There are 206 bones in the human skeleton (excluding teeth) that provide shape and support for the body, act as levers for muscles, and protect vital organs. When a bone has been badly damaged from any cause, such as arthritis, malformation since birth or abnormal development and damage from injury, a bone replacement would become necessary to restore function to the human body. Although the history of using man-made materials to replace a human’s hard tissues such as bone and teeth goes back to 2700 BC, where ancient civilizations used gold in dentistry, the actual development of man-made material as bone replacement has only happened within the past 100 years Citation[1].
In order to have a good hard tissue replacement in the body, we need a material with the following requirements:
• Good corrosion resistance to withstand the harsh environment in the body;
• High strength and mechanical stability to handle the load during daily activities;
• Good biocompatibility and nontoxicity, so it will not be rejected by the body.
The best materials that can offer all of the above are metallic materials. So far, stainless steel, cobalt-based alloys and titanium-based alloys have been considered as the most popular bone replacement (implant) materials Citation[1]. Stainless steel implants have reasonable corrosion resistance and biocompatibility, with high strength and ease of production, while maintaining a low cost.
Cobalt alloys are one of the first materials that successfully combined strength and biocompatibility for biomedical applications. Cobalt itself is not a particularly biocompatible material, but the addition of 15–30% chromium will create a passivating oxide film that is very stable in the body. Cobalt–chromium alloys offer very good corrosion resistance, but they are brittle and difficult to fabricate.
Titanium and its alloys are of particular interest for biomedical applications because of their outstanding biocompatibility, as well as their little or non-reaction with tissue surrounding the implant. Titanium also offers the lowest density compared with the other two classes of metallic biomaterials, and that makes it more attractive as a bone replacement. This is predominantly because bone has a low density and the replacement must match its density to maintain the distribution of body weight in balance. The average bone density is approximately 1.5 g/cc while the density of steel, cobalt and titanium are 7.8, 8.9 and 4.5 g/cc, respectively. It appears that the closest density to bone belongs to titanium alloys.
Stiffness, modulus of elasticity or moduli of biomedical materials, which are the material’s tendency to be deformed elastically (nonpermanently) under loading, are further important factors in choosing the right bone replacement. The discrepancy between the stiffness and/or modulus of bone, and that of the alloy used for implant to support the structural loads, causes the metallic devices implanted in the body to take a disproportionate share of the load. Consequently, the actual load experienced by bone will be proportionally lower, leading to the phenomenon known as ‘stress shielding’.
Stress shielding refers to the reduction in bone density as a result of removal of normal stress (load) from the bone by an implant that is stiffer than bone. This is explained by Wolff’s law that states, “the bone in a healthy person or animal will adapt to the loads it is placed under” Citation[2]. If loading on a particular bone increases, the bone will remodel itself over time to become stronger to resist that sort of loading, while if the loading on a bone decreases, the bone will become weaker owing to turnover. It is also less metabolically costly to maintain, and there is no stimulus for continued remodeling that is required to maintain bone mass Citation[3].
The stress-shielding phenomenon, which leads to the deterioration of bone quality, causes a decrease of bone thickness, bone mass loss and osteoporosis (bone resorption) which eventually loosen the prosthetic device, and can result in the need for revision surgery and replacing the implant with a new one. This is one of the main factors which limit the lifetime of the implant in the body.
Within the three categories of metallic alloys for biomedical implants, titanium has the lowest moduli of 105–125 GPa, while steel has a moduli near 205 GPa, and the cobalt alloys have the highest moduli of 240 GPa. Bone has a modulus of approximately 17 GPa (between 7 and 30 GPa in general, depending on the age and health condition of the person). As a result, titanium alloys have become the most popular implant alloy owing to their closest moduli or stiffness to that of bone, and the less potential to stress shielding.
A number of attempts have been made on decreasing the moduli or stiffness of implantable alloys. The introduction of α-β-titanium alloys with moduli values approximately half that of stainless-steels or cobalt–chromium–molybdenum alloys, and the introduction of metastable β-titanium alloys with elastic modulus values in the range of 70 GPa, are some of those. Since the moduli of bulk materials could not go any lower, new attempts have been made on making the implants out of advanced composite materials such as polymers reinforced with glass or carbon fibers, or the use of porous coatings on bulk metallic implants, such as the stems of total hip replacements.
Some studies reported that the new biomimetic carbon fiber-reinforced polymer-composite (CF/PA12) stem for a total hip replacement, with moduli or stiffness close to that of the bone, generates a better bone-density pattern compared with the titanium-based stem, indicating the effectiveness of the composite stem to reduce bone resorption caused by stress-shielding phenomenon Citation[4]. In 2000, de Santis et al. reported the use of a composite hip prosthesis made from polymer reinforced with carbon and glass fibres with a moduli of 40 GPa along the hip stem axis and 10–60 GPa in the tip–head direction. de Santis noted that the tailored rigidity of the new composite material matches the mechanical behavior of the surrounding femoral bone model as a function of the plie-stacking sequence, leading to a more physiological strain distribution in the proximal femur, while the high strength ensures no fracture Citation[3].
Other efforts were focused on the two categories of proximally porous coated or fully porous-coated cementless implants. While both have been widely used, there remains debate regarding differences in clinical outcome scores, relative incidence of thigh pain and development of stress shielding Citation[5]. In this prospective, randomized, multicenter-blinded clinical trial comparing a proximally porous-coated titanium taper stem design to a fully porous-coated cobalt chrome stem design, no differences in clinical outcome scores were observed, and the incidence of thigh pain and progressive femoral bone loss was seen with both component designs and materials Citation[5].
To reduce failure rates and substantially improve implant function, new implantable biomaterials must be developed. The development of new implantable materials is costly, time-consuming and extremely difficult; however, new materials are needed in order to optimize implant function, incorporate emerging technologies and substantially improve patient care.
A new effort that I have reported on, has resulted in a new material that mimics the elastic modulus of natural bone. This new material called ‘composite metal foam’ is a porous material that can be made out of stainless steel, cobalt chromium or titanium. The modulus of elasticity of stainless steel composite foam is reported to be approximately 10–15 GPa Citation[6]. It is notable that the density of the material is approximately a third of the bulk material’s density from which they are made. For example, the density of a stainless steel composite foam is approximately 2.6–2.7 g/cc which is closer than any other metallic biomedical alloy to the density of bone (1.5 g/cc). Moreover, the porous nature of the material will help anchoring it into the surrounding tissue. The composite metal foams are thought to be the new materials for next-generation biomedical and dental implants. Currently, the researchers are still working to further evaluate the biological response of composite metal foams Citation[101].
Financial & competing interests disclosure
The author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.
References
- Handbook of Materials for Medical Devices. Davis JR (Ed.). ASM International, OH, USA (2003).
- Wolff J. The Law of Bone Remodeling. Springer, NY, USA (1986).
- de Santis R, Ambrosio L, Nicolais L. Polymer-based composite hip prostheses. J. Inorg. Biochem.79, 97–102 (2000).
- Bherara H, Bureau MN, L’Hocine Y. Bone remodeling in a new biomimetic polymer-composite hip stem. J. Biomed. Mater. Res. Part A.92(1), 164–174 (2010).
- MacDonald SJ, Guerin JS, McCalden RW, Bourne RB, Barrack RL. Proximally versus fully porous-coated femoral stems: a multicenter randomized trial. Clin. Orthop. Relat. Res.468(2), 424 (2010).
- Vendra L, Rabiei A. Evaluation of modulus of elasticity of composite metal foams by experimental and numerical techniques. Mater. Sci. Eng. A.527(7–8), 1784–1790 (2010).
Website
- Afsaneh Rabiei’s website www.mae.ncsu.edu/homepages/rabiei/index.html