The Rationale for Silicon Nitride Bearings in Orthopaedic Applications

The history of silicon nitride (Si3N4) has been described previously; detailed analyses of particles of meteroritic rock have been shown to contain silicon nitride crystals, suggesting that this material exists naturally in the galaxy.1 Synthetic Si3N4 was probably developed by Deville and Wöhler in 1859. Commercial interest in this material increased in the 1950s, as the material properties of silicon nitride were better understood, and its application in internal combustion engines was contemplated. Today, industrial applications of silicon nitride ceramics and related composites are common; these include bearings, turbine blades, and glow plugs, related to the fact that silicon nitride has high fracture toughness, strength, and attractive wear properties.2 Ceramic ball bearings made of silicon nitride, for example, have been used in technical applications, and their extreme strength has been validated using a number of techniques.3 These material qualities have led many investigators to speculate that silicon nitride may also have applicability in biomedical fields, especially since it exhibits biocompatibility4, 5 and is visible on plain radiographs as a partially radiolucent material. Because of the fortuitous combination of these properties, silicon nitride has been investigated for applications in skeletal repair, and for the bearings of prosthetic replacements of arthritic hip and knee joints. Since clinical data are yet sparse, this review will attempt to give the reader an overview of the rationale for the use of silicon nitride in biomedical, specifically orthopaedic applications. As of the present time, surgical screws, plates, and bearings for use in prosthetic hip and knee joints have been developed and tested, using silicon nitride as the source material.6-8 Cervical spacers and spinal fusion devices made of silicon nitride composites are presently in use, although clinical results have yet to be reported given the relatively short follow-up time. The goal of this chapter is to examine the scientific rationale to support the adoption of silicon nitride ceramics for use in biomedical, specifically, orthopaedic applications.