Spinal interbody cages are provided that include a bulk interbody cage, a top face, a bottom face, pillars, and slots. The pillars are for contacting vertebral bodies. The slots are to be occupied by bone of the vertebral bodies and/or by bone of a bone graft. The spinal interbody cage has a Young's modulus of elasticity of at least 3 GPa, and has a ratio of the sum of (i) the volumes of the slots to (ii) the sum of the volumes of the pillars and the volumes of the slots of 0.40:1 to 0.90:1.