Optimizing Surface Topographies for 3D Printed Metallic Orthopedic Implants

Powder bed fusion (PBF), a popular metal 3D printing technique utilizing Selective Laser Melting (SLM) or Electron Beam Melting (EBM), has revolutionized the biomedical industry by providing uniquely customizable and precisely complex orthopedic implants that other production methods cannot offer. Titanium and cobalt alloys (Ti6Al4V and Co28Cr6Mo) are popular biocompatible materials utilized in orthopedic implants fabricated via PBF due to their high strength, low density, corrosion resistance, and suitability for osseointegration. Once implanted, the surfaces of these devices dictate short and long-term stability and durability through osseointegration and minimizing premature structural failure. While surface roughness and porosity can enhance osseointegration and stability, these surface topographies create areas for stress to concentrate. Implant strength and durability is then lowered, leading to an inherent trade-off between surface topography and the lifespan of orthopedic implants. This research aims to systematically evaluate the impact of relatively new and relevant surface topographies on short and long-term mechanical performance while applying appropriate American Society for Testing and Materials (ASTM) standards. To predict the short-term stability of an implant, Ti6Al4V devices with varying surface topographies manufactured through SLM underwent expulsion, subsidence, and shear testing. These mechanical tests evaluate the effects of topography, device size, porosity, and applied normal force at the bone-implant and solid-porous interface, while identifying trade-offs in static mechanical performance at these interfaces. When varying surface topographies, normal force was the most dominant predictor of expulsion, and a bigger device with more surface features displayed higher expulsion forces at increasing normal forces. When keeping a consistent gyroid topography and changing the unit cell size and wall thickness, the overall porosity was the statistically significant variable impacting expulsion, subsidence, and shear testing. A modeled 65% porous gyroid presented the best overall performance characteristics within the error of measurements. On the other hand, to predict long-term strength and durability, devices with varying material (Ti6Al4V versus Co28Cr6Mo), manufacturing techniques (SLM, EBM, and wrought), and surface finishes (“as printed”, blasted, machined, polished, and added surface porosity) were subjected to fatigue testing. Although varying fabrication methods impacted microstructure, data confirmed that refining the surface finish through mechanical post-processing was the predominant factor in improving fatigue strength across all sample groups. A critical surface roughness of about 0.2 µm was identified in which further decreasing the surface roughness was relatively ineffective in increasing fatigue strength. Conversely, further increasing gyroid thickness on the surface of SLM Ti6Al4V samples had a limited impact in decreasing fatigue strength. In conclusion, this research highlights potential tradeoffs for optimizing stability and lifespan of orthopedic implants through design and manufacture.