Titanium and Titanium Alloys as Biomaterials

Chemical Formula

Ti and Ti6Al4V

Topics Covered

Background

Physiological Behaviour

Mechanical Suitability

Applications

Orthopaedic Implants

Dental Applications

Background

Titanium and some of its alloys are used as biomaterials for dental and orthopaedic applications. The most common grades used are commercially pure titanium and the Ti6Al4V alloy, derived from aerospace applications.

Physiological Behaviour

These materials are classified as biologically inert biomaterials or bioinert. As such, they remain essentially unchanged when implanted into human bodies. This is no doubt a result of their excellent corrosion resistance.

The human body is able to recognise these materials as foreign, and tries to isolate them by encasing them in fibrous tissues. However, they do not illicit any adverse reactions and are tolerated well by the human body.

Furthermore, they do not induce allergic reactions such as has been observed on occasion with some stainless steels, which have induced nickel hypersensitivity in surrounding tissues.

The surface of titanium is often modified by coating it with hydroxyapatite. Plasma spraying is the only commercially accepted technique for depositing such coatings. The hydroxyapatite provides a bioactive surface (i.e. it actively participates in bone bonding), such that bone cements and other mechanical fixation devices are often not required.

Mechanical Suitability

Titanium and its alloys possess suitable mechanical properties such as strength, bend strength and fatigue resistance to be used in orthopaedics and dental applications. This is part of the reason why they have been employed in load-bearing biomedical applications in stead of materials such as hydroxyapatite, which displays bioactive behaviour.

Other specific properties that make it a desirable biomaterial are density and elastic modulus. In terms of density, it has a significantly lower density (table 1) than other metallic biomaterials, meaning that the implants will be lighter than similar items fabricated out of stainless steel or cobalt chrome alloys.

Table 1. Densities of selected biomaterials and cortical bone.

Material

Density

Elastic Modulus

Cortical Bone

~2.0 g.cm-3

7-30

Cobalt-Chrome alloy

~8.5 g.cm-3

230

316L Stainless Steel

8.0 g.cm-3

200

CP Titanium

4.51 g.cm-3

110

Ti6Al4V

4.40 g.cm-3

106

Having a lower elastic modulus compared to the other metals is desirable as the metal tends to behave a little bit more like bone itself, which is desirable from a biomechanical perspective. This property means that the bone hosting the biomaterial is less likely to atrophy and resorb.

Applications

Orthopaedic Implants

Titanium is commonly used in orthopaedic implants such as joint replacements and bone pins, plates and screws.

Figure 1 shows the various components of a total hip replacement. On the left is the femoral stem made of a titanium alloy. The long round section fits down into the thigh bone or femur. The white section is a hydroxyapatite coating to encourage bone bonding to the implant. This section is also macrotextured to provide surface features for the bone to mechanically interlock with. The ball on top of the femoral stem is called the femoral head. It is made of zirconia ceramic and fits into the hip joint in the pelvis.

The hemispherical item on the right is the acetabular cup, also made from titanium alloy. It is coated in a porous alumina ceramic, to allow bone ingrowth for stabilisation. A ultra high molecular weight polyethylene (UHMWPE) liner fits inside the acetabular cup and provides the articulating surface for the femoral head.

AZoM - Metals, Ceramics, Polymer and Composites : Titanium and Titanium Alloys as Biomaterials, total hip replacement prosthesis

Figure 1. Implant components for a total hip replacement (photo courtesy of Dr. Karlis Gross).

Figure 2 shows a prototype total knee replacement prosthesis, similar in design to many commercial implants. It consists of titanium alloy upper and lower structural components. A zirconia wear surface has been fabricated for the upper section. Similar to the hip prosthesis, this articulates against a UHMWPE insert on the lower section.

AZoM - Metals, Ceramics, Polymer and Composites : Titanium and Titanium Alloys as Biomaterials - Knee prosthesis

Figure 2. A total knee replacement prosthesis (photo courtesy of Dr. Besim Ben-Nissan).

Other orthopaedic applications for titanium-based materials include bone pins, plates and screws, used for repairing broken bones etc.

Dental Applications

Titanium pins and posts are used to secure dental implants. They use threaded fixtures to secure them into the jaw.

Titanium superstructures are now being investigated as an alternative to other metals such as gold for implants such as polymer based dentures (figure 3).

AZoM - Metals, Ceramics, Polymer and Composites : Titanium and Titanium Alloys as Biomaterials, dental implants, dentures,

Figure 3. Side view of a superplastically-formed, titanium alloy, cantilevered superstructure, attached to dental plaster analogues in a plaster model of a patients jaw.

 

Source: AZoM.com

 

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