This process occurs when tiny particles are generated at the bearing couple of a joint replacement. Since most joint replacements use polyethylene (a type of plastic) as one of the bearing surfaces, and since this is usually the softer material, most of the tiny particles will be polyethylene. The ones that cause the most trouble are so small that their size is measured in microns ( thousandths of a millimeter) or even nanometres (thousandth of a micron). Aseptic loosening can also be caused by tiny particles of cement, metal or ceramic. It is the size and number of particles that counts rather than the actual material of which they are made. A typical joint replacement will generate millions of these particles every year, even if functioning as designed. Very little plastic will be worn away as the particles are so very small. The plastic wear will therefore not be detectable by ordinary X-rays. Everyone believes that all is well ...

The particles attract a cell type called a macrophage. These cells try to engulf and digest the particles that are seen as 'foreign' to the body, much as they would a bacterium. i.e. this is a natural and essential part of the human body's defence mechanism. Unfortunately plastic causes macrophages a fatal case of indigestion and these cells die, releasing enzymes and other chemicals. It is these enzymes and chemicals that cause the bone to be eaten away. If only a few macrophages die and release their chemicals, little harm is done. The problem is most joint relacements release millions or billions of particles so a corresponding number of macrophages may be attracted and die. It is all about numbers

Aseptic loosening shows on X-rays as lines appearing around the prostheses or isolated cavities. Click here to see an example

When the problem was first observed affecting early joint relacements in the 1960's ,70's and 80's; it was initially poorly understood. Microscopes to see the tiny particles were not available. Many thought the problem was caused by acrylic bone cement and the term 'cement disease' was used. The North American market was therefore dominated by cementless designs in an effort to avoid the problem (a situation that remains!).

When cementless designs began to fail as well, this was labelled osteolysis. Osteolysis is the same basic process as aseptic loosening, just a slightly different pattern.

In summary, all joint replacements whether cemented or uncemented are at risk of failing as a result of aseptic loosening/osteolysis. This is especially true of designs using polyethylene as one of the bearing surfaces. Bad designs or poor technique can greatly accelerate the rate of particle production and therefore increase the risk. The risk is time dependant and therefore older patients will die before aseptic loosening/osteolysis becomes a problem. This group is also less active and will generate fewer particles than a younger patient per annum because they will tend to use their joint replacement less. Good designs (cemented or uncemented) and good surgical technique, in contrast, will give survival rates of >95% at 18 to 20 years.