Biomedical Engineering Reference
In-Depth Information
wear resistance and strength [RIT 96]. The main drawbacks of
pyrolytic carbons are the lack of bone fixation leading to the loosening
of joint prostheses, the apparition of cavitation and pitting due to
blood flow and impact stress resulting from the implant failure, and
despite their good hemocompatibility, the platelet adhesion on the
implant surface which may induce thrombus formation [BOE 11].
3.3.3.
Bioactive ceramics
Bioactive implants elicit a specific biological response at the
material surface leading to the formation of a chemical bond between
the material interface and the tissue. Some materials have
demonstrated the ability to develop bioactive fixations to tissue with
various mechanisms and rates of bonding, and different thicknesses
and strengths of the interfacial zone. These materials include bioactive
glasses and glass-ceramics, hydroxyapatite (HA) and some
composites. They develop at their surface a layer of hydroxyl
carbonate apatite which is chemically and structurally equivalent to
the mineral phase of bone. Due to its equivalence, the layer can bond
with collagen fibrils and thus to bone. The surface reaction is
dependent on the material composition and small changes can totally
suppress the bioactive property. In bioactive fixation, the adherent
interface resists to high mechanical forces. Indeed, the adhesion
strength of the interfacial zone is equivalent to or even greater than the
cohesive strength of the only material or the only tissue. For example,
when testing adhesion in a pull-off model, the failure of the device
appears in the implant when using Bioglass® and in the bone when
using glass-ceramic Cerabone®. In addition, the bioactive fixation is
15-40 times stronger than the fixation developed with a non-bioactive
material (morphological fixation) like alumina where the failure of the
device occurs in the interfacial zone [HEN 98].
Common inorganic glasses are developed from the SiO
2
-CaO-
Na
2
O system. Bioactive glasses are single phase vitreous materials
which are based on a stable vitreous silica-rich matrix (38-65% SiO
2
)
containing oxide in specific amounts, such as Na
2
O (15-30%), CaO
