Physical Properties of Materials
Strength
Certainly, an orthopaedic implant should be designed to be strong as possible. Even in everyday activities, you will place high levels of mechanical stress on your bones and joints. The ideal implant should be able to withstand these stresses day to day for years without breaking or permanently changing shape. An implant should also be designed to withstand the fatigue effect of the accumulation of these repeating stress cycles for an acceptable period of time (“service life”).
Flexibility
While strength characteristics of implants are important, they must also be somewhat flexible to avoid shielding of bones from stress (“stress shielding”). When stress is applied to a stiff orthopaedic implant, the implant will carry most of the stress and the bone may start to resorb and may become less dense and weak. On the other hand if stress is applied to a less stiff or more flexible implant, some of the stress can be shared with the surrounding bone. This will help to keep the bone active and strong.
Resistance to wear
Artificial joints are designed to replace surfaces that move relative to each other (articulating surfaces). Any time two parts move relative to each other, there may be wear. When an implant wears, tiny particles of the material are removed from the surface. Generally, the harder the material, the more resistant it is to wear. This property is important in designing long-lasting implants. The choice of two materials that articulate (rub) against each other in an implant is also important in minimizing wear. Many combinations of materials are used today for implants, including metal on polyethylene, metal on metal, ceramic on polyethylene, and ceramic on ceramic.
Resistance to corrosion
Highly corrosive materials like Ti-6Al-4V alloy, Cobalt-Chromium-Molybdenum alloys, and stainless steels are used in the manufacture of orthopaedic implants. These implants, however, may react with the body fluid and produce metal ions and corrosion products. The amount of corrosion produced from these implants is very limited.
Biocompatibility
Biocompatibility refers to the way materials interact with your body. Some materials – lead and mercury for example – are naturally harmful when taken into the body, so they are not suitable for implanting. Other materials are not suitable to implant because the body fluids cause them to break down, either weakening them, or causing corrosion or other byproducts. Some materials may cause irritation and the implant site may experience some inflammation. Biomaterials used in Zimmer orthopaedic implants are or have been tested for biocompatibility before they are distributed.
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