Originally posted on 1/4/2007
Donna M. Meyer, A. Tillinghast, N. C. Hanumara, A. Franco, Bio-Ferrography to Capture and Separate Polyethylene Wear Debris from Hip Simulator Fluid and Compared With Conventional Filter Method. Journal of Tribology, Vol. 128, April 2006, 436-441.
Introduction and Importance of Capturing and Separatation of Polyethylene Wear Debris
I found this manuscript a few weeks ago after noticing a news release about the topic and the first author Donna Meyer. Donna is an assistant professor at the University of Rhode Island in the Department of Mechanical Engineering and Applied Mechanics. She indicates her funding sources for this study to be NSF and NIH grants.
The topic is important because as we’ve seen in previous posts, although the recent improvements in polyethylene have resulted in less catastrophic fatigue wear, the nature of two disparate surfaces in motion against each other is to produce wear particulate. This particulate can then travel to the surrounding bony tissues and result in osteolysis, or the loss of bone. Bone loss can then lead to loosening of the implant and ultimately end in replacement of the implant (for example, in Sweden, 70% of hips and 44% of knees are revised as a result of asceptic loosening(Sundfeldt, et al., 2006)). Therefore, identifying the type, size, shape, and quantity of particulate produced by these materials under in vivo-like conditions is paramount to optimizing the materials and their wear behavior.
In the past, the identification and characterization of wear particulate has mostly been performed using filtration techniques. However, the potential for loss of the smallest particles is an issue. Submicron-sized particles are thought to be more detrimental for bone retention, thus identification of these is particularly important. Historically, the size distribution of wear particles produced in hip and knee total joint replacements were different due to different motions (kinetics and kinematics) and wear processes taking place in the two joints. The improvements in polyethylene may have lessened those differences, but few reports are available on this at this time.
So, excellent timing, I say, to Dr. Meyer! Let’s get down to today’s manuscript, shall we?
Methods and Materials
Adapting a technique developed for the aircraft industry, ferrography, Dr. Meyer and her team used erbium chloride (ErCl3) to marginally magnetize particles (cleaned of biological materials via previous enzymatic digestion) in diluted fluid from a hip wear simulator. Very briefly, the fluid was then passed through a ferrographic device described as a glass slide mounted at an angle to a induced magnetic field. Because of the angle the particles were separated by size (larger particles attracted at greater distances from the magnetic field, smaller particles attracted at lesser distances from the magnetic field). The magnetic field then held the particles in place while the fluid continued on, leaving glass slides with wear particles arranged in size gradients and appropriate for examination via light microscopy (Dr. Meyer used a bi-chromatic illumination scheme) and scanning electron microscopy.
Results
The particles found were either spherical or pill-shaped, 0.1-2 microns in size (with the occasional 20 micron particle), and often agglomerated into large groups. In comparison, particles found by filtration were mostly spherical with a size range of 0.1-20 microns (if agglomerates were found, it was not noted). To additionally validate the technique, several tests were done with purchased particles (3-45 microns, nominal size range). These particles were found via bio-ferrography and seen to form agglomerates.
Discussion
Dr. Meyer spends time discussing the causes of the particle shape and agglomeration. She postulates that the shape is a result of the rolling up of flat debris trapped in the wear track. She then indicates that the agglomeration probably occurred during the preparation of the fluid for bio-ferrography, but if it did occur during the wear process itself, the size of the agglomerations would preclude their involvment in the currently held theory of particle-induced osteolysis.
I for one, agree, and believe that the agglomeration could be avoided by diluting the fluid (assuming no other analysis of said fluid is neccesary) with a non-aqueous medium. In other filter-based tests (such as M. Scott, et al., 2005), agglomeration has been avoided by using hydrochloric acid to digest the biologic materials present in the fluid. However, if the agglomeration occurs as a result of the addition of the ErCl3 . . . It is difficult to identify the source of the agglomerations because the manuscript does not indicate if ErCl3 was used with the filteration tests which were not reported to have agglomerations. But the occurrence of the agglomerations is troubling, as they confound the ability to count the particles.
Kudos
I found this manuscript very interesting, indeed. Dr. Meyer has shown significant insight in applying this technique to the field of implant wear testing. Bravo! We need more scientists like her!
Now it is your turn!
A note about the text formating used in journal club posts
