Interface contact behavior of 3D printed porous surfaces

When a 3D printed implant integrates a thin surface lattice, the geometry of this porous region controls the bone-implant interface, affecting both short-term implant stability and long-term osseointegration. In intervertebral devices, for example, high expulsion resistance and propensity to subside are imperative in reducing implant migration and loss of disc height. Moreover, the shear strength of the porous-solid metal interface is critical to prevent metal-to-metal decohesion prior to and after osseointegration. While lap shear and subsidence tests are both governed by ASTM standards, expulsion testing is not standardized, and the tradeoffs in the potential implant failure modes have not been thoroughly investigated to find an optimal porosity satisfactory in all three performance tests. In this multi parameter study, we perform a series of experiments on 3D printed porous gyroid surfaces to understand the interplays and inherent tradeoffs between porosity, expulsion resistance, propensity to subside, and porous layer strength. Porosity was the only significant factor affecting expulsion, subsidence, and ultimate shear strength. As porosity increased, expulsion resistance of the surface porosity sample increased, resulting in samples that are harder to push out of a constrained bony cavity; however, shear strength and propensity to subside both decreased, resulting in a weaker metal-to-metal porous layer adhesion and equivalent penetration into the Sawbone surface at lower forces. Within the error of the measurements, the 6 × 6 × 6 0.75 mm wall thickness gyroid (65% modeled porosity and 62% measured porosity) design presented the best overall performance characteristics.