  Recently, several episodes of fracture of zirconia ceramic femoral heads of
total hip prostheses have alarmed the medical and scientific community
regarding aging problems in zirconia prostheses. Such fractures cause immediate
local tissue reactions, which require urgent medical intervention to prevent
further complications. As a result, it has been promoted that yttria-stabilized
zirconia (Y-TZP) hip prostheses be substituted by alumina and alumina/Y-TZP
ceramics. In the present investigation, we have found an upper limit of Y-TZP
concentration in alumina/Y-TZP composites (16 vol.%) to avoid future aging
problems. This limit coincides with the percolation threshold measured by
infrared (IR) reflectance in a series of alumina/Y-TZP composites.
