**3.3 The inert behaviour**

Despite the great progress achieved in orthopaedic biomaterials, fixation of implants to the bone host remains a problem. As titanium has an inert behaviour, the body tries to encapsulate the Ti-based implant. However, titanium does not bond directly to bone resulting in micro-movements and, eventually loosening of the implant. Undesirable

Titanium as a Biomaterial for Implants 155

Although this benefits, there are some problems in the manufacture of the composite

One processing problem is that, as Ti is stable in vacuum or reducing atmospheres and HA is stable only in oxidizing atmospheres (Weng et al., 1994), sintering of this type of composites is difficult. Also, there are some reports indicating intense HA decomposition at temperatures lower than the decomposition temperatures of the monolithic powders, due the interaction with the Ti powder (Yang et al., 2004), which declines its bioactivity and mechanical properties According to literature the Ca titanates CaTiO3 and CaTi2O5 are formed through reactions between HA and TiO2 in vacuum, both when the titanium oxide was intentionally added or when it resulted from the oxidation of metallic Ti in Ti-HA

Another problem of porous metals is its fatigue behaviour. The porosity of most implants is usually determined to compromise between maintaining the mechanical strength of the implant while still providing adequate pore size for tissue ingrowth. Although optimum pore size required for implant fixation remains undefined, the consensus is that in order to optimize mineralised bone ingrowth, pore sizes between 100 and 400 m are necessary (Cameron et al., 1976). A major concern with the use of porous implants in highly loaded applications is the effect the porous matrix might have on fatigue strength. Ti alloys experienced drastic reductions in fatigue strength till one-third that of the solid alloy equivalent shape (Wolfrth & Ducheyne, 1994). Stress intensification due to these pores are major sources of weakness in the fatigue strength. This is sometimes referred to as the nocth effect. To achieve a functionally strong implant, porous implant design needs to account for

Nevertheless titanium-hydroxyapatite porous structures are promising biomaterials to be

As an artificial hip joints need to be designed to withstand the loads that they are expected to bear without fracture or fatigue, stress analysis is therefore required to ensure that all components of the device operate below the fatigue limit. For simple calculations, simple analytical calculations usually suffice. Unfortunately, analytical solutions are limited to

Implants as a hip joint involve some combinations of material or geometry non-linearity, complex geometry and mixed boundary conditions. Applying analytical methods to such a problem would require so many assumptions and simplifications. An alternative is the use of approximate or numerical methods. The most popular numerical method for solving problems in continuum mechanics is the finite element method (FEM), also referred to as

FEA uses a complex system of points called nodes which make a grid called a mesh. The complex structure is divided into a large number of smaller parts, or elements, with interconnecting nodes, each with geometry much simpler than that of the whole structure. This mesh is programmed to contain the material and structural properties which define how the structure will react to certain loading conditions. Nodes are assigned at a certain density throughout the material depending on the anticipated stress levels of a particular area. Regions expected to receive large amounts of stress usually will have a higher node

linear problems and simple geometries governed by simple boundary conditions.

material and some doubts about its biocompatibility.

composites (Yang et al., 2004).

these losses in metal strength.

used as replacement implants.

**5. Finite element analysis** 

finite element analysis (FEA).

density than those which experience little or no stress.

movements at the implant-tissue interface results in failure cracks of the implant. As osseointegration starts with the cellular stage and continues with the nucleation of mineral and the structuring of the new vital bone, the overall required time is varying in a broad range. A proposed solution for a better control of osseointegration is the bioactive fixation.

One approach to improving implant lifetime is to coat the metal surface with a bioactive material that can promote the formation and adhesion of hydroxyapatite (Ca10 (PO4)6(OH)2), the inorganic component of natural bone.

The application of bioactive coatings to titanium-based alloys enhance the adhesion of Tibased implants to the existing bone, resulting in significantly better implant lifetimes than can be achieved with materials in use today (Katti, 2004). Typically, several silicates glasses or hydroxyapatite (HA) are used as bioactive coatings. Some properties of hydroxyapatite are shown in Table 5.


#### Table 5. Hydroxyapatite properties

An ideal bioactive coating would bond tightly both to the bone and the metal. Two problems arise when attempting to coat metals with ceramics. For one side, the thermal expansion coefficients of the ceramic and metal are usually different, and as a result, large thermal stresses are generated during processing. These stresses lead to cracks at the interface and compromise coating adhesion. In addition, chemical reactions between the ceramic and metal can weaken the metal in the vicinity of the interface, reducing the strength of the coated system. This problem is particularly important when coating Ti alloys, due to their high reactivity with most oxide materials. However, bioactive ceramics coatings on Ti implants further improves the biocompatibility of these implants.

### **4. Titanium-Hydroxyapatite composite**

Biocomposite materials have been developed in order to combine bioactivity of ceramics and mechanical properties of metals. Hydroxyapatite (HA) is known for its weakness and brittles (see Table 5) but has an excellent biocompatibility and is a bioactive material. When HA is added to titanium, an improvement of the biomaterial chemical properties occurs. New developments try to aggregate hydroxyapatite as a second phase to the Ti alloy, with powder metallurgy techniques (P/M). In this composite material, particles of HA are incorporated in a porous titanium matrix providing points of good bone reaction. These solutions allow improved adhesion strength of the load bearing metallic component to the bone, resulting in shorter healing periods as well as predictable behaviour of the implant for longer periods of time.

movements at the implant-tissue interface results in failure cracks of the implant. As osseointegration starts with the cellular stage and continues with the nucleation of mineral and the structuring of the new vital bone, the overall required time is varying in a broad range. A proposed solution for a better control of osseointegration is the bioactive fixation. One approach to improving implant lifetime is to coat the metal surface with a bioactive material that can promote the formation and adhesion of hydroxyapatite (Ca10 (PO4)6(OH)2),

The application of bioactive coatings to titanium-based alloys enhance the adhesion of Tibased implants to the existing bone, resulting in significantly better implant lifetimes than can be achieved with materials in use today (Katti, 2004). Typically, several silicates glasses or hydroxyapatite (HA) are used as bioactive coatings. Some properties of hydroxyapatite

An ideal bioactive coating would bond tightly both to the bone and the metal. Two problems arise when attempting to coat metals with ceramics. For one side, the thermal expansion coefficients of the ceramic and metal are usually different, and as a result, large thermal stresses are generated during processing. These stresses lead to cracks at the interface and compromise coating adhesion. In addition, chemical reactions between the ceramic and metal can weaken the metal in the vicinity of the interface, reducing the strength of the coated system. This problem is particularly important when coating Ti alloys, due to their high reactivity with most oxide materials. However, bioactive ceramics coatings

Biocomposite materials have been developed in order to combine bioactivity of ceramics and mechanical properties of metals. Hydroxyapatite (HA) is known for its weakness and brittles (see Table 5) but has an excellent biocompatibility and is a bioactive material. When HA is added to titanium, an improvement of the biomaterial chemical properties occurs. New developments try to aggregate hydroxyapatite as a second phase to the Ti alloy, with powder metallurgy techniques (P/M). In this composite material, particles of HA are incorporated in a porous titanium matrix providing points of good bone reaction. These solutions allow improved adhesion strength of the load bearing metallic component to the bone, resulting in shorter healing periods as well as predictable behaviour of the implant for

on Ti implants further improves the biocompatibility of these implants.

It has the less solubility in body fluids media, so it is impossible to have Ca2+ or PO43+ ions in water (PH=7)

Hardness (Mohs) 5 Density (g/cm3) 3.1 Elastic Modulus (GPa) 100 Ultimate Tension Stress (MPa) 100 Compression Stress (MPa) > 50 (good) Toughness KIC (MPa m1/2) 1

the inorganic component of natural bone.

Solubility

**4. Titanium-Hydroxyapatite composite** 

longer periods of time.

Table 5. Hydroxyapatite properties

are shown in Table 5.

Although this benefits, there are some problems in the manufacture of the composite material and some doubts about its biocompatibility.

One processing problem is that, as Ti is stable in vacuum or reducing atmospheres and HA is stable only in oxidizing atmospheres (Weng et al., 1994), sintering of this type of composites is difficult. Also, there are some reports indicating intense HA decomposition at temperatures lower than the decomposition temperatures of the monolithic powders, due the interaction with the Ti powder (Yang et al., 2004), which declines its bioactivity and mechanical properties According to literature the Ca titanates CaTiO3 and CaTi2O5 are formed through reactions between HA and TiO2 in vacuum, both when the titanium oxide was intentionally added or when it resulted from the oxidation of metallic Ti in Ti-HA composites (Yang et al., 2004).

Another problem of porous metals is its fatigue behaviour. The porosity of most implants is usually determined to compromise between maintaining the mechanical strength of the implant while still providing adequate pore size for tissue ingrowth. Although optimum pore size required for implant fixation remains undefined, the consensus is that in order to optimize mineralised bone ingrowth, pore sizes between 100 and 400 m are necessary (Cameron et al., 1976). A major concern with the use of porous implants in highly loaded applications is the effect the porous matrix might have on fatigue strength. Ti alloys experienced drastic reductions in fatigue strength till one-third that of the solid alloy equivalent shape (Wolfrth & Ducheyne, 1994). Stress intensification due to these pores are major sources of weakness in the fatigue strength. This is sometimes referred to as the nocth effect. To achieve a functionally strong implant, porous implant design needs to account for these losses in metal strength.

Nevertheless titanium-hydroxyapatite porous structures are promising biomaterials to be used as replacement implants.
