Advancing Biomechanics in Cervical Arthroplasty: Surface Topography and Case Depth Characterization of Hardened Titanium Implants

Introduction: Cervical disc arthroplasty (CDA) devices featuring metal-on-metal (MoM) endplate articulation have shown significant long-term neurological success after CDA. Current MoM devices are made of either 90% Ti-6Al-4V alloy and 10% Ti-Carbide (Ti64/TiC), or Cobalt-Chromium-Molybdenum alloy (CoCrMo). We hypothesized that utilizing pure Ti-6Al-4V alloy with heat-treat (HT) surface hardening could offer a biomechanically superior alternative while also ensuring safety of the implant surface.

Methods: Four cervical disc implants composed of HT-hardened Ti-6Al-4V from distinct methods (HT1, HT2, HT3, HT4), one disc composed of Ti64/TiC, and one disc composed of CoCrMo underwent mechanical tests for surface roughness (ASTM B46.1-2009), nanoindentation hardness (ASTM E2546), and hardness and modulus profile of case depth layers.

Results: All discs were cut and surface finished to 0.4(µm) Ra before heat treatments were applied. Surface roughness was measured after heat treatments were applied to assess for any change in surface roughness induced by the HT process. The surface roughness of the HT-hardened discs all improved after HT: 0.06 (HT1), 0.2375 (HT2), 0.2725 (HT3) and 0.125 (HT4). Surface nanoindentation revealed a hardness of 11.53GPa (HT1), 10.14GPa (HT2), 15.89GPa (HT3) and 19.72GPa (HT4), significantly higher than 5.58GPa and 9.84GPa in Ti64/TiC and CoCrMo discs, respectively.
The hardness decreased from surface to substrate gradually for all four HTs, with plateaus occurring at depths of 30µm (HT1), 40µm (HT2), 100µm (HT3), and 40µm (HT4). These depths were significantly higher than the optical case depths of heat treatment. The modulus of elasticity also showed a similar decreasing profile.

Conclusion : Our study demonstrates HT-hardened Ti-6AL-4V discs have a significantly enhanced surface topography compared to the Ti64/TiC and CoCrMo implants currently in clinical practice. Along with significantly improved indentation hardness, the deeper hardness relative to optical case depth is suggestive of an strong integration between the treated surface and underlying base substrate.