Biomaterials are classified into three groups with respect to their biocompatibility, i.e. the ability of a material to perform with an appropriate host response in a specific application, as suggested by Oborn:
1. Biotolerant materials CoCr, bone cement and stainless steel. A connective tissue layer surrounds those implants in that way a fibrous tissue encapsulates the material and there is not a contact between the material and the bone.
2. Bioinert materials such as titanium, alumina, zirconica and carbon materials. These materials are characterized by an ability to form direct contact between themselves and the surrounding bone. A stable oxide layer surrounds the material surface.
3. Bioactive materials implants such as calcium-phosphate ceramics and certain glass ceramics. A direct chemical bond between the implant and the surrounding bone characterizes this group of material.

Cited from Guy Sovak PhD Thesis:
Bone Tissue Reaction to a Novel Titanium Nitride (TiN)
Coating on Titanium Alloy (Ti-6al-4V) Implants
and the Effect of Growth Factors and
Cell Adhesion Molecules.