Comparative in Vitro Analysis of Wear Particles Generated by a Viscoelastic Disc Versus 2 Articulating Total Disc Replacements.

Abstract

Wear debris is a known contributor to orthopaedic implant failure, particularly in joint arthroplasty. The wear characteristics of spinal total disc replacement (TDR) remain under-investigated. Spinal TDRs have been shown to produce wear particles that elicit strong inflammatory reactions. Submicron debris, in particular, is associated with osteolysis and implant loosening. Viscoelastic TDR (VTDR) devices have emerged to address these risks. Five AxioMed Freedom Lumbar Disc (FLD) devices underwent 30 million cycles (10 million each in flexion-extension, lateral bending, and axial rotation) of wear testing in phosphate-buffered saline solution at 37°C using an MTS servohydraulic system. Wear fluid samples were collected every 5 million cycles and analyzed using scanning electron microscopy and laser diffraction. A 30-million device cycle count simulates 240 years of lumbar bending. Wear rates were calculated in milligrams per million cycles (mg/MC). Comparative data for CHARITÉ (DePuy Synthes) and prodisc L (Centinel Spine) discs were obtained from the United States Food and Drug Administration (FDA) Summary of Safety and Effectiveness Data. The AxioMed device showed a mean wear rate of 1.7 mg/MC, in comparison to 5.7 mg/MC for the prodisc L. The number-average particle diameter was 1.9 μm, with a mass-average particle diameter of 49 μm, which was notably larger than those reportedly produced by the CHARITÉ (0.2 μm) and prodisc L (0.4 μm) devices, which is promising because larger particles (>1.0 μm) are less likely to induce inflammatory responses. No mechanical failures were observed during the 30 million cycles. The AxioMed 1-piece VTDR device demonstrated a lower wear rate and larger, less biologically reactive, particles compared with articulating TDRs, suggesting a reduced risk of osteolysis and longer implant lifespan. No mechanical failures were observed, even after each 10-million-cycle interval, which simulates approximately 80 years of lumbar-bending motions. This study focused on particle size; further work is warranted to characterize composition and particle burden. This 1-piece VTDR may offer a safer and more durable alternative for motion-preserving lumbar spine surgery. Further clinical and retrieval studies are warranted.