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|Ceramics, the biomaterials of choice as implants|
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The number and the complexity of medical interventions including implant and prosthetic surgery has dramatically increased in recent years due to:
As a result there is an ever increasing need for new surgical techniques, products and « biomaterials », a term which includes all materials which interact with biological systems to treat, strengthen or replace tissue, an organ or a bodily function. Metals and polymers are widely used, and ceramics, although more recently developed, have also found many applications as shown in table 1. Bioceramics can be used to either directly substitute bone or to solve a weakness in a particular function (e.g. hip prosthesis).
IMITATION OF HUMAN BONE
Bone is a composite material (60% HAP, 40% collagen fibres) characterised by high flexural strength (sf ± 120 MPa), low elastic modulus (E ± 18 GPa) and compression strength similar to sf (sc ± 150 MPa). Depending on the porosity, bone is described as compact (pore volume VP ~ 65%, diamètre, dp ~ 190 to 230 µm) or spongy (dp ~ 500 to 600 µm).
Although bone grafts of human origin (auto or allografts) or animal origin (xenografts) are attractive in many cases they present major disadvantages as summarised in table 2.
One alternative to natural bone is coral and mother-of-pearl, which are both composed of porous aragonite (CaCO3). However, the most promising solution is to use synthesised ceramics with chemical compositions close to that observed in the human body and to fabricate materials with controlled pore size and pore interconnection diameters.
APPLICATION OF BIOCERAMICS AS STRUCTURAL SOLUTIONS – PROSTHESES AND OTHER IMPLANTS
Depending on the type of ceramic under consideration, different interactions will occur with biological tissues. Three classifications of bioceramics can be considered:
In table 3 a number of material couples in hip prostheses are compared in terms of their market share and their wear behaviour as obtained in tests of 1 million cycles (1 Mcycle) which simulates a typical year’s walking. It can be seen that the ceramic – ceramic couple had extremely low wear, an enormous advantage compared to couples containing metal or polymer components.
In couples containing polyethylene (PE) acetabular cups, submicron PE debris created by wear is digested by macrocells and induces the presence of antibodies. This leads to a degeneration of bone around the prosthesis, which then becomes loose. In the case of metals, submicron debris is also formed which is slowly assimilated by the body. Danger lies in the toxicity of some alloys that are used, particularly those containing chromium or cobalt.
Ceramic parts also wear to some extent; alumina by grain pull-out (seemingly well assimilated by the body) and zirconia by progressive destabilisation of the cubic phase.
Many of the materials discussed above are being researched in the regions of the Euroceram network. Some details are presented in the following sections but for more information on current developments contact the regional contact points listed on the cover of this newsletter.
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