Commentary & Perspective on
"Surface Damage in Machined Ram-Extruded and Net-Shape Molded Retrieved Polyethylene Tibial Inserts of Total Knee Replacements"
by Aivars Berzins, MD, et al.

Commentary & Perspective by
Thomas P. Schmalzried, MD*,
Joint Replacement Institute at Orthopaedic Hospital,
Los Angeles, CA

It is generally agreed that the optimal properties of a polyethylene used in total knee arthroplasty are different from those needed in hip components. Consequently, there has not been a rapid acceptance of more extensive cross-linking of polyethylene for use in total knee replacements. Furthermore, due to the greater variability in geometry, conformity, loading, kinematics, and modularity of total knee prostheses, any single polyethylene "formulation" may not be optimal for all applications in total knee arthroplasty.

The conclusion of the study by Berzins et al.—that an equivalent degree of surface damage (particularly delamination) occurs after a shorter in vivo service life with machined polyethylene components in comparison with molded ones—is well supported by their data. It is unfortunate that the dates of sterilization of these components were not made available. The importance of the duration of shelf life (following gamma sterilization and packaging in air) on the in vivo performance of polyethylene tibial components was recently the topic of a lead article published in The Journal¹. It is unfortunate that Berzins et al. were unable to analyze the relationship between the duration of shelf life and the surface damage on the retrieved specimens in their study. Similarly, it would be interesting to know the relationship between the level of oxidation in these components and the amount of surface damage. I suspect that the machined components had a higher level of oxidation.

It is generally accepted that oxidation of a polyethylene component reduces its strength and ductility. Oxidation results from gamma irradiation, which remains the most common method of sterilization of the polyethylene used in total knee components. Gamma radiation breaks covalent bonds in the polyethylene, and the resultant free radicals can combine with available oxygen. This oxidation reaction continues over time, and the level of oxidation increases in direct proportion to the length of time that the component is exposed to air (oxygen) while it is "sitting on the shelf" prior to implantation. A lower rate of oxidation continues in vivo². The highest level of oxidation is typically found 1 to 2 mm below the surface of the component. Unfortunately, this depth coincides with the location of the peak stresses in polyethylene tibial bearings. Because of this, oxidation has a more detrimental effect on polyethylene components in knees than it does on those in hips. High levels of oxidation have been associated with accelerated wear due to delamination (subsurface defects), and other fatigue-related phenomena such as pitting and cracking³. Oxidation may also contribute to greater relative motion of modular inserts, leading to an increase in backside wear.

Components with low levels of oxidation have a greater tolerance to the subsurface peak stresses of polyethylene tibial components (resulting in less fatigue failure and delamination). Several observations indicate that net-shape molded components are more oxidation-resistant following sterilization with gamma irradiation in air than machined components are. Lower levels of oxidation may be the key to the better in vivo performance of molded components, which contain polyethylene with moderate cross-linking and relatively little oxidation. To further reduce oxidation, several manufacturers are using gamma irradiation to sterilize polyethylene and are packaging the components in an oxygen-free ("gamma inert") environment. While it is not known what effect this method will have on the in vivo aging of polyethylene, several analyses indicate that it is highly effective in eliminating oxidation in the packaging. The conclusions of the study by Berzins et al. are limited to components sterilized with gamma irradiation in air that have some (variable) aging related to shelf life. Additional retrieval analyses are needed to determine the in vivo performance of machined components that are sterilized and packaged in "gamma inert" environments.

*The author did not receive grants or outside funding in support of the research or preparation of this manuscript. He did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated.

References

1. McGovern TF, Ammeen DJ, Collier JP, Currier BH, Engh GA. Rapid polyethylene failure of unicondylar tibial components sterilized with gamma irradiation in air and implanted after a long shelf life. J Bone Joint Surg Am. 2002;84:901-6.
2. Schmalzried TP, Callaghan JJ. Wear in total hip and knee replacements. J Bone Joint Surg Am. 1999;81:115-36.
3. Schmalzried TP, Campbell P, Schmitt AK, Brown IC, Amstutz HC. Shapes and dimensional characteristics of polyethylene wear particles generated in vivo by total knee replacements compared to total hip replacements. J Biomed Mater Res. 1997;38:203-10.