Surface Finishing

Surface finishing is one of the main factors in the design of implants that significantly affects the longevity of THR (Jamali et al., 2006). The surface finishing (surface roughness) of the head and cup is required to provide good function, whereas the surface finishing of the stem remains debatable (Zhang et al., 2008). The finish of a metallic stem can be smooth-polished surfaces, roughened-blasted surfaces, or geometrically textured surfaces (Crowninshield, 2001).

Materials Utilized in Artificial Hip Joint Components

The following section presents a brief review' on the materials used in artificial hip joint components. The materials can be classified into four main groups, namely, metals, polymers, ceramics, and composites. Each group has strengths and weaknesses.

Metals

St, Co-Cr-Mo alloys, and Ti alloys are the most commonly used metals for implant designs (Khanuja et al., 2011). St is advantageous in terms of cost and processing availability (Long & Rack. 1998). However, given that St-based prostheses are prone to corrosion and fracture, Co-Cr-Mo alloys and Ti alloys are more frequently used in prostheses (Musolino et al., 1996). Co-Cr alloys are stronger than St and Ti alloys and have better corrosion resistance than St (Manicone et al., 2007). Ti alloys have lower Young’s modulus, better biocompatibility, and more corrosion resistance than St- and Co-based alloys (Long & Rack, 1998). However, Ti alloys have poor shear strength and wear resistance (Long & Rack, 1998).

Polymers

Polymers are long-chain high molecular weight materials that consist of repeating monomer units (Loser & Stropp, 1999). Orthopedic implants made of polymeric material can be classified into two groups: temporary (bioresorbable or biodegradable) and permanent (long-term implant). Permanent polymeric implants are commonly produced using polyethylene, urethane, and polyketone, w'hereas temporary polymeric implants consist of polycaprolactone, polylactide, and polyglycolide. Sir John Charnley developed a low-friction joint w'ith a polymeric acetabular cup made of polytetrafluoro- ethylene (PTFE) and a small metallic femoral head; however, PTFE has been replaced by ultrahigh molecular weight polyethylene, w'hich has excellent energy absorption and low coefficient of friction (Long & Rack. 1998; Slouf et al., 2007).

Ceramics

Ceramics are inorganic materials composed of metallic and nonmetallic elements (Asthana et al., 2006; Mackenzie. 1969). Ceramics are widely used in engineering, particularly in the aviation and automotive industries. In addition, ceramic materials have good biocompatibility, and thus they are suitable for medical devices and hard tissue replacement. Ceramics, including HA, alumina, and zirconia, have orthopedic applications.

HA, with the chemical formula of Ca10(PO4)6(OH)2, is a crystalline molecule that consists of phosphorus and calcium (Saithna, 2010). HA is the main mineral component (65%) of human bone (Havlik, 2002). This compound exhibits significant properties such as excellent biocompatibility, bioactivity, nontoxicity, and unique osteoinductivity for orthopedic applications (Ohgaki & Yamashita, 2003; Pramanik & Kar, 2013). The brittleness of HA and its lack of mechanical strength limit its application in implants (Aminzare et al., 2012). Therefore, HA can be used as a composite material by reinforcing with other materials or can be applied as a coating on the surface of implants (Aminzare et al., 2012). The HA coat creates a firm fixation by forming a biological bond between the host bone and implant (Singh et al., 2004). Thus, cementless implants coated with HA have higher survival rate than the uncoated implants (Singh et al., 2004).

Calcium silicate (CS) (CaSiO,) is a highly bioactive material that induces the formation of an HA layer on its surface after soaking in simulated body fluid or human saliva. Hence, CS is an appropriate material for bone filling, implant, and bone tissue regeneration because of its osseointegration properties. However, similar to HA. CS has low fracture toughness and load-bearing capacity, thus limiting its application in the human body. Therefore, numerous studies have endeavored to enhance the load-bearing capacity and toughness of CS by reinforcing it with other materials such as alumina (Shirazi et al., 2014), carbon nanotube (Borrmann et al., 2004), graphene oxide (Xie et al., 2014), and reduced graphene oxide (Mehrali et al., 2014). In addition, CS is applied as a coating layer on metallic implants to increase their surface bioactivity and to provide a good bond with the bone and a firm fixation.

Alumina is the most stable and inert ceramic material that has been utilized in orthopedic implants (Shikha et al., 2009). Alumina is a polycrystalline ceramic that contains aluminum oxide, which is extremely hard and ranks third after diamonds and silicon carbide, and is also a scratch resistant material (Jenabzadeh et al., 2012). Alumina has a Young’s modulus of 380 GPa, which is approximately twice as much as that of St (Hannouche et al., 2005). Zirconia, a crystalline dioxide (ZrO,) of zirconium, has good chemical and dimensional stability, wear resistance, mechanical strength, and toughness, in addition to the following characteristics: Young’s modulus similar to that of St; tensile strength, between 900 MPa and 1,200 MPa; and compressive strength. 2,000 MPa (Piconi & Maccauro, 1999). A molecularly stable zirconia can be achieved by mixing it with other metallic oxides, such as MgO, CaO, or Y203 (Manicone et al.. 2007). Despite the difficulty in stabilizing zirconia with Y20, sintering, this combination presents better mechanical properties than other combinations (Manicone et al., 2007). A biomedical grade of zirconia that was

Scheme of the aging process. (Reproduced with permission from Chevalier (2006). Elsevier.)

FIGURE 1.12 Scheme of the aging process. (Reproduced with permission from Chevalier (2006). Elsevier.)

proposed in 1969 for orthopedic implants and for replacement of Ti and alumina implants has comparable brittleness with that of alumina, thus preventing implant failure (Chevalier, 2006). However, zirconia aging and surface grinding have detrimental effects on its properties and toughness (Figure 1.12) (Kosmac et al„ 1999; Luthardt et ah, 2002).

Composites

Composites are engineered materials composed of two or more constituents. Currently, composite materials have been used in different fields of engineering, such as biomedical engineering, to produce new devices and implants (De Oliveira Simoes & Marques, 2001). The properties of composites can be modified according to different requirements; moreover, composites overcome the limitation of using single-phase material with the use of combined materials (Evans & Gregson, 1998). Therefore, these materials have better biological and mechanical compatibilities with body tissues and optimal strength and durability (Evans & Gregson, 1998). Orthopedic composites can be classified into polymer composites, ceramic composites, metal composites, and functionally graded materials (FGMs). In polymer composites, biocompatible polymers are applied as matrix with the reinforced materials (particulates, short or continuous, woven fibers (fabric), and nanofillers), regardless of the curing process (thermoset and thermoplastic). Thermoset polymer composites with low Young’s modulus and high strength have been implemented in femoral prostheses and fixation devices (Scholz et al., 2011). However, their performance in fixation devices is better than that in femoral prostheses (Evans & Gregson, 1998). Moreover, thermoplastic polymer composites have been used in acetabular cups and artificial knee joint bearing.

Composite materials made of ceramics and metals are categorized based on the matrix and reinforcing materials into ceramic-metal composites (CMCs) and metal-ceramic composites (MCCs). The significant change in mechanical properties is caused by the inclusion of ceramic or metal particles into the metal or ceramic matrixes (Rodriguez-Suarez et ah, 2012). Therefore, CMCs and MCCs possess superior stiffness, fracture, fatigue, tribological, and thermal properties to their monolithic ceramic and metal counterparts because of the overlapped strengths and weaknesses of the ceramics and metals (Mattern et ah, 2004). Accordingly, conventional and monolithic materials (ceramics and metals) can rapidly change with these composites in various engineering applications such as in the aerospace and automobile industries (Sahin, 2005).

FGMs are special groups of composite materials that incorporate continuous change (gradient) or stepwise change (graded) in their microstructure and properties as shown in Figure 1.13 (Miao & Sun, 2009). This concept was obtained from their natural biological structures (Pompe et ah, 2003). Adapting materials with specific structural, compositional, morphological, and mechanical properties has emphasized that FGMs can be utilized in the design of new prostheses. The mechanical properties of FGMs can be optimized and controlled by adjusting the volume fraction of each material phase (Nie & Batra, 2010). In addition, the FGM-based implants provide better load-bearing capacity, fracture toughness, and wear resistance than

A typical FGM structure

FIGURE 1.13 A typical FGM structure: (a) gradient and (b) graded.

their monolithic ceramic or metallic counterparts (Miao & Sun. 2009; Mishina et al.. 2008; Zhang et ah, 2012).

 
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