The majority of implants are made of a cobalt chrome molybdenum (CoCrMo) casting alloy (according to ISO 5832-4). Casting molds are produced from reproducible wax models which are, via several steps, coated with a ceramic layer. After the wax has been melted out of these ceramic molds then the liquid CoCrMo alloy is carefully poured inside them. Once this has cooled the ceramic mold is removed from the casting blank and the components are checked for defects. Only then are they machined further and sent for finishing (i.e. ground, polished, coated, etc. – as applicable).
cut-off grade % (mass-fraction)
|Chromium||26.5 to 30|
|Molybdenum||4.5 to 7|
The raw titanium (TiAl6V4) alloy material (according to ISO 5832-3) is machined – usually by milling and / or turning before being sent for finishing (i.e. ground, polished, coated, etc. – as applicable)
cut-off grade % (mass fraction)
|Aluminium||5.5 to 6.75|
|Vanadium||3.5 to 4.5|
Ultra high molecular weight polyethylene, or UHMWPE (according to ISO 5834-2), starts out as a powder which is compression molded to make rods. These are then heat-treated and cut into pucks or cubes for stress reduction before product-specific processing. The resultant implants are placed in gas-permeable packaging and sterilised using ethylene oxide (EtO).
This is a version of UHMWPE (according to ISO 5834-1) which has been crosslinked to decrease wear rates. The rod material (formed from compression molding GUR 1020 powder) is gamma-irradiated with 75kGy in the atmosphere at room temperature. The rod is then heated to 150°C ± 2°C for 10 hours before being allowed to return to room temperature.
This is a vitamin E stabilised version of the crosslinked implacross® UHMWPE material (hence implacross® E also complies with ISO 5834-1). In this instance however 1000 ppm of vitamin E is thoroughly mixed with the GUR 1020 UHMWPE powder prior to compression molding into plate form. The plate is cross-linked by gamma-irradiation (50 kGy and 75 kGy respectively) before being heat-treated. The use of vitamin E is intended to improve the long-term stability of the UHMWPE by bonding with the free-radicals that may be left behind after the gamma-irradiation or cross-linking steps. The vitamin E which is mixed in at the powder stage has an affinity for these free-radicals and bonds with them before the free-radicals can bond with oxygen (oxidation) which shortens the UHMWPE molecule and damages the mechanical properties of the UHMWPE molecule.