UHMWPE (Ultra High Molecular Weight Polyethylene) implants have been used in the human body for quite a long time. Application necessitates that usage over a very long period of time (like 30 years or more) should not wear them out. However, UHMDPE being Polyolefinic in nature is liable to get degraded due to oxidative degradation. In fact, the gamma sterilization done before they are implanted in the human body can initiate oxidative degradation.
It is found that the degradation is more prevalent at some depth of the implant as compared to the surface. Greater availability of Oxygen facilitates more cross linking on the surface. The cross linking does not affect the wear adversely, but any breakage of the molecular chain could cause an increased wear. Therefore it is essential that these products should be tested for accelerated ageing before they are used for implants.

An accelerated ageing test conducted for the evaluation of gamma sterilization on the performance of medical grade UHMWPE, used conditions of heating rate of 0.6°C/min to 80°C for either 11 or 23 days. This was considered to be equivalent to 4 to 6 or 7 to 9 years of shelf ageing, respectively.
An alternative accelerated ageing method involves use of a pressurized vessel. Using this technique, with a pressure of 5 atmospheres oxygen and temperature of 70°C, 5 to 10 years of shelf ageing could be simulated in as little as one week. The technique also produces the subsurface oxidation maximum, which is often observed for shelf aged components.
An ASTM standard for accelerated ageing of UHMWPE has recently been published (ASTM F2003-00). It incorporates both method A (21 days at 80°C in air) and method B (14 days at 70°C in 5 bar oxygen). However, inter-laboratory studies have identified that although materials can be ranked successfully by both these methods, there is poor inter-laboratory reproducibility (especially for method B).

Oxidation is perhaps the most popular current method for assessing damage to UHMWPE following gamma sterilization and ageing. Unfortunately, a wide range of differing techniques has been developed and therefore it is often difficult to compare results between various workers. The term 'Oxidation Index' is often used but an ASTM standard addressing the definition of this has only recently been drafted. It refers to comparison of the peak or peaks in the FTIR spectra in the region 1650 to 1850 cm-1 (corresponding to carbonyl C=O groups) and a reference peak, which is independent of specimen degradation.

Density depth profiles through UHMWPE are obtained using a density gradient column containing two liquids (like water and ethanol) and essentially provide the same information with regard to extent of oxidation as FTIR. Density has been shown to correlate well with oxidation index. The advantages of this method are that results are directly comparable between workers, low capital outlay for equipment as compared with FTIR and relatively quicker results. Other factors influencing density could be starting material and processing conditions.

There is another technique for determination of cross link density to ascertain the suitability of UHMDPE for implants. It is important to understand the balance that occurs between chain scission due to oxidation and internal crosslinking reactions in gamma sterilized UHMWPE. In terms of wear performance, oxidation is considered to have a detrimental effect, whereas crosslinking is considered beneficial.

The majority of methods for measurement of crosslink density has been modified from ASTM2765-90 or ASTM2765-95 (method C) and involve immersing samples in hot xylene. If crosslinking occurs within UHMWPE a 3-dimensional gel phase is formed. The non-crosslinked portion of the UHMWPE (the sol phase) can be dissolved in hot xylene, leaving the gel phase, from which the crosslink density can be determined. The main reason from modification of the ASTM standard is the need to use a small sample size to achieve adequate information on variation of crosslink density with depth into the sample. The drawback of using small samples is that, although the gel content can be measured with reasonable accuracy, errors may be high when determining the 'swell ratio' of the UHMWPE.

It is clear that accelerated ageing of UHMWPE can be achieved by a number of protocols, based on either elevated temperature alone, or elevated temperature combined with elevated pressure in oxygen. With regard to comparison of materials and sterilization processes, any of these techniques would seem appropriate if performance is to be ranked merely by oxidation resistance (measured by density change or oxidation index). Therefore either of the two methods proposed by the ASTM would be appropriate. It would seem sensible, however, to include a standard reference material as a control, to be sterilized and aged under identical conditions. Evidence in the literature suggests that care must be taken to ensure that test materials are always treated in a similar manner prior to and during accelerated ageing.

If wear testing is to be conducted following accelerated ageing, then ageing parameters need more careful control. Wear performance may be affected by the location of the oxidation i.e. whether on the surface or subsurface. In this case accelerated ageing needs to be capable of producing the same oxidation depth profile as observed for shelf ageing. This is difficult because the exact depth profile for shelf ageing of a new material or new process is not yet known. Accelerating ageing protocol will need to be based on well-characterized materials where data for shelf ageing exists. It is a matter of not only simulating the oxidation of the UHMWPE, but also the level of internal crosslinking, since both are understood to effect wear performance. Further ongoing work in this area continues to establish factors, which control the depth of maximum oxidation and the balance between oxidation and crosslinking for accelerated ageing compared with shelf ageing.