Additive manufactured osseointegrated screws with hierarchical design

Bone screws are devices used to fix implants or bones to bones. However, conventional screws are mechanically fixed with thread and often face long-term failure due to poor osseointegration. To improve osseointegration, screws are evolving from solid and smooth to porous and rough. Additive manufacturing (AM) offers a high degree of manufacturing freedom, enabling the preparation of predesigned screws that are porous and rough. This paper provides an overview of the problems currently faced by bone screws: long-term loosening and screw breakage. Next, advances in osseointegrated screws are summarized hierarchically (sub-micro, micro, and macro). At the sub-microscale level, we describe surface-modification techniques for enhancing osseointegration. At the micro level, we summarize the micro-design parameters that affect the mechanical and biological properties of porous osseointegrated screws, including porosity, pore size, and pore shape. In addition, we highlight three promising pore shapes: triply periodic minimal surface, auxetic structure with negative Poisson ratio, and the Voronoi structure. At the macro level, we outline the strategies of graded design, gradient design, and topology optimization design to improve the mechanical strength of porous osseointegrated screws. Simultaneously, this paper outlines advances in AM technology for enhancing the mechanical properties of porous osseointegrated screws. AM osseointegrated screws with hierarchical design are expected to provide excellent long-term fixation and the required mechanical strength. Graphic abstract