Introduction to bone and tissue

12.2.2 Introduction to bone and tissue

An understanding of the structure of bone is important when considering the repair of fractures and the replacement of joints. In repairing bone fractures it is necessary to hold the bone together while

Case examination of biomaterials, sports materials and nanomaterials 585 Table 12.2 The mechanical properties of some natural and biomaterials.

Material Elastic

Fracture Fatigue modulus

Tensile

Elongation (%)

toughness strength (GN m −2 )

strength

(MN m −3/2 ) (MN m −2 ) Austenitic

(MN m −2 )

40 100 200–250 stainless steel Cobalt–Chromium

100 600 Ti–6Al–4V

– Glass fiber

2.8 55 8 – 20–30 Bone cement

Silicone rubber

2–12 Bone (cancellous)

Bone (cortical) 7–25

2–12 Tooth enamel

– Tooth dentine

– Collagen, tendon, wet

the natural healing process takes place, usually for a few months. Metallic alloys have been univer- sally used for plates, pins, screws, etc. They are considered to have the required strength and rigidity together with sufficient corrosion resistance. Generally, these fixtures are considered temporary until the bone has joined, after which they are removed. The main reason for this is that the very rigidity with which the bone is held to allow healing will eventually lead to progressive weakening of the bone structure.

The biological structure of bone is reproduced in Figure 12.1. In material science terms, the apparent complexity of bone can be described as a composite made up of a matrix of collagen (polymer) reinforced with approximately 50% volume fraction of hydroxyapatite (ceramic) nanometer-scale crystals. Most bones are made of a porous cellular structure (cancellous bone) covered with a denser compact shell. The porosity and density of the cancellous bone varies with location in the body, depending on the stress state in that region. Low-density regions have a relatively open cell structure and high-density regions more closed. The density determines the strength and stiffness of the bone which grows and develops to support the stress imposed on it. This may be uniaxial, when the cell walls will be oriented and thicker along this direction, or more uniform when the cells are roughly equiaxed.

Bone is not a static entity but dynamic in nature, continually undergoing remodeling, where ‘old’ bone is resorbed and replaced by new bone. Various factors control the process but extremely important in stimulating the bone-producing cells (osteoblasts) is the application of stress. In bone, the mineral phosphate material is slowly dissolved and resorbed by the body and when subjected to normal stress is replenished by new bone material synthesized by the osteoblasts. This recycling process ensures

a healthy, strong bone structure with ageing. Bones for which the major stress is carried by metal implants do not show the same tendency for replenishment, so that the bone surrounding the implant is resorbed without replenishment, leading to a loosening of the implant. A rigid metal plate attached to

a bone that has healed will nevertheless carry the majority of the load. To avoid this ‘stress shielding’

586 Physical Metallurgy and Advanced Materials Direction of

Hydroxyapatite crystal

Figure 12.1 The hierarchy of structure in bone. At the molecular level (a), polarized triple helix of tropocollagen molecules assembles into a microfibril (b), with small gaps between the ends of the molecules into which small (5 by 30 nm) crystals of hydroxyapatite later form. These microfibrils assemble into larger fibrils (c), which then form the layers in the osteon (d – part cut away to show the alternating orientation of fibers in the annular layers). Osteons form in association with each other (e), forming bone ( f ). The cells which are responsible for most of this process, the osteocytes, are shown sitting between the layers of the osteon (d) (after Vincent, 1990; courtesy Institute of Materials, Minerals and Mining).

leading to bone weakness, it is recommended that the surgeon removes the holding plates soon after the fracture has healed.

With the aim of trying to avoid the removal operation, alternative approaches are being considered for bone plates, screws, etc., including the use of biodegradable, or resorbable, materials such as polymers or composites, which could be resorbed into the surrounding tissue or dissolve completely over a period of time after the fracture has healed. However, the combination of strength, ductility, toughness, rigidity and corrosion resistance of metals is hard to match with the non-metallics.

The large difference in elastic modulus between competing biomaterials and bone (see Table 12.2) is evident. Of the various metals, titanium and its alloys are clearly the most suitable and are being increasingly used. Titanium does, however, have a tribological weakness but the application of coatings and surface engineering is being increasingly adopted to overcome this problem.

Tissues include skin, tendons, ligaments and cartilages. Skin has the dual property of keeping the body fluids in while allowing the outward movement of moisture through a porous membrane, which is important in cooling and maintaining the body temperature. Skin also protects against infections, such as bacteria, but is not, of course, particularly strong. It is made up of layers including an outer epidermis and an inner dermis, a dense network of nerve and blood vessels. It is therefore virtually impossible to make an artificial skin from biomaterials to match this complexity. Nevertheless, skin replacements have been made from polymers with an open structure which provides a basic framework onto which real skin is able to grow. Moreover, with a biodegradable polymer the framework degrades as the new tissue regrows. The porous film can be coated with silicone rubber to provide infection protection and retain fluids while the skin grows. When sutured in place, tissue-forming cells (fibroblasts) migrate into the porous polymer framework to generate new skin layers. For severe burns, artificial skin can

Case examination of biomaterials, sports materials and nanomaterials 587

Acetabulum

Polyethylene Acrylic

Femur

cement Femoral head

Acrylic cement

Figure 12.2 Schematic diagram of a replacement hip joint.

be made by growing epidermal skin cells within a biodegradable collagen mesh in a culture medium. The synthetic skin can then be grafted onto the patient. Other biodegradable products include the

copolymers lactic acid–glycolic acid and lysine–lactic acid. The adhesion of the polymer framework can be improved by incorporating an adhesive protein fibronectin.

Other tissues such as ligaments and cartilages are largely elastic filaments of fibrous proteins. Synthetic substitutes have included Dacron polyesters, PTFE fibers and pyrolyzed carbon fibers, with mixed success. The fibers may be coated with polylactic acid polymer, which breaks down in the body to be replaced by collagen. At this stage such techniques are relatively new, but it does suggest that in future the growth of cells in a culture vessel may possibly supply complex biomaterials for various implanted functions. This approach is termed tissue engineering and, together with biometrics,

i.e. the mimicking of the working of biological systems, offers a way of producing materials which totally synergize with the human body.