Cobalt compounds used
Hip and knee prosthetics, dental implants.
To be a suitable material for use in the replacement of human joints materials must have the following properties:
- High level of bio-compatibility
- Adequate strength and wear resistance
- Not be affected by body fluids
- Be able to mirror the physical and mechanical properties of the replaced joint
Cobalt-chromium alloys meet all four of these specifications explaining their popularity. Although new technologies utilise ceramics and titanium alloys due to reduced weight, cobalt-chromium alloys are still an important part of the sector.
Cobalt chromium alloys are one of the major materials in the creation of orthopaedic implants. Following the use of several other metals including platinum and gold, in 1924 it was concluded that cobalt-chromium offered the most ideal set of properties for use in prosthetics.
In 1937 Vitallium, a cobalt-chromium-molybdenum alloy, was used in prosthetics offering good strength, corrosion resistance and bio-compatibility.
Typical Vitallium alloys contain 62% cobalt, 30% chromium 5 % molybdenum. These alloys are mainly used where wear resistance is key due to their heavy weight and low ductility. They are therefore commonly used in replacing bone where a load will be placed for an extended period, such as at the knee or the hip. In modern medicine, cobalt-chromium alloys are still used in cemented and porous forms.
In 1951 Titanium alloys were introduced offering low weight and a modulus of elasticity closer to bone. Fracture plates and Intramedullary rods are common applications for titanium alloys. Cobalt-chromium alloys are often used in combination with titanium alloys to significantly lower micro-motions and fretting corrosion (Grupp, et al. 2010).
Prosthetic devices were traditionally cast using the lost wax process but the need for higher strength and better fatigue properties led to the development of wrought versions of the alloys. The lower carbon content of these alloys resulted in improved hot workability/forgeability but lower strength than cast alloys. Thermo-mechanical forging techniques were developed that allowed the required strength of the as-forged component to be achieved.
In response for the need of improved properties, alloys produced by powder metallurgy have been developed in the past few years.
Grupp TM, Weik T, Bloemer W, Knaebel HP. Modular titanium alloy neck adapter failures in hip replacement - failure mode analysis and influence of implant material. BMC Musculoskelet Disord. 2010 Jan 4. 11(1):3