|
Year |
2007 |
|---|---|
|
Question Number |
205 |
|
Category |
Hip and Knee Reconstruction |
Question
Which of the following polyethylene inserts has the highest propensity for crack propagation under cyclic loading?
1- Highly cross-linked polyethylene that has been remelted
2- Highly cross-linked polyethylene that has been annealed
3- Moderately cross-linked polyethylene that has been annealed
4- Conventional polyethylene that has been remelted
5- Conventional polyethylene that has been annealed
Figures
Answer (click for answer)
The correct answer is 1- Highly cross-linked polyethylene that has been remelted
Crosslinking of polyethylene in total joint replacements has become a popular source of test questions, especially in regards to the biomechanical properties of the various types of polyethylene. Conventional polyethylene is that which has been sterilized by a method other than radiation or by very low dose radiation. Exposure of conventional polyethylene to higher doses of gamma radiation causes crosslinking of the polyethylene. The degree of crosslinking is dependent upon the dose of radiation. Thermal processing of polyethylene reduces the concentration of free radicals to prevent oxidation. Thermal processing that occurs below the melting point is called annealing while that which occurs above this temperature is called remelting. While increasing crosslinking and decreasing the free radical concentration both slow polyethylene degradation and prolong its lifetime under cyclic loading, these processes carry certain drawbacks. While increasing crosslinking prevents degradation, it lowers ductility and resistance to crack propagation in polyethylene. A similar pattern is seen with thermal processing. Increasing the temperature of thermal processing decreases free radical concentration but also decreases ductility and resistance to crack propagation. When looking at the possible answers to the question, the processing that would decrease crack resistance (i.e., increase crack propagation) would be the highest level of crosslinking at the highest temperature of thermal processing (#1).
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