Abstract
Background
Polyethylene (PE) as a bearing material for total joint replacements (TJR) represents the golden standard for the past forty years. However, over the past decade it becomes apparent that PE wear and the biological response to wear products are the limiting factor for the longevity of TJRs. For this reason research has focused onto PE wear particle analysis. A particle analysis highly depends on the methodological work and results often show discrepancies between different research groups. From there, our hypothesis was, that an often unattended influencing factor is the optical magnification which has been used for particle analyses.
Material and Methods
In the present study samples of a previous conducted knee wear simulator test were used. Wear particles were isolated from the bovine serum using an established method1. Briefly the serum was digested with hydrochloric acid and a continuous stirring and heating. Particles were filtered onto 20nm alumina filters and analyzed using high resolution field emission gun scanning electron microscopy (FEG-SEM). Filters were analyzed on the same points using three different magnifications: 5000, 15000 and 30000. To describe the size and morphology of the particles the equivalent circle diameter (ECD), aspect ratio (AR), roundness (R) and form factor (FF) were specified according to ASTM F 1877-05. The estimated total number (ETN) of particles was calculated based on the number of particles recovered on the filter, the analyzed area, the dilution, evaporation and the total serum volume.
Results
The results showed significant differences between the different magnifications. Examples of the analyzed pictures are depicted in Fig. 1. The results are summarized in Tab. 1. In particular the size of the particles highly depends on the choosen optical magnification which becomes apparent in a more than twofold increase when comparing wear particles at magnification of 5k or 30k (p≤0.001). The mean particle diameter distribution (Fig. 2) also shows a shift in the distribution of wear particles: A higher magnification results in a higher fraction of smallest particles (e.g. over 50 percent between 0–0.2µm with magnification 30.000 compared to only 3 percent with a magnification of 5.000) and nearly no particles above 1µm could be verified. The results regarding the particle morphology show smaller but significant differences. The ETN of particles quadruplicates when comparing results with magnifications of 5.000 and 30.000.
Conclusion
This study shows great differences in particle size, which can be directly attributed to the differences in optical magnification. According to ASTM F1877-05 a magnification of 10.000 for the analysis of wear particles between 0.1–1µm is recommended. However, results show that this magnification is not sufficient especially for particle sizes below 0.2µm, which account for the greatest number of particles. To the authors opinion a more detailed recommendation concerning the magnification is needed. Additional, a standardized evaluation system which takes the magnification into account is necessary to allow comparison of different research groups.
