So that the metal can join with my skeleton, part of the stem and shell have a coating of a substance called commercially-pure titanium. Bone can fuze with this porous layer, no cement needed, joining the natural with the artificial. “Bone looks at this surface and kinda sees itself,” Dorr, who’s replaced about 7,000 hips and 5,000 knees, explains. A few weeks post-op, immature bone will have already started growing into the metal. “I never figured out why bone is so dumb,” Dorr lightheartedly says. Grow into the titanium? Sure, why not.