While it certainly feels like our bones are hard, they aren't. In fact they are actually not dead, un-reactive material either. Many don't realize that bones are flexible, to a point, and they are very much living tissue. The familiar and visible exterior of the human bone is merely a casing that protects an important network of nerves, blood vessels and material that allows growth of the bone as well as the marrow. Until now, science has been replacing broken bones with solid prosthesis or hollow ones without any way for surrounding tissue to take hold within the prosthesis. Until now. An International team from the Institute of Ceramic Technologies and Systems in Germany and doctors from the Technical University of Dresden, as well as several companies got together to create a viable filling for the hollow portion of bone replacements. It's called TiFoam, or Titanium Foam.
TiFoam is flexible yet rigid, just as natural, healthy bones are. The flexibility comes from the flexibility of the casing and the way the inner material allows bend and shock absorption. TiFoam is similar and even looks like the way the inside of a healthy bone should look. The hollow areas, promote growth of nerves as well as vessels. They work especially well in replacing bones that are used to loads - like the femur and tibia.
The technology comes from powder metallurgy molding. The way it's done is that open cell polyurethane foams are injected with titanium powder and a material like an epoxy that binds the powders within the polyurethane. Once the binding agents solidify, the PU is dissolved and what remains is a network of titanium lattice that looks identical to the inner lattice of bones. The result is a replacement that is durable as well as rigid. By being rigid, the bone allows transmission of forces along a prescribed path. The flexibility allows the dissipation of forces that are absorbed at the molecular level and prevents fractures.
The impressive thing about this is that the level of technology humans have been able to advance in medicine, communication and materials, as well as almost every facet of our daily complex lives, is a testament to the direction of our species' evolution. We have been able to comprehend, and invent whatever we need.