Abstract

Due to its excellent mechanical properties, good biocompatibility, corrosion resistance and excellent chemical stability oxide ceramics (alumina and zirconia) have been qualified as a material for a growing range of medical applications such as prostheses, dental materials, femoral heads, among others. It is often used in its dense form, despite there are some applications where the use of porosity has proven to be beneficial. The use of dense ceramic prostheses can pose the problem of stress shielding (reduction in bone density as a result of removal of normal stress from the bone by an implant) due to the mismatch in Young’s modulus (YM) relative to the bone, which is significantly lower. Porosity have the ability to reduce the YM of the ceramic, reducing the mismatch to that of the bone, and at the same time, exhibits the potential of bone ingrowth in implants, depending on porous parameters such as pore size interconnectivity and porosity. Four levels of pore sizes was described by Smiske et al. as heaving specific features: 1) the range between 1-100 microns are similar to porous bone structure and must be present in biomaterials for biomimetic principles; 2) the range between 100-350 microns is optimum for bone ingrowth; 3) the range between 350-1000 microns is useful to decrease the ceramic YM and to reduce the stress shielding; 4) the range of 350 to 3500 microns is usefull for mechanical attachment of a porous implant during the medical surgery. The implant should therefore include these types all these types of porosity ranges rather than being dense or having homogeneous porosity. Together with the type of porosity, the pore connectivity also impacts the mechanical properties and cell/tissue ingrowth in an implant. The gradation in porosity size and shape across the implant volume follows the philosophy of the functionally graded materials (FGMs). FGMs offer the advantage of tailoring materials with specific structural, compositional, morphological, and mechanical properties. The early research and development of FGMs was driven by the need of reducing the thermal stresses developed in thermal barrier coatings on high temperature alloys. Now FGMs can be found in various material/phase combinations. FGMs have also been used as biomaterials for dental and orthopedic applications. FGMs have been reviewed by some researchers.