone substitute and incorporating gentamicin-microspheres was further developed to study the local release of the drug in a simulated environment of the bone. The gel retained the antibiotic activity and showed suitable properties to be applied as bone scaffolds offering prolonged release of the drug and enhanced cellular penetration (Dorati et al., 2017a).

In attempt to solve the problem associated with the High IV doses of Hydrochloric norvancomycin (HNV), HNV-loaded PLGA microspheres were successfully developed using double emulsion solvent evaporation method. HNV-PLGA microspheres showed maintained controlled release, lacking burst effect, over a period of 7 days. The antimicrobial studies demonstrated that the encapsulation of HNV enhanced its antibacterial action and effectively enabled the inhibition of the growth of microorganisms. The authors concluded that the use of HNV-loaded PLGA microspheres met the requirements for local anti-infection to prevent bone destruction (Yang et al., 2017).

In another study, Poly (ε-Caprolactone) microparticles loaded with vancomycin were prepared. The prepared microparticles showed prolonged release of vancomycin from the formulations where the local delivery to the affected site allowed massive reduction of the dose compared to the systemic one and limited the side effects. The in-vivo study was performed in a rabbit model where chronic osteomyelitis was induced and confirmed a significant killing of the causing bacteria after 11 days (Le Ray et al., 2005).

3.3. Scaffolds combined with particulate system

Combining the bulk scaffold with particulate system can offer the advantages of both systems. Scaffold can be osteoconductive by occupying the dead space in the bone structure and enhancing the process of osteogenesis, while the particulate system can deliver the antibiotic with tunable release kinetics without being dependent on the degradation of the scaffold (Posadowska et al., 2016; Mahmoudian & Ganji, 2017).

A biodegradable scaffold was developed by Dorati and coworkers (2017a) in order to induce prolonged local release of gentamicin at the infected bone tissue for the treatment of osteomyelitis. The prepared scaffold was an injectable, thermosetting composite scaffold based on a polymeric solution of chitosan supplemented with bovine bone substitute particulate. Chitosan solution can form thermosetting at the physiological environment of 37 ºC in the presence of β-glycerophosphate, which acts as inducer to stimulate gel formation in addition to being able to enhance bone regeneration. The supplementation of bovine bone substitute particulate affected the physicochemical properties of the prepared scaffolds namely; its mechanical properties, interconnectivity, porosity, and biological activity. After lyophilization of the prepared scaffolds, solid composite 3D scaffolds were obtained having high water retention affinity and hydrophilic nature, which was able to enhance cell attachment and infiltration. The polymeric components of the composite scaffolds showed no remarkable toxicity caused by degradation products during in-vitro incubation in simulated physiological conditions. Additionally, the scaffolds had excellent physical stability in-vitro.

Vancomycin was incorporated into composite system based on hydroxypropyl methylcellulose (HPMC) microparticles and chitosan thermosensitive hydrogel for the local treatment of osteomyelitis. Mixtures of vancomycin hydrochloride and HPMC were spray dried to prepare vancomycin-loaded HPMC microparticles in the first step of preparation. Later, the microparticles were incorporated in an injectable chitosan/glycerophosphate solution (free-flowing at room temperature) which form a semi-solid gel at body temperature. The system was able to retard the release of vancomycin (Mahmoudian & Ganji, 2017). In another study, injectable system composed of gentamicin-loaded PLGA NPs, embedded in the polysaccharide gellan gum hydrogel was fabricated. This system ensured less-invasive administration technique compared to the surgical intervention required for the implants. The NPs ensured sustained and controlled drug release, while the hydrogel enabled localized drug release by keeping the NPs at the infected area. The system was evaluated regarding injectability and handling which proved good performance (Posadowska et al., 2016).

3.4. Bone grafts loaded with antibiotics

One of the potential procedures used in the treatment of chronic osteomyelitis is utilizing a bone graft loaded with antibiotics to be implanted in the defect area. That graft can provide a high and effective concentration of antibiotics at the site of defect with the avoidance of systemic side effects in addition to facilitating bone formation through acting as scaffold (Fölsch et al., 2020). Bone grafts can either be autologous if a bone from the donor is transported to another location in the same donor (Miller & Chiodo, 2016) or allogenous if a bone from the donor is transported to different receiver (Kumar et al., 2013). Either autologous or allogenous bone grafts may be used depending on the location of the infection, the size of the bone defect and the availability of an internal bone bank. Generally, three types of bone autologous grafts are identified; fresh/fresh-frozen, freeze-dried and demineralized freeze-dried (Anagnostakos & Schröder, 2012).

It was previously reported in the literature that incorporation of antibiotics may be made by mixing with the bone graft manually, or by immersing the bone grafts into solutions containing the antibiotic for various time periods (Winkler et al., 2000; Ahmed et al., 2018). In a study performed by Fang et al. (2021), a new antibacterial matrix was designed through crosslinking demineralized extracellular cancellous bone with vancomycin. The in-vitro results showed that the matrix had sustained release of vancomycin over 6 weeks accompanied with enhancing osteogenesis in the tested mesenchymal stem cells and osteoblasts. The in-vivo studies in rat model confirmed its antibacterial activity in addition to enhancing the osteogenesis in the defect area. In another study, bone grafts were obtained from fibrous tissue, cartilage and cortical layer that were extracted from the femoral heads of patients. The grafts were saturated with vancomycin and the in-vitro study showed that the release of the drug was controlled over a prolonged period, which helped in preventing the development of resistance to vancomycin (Melicherčík et al., 2012).

4. Challenges facing the development of local drug delivery systems for managing osteomyelitis

However, there are some commercially available cements, either incorporating antimicrobial agents or lacing them to allow surgeons to impregnate them with appropriate antimicrobial agents during operation, they still do not justify the tremendous amount of research-work and time invested in this area including developing different categories of products and delivery systems, controlling their drug release patterns and testing them in animal models neither they satisfy all patients’ needs.

From clinical point of view, the commercially available cements incorporating antimicrobial agents do not satisfy the variety of personalized conditions facing surgeons namely, the patients’ ages and their general health conditions, the severity/chronicity of their infection-conditions, the type and volume of the involved bones’ tissues which affect the required dose of the antimicrobial agent that can eradicated the infection and do not deteriorate the patients general health conditions if absorbed to the system through the close muscular tissues. This is further complicated by the diversity of the infectious microorganism causing bone infections and the continuous development of resistance and the needs for diversity of antimicrobial to be incorporated in delivery products besides the continuous introduction of new chemical moieties.

On the other hand, however, the commercially available blank cements allow surgeons to incorporate the proven effective antimicrobial agents in the required strengths during the operation, the in-situ developed scaffolds are not standardized with respect to the rate of drug release which does not guarantee the delivery of sufficient amount of the drug to keep its concentration above the minimum inhibitory concentration (MIC) for the specified infectious organism during the whole treatment course. They also are not tested for being not dose dumbing for the specific antimicrobial agent to avoid fast depletion of the scaffold and/or the toxicity effect on patients. The previous analysis leads to continuous personalized research studies in many orthopedic departments of hospitals and clinics seeking beneficiary treatments for the wellbeing of their patients (Weinrauch et al., 2007; Kammerlander et al., 2011; Cazzato et al., 2014; Dzyuba et al., 2016; Luo et al., 2016).

Furthermore, all the commercially available scaffolding material, whether medicated or blank, require surgical procedures for application into the infection site. If surgical debridement is not part of the treatment strategy, applying the local implant may be unfavorable option for many patients due to significant extra costs, the required recovery period and the probability of development of infection recurrences if the scaffold did not completely eradicate the infection which dictate reoperation. Moreover, the continuous developed resistance against potential antibiotics may impede their use with the biomaterials to achieve the required minimum inhibitory concentration (MIC) in the infected bone (Kavanagh et al., 2018).

From the industrial perspective, personalized medicine based on developing customized scaffolds for filling the dead spaces in patients’ infected bones are not feasible neither will be creating the huge variety of required strengths of different antimicrobial agents per each scaffolding materials. Creating the personalized scaffold shape and volume is better achieved using the advanced technology of 3 D printing (Ford & Cassat, 2017) in hospitals and clinics. From the industrial point of view this can be solved by the blank scaffolding cements which, in the same time, allow incorporating different variety of antimicrobials and different strengths. This leads again to the shortcoming of the lack of standardization of the developed scaffolds.

From the researchers’ overview, to be able to create a multipurpose scaffolding materials for managing different stages of osteomyelitis, many areas has to be tackled. The most important of which is creating reliable osteomyelitis models that imitates features of osteomyelitis conditions in humans’ bones which, continues to be a challenge to scientists (Roux et al., 2021). The current animal models employed to assess the efficacy of scaffolds for elimination of infection and regeneration of bone should be fully detailed and reproducible, with well-defined gold standard for the diagnosis of osteomyelitis, there originating sources and stage of infection in order to be able to simulate the clinical scenarios (Reizner et al., 2014). Furthermore, scaffolding materials have to offer multipurpose including eradication of infections, assisting the proliferation and healing of the bone tissues besides acting as internal mechanical supports.

5. Future perspectives

New strategies are highly encouraged to develop and commercially distribute antibiotics-loaded delivery systems that meet different patients’ needs for the local treatment of osteomyelitis whether surgical debridement is part of the treatment strategy or not. Currently, there are plenty of biomaterials with different characteristics that can be adjusted to achieve controlled-release of antibiotics and keep the concentration above the MIC yet do not dump the loaded amount of the drug in a much higher rate than required to allow the delivery system to be effective for longer period of time (Elkasabgy et al., 2019; Abdel-Salam et al., 2020). Among those biomaterials namely; ploy (lactic acid), poly (lactic co-glycolic) and chitosan either alone or in combination with bioactive glass can offer many advantages including: (1) being biocompatible, they do not initiate hypersensitivity reactions after application, (2) being biodegradable, they do not need postoperative surgery to remove the empty scaffold besides the fact that with research effort their rate of degradation can be adjusted to simulate the rate of bone regeneration (3) being polymeric in nature they can be designed to undergo in-situ gelling upon injection to the affected bone site or in its close vicinity which will be advantageous if surgical debridement is not required, (4) being injectable will allow surgeons to give multiple injection of the controlled release delivery system to the operated site of infection during the course of treatment to adjust the in-situ concentration above the MIC without having to re-operate for the re-application of the cement or to continue the course of antimicrobial therapy, systemically with all the risks of the systemic therapy on the general health condition of patients specially most of them are elderly, (5) having the nano-size bioactive glass (NBAG) will allow them to acquire more stiffness upon the in-situ setting of the polymeric materials which act as internal mechanical support specially in cases when long bones are involved. Besides the osteoinductive and osteoconductive properties of NBAG, these characteristics can be improved by incorporating antiresorptive drugs in a combination therapy to promote bone healing. With the financial support of industrial companies a well-designed researches specially in the field of developing animal models for osteomyelitis, more studies including the application of the injectable scaffolding materials loaded with different antimicrobials on multiple animal species so that a good reflections on possible human manifestations and responses to therapies may be drown which allow for clinical experimentation and finally commercial production.

6. Conclusion

Different local drug-delivery systems for the treatment of osteomyelitis were discussed in the current review. As a conclusion of the above discussion, it is very important while designing a new local antibiotic delivery system for the management of osteomyelitis, either in its early stages or after surgical debridement of the dead tissues, that these systems should have as many desirable features as possible. These features include being biodegradable, biocompatible, can enhance the regeneration of new bone tissues, being able to provide mechanical support to the affected long bone during the healing period and can offer sustained drug release. Finally, if made injectable it would provide two major advantages namely; reducing the level of invasion and wins patients’ compliance and allowing multiple injection to the affected part after surgical debridement over the treatment period to avoid the systemic therapy for long time which leads to many side effects and toxicity.

Disclosure statement

The authors of the current review declare that there is no financial, ethical, copyright or any other conflict of interest in the current work.