Ceramic biomaterials for orthopaedic prostheses

This article presents "bio-inert" ceramics for orthopedic implants (devices implanted in the human body to restore joint function). Other inorganic materials used for prosthesis coatings or bone substitutes ("bio-active" ceramics such as calcium phosphates, glasses or cements) are the subject of specific articles in Techniques de l'Ingénieur, and will deliberately not be discussed here.

While the use of ceramics of natural origin as biomaterials (i.e. bioceramics) dates back several millennia (use of mother-of-pearl as a dental substitute by the Mayans, for example), the controlled use of technical ceramics in contact with the human body came much later (porcelain for the manufacture of dental crowns in the 18th century, plaster of Paris for bone filling in the 19th). It was only in the mid-twentieth century that technical ceramics specifically dedicated to orthopedics appeared: in 1965, alumina (aluminum oxide) was patented for use as a head and cup for hip prostheses. This was the first use of a so-called structural bioceramic (with high mechanical properties). Zirconia and alumina-zirconia composites, with even better mechanical properties, followed for the same applications.

Alumina, zirconia and their composites are considered "bio-inert" ceramics, as they do not bond directly with bone. Indeed, after implantation of a bio-inert ceramic, a fibrous capsule forms, isolating the implant from the bone and limiting its integration. For this reason, these materials are not used for bone filling and rarely come into direct contact with bone (except in the case of zirconia dental implants, which require special surface treatments).

The creation of friction surfaces is therefore the most important application for inert bioceramics in orthopaedics. The use of bioceramics reduces wear on prostheses. Their main use is in the manufacture of components for hip prostheses (heads and cups), but they have also recently been used in certain knee prostheses and mobile cervical prostheses... The clinical successes associated with the use of ceramics have led to the implantation of over 600,000 zirconia hip replacement heads, over 3.5 million alumina heads and almost 2 million alumina-zirconia composite heads since the beginning of their use. The major drawback of ceramics is their intrinsic brittleness (in the mechanical sense of the word: they break before plastic deformation). As a result, the early days were sometimes chaotic (up to 13% fracture rate for certain series of heads in the late 1960s). The rupture rate of alumina heads is now very low (less than 0.01%), and that of alumina-zirconia heads even lower. This failure rate is therefore negligible compared to the overall failure rate of hip prostheses (around 15% at 25 years), which is generally...