BIOLOX?delta is a new high performance ceramic composite material with outstanding material properties which are based on complex reinforcing mechanisms. In particular, the fracture toughness is increased by 2 reinforcing ingredients, i.e. zirconia and oxide platelets. It is shown that the improved material properties in comparison to pure alumina leads to significant higher reliability of the components for surgery. For materials containing zirconia as a reinforcing element the issue of stability in hydrothermal environment is of particular interest. For this purpose an extreme test setup has been chosen where severe hydrothermal treatment in water vapour at 134°C and cyclic mechanical loading at a very high stress level are combined. All specimens survived these test conditions which simulate a multiple of the expected in-vivo loading. An analysis of the phase composition shows that the reinforcing mechanisms have been thoroughly activated regardless of environmental conditions. From this outstanding result it is concluded that BIOLOX?delta provides highest probability of long term durability in well designed artificial joint systems.