**3. Titanium and titanium alloys as bone implant materials**

The applications of titanium in modern society, such as aviation and military defence, have been exploited widely. Titanium components have also been used in biomedical devices, in‐ cluding screws, plates, and hip and knee prostheses, for either bone fractures or bone re‐ placement. These proven applications can be attributed to the distinctive properties of titanium and its alloys; properties such as high strength to density ratio and high corrosion resistance that enable their use as bone substitutes under load bearing conditions. Moreover, titanium exhibits a high tensile strength that is not featured in polymer or ceramic biomate‐ rials. However, the long term inertness of titanium towards human tissues after implanta‐ tion is a major drawback, as this means a lack of direct chemical bonding between the implant and host tissues [5-6].

Another concern regarding the use of solid titanium is that the dense structure is unable to support new bone tissues in growth and vascularisation. In addition, titanium has a much higher elastic modulus than natural bone, *i.e.,* 5 GPa and 110 GPa for bone and dense titanium, respectively [7-8]. This biomechanical mismatch causes stress shielding and, eventually, may lead to aseptic loosening that results in additional surgery after 10-15 years of implantation [9]. The development of porous titanium may potentially overcome problems of this nature.

**2.3. Criteria of ideal bone implant**

24 Titanium Alloys - Advances in Properties Control

tem if they are appropriately designed.

corrosion resistance.

implant and host tissues [5-6].

An ideal bone implant material should be osteoconductive, osteoinductive and should have osseointegration ability [3]. Furthermore, other key criteria for excellent implant perform‐ ance include biocompatibility and mechanical compatibility. In addition, any implant waste after degradation should not cause harmful effects to the body. Recent trends in bone tissue engineering studies have revealed that bone implants may also serve as a drug delivery sys‐

Osteoconduction is a process by which bone is directed to conform to a material's surface, while osteoinduction is the ability of an implant to induce osteogenesis. An inductive agent will stimulate undifferentiated cells to form preosteoblasts [3]. According to Branemark *et al*. [4], osseointegration could be defined as the "continuing structural and functional co-exis‐ tence, possibly in a symbiotic manner, between differentiated, adequately remodelled, bio‐ logic tissues, and strictly defined and controlled synthetic components, providing lasting,

In the context of orthopaedic implants, the development of a drug delivery system is still at an early developing stage. The promising concept of using an implant as part of a drug de‐

In addition to high mechanical strength, the Young's modulus is a critical mechanical prop‐ erty in an artificial device when designing materials for bone implants. Other fundamental requirements for an ideal orthopaedic biomedical implant include high wear resistance, good fatigue properties if used under cyclic loading, no adverse tissue reactions, and high

The applications of titanium in modern society, such as aviation and military defence, have been exploited widely. Titanium components have also been used in biomedical devices, in‐ cluding screws, plates, and hip and knee prostheses, for either bone fractures or bone re‐ placement. These proven applications can be attributed to the distinctive properties of titanium and its alloys; properties such as high strength to density ratio and high corrosion resistance that enable their use as bone substitutes under load bearing conditions. Moreover, titanium exhibits a high tensile strength that is not featured in polymer or ceramic biomate‐ rials. However, the long term inertness of titanium towards human tissues after implanta‐ tion is a major drawback, as this means a lack of direct chemical bonding between the

Another concern regarding the use of solid titanium is that the dense structure is unable to support new bone tissues in growth and vascularisation. In addition, titanium has a much higher elastic modulus than natural bone, *i.e.,* 5 GPa and 110 GPa for bone and dense titanium, respectively [7-8]. This biomechanical mismatch causes stress shielding and, eventually, may lead to aseptic loosening that results in additional surgery after 10-15

livery system could be described as the integration of therapeutic agents and devices.

specific clinical functions without initiating rejection mechanisms".

**3. Titanium and titanium alloys as bone implant materials**

The development of new titanium alloys has been extensively explored. Usually Al, Sn, O, C, N, Ga, and Zr are used as α stabilizers, while V, Mo, Ta, Nb, and Cr are used as β stabiliz‐ ers [10]. Titanium alloys such as Ti6Al4V with aluminium and vanadium as α and β stabiliz‐ ing elements have been widely used as implant materials. These first generation biomedical titanium alloys, however, have revealed that the release of Al and V metal ions is harmful to the human body [11]. The decisive requirement of a biomedical implant is its biocompatibili‐ ty in the human body. Thus, alloying elements must be carefully chosen to reduce any bio‐ logically adverse impacts. Alloying elements that attract biomedical applications are Ta, Nb, and Zr due to their non-cytotoxicity, good biocompatibility, high corrosion resistance and their complete solid solubility in titanium [10].

Beta alloys that have higher β stabilizers content are attracting great interest for bone im‐ plant applications due to their low elastic modulus. Beta alloys that have been studied for bone implant applications include Ti50Ta20Zr, Ti64Ta, Ti13Nb13Zr, Ti42Nb, and Ti30Zr10Nb10Ta. Studies conducted by Obbard *et al.* [12] showed that by adjusting the concentration of β stabilizer Ta, the elastic modulus could be reduced. In this fashion the compliance mismatch between the implant and bone would be reduced, leading to lesser stress shielding.

Alpha-beta alloys may have some advantages over β alloys, namely lower density and high‐ er tensile ductility. Some studies have succeeded in the production of alpha-beta alloys with a porous structure. The porous structure serves as an anchorage for bone in-growth and ex‐ hibits a lower elastic modulus, while the α and β phases provide sufficient mechanical strength for load bearing applications.

The development of porous titanium alloys with a variety of alloy components has brought about many improvements in bio-mechanical properties. For example, porous Ti10Nb10Zr with 69% porosity exhibited a tensile strength of 67 MPa, while pure Ti and pure Ta scaf‐ folds with the same porosity demonstrated lower strengths of 53 MPa and 35.2 MPa, respec‐ tively [13]. Xiong *et al.* [14] reported that the elastic moduli of porous Ti-26Nb alloys with porosity of 50, 60, 70, and 80% were 25.4, 11.0, 5.2 and 2.0 GPa, respectively, while the pla‐ teau strength ranged from 180 MPa to 11 MPa.
