## 1 Introduction  
Orthopedics is a multibillion-dollar global industry that is mainly focused on the diagnosis, treatment, and prevention of musculoskeletal system via surgical and nonsurgical therapeutic interventions. Orthopedic devices have gained a considerable amount of attention for the treatment of bone, joint, and muscular injury. According to the American Academy of Orthopedic Surgeons (AAOS), musculoskeletal problems are the most prevalent medical conditions, and more than 130 million patients need medical attention each year [1]. Reports have also suggested that the annual cost of musculoskeletal disorders in the US is about $510 billion [2]. Therefore, there is a great demand for orthopedic devices in this industry. Current metallic biomaterials have been used to repair or replace existing damaged bone tissues due to their mechanical strength and fracture toughness [3]. However, these metallic biomaterials release toxic metallic particles through a corrosion process [4; 5; 6; 7; 8]. These toxic corrosion particles lead to inflammatory responses, which reduce the biocompatibility of implants and result in tissue damage [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. Several foundational studies on humans have suggested that magnesium (Mg)-based alloys be used for orthopedic implants. These implants were abandoned due to their rapid degradation and accumulation of hydrogen gas in adjacent tissues at the site of implant [16; 17]. However, magnesium (Mg) was rediscovered as a biodegradable material in orthopedic applications due to its biocompatibility and structural stability [18]. These biodegradable implants (after supporting the diseased tissues and bone fractures) degrade in the physiological environment of the body. Therefore, this eliminates the need for a second surgery to remove implants. However, unpredictable degradation behavior, the formation of gas cavities at the junction of the implant and the surrounding tissues, and their compromised structural integrity in an in vivo environment makes Mg-based implants challenging [19]. Some studies have reported a disparity between in vitro and in vivo corrosive environments for Mg-based alloys [20; 21]. Moreover, the surface characteristics, biocompatibility, and toxicity of the corrosion products released from Mg-based implants in the form of MgO/Mg(OH)\({}_{2}\) nanoparticles (NPs) (at the junction of implants and surrounding tissues) are not completely understood. Therefore, there is a need to control the corrosion rate of these implants for better performance for an adjustable amount of time. The purpose of this study, therefore, was to determine the in vitro cytotoxicity of magnesium oxide (MgO) and magnesium hydroxide (Mg(OH)\({}_{2}\)) nanoparticles (NPs) in human fetal osteoblast (hFOB) 1.19 cells to understand an acceptable concentration of release from biodegradable Mg-based alloys to develop a sustainable implant for future in vivo studies. In this study, a surface characterization of MgO/Mg(OH)\({}_{2}\) NPs was performed to evaluate the size of the nanoparticles. Further, we systematically investigated the degradation behavior of various concentrations (1 mM, 5 mM, 10 mM, 50 mM, and 100 mM) of MgO/Mg(OH)\({}_{2}\) NPs through an immersion test in Dulbecco's Modified Eagle's Medium (DMEM), and the change in the pH values from day 1 to 7 was measured. After seven days, we investigated the elemental depositions on MgO/Mg(OH)\({}_{2}\) NPs for the higher concentration (100 mM) through scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDX) mapping. Since the results of the pH values obtained from the immersion test between 50- and 100-mM concentrations of MgO/Mg(OH)\({}_{2}\) did not show a significant difference, we therefore optimized the 50-mM concentration as the highest concentration for further experiments. A live/dead assay and SEM were performed to study the cytotoxicity and morphological changes in the hFOB 1.19 cells in response to various concentrations (1 mM, 5 mM, 10 mM, and 50 mM) of MgO/Mg(OH)\({}_{2}\) NPs.