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Ceramics, the biomaterials of choice as implants
F. CAMBIER
BCRC
4, avenue Gouverneur Cornez
7000 Mons
Belgique

The number and the complexity of medical interventions including implant and prosthetic surgery has dramatically increased in recent years due to:

  • increased life expectancy resulting in increased incidence of degenerative diseases such as osteoporosis.
  • lifestyle changes including increased participation in sport causing serious trauma injuries, or excessively sedentary lifestyles resulting in degenerative osteoarthritis.
  • the refusal of the public to tolerate the slightest limitations in mobility.
  • recent developments in surgical procedures and materials that have allowed procedures to take place that would not have been possible a few years ago.
  • 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).

    TABLE 1 : USE OF BIOCERAMICS AS IMPLANT MATERIALS IN THE HUMAN BODY
    APPLICATION CERAMIC
    Dental Porcelain, alumina, zirconia
    Articulation Alumina, zirconia, hydroxyapatite (HAP)
    Bone restauration HAP, tricalcium phosphate (TCP), bioglasses
    Implants in the inner ear Bioglasses, HAP
    Cardiac valves Carbon
    Ocular implants HAP
    Reinforcement of protheses Ceramic fibres

    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.

    TABLE 2 : DISADVANTAGES OF GRAFTS OF HUMAN OR ANIMAL ORIGIN
    TYPE OF GRAFT DISADVANTAGES
      Autografts
  • 2 simultaneous operations
  • longer surgery times
  • general anaesthtic required
  • (donor is the recipient)  
  • increased blood loss
  • high pain levels, extra care required
  • possible infection of both sites
  • limited quantity of graft material available
  •   Allografts
  • risk of bacterial or viral infection
  • variable quality of grafts (often osteoporosis because of elderly donors
  • (donor is a cadavre)  
  • time limit on conservation
  • high cost of grafts
  •   Xenografts
  • major infection risks from:
  • (donor is an animal)   – elimination of the organic phase
    – purification of the mineral phase

    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:

  • Inert bioceramics: stable with respect to time, their role is essentially mechanical (e.g. alumina, zirconia, dense vitreous ceramics).
  • Resorbable bioceramics: assimilated by the surrounding live tissue, they serve as short-lived fillers (e.g. b-TCP, plaster of Paris, aragonite, certain bioglasses).
  • Reactive bioceramics: acting as an interface between the implant and surrounding tissue, they stimulate an osseous response by inducing chemical or biomechanical bonding (e.g. porous HAP, bioglasses).
  • 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.

    TABLE 3: COMPARISON OF DIFFERENT MATERIAL COUPLES FOR HIP PROSTHESES
     Couple  Metal  PE* Al2O3  PE ZrO2  PE Metal  Metal Al2O3  Al2O3 ZrO2  Al2O3
    Market (1990) 76 13 8 Non existent 3 < 1
    Wear
    (mm3/Mcycle)
    60 15 15 Enormous 1 0.01
    * PE : high density polyethylene

    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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