Page 48 - Materials Australia - April 2019

FEATURE – Additive Manufacturing
Orthopaedic Materials
Source: Cameron Chai
While the human body continues to evolve, it is perhaps doing so at a rate slower than the rate at which our population is ageing. The increasing age of our society, due in part to advances in medical science, places more stress on our healthcare system as well as our own bodies. This is one of the main factors driving an increase in the number of orthopaedic implants being used. Other contributing factors are more sedentary lifestyles, increasing levels of obesity, increasing prevalence of orthopedic injuries or diseases and technological innovations. Materials science plays a key role in improving the quality of life for many people and, according to Allied Market Research, will drive a market estimated to be worth $47,261 million in 2016, and to a predicted value of $74,796 million by 2023.
Orthopedic implants are defined as medical devices surgically implanted inside the body to restore bone function by reinforcing or replacing a damaged structure. These implants may be implanted permanently or removed at a later date after sufficient healing has taken place. The main types of orthopaedic implants that have fueled this growth are knee, hip, shoulder, and other (bone or joint) implants.
Biomaterials
The human body is a complex system and a harsh environment for materials. As such, surgeons can’t just implant any material. In fact, there are only a handful of materials that are accepted by organisations such as the US FDA (Food and Drug Administration) and the Australian TGA (Therapeutic Goods Association) for use in medical devices.
The physiological conditions encountered in the human body are extremely corrosive and any ions that may leach out of an implant could have detrimental effects on not only surrounding tissue but also wider reaching systems, such as the nervous system. Furthermore, the human body is extremely intelligent. It can detect foreign materials and will attempt to reject or isolate them to protect itself from their potential ill-effects. Hence, any material implanted must be ‘biocompatible’ and in some instances ‘bioactive’ is an even better solution.
Biocompatible materials are either bioinert or bioactive and do not elicit any toxic effect. Bioinert materials remain essentially inert when implanted into the human body and remain unchanged during their service period. This category includes polymers such as UHMWPE (ultra-high molecular weight polyethylene) as well metals and ceramics as outlined in Table 1 (on page 49).
Bioinert materials are recognised as foreign by the human body
and will become encapsulated by fibrous tissue when implanted. Depending on the application, bioinert materials can retard the integration of the implant which can potentially lead micromovements and implant loosening, ultimately requiring surgical revision.
The most common bioactive material is hydroxyapatite (Ca10(PO4)6(OH)2). This calcium phosphate ceramic is analogous to the mineral component of bone. In bone, hydroxyapatite provides stiffness, while the collagen matrix contributes a degree of flexibility. The composite composition of bone results in a fracture resistant structure. Being bioactive means that the body will happily integrate with these materials.
Bone is a dynamic structure that is constantly growing thanks to
48 | APRIL 2019
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