Orthopaedic implants
In the past few years, the demand for orthopaedic implants has risen exponentially, due to a substantial increase in bone cancer, accidental fracture and joint disorders [1]. The National Institutes of Health has reported that the demand for orthopaedic implants is increasing rapidly with an increase in bone diseases and injuries caused by the world’s ageing population and changes in lifestyle. According to an Allied market research survey, the global demands for medical implants was estimated at $77,738 million in 2016 and expect to achieve $124,154 million between 2017 to 2023 with a compound annual growth rate (CAGR) of 6.9% [2].
An orthopaedic implant is a device used to repair or replace an injured bone through internal fixation. Generally, implants are classified as permanent and temporary implants based on their applications. These permanent implants, such as the artificial hip and knee joint (figure 1a) are expected to serve throughout the life span of the patient. The implants must be able to support body weight during body movement and retain their shape and size under a repeatedly applied mechanical load [3]. On the other side, the temporary implants such as screws, plates, pins and intramedullary nails (figure 1b) are used for fixation of structure and do not carry the weight of the body. The role of these temporary implants is to compress and align the injured bone, which stimulates the healing process of bone tissues [4–7].
The Metal and alloys have widely used as a suitable material for permanent and temporary implants due to their excellent mechanical characteristics, reasonable biocompatibility and ease of manufacturing [8,9]. At present, 316L stainless steel (316L SS), Cobalt-Chromium (Co-Cr), Titanium (Ti) and Magnesium (Mg) alloys are used for implant applications [10,11]. Still, the metallic implants have several drawbacks like higher corrosion, wear and dispersion of metallic ions. One another issue of stress-shielding effect is also observed due to the mismatch of Young’s modulus that restricts the application of these alloys in orthopaedics [12–14]. The advantages, limitations and applications of various metallic alloys used for orthopaedic implants are briefly described in Table 1. The drawbacks such as higher Young’s modulus, lack of biocompatibility and lower wear resistance encourage to do further research with imaginative advancements to enhance the properties of the metallic implant [15–17].
The properties of any metal or alloy can be improved by reinforcing through nanomaterials. In nanomaterials a large number of grain boundaries associated with a unique atomic structure that offers high strength and hardness with low brittleness [18,19]. The nanomaterials provide increased surface area with desired mechanical, biocompatible and physiochemical properties such as adhesion, proliferation and osseointegration. Nanomaterials are broadly employed to develop nanocomposites (NCs) for orthopedic applications due to these exceptional properties [20,21]. Therefore, a broad area of research is available for the development of metallic NCs [22]. The review focuses on the metallic NCs developed for implant applications and their fabrication techniques. It unfolds the effect of nanosized strengthening and fabrication technique on the material properties of orthopedic implants. Recent issues and challenges in the field of metallic nanocomposite for implant applications are also described and addressed to set up a significant forum for this rapidly growing area of research.