Infection of bone and soft tissue is one of the biggest problems in orthopedic daily practice, which mostly leads to progressive inflammation and even bone destruction [1]. Osteomyelitis is the infection and inflammation of bone and bone marrow tissue, usually begins as an acute infection, but it may evolve into a chronic condition. It can be subdivided into hematogenous or posttraumatic spread of bacteria, whereas the incidence of posttraumatic osteomyelitis is around 80% [2]. The treatment of chronic osteomyelitis is a challenging problem, since germs with variable drug sensitivities exist. The most common pathogens are coagulase negative staphylococci and Staphylococcus aureus [3]. There is still no evidence-based guidance on the treatment of chronic osteomyelitis and there is no single-treatment regimen or surgical procedure that is appropriate for all patients [2]. Besides the use of antibiotics, sufficient debridement seems to be the most important therapy for osteomyelitis [2].

In the presence of necrotic bone, a biofilm can develop and reduce the efficacy of antibiotic therapy by 10³ folds [4]. Bacteria may persist in a biofilm-based colony or be intra-cellularly concealed within osteoblasts, these characteristics render bacteria a greater resistance to the host’s immune response and antibiotic therapy [5]. Biofilm is any group of microorganisms in which cells stick to each other and adhere to a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS) [6]. Suitable drugs are not yet available for the eradication of such biofilm-producing bacteria.

Iron is an essential but potentially hazardous biometal. Mammalian cells require sufficient amounts of iron to satisfy metabolic needs or to accomplish specialized functions. Iron is an essential trace nutrient required for the active sites of many electron transfer and oxygen transport enzymes. In contrast, iron is also a catalyst for reactive oxygen species (ROS). Organisms must acquire iron but also must protect against oxidative damage. Ferritin is a naturally designed nanomachine for disposing cellular irons. Ferritin sequesters iron and maintains it in a biologically safe form. Ferritin also releases stored metals in response to cell needs. Thus, ferritin plays important roles in detoxification and reservation, and is vital for maintaining iron homeostasis in living cells. Ferritin genes are ubiquitously present among bacteria, plants, and animals, suggesting that they are essential genes for most organisms [7–9]. The iron oxide particle size of these agents varies widely, and influences their physicochemical and pharmacokinetic properties, and thus their clinical applications. Iron oxide nanocrystals (IONPs) are superparamagnetic. Many IONPs, including superparamagnetic iron oxides (SPIOs), are encapsulated by hydrophilic polymers to improve the stability and solubility. SPIOs have been used in clinical applications as a contrast agent for magnetic resonance imaging (MRI) [10, 11]. IONPs can be safely administered in the body because they do not raise concerns of heavy metal uptake and accumulation.

The healing effect of heat has been well-known since ancient times, it have been used to cure cancer [12] and infection [13]. The heating of biological tissue can be achieved by several physical mechanisms including microwave irradiation [14, 15], Ohmic heating [16], optical laser irradiation [17] or water bath heating [18, 19]. With these heating methods, it remains a challenge to control the spatial extent of heating in tissue. Magnetic particles hyperthermia improves the precision of heating by embedding the heating source into tissue and heating it using an external alternating magnetic field [20]. Many clinical trials have demonstrated the efficacy and safety of magnetic hyperthermia for prostate cancer and gliomas; patients can tolerate this therapy without discomfort or serious side effects [21–23]. However, there is no research for magnetic hyperthermia in the treatment of osteomyelitis. The aim of this study is to study the heating effect of magnetic nanoparticles-induced hyperthermia on destroying biofilm and promoting antibiotic efficacy for the treatment of peri-implant osteomyelitis.

Transmission electron microscope (TEM) examination of Fe₃O₄ magnetic nanoparticles was illustrated in the Fig. 3. In the presence of small amounts of stabilizer and dispersant, hydrophilic and stable Fe₃O₄ nanoparticles were synthesized (Fig. 3a); the formation of F₃O₄ nanoparticles did not significantly affected by the changes in the injection temperature (Fig. 3b).

The crystal structure of Fe₃O₄ nanoparticles was confirmed by XRD spectroscopy. As shown in Fig. 4a, the diffraction peaks at 2θ = 30.4°, 35.5°, and 43.2° can be well indexed to (220), (311), and (400) planes of the inverse cubic spinel structure of Fe₃O₄ (JCPDS card no. 75–1610), respectively; according to the reflection peak positions and relative intensities, which confirms the formation of the Fe₃O₄ nanoparticles. The sample formed under a magnetic field is pure Fe₃O₄ without any impurity phases [1, 25]. Figure 4b was the hysteresis loops of Fe₃O₄ nanoparticles at room temperature. As shown in Fig. 4b, the hysteresis loops show that saturation magnetizations of Fe₃O₄ was 1.0 emu/g at the field of 1.5 KOe, and the absence of coercivity and remanence indicates that the as-prepared Fe₃O₄ has superparamagnetic properties above the blocking temperature. The TEM image in Fig. 4c shows that the average size of as-synthesized Fe₃O₄ magnetite nanoparticles is ca. 16 nm with narrow size distribution.

Heat production is determined by the magnetic properties of Fe₃O₄ nanoparticles, its concentration, and the strength of the AMF. Therefore, we examined the heating effect of AMF on medium containing Fe₃O₄ nanoparticles by thermography. The temperature increased time-dependently, and we adjusted the required temperature dependent on the concentration of Fe₃O₄ nanoparticles and the magnitude of the electric current used to generate AMF to generate a temperature of 75 °C, and we adopted these conditions for the subsequent assays. In this study, the inserted implants can be heated to 75 °C by magnetic heating (Fig. 5). When the implants were heated to 75 °C either for 90 s or 120 s (each 30 s heating with a rest for 1 min), there were no significant thermal damage noted in these two testing conditions (Fig. 5).

All animals survived after the surgical procedures and no postoperative skin infection or fever was observed. Figure 6 showed the histological, histopathological staining and microbiology assessment from osteomyelitis rats after 40 days infection (Fig. 6. Left upper). In this figure, uninfected group possesses normal trabecular patterns of cancellous bone (upper: Fig. 6 Left upper: A and B); while in the acute suppurative changes of osteomyelitis, there was devitalized lamellar bone with scalloped edges and absence of stainable osteocytes and osteoblasts, edema and granulocytic infiltration of surrounding tissues are obvious (middle: Fig. 6 Left upper: C-E). In the chronic osteomyelitis, there was the irregular fragment of devitalized bone surrounded by dense fibrous tissue heavily infiltrated by plasma cells, lymphocytes, and only a few granulocytes (lower: Fig. 6 Left upper: F-H).

In the histopathological staining, we verified the pathomorphological characteristics of our infected rat femur (Fig. 6 Left lower). As shown here, GRAM-staining revealed the presence of bacteria within the defect site as Gram (+) Staphylococcus aureus bacteria could be detected (Fig. 6 Left lower: B and C). Cango red staining (Fig. 6 Left lower: A) and SEM image (Fig. 6 Left lower: D) showed the biofilm forming completed after 40 days induction of osteomyelitis.

All smears taken from the experimental sites showed characteristic accretion of S. aureus infection as depicted in Fig. 3. In the control group, there is few characteristic CFUs on the agar plate (Fig. 6 Right: A). On the other hand, tissue from the affected area, intramedullary liquid from the infected femur and the implants from the infected animals all showed characteristic growth of CFUs on agar plates (Fig. 6 Right: B-D).

After the inoculation of Staphylococcus aureus, tissue necrosis with active inflammation and even biofilm formation were clearly demonstrated in all groups; while, the biofilm formation were found both at the surface of the inserted pin and the inner surface of bony trabeculae. Different treatment modalities did not change the histopathological features of the induced osteomyelitis; while massive necrosis of tissue was observed in the group V which was treated with intramuscular injection of vancomycin and magnetic particles hyperthermia [VC(i.m.) + M + IOHA] (Fig. 7).

In our study, we demonstrated that systemic administration of vancomycin [VC (i.m.)] did not show any effect on eradication of the bacteria when compared with the positive control group; similar event observed in the presence of magnetic nanoparticles [VC (i.m.) + M]. But, local administration of vancomycin into the femoral canal [VC (i.m.) + VC (f.c)] did enhance the eradication of bacteria; similar event was observed in the presence of magnetic nanoparticles when magnetic field was applied [VC (i.m.) + M + IOHA] (Fig. 8).

In the peri-implant osteomyelitis, systemic administration of vancomycin [VC (i.m.)] with/or without the presence of magnetic nanoparticles [VC (i.m.) + M] did not affect the percent bone volume; but, when local administration of vancomycin into the femoral canal [VC (i.m.) + VC (f.c)] or magnetic field was applied in the presence of magnetic nanoparticles [VC (i.m.) + M + IOHA]. The percent bone volume (BV/TV) was significantly higher than that of the positive control (Fig. 9).

Currently, there are no evidence-based guidelines in terms of the treatment of chronic osteomyelitis [2]. It is difficult to achieve complete remission, some studies report a failure rates of 20–60% [26]. The aim of treatment is to improve quality of life through either a curative or a palliative strategy. Curative management strategies usually comprise of a combination of complex surgical procedures and tailored adjuvant antibiotic therapy [27]; on the other hand, palliative treatment strategies are less invasive and typically involve to the use of chronic suppressive antibiotic therapy [28].

Hyperthermia (HT) is a minimally invasive method for cancer therapy in which the target tissues are annealed to about 42–46 °C. Heat treatment in such a temperature range is cytotoxic for tumor cells because of their disorganized and compact vascular structure that promotes an unfavorable microenvironment inside them. Although HT is a promising approach to cancer therapy, the inevitable technical problem with hyperthermia is the difficulty in heating only the local tumor region to the intended temperature without damaging the surrounding healthy tissue [30]. Magnetic nanoparticles are increasingly used for clinical applications such as drug delivery, magnetic resonance imaging and magnetic fluid hyperthermia. Magnetic nanoparticles (MNPs) have been used for HT treatment in an attempt to overcome this obstacle [23, 31]. The magnetic nanoparticles have been focus on many research recently because they possess attractive properties which could be potential used in biomedicine, tissue specific targeting, magnetic resonance imaging, defect sensor and environmental remediation, etc. [32–36].

In this study, the inserted implants can be heated to 75 °C by magnetic heating; while, there were no significant thermal damage noted in this testing condition (Fig. 5) Due to magnetic particles had better dispersion, magnetic nanoparticles (MNPs) heating sources was related to the better distribution of heating intensity in tissue [37, 38]. We did observed that after inoculation of Staphylococcus aureus, tissue necrosis with active inflammation and even biofilm formation were clearly demonstrated in all groups; the biofilm formation were found both the at the surface of the inserted pin and the inner surface of bony trabeculae. Different treatment modalities did not change the histopathological features of the induced osteomyelitis; while massive necrosis of tissue was observed in the group V which was treated with intramuscular injection of vancomycin and magnetic particles hyperthermia [VC(i.m.) + M + IOHA] (Fig. 7). As shown above, since this parameter did not cause tissue damage, we implied that the induced hyperthermia can enhance the destruction of Staphylococcus aureus.

In this study, rat osteomyelitis model was built by implanting metallic needle into the marrow cavity of distal femur after injecting 10⁸ CFU/0.5 ml of bacterial suspension. In this model, intramuscular injection of vancomycin [VC (i.m.)] did not show any effect to treat osteomyelitis. In the group of intramuscular injection of vancomycin with magnetic environment also could not make biofilm collapse and even to enhance bacteria growth [VC (i.m.) + M]. By raising the temperature of implant, adding magnetic nanoparticles to support hyperthermia that could destroy biofilm and enhance efficacy of vancomycin significantly [VC (i.m.) + M + IOHA] (Fig. 8). In the peri-implant osteomyelitis, local administration of vancomycin into the femoral canal [VC (i.m.) + VC (f.c)] or magnetic field application in the presence of magnetic nanoparticles [VC (i.m.) + M + IOHA] did prevent the bone loss associated with infection (Fig. 9).

A number of animal models were created in order to study the treatment effects of bacterial infections. Staphylococcus aureus strain for bacterial inoculation was used in almost all of them, because this is the most common pathogen in the setting of osteomyelitis [3, 39]. Furthermore, rats and rabbits were used in most studies because they are big enough to create stable long bone infections without giant efforts; while on the other hand they are small enough to be housed and handled easily [40–43]. Bone and intramedullary bacterial infections are one of the most serious complications of the surgical repair of fractures. Treatment of implant-related chronic osteomyelitis is often difficult and usually consists of implant removal, extensive surgical debridement, and prolonged antibiotic use. To reduce the incidence of implant-related infections, several biomaterial surface treatments with integrated antibiotics, antiseptics, or metal ions have been developed for implants to reduce implant-related infections [44, 45]. In the current study, we developed a rat model for studies related to improve the treatment of chronic osteomyelitis.

Biofilms are universal, occurring in aquatic and industrial water systems as well as a large number of environments and medical devices relevant for public health. Recent clinical research has implicated biofilms in the exacerbation of wounds [46, 47]. Biofilm formation, which decrease activity of antibiotics, is a crucial step in the pathogenesis of many subacute and chronic bacterial infections, including infection after artificial joints surgery. In this study, we developed a novel antimicrobial thermotherapy platform to treat chronic osteomyelitis. Previous studies always focused on a successful application of hyperthermia in cancer therapy [48, 49]. However, to our knowledge, this is the first in-vivo application of thermotherapy for osteomyelitis. Because of differences in the temperature sensitivity of Staphylococcus aureus compared to cancer cells, transient higher temperature must be safe for realization of this technology as an effective therapeutic adjuvant for infectious pathogens. Our result showed temperature up to 75 °C is still not causing tissue damage. More important, thermotherapy may be successfully combined with standard-of-care antibiotic treatment for polymicrobial biofilm infections and enhance antibiotic penetration by disrupting the biofilm polysaccharide matrix. This may significantly enhance the efficacy of a given antimicrobial effect and facilitate reduced drug dose, thus providing a valuable tool for the treatment of fragile patients who cannot tolerate high dose systemic antibiotics.

In this study, we use the needle implantation and injection of methicillin-sensitive Staphylococcus aureus bacterial suspension to create an animal model of osteomyelitis which mimic the clinical situation as chronic infection after implantation of artificial materials. Due to biofilms decreased the efficiency of antibiotics, using systemic antibiotic only to treat osteomyelitis did not show therapeutic difference with that of the positive control group. Here, we developed a new treatment modality to improve antibiotic efficacy in the treatment of chronic osteomyelitis. As the smaller size of magnetic naoparticles and their better dispersion in the infected surroundings, nanoparicles-induced hyperthermia can be evenly distributed to the whole surface of biofilm; the biofilms could be destroyed by magnetic nanoparticles hyperthermia and therapeutic effect of systemic antibiotics could be enhanced.