Biomedical Engineering Reference
In-Depth Information
Hydroxyapatite is a naturally occurring ceramic mineral present as a struc-
tural component of bone. Hydroxyapatite (Ca
10
(PO
4
)
6
(OH)
2
) does not
break down under physiological conditions unlike the other calcium phos-
phates, which are considered bioresorbable. It is stable at physiological
pH and actively takes part in forming strong chemical bonds with the sur-
rounding bone. This property is exploited for rapid bone repair after major
trauma or surgery. While its mechanical properties have been found to be
unsuitable for load-bearing applications such as orthopedics, hydroxyapa-
tite is used as a coating on materials such as titanium and titanium alloys.
Hydroxyapatite contributes the bioactive properties, while the metallic
component bears the load. Such coatings are typically applied by plasma
spraying at high processing temperatures. Careful control of processing pa-
rameters is necessary to prevent thermal decomposition of hydroxyapatite
into other soluble calcium phosphates.
Silicate-based materials with calcium and phosphate in proportions
mimicking natural bone composition have also been developed. After im-
plantation in bone tissue, these materials resist bonded to bone due to the
surface reaction. Various kinds of bioactive glasses and glass-ceramics with
different properties such as high mechanical strength, and high machina-
bility have been developed. Bioactive glass and ceravital (additionally con-
tains oxides of magnesium and potassium) are the most prominent. Bioac-
tive glass is shown to bond to bone as well as soft tissue. They are used in
nonload-bearing applications such as in the middle ear, and alveolar ridge
maintenance implants.
3.
Resorbable ceramics.
This class of ceramics degrades upon implantation
to the host. Examples include a variety of phosphates (calcium, tricalci-
um, aluminum-calcium, zinc sulfate-calcium), oxides (zinc-calcium-phos-
phorous, ferric-calcium-phosphorus), corals (mostly calcium carbonate),
and plaster of Paris (calcium sulfate dihydrate). They are used in implants
where endogenous tissues gradually infiltrate as the implants degrade and
are absorbed by the body. Degradation could be caused by physiologic
environment such as pH, physical disintegration such as grain boundary
attack, or biological processes such as phagocytosis. They are useful in the
replacement or repair of damaged bone by trauma or disease, coating of
metal implants to promote bone in-growth, repair and fusion of vertebrae,
repair of herniated disks, repair of maxillofacial and dental defects, and
drug delivery.
Calcium phosphate compounds are abundant in living systems. Ap-
atites constitute the principal inorganic phase in normal calcifi ed tissues
such as enamel, dentin, and bone. There are several calcium phosphate
ceramics that are considered biocompatible and of these, most are resorb-
able when exposed to physiological environments. The calcium phosphates
attain their biocompatibility through the release of Ca
2+
and (PO
4
)
−3
ions
to the surrounding tissue after implantation, and the ion release rate is
determined by the composition, structure, porosity, and other factors, and
thus can be controlled somewhat by manipulation of the material during
processing. The in vivo response to these materials is also a variable. The
