Osteoconductive Properties of Silicon Nitride

Bioceramics such as alumina and zirconia have been implanted in humans for several decades as bearings in synthetic hip and knee joints. In addition to the observed low wear of highly polished ceramic bearings, it was accepted that these materials are inherently bioinert (i.e., they remain stable in the body without biological activity), thereby contributing to implant longevity. In this chapter, the belief that bioceramics are bioinert is challenged by showing that complex surface chemical reactions at the implant’s surface affect cell metabolism and differentiation. These reactions and experimental modulation of silicon nitride’s surface chemistry provide new insights and knowledge related to the bioactivity of orthopedic implants. The surface interactions of silicon nitride with cells are consistent with the immune system’s reaction to implanted devices. These defensive mechanisms consist of dissolution, encapsulation, or integration of the foreign material. In contrast to orthopedic metals and oxide ceramics, silicon nitride actively promotes attachment, differentiation, and proliferation of bone cells. It has a zwitterionic surface similar to hydroxyapatite, facilitating the deposition and motility of osteoblasts. Surface reactions are triggered when silicon nitride is exposed to biological fluids, releasing trace amounts of silicon and nitrogen, which contribute to a highly bone-friendly surface containing osteopromotive moieties that enhance bone formation. Because of these properties, silicon nitride has the potential to not only be utilized as orthopedic implants but also contribute to new and improved treatments of a variety of skeletal diseases and conditions with faster and stronger bone integration.