MORF Method for Assessment of the Size and Shape of UHMWPE Wear Microparticles and Nanoparticles in Periprosthetic Tissues
Abstract
PURPOSE OF STUDY Aseptic loosening of total joint replacement (TJR) due to wear of ultra-high molecular weight polyethylene (UHMWPE) is regarded as one of the major problems in the field of arthroplasty. This work describes a newly developed method, called MORF, which completely describes the morphology of UHMWPE wear particles.
The differences in wear particle morphology may help to elucidate individual differences in TJR failures. MATERIAL AND METHODS During the years 2002-2010, a set of 47 typical damaged periprosthetic tissues, coming from 16 TJR revisions, was collected.
Isolated on polycarbonate (PC) filters were quantified. Quantification of the particles consisted in determination of their concentration and description of their morphology by means of the newly developed MORF method.
Firstly, the micrographs of isolated UHMWPE particles were obtained with a scanning electron microscope (Quanta 200 FEG; FEI) at two magnifications: x1200 and x6000. Secondly, both high- and low-magnification micrographs were processed by a standard image analysis software (program NIS Elements; Laboratory Imaging) in order to obtain basic morphological descriptors.
Finally, the results from image analysis of high- and low- magnification micrographs were combined by means of our own program MDISTR in order to obtain correct particle sizes and shapes. RESULTS In the first stage, the method was applied to 25 samples and yielded an average particle size of 0.51 μm.
In the second stage, the method was further improved in order to calculate not only the size of particles but also the shape of descriptors. The improved method was applied to eight samples and gave an average size of particles (equivalent diameter, D) in the range of 0.27 - 0.60 μm, circularity (C) of 0.66-0.85 and elongation (E) of 1.75-1.79, suggesting that the great majority of particles were approximately spherical.
Finally, in the third stage, the MORF method was applied to two exceptional samples which contained extremely small wear particles (D = 18.5 nm and 21.2 nm). The shape of these small wear nanoparticles (C = 0.97 and 0.93; E = 1.29 and 1.35) was even more spherical than that of wear microparticles described above.
This was one of the first two studies which proved the presence of UHMWPE wear nanoparticles in vivo. DISCUSSION Our newly developed MORF method described in this contribution yields both size and shape descriptors of UHMWPE wear particles, with sizes from 0.1 to 10 μm, which are regarded as most biologically active.
The main objective of the method is to yield the highest accuracy. This is achieved by parallel analyses of high- and low-magnification micrographs taken of the same sample.
In the end, the two analyses are combined together in order to obtain the correct and complete size and shape description of all particles in the sample. The morphology of UHMWPE wear particles influences TJR lifetime both directly (size and shape of the particles is related to their biological activity) and indirectly (for the given total volumetric wear, size and shape of the particles influence their concentration, which is associated with the biological response of the organism).
CONCLUSION The authors have developed a new method which yields a complete description of the size and shape of UHMWPE wear particles in periprosthetic tissues. The method, which was called MORF, can be applied to studies of TJR failures and also used to evaluate the quality of different UHMWPE components of TJR.
The method is quite universal and therefore can be used not only for analyses of wear particles, but also for other types of particles, such as microparticles in polymer blends or inorganic/metallic nanoparticles.