Shapes and dimensional characteristics of polyethylene wear particles generated in vivo by total knee replacements compared to total hip replacements.

Periprosthetic tissue was obtained at revision surgery from 10 posterior cruciate retaining total knee replacement cases (five different manufacturers). The tissues were hydrolyzed and polyethylene particles were isolated from each case. Individual particles were analyzed by scanning electron microscopy combined with computerized image analysis. For comparison, periprosthetic tissues from 10 total hip replacement cases (six different manufacturers) were processed and analyzed simultaneously with identical methods. The morphologies of the isolated polyethylene particles from total knee specimens were distinctly different. There was more variety of size, shape, and texture in the total knee particles. Submicron granules were less prevalent than in hip specimens. Larger flake-shaped particles, some measuring several microns in length and width, were commonly seen in knee specimens but not hip specimens. The overall average area of particles from the total knees (1.2 microns2) was twice that of total hips (0.61 micron2) (p = 0.049). The average perimeter (p = 0.026) and length (p = 0.026) of total knee particles was also greater than the total hip particles. The proportion of the smallest polyethylene particles (those averaging 0.2 micron2) in the total knee specimens was significantly less than that in total hip specimens (p < 0.0001). Although the large flake-shaped particles were visually striking, it is important to note that the majority of the wear particles from the total knee cases were also submicron. These differences in wear particle morphology and size are due to differences in the wear mechanisms of total knees and total hips. Size is only one parameter in wear particle bioreactivity. Other factors include particle shape, surface area, and possibly size/shape variability, as well as surface chemistry and particle concentration. Polyethylene wear particles are not unidimensional; they have complex and variable shapes. The combination of morphologic description and quantitative image analysis used in this study defines several differences in polyethylene wear particles from different sources.