Investigation of articular cartilage and counterface compliance in multi-directional sliding as in orthopedic implants

As the use of total-joint replacement devices has become widespread, debris-induced osteolysis from ultra-high molecular weight polyethylene (UHMWPE) is still the primary challenge to lengthening the pain-free lifetime of these implants. Investigators have long suspected that the natural cartilage-on-cartilage interface found in synnovial joints is an optimal system, but technology has not advanced to the point of producing such an implant for use in total-joint replacement. The authors have investigated the performance of such a joint by developing an in vitro model of a cartilage-on-cartilage system. Bovine articular cartilage was fabricated into pins and subjected to multi-directional sliding in the Dual Axis Wear Simulator (DAWS), a machine developed by the authors to simulate conditions in an in vivo joint. The pins were worn against a rigid stainless steel counterface, similar to that found in conventional total-joint implants, and these results were compared to wear against a compliant polymeric counterface. The rigid counterface produced cartilage wear amounts with a mean of 25.65 mg after 100,000 cycles of 39 N loading and a 6.4-mm square wear path. However, use of the compliant counterface yielded a mean wear value of 9.97 mg for the same testing conditions. It is likely that the mechanism responsible for this wear involves the generation and subsequent depletion of a fluid layer at the interface between the cartilage pin and the counterface such that the majority of material removal occurred during the fluid deficient portions of the wear cycle. This difference in wear amounts was attributed to the substantial difference in contact pressures between the rigid and compliant counterfaces. To estimate the contact pressures in each case, the storage and loss moduli were measured for cartilage samples through dynamic mechanical analysis (DMA). It was found that over a frequency range from 0.1 to 10 Hz, the storage modulus ranged from 470 to 1010 kPa, while the loss modulus increased more modestly, from 176 to 249 kPa. The results of wear testing and the measurement of the viscoelastic properties of articular cartilage suggest the possibility of successful cartilage-on-metal implant systems where the contact pressures are kept low through joint geometry or the use of a biocompatible compliant surface on the metallic component.