**1. Introduction**

In the fourth chapter the relationships between processing parameters, morphology of the microstructure characterized by quantitative stereological parameters of microstructure and mechanical properties of selected high strength two-phase titanium alloys are summarized.

In the next chapter the formability of titanium alloys is discussed on the example of Ti-6Al-4V alloy. Forming limit diagrams are determined experimentally in the hydroforming bulge test

In the final chapter the influence of the radiation and others factors like plastic deformation and thermo-chemical treatment on the structure and crystal lattice defects in binary titanium

**Prof. Jan Sieniawski**

Faculty of Mechanical Engineering and Aeronautics

of the Rzeszów University of Technology

and numerically, using finite element method and various constitutive models.

alloys is investigated.

VIII Preface

Metallic implants are commonly used in the orthopedic field. Despite the large number of metallic medical devices in use today, they are predominantly make up of only a few metals. Metallic alloys such as titanium continue to be one of the most important components used in orthopaedic implant devices due to favorable properties of high strength, rigidity, fracture toughness and their reliable mechanical performance as replacement for hard tissues. Ortho‐ paedic implants are medical devices used for the treatment of musculoskeletal diseases and may consist of a single type of biomaterial or comprise a number of different biomaterials working together in modular parts. Prime examples of titanium implants used in orthopaedics would include prosthetic hip and knee replacements for various types of arthritis affecting these joints, spinal fusion instruments for stabilizing degenerate and unstable vertebral segments, and fracture fixation devices of various types such as plates, screws and intrame‐ dullary rods. Although titanium based implants are typically expected to last ten years or more, however longevity is not assured and the lack of integration into the bone for long-term survival often occurs and leads to implant failure. Revision surgery to address such failure involves increased risk, complications and costs. The main reason for the failure of these implants is aseptic loosening which accounts for 60 to 70% of the cases for revision surgery. The success of implants is dependent on firm bonding or fixation of implant biomaterial to bone, for optimal function and lastingness. Therefore one of the key challenges in bone healing and regeneration is the engineering of an implant that incorporates osseointegration with enhanced bioactivity and improved implant-host interactions so as to reduce biological related implant failure.

#### **1.1. Development of titanium alloys for use in orthopaedics**

Titanium alloys, originally used for aeronautics, garnered attention from the biomedical field, due to their biocompatibility, low modulus of elasticity, and good corrosion resistance.

© 2013 Wang and Poh; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Wang and Poh; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Nonetheless, it was the osseointegration phenomenon due to the presence of a naturally formed oxide layer on the titanium surface that sparked development of titanium for use in orthopaedics [1]. Titanium alloys are often used in non-weight-bearing surface components such as femoral necks and stems (Figure 1), as they have lower modulus of elasticity resulting in less stress shielding of bone [2]. Nonetheless the osseointegrative bioactivity is still often not sufficient to attain true adhesion between the implant and bone, which may ultimately lead to mechanical instability and implant failure [3].

and commercial purity Ti are currently the most popular materials for implantation purposes. Commercial purity Ti has been tested to be inferior considering tensile strength, while Al and V have been shown to be unsafe. Currently researchers are still trying to develop other grades of alloys, such as Ti-6Al-7Nb and Ti–15Sn–4Nb–2Ta–0.2Pd. The most Ti alloys researched upon

Titanium Alloys in Orthopaedics http://dx.doi.org/10.5772/55353 3

Each manufacturer of titanium implants has his own differing theories on implant designs for specific orthopaedic applications. Generally there are certain guiding principles that will affect the ultimate viability of an implant. The design of the implant has to take into account biomechanical and biological factors that may affect its success. Conformity to native anatomy, material properties and mechanical strength appropriate for the targeted function and environment are some of the considerations that come into play. Despite the benefits and successes of these medical devices, their use is not without risk of adverse effects. Titanium implants generally develop an oxide layer which allows it to integrate with living bone tissue. However, the body can have adverse reactions to titanium like fibrosis and inflammation which may affect its long term functional performance [4]. Success in the application of an orthopaedic implant would depend on various factors and implants may fail due to physio‐

Generally there are two types of implant-tissue responses [5-7]. The first type is the response of the hosts' tissues to the toxicity of the implanted material. Implanted material may be toxic or release chemicals that could damage the surrounding tissues. The second response which is also the most common is the formation of a nonadherent fibrous capsule between the implant and the hosts' tissues termed fibrosis. This is a natural response to protect the body from a foreign object which may eventually lead to complete fibrous encapsulation [8]. Typically implants are intended to stay fixed in the human body for a long time and bone is expected to grow into the surface of the implant. Unfortunately this does not always happen. Fibrosis referred to as foreign body reaction, develops in response to almost all implanted biomaterials and consists of overlapping phases similar to those in wound healing and tissue repair processes [9]. Despite the biocompatibility of metallic implants used, titanium materials are generally encapsulated by fibrous tissue after implantation into the living body [10]. Cells trapped between the implant and the fibrous capsule also lack general housekeeping tissue functions like removing apoptotic or necrotic cells which can also promote chronic inflamma‐ tion [11]. Not only that the ECM (extra cellular matrix) secreted by fibroblast is different from the bone matrix formation generated by osteoblast, in the long run this ECM layer may lead to micromotion and the generation of wear particles on the surfaces of the implant [12]. The resulting titanium debris may play a leading role in the initiation of the inflammatory cascade leading to osteolysis [13]. Eventually this causes aseptic loosening as the bonds of the implant to the bone are destroyed by the body's attempts to digest the wear particles. When this occurs the prosthesis becomes loose and the patient may experience instability and pain. Revision surgery to resolve this would entail further costs and morbidities to the patients. For bone tissue, direct osteoblast attachment on metal is important to prevent aseptic loosening of the

are the (α + β)-type alloys for their strength and ductility.

**1.2. Bioactivity of titanium alloys**

logic reasons such as aseptic loosening.

**Figure 1.** a) Titanium stem and (b) surface elemental analysis.

The mechanical properties of suitable titanium alloys based on Young's moduli should be similar to that of cortical bone. Cortical bone also termed compact bone is the major and most important constituent of the human skeleton and is crucial for bone functions including organ protection, movement, support etc. Young's moduli of β-type titanium alloys are substantially smaller than those of the α- and (α + β)-type alloys. This has brought on the discovery of harmless low-rigidity Ti alloys such as Ti-13Nb-13Zr, Ti-12Mo-6Zr-2Fe, Ti-15Mo-5Zr-3Al, Ti-15Mo, Ti-35Nb-7Zr-5Ta and Ti-29Nb-13Ta-4.6Zr. Nonetheless there are both advantages and disadvantages of the application of these titanium alloys. These alloys have proved to be effective in preventing bone atropy and enhancing bone remodeling, however the high amount of spring back and low fatigue strength make them undesirable as implant material. Ti-6Al-4V and commercial purity Ti are currently the most popular materials for implantation purposes. Commercial purity Ti has been tested to be inferior considering tensile strength, while Al and V have been shown to be unsafe. Currently researchers are still trying to develop other grades of alloys, such as Ti-6Al-7Nb and Ti–15Sn–4Nb–2Ta–0.2Pd. The most Ti alloys researched upon are the (α + β)-type alloys for their strength and ductility.

### **1.2. Bioactivity of titanium alloys**

Nonetheless, it was the osseointegration phenomenon due to the presence of a naturally formed oxide layer on the titanium surface that sparked development of titanium for use in orthopaedics [1]. Titanium alloys are often used in non-weight-bearing surface components such as femoral necks and stems (Figure 1), as they have lower modulus of elasticity resulting in less stress shielding of bone [2]. Nonetheless the osseointegrative bioactivity is still often not sufficient to attain true adhesion between the implant and bone, which may ultimately

The mechanical properties of suitable titanium alloys based on Young's moduli should be similar to that of cortical bone. Cortical bone also termed compact bone is the major and most important constituent of the human skeleton and is crucial for bone functions including organ protection, movement, support etc. Young's moduli of β-type titanium alloys are substantially smaller than those of the α- and (α + β)-type alloys. This has brought on the discovery of harmless low-rigidity Ti alloys such as Ti-13Nb-13Zr, Ti-12Mo-6Zr-2Fe, Ti-15Mo-5Zr-3Al, Ti-15Mo, Ti-35Nb-7Zr-5Ta and Ti-29Nb-13Ta-4.6Zr. Nonetheless there are both advantages and disadvantages of the application of these titanium alloys. These alloys have proved to be effective in preventing bone atropy and enhancing bone remodeling, however the high amount of spring back and low fatigue strength make them undesirable as implant material. Ti-6Al-4V

lead to mechanical instability and implant failure [3].

2 Titanium Alloys - Advances in Properties Control

**Figure 1.** a) Titanium stem and (b) surface elemental analysis.

Each manufacturer of titanium implants has his own differing theories on implant designs for specific orthopaedic applications. Generally there are certain guiding principles that will affect the ultimate viability of an implant. The design of the implant has to take into account biomechanical and biological factors that may affect its success. Conformity to native anatomy, material properties and mechanical strength appropriate for the targeted function and environment are some of the considerations that come into play. Despite the benefits and successes of these medical devices, their use is not without risk of adverse effects. Titanium implants generally develop an oxide layer which allows it to integrate with living bone tissue. However, the body can have adverse reactions to titanium like fibrosis and inflammation which may affect its long term functional performance [4]. Success in the application of an orthopaedic implant would depend on various factors and implants may fail due to physio‐ logic reasons such as aseptic loosening.

Generally there are two types of implant-tissue responses [5-7]. The first type is the response of the hosts' tissues to the toxicity of the implanted material. Implanted material may be toxic or release chemicals that could damage the surrounding tissues. The second response which is also the most common is the formation of a nonadherent fibrous capsule between the implant and the hosts' tissues termed fibrosis. This is a natural response to protect the body from a foreign object which may eventually lead to complete fibrous encapsulation [8]. Typically implants are intended to stay fixed in the human body for a long time and bone is expected to grow into the surface of the implant. Unfortunately this does not always happen. Fibrosis referred to as foreign body reaction, develops in response to almost all implanted biomaterials and consists of overlapping phases similar to those in wound healing and tissue repair processes [9]. Despite the biocompatibility of metallic implants used, titanium materials are generally encapsulated by fibrous tissue after implantation into the living body [10]. Cells trapped between the implant and the fibrous capsule also lack general housekeeping tissue functions like removing apoptotic or necrotic cells which can also promote chronic inflamma‐ tion [11]. Not only that the ECM (extra cellular matrix) secreted by fibroblast is different from the bone matrix formation generated by osteoblast, in the long run this ECM layer may lead to micromotion and the generation of wear particles on the surfaces of the implant [12]. The resulting titanium debris may play a leading role in the initiation of the inflammatory cascade leading to osteolysis [13]. Eventually this causes aseptic loosening as the bonds of the implant to the bone are destroyed by the body's attempts to digest the wear particles. When this occurs the prosthesis becomes loose and the patient may experience instability and pain. Revision surgery to resolve this would entail further costs and morbidities to the patients. For bone tissue, direct osteoblast attachment on metal is important to prevent aseptic loosening of the metal implant caused by fibroblast layer attachment. Fibrosis can also cause osteoclastindependent bone resorption by fibroblast-like cells. It has been shown that fibroblast-like cells, under pathological conditions, not only enhance but also actively contribute to bone resorption [14]. Successful implant integration into the surrounding tissue is highly dependent on the crucial role of native cells, chiefly osteoblast attaching to the implant surface. Therefore one of the key challenges in orthopaedics is the engineering of an implant with enhanced osseointe‐ gration properties to reduce implant failure rates.

chemical techniques, organic molecules such as growth factors, peptides or enzymes are incorporated to the implant layer to affect specific cellular responses [17]. While in physico‐ chemical techniques, the incorporation is achieved with inorganic phases such as calcium phosphate which may increase the biochemical interlocking between bone matrix proteins and surface materials thereby enhancing bone-bonding [16]. Many implant modifications may combine both physical and chemical engineering methods. In the following sections we will discuss some of the more popular strategies used to enhance implant integration and bone-

Titanium Alloys in Orthopaedics http://dx.doi.org/10.5772/55353 5

Calcium phosphate coating has been widely used in the orthopaedic field due to their similarity with the mineral phase of bone [18] and are known for their bioactive properties which are beneficial in bone-bonding [19]. As calcium phosphate generally lacks the mechan‐ ical strength for use as bulk materials under loading conditions, they are often coated onto the surface of metallic implants. There are several studies published which have shown the favorable use of calcium phosphate coatings in increasing the biocompatibility of bone-implant interface, implant anchorage and integration [20]. The calcium phosphate layer functions as a physiological transition between the implant surface and the hosts' tissues which guides bone formation along the implant surface and the surrounding tissues. One of the most successful method for the application of calcium phosphate coatings is via the plasma-spraying method due to its advantage of extensive coating capability and high deposition rate. However despite numerous findings [21] that report the beneficial osteoinductive properties of plasma-sprayed calcium phosphate coatings, there are still some concerns regarding its use. Plasma-sprayed coatings are not uniform and there is poor control over thickness and surface topography, which may result in implant inflammation when particles are released from these heteroge‐ neous coatings. To overcome these drawbacks, various other deposition strategies have been developed and employed such as biomimetic, electrophoretic and electrospray deposition etc. However care should be taken when comparing the efficacy of each of these methods which would require a comprehensive evaluation of both biological response and clinical perform‐ ance. Although calcium phosphate coatings have been shown to be beneficial in enhancing bone-bonding, there is still no general consensus on the use of calcium phosphate coating systems. The main problems include large variation in the quality of calcium phosphate coatings, even between different batches and market forces which offer other cheaper alter‐

Surface modification of implant materials with growth factors and peptides is gaining popularity in the recent years [23, 24]. Various therapeutic biomolecules of interest can be immobilized onto implant surfaces to enhance the bone-implant interface interactions. Currently more popular approaches would include the immobilization of bone growth factors such as bone morphogenetic proteins (BMPs) to enhance osteogenesis and the deposition of peptide sequences to induce specific cellular functions. Growth factors

bonding.

natives [22].

**2.2. Organic coatings**

**2.1. Inorganic coatings**

**Figure 2.** Schematic figure of a hip implant. The femoral neck is the region at risk of compromised vascularity. Arrows indicate area of compromised vascularity where osseiointegration fails to take place.
