Evaluating the Wear of a Viscoelastic Spinal Disc in the Context of Hip and Knee Arthroplasties

The study by Chin and colleagues provides valuable data on the wear debris generated by a nonarticulating, viscoelastic spinal disc, including particle morphology, compared with 2 designs that feature metal-on-polyethylene articulating surfaces. It is useful to interpret their findings in a context similar to that which was developed in the study of total hip and knee arthroplasties. The general conditions under which wear occurs in hip and knee arthroplasties comprise 4 general modes, as follows1: Mode 1: two bearing surfaces rubbing together, as intended. Mode 2: a bearing surface rubbing against a non-bearing surface. Mode 3: two bearing surfaces rubbing together, but with third-body debris interposed. Mode 4: two non-bearing surfaces rubbing together. Since a viscoelastic disc does not have bearing surfaces, Modes 1, 2, and 3 do not apply. Motion is achieved by deformation of the disc, and any rubbing between surfaces is presumably unintentional. Nevertheless, it is appropriate to compare the wear of a viscoelastic disc to that of an articulating disc, since the important question is the amount and morphology of the wear debris that will be released to the surrounding tissues. In hip arthroplasty, a review of studies comparing the incidence and intensity of the reaction of periarticular tissues to debris burden showed that a wear rate of 0.1 mm/year for a polyethylene socket represented a “threshold,” below which clinically important osteolysis seldom occurred2. As the wear rate increased to ≥0.2 mm/year, failure due to extensive osteolysis became increasingly frequent. Thus, the 0.1-mm/year threshold provided a quantitative target for those developing the highly wear-resistant cross-linked polyethylenes. The reductions in wear rates compared with historical “conventional” polyethylene were ≥90%, and there have been few reports of failure due to osteolysis3. A 32-mm polyethylene hip socket that is wearing at 0.1 mm/year over one-half of its internal bearing surface releases about 40 mm3/year of wear debris. It is promising that the volume of wear of each of the 3 discs compared by Chin and colleagues was well below this threshold value. However, the nature of the adverse reactions to wear particles will depend on their concentration within tissues, and on the types of tissues exposed (e.g., capsular membrane in a hip compared with neural tissues in the spine)4. The incidence and intensity of the adverse reactions may be substantially different with spinal discs than with a hip or knee arthroplasty, such that a lower wear-rate threshold may be desirable with spinal discs. Chin and colleagues present a thorough listing of the limitations of their study, including testing in saline solution rather than blood serum, as recommended according to relevant ASTM International and ISO standards. The recommendation for using serum arose from the results of tests that compared the wear of polyethylene pins against metal and ceramic counterfaces in a variety of lubricants, which showed substantially different amounts and types of wear in saline solution versus bovine blood serum (the latter being a model for the fluid in a joint following arthroplasty)5. However, the wear of a viscoelastic component as reported in the current study may not be as sensitive to the type of lubricant as with Mode-1 wear in a hip or knee arthroplasty. Comparative testing would be useful to assess this. Finally, the authors state that the wear of the viscoelastic discs in the present study occurred adjacent to a flash ring on the polymer core that was formed during the molding process. It is not clear what, if anything, was rubbing against the core during the test, but it suggests that this type of Mode-4 wear could be minimized by suitable modifications to the fabrication process to eliminate the flash ring and/or by minimizing contact between the polymer core and the surrounding tissues. If there was no such contact, then the shedding of debris may have been due to fatigue of the elastomer, rather than actual wear, and additional testing should be focused on the potential for fatigue damage.