Biomedical Engineering Reference
In-Depth Information
electron microscopy revealed an interfacial hierarchy that consisted of a 20-40 nm
thick proteoglycan layer within four nm of the titanium oxide, followed by col-
lagen bundles as close as 100 nm and Ca deposits within fi ve nm of the surface
[Healy et al., 1992a]. To reach the steady-state interface described, both the oxide
on titanium and the adjacent tissue undergo various reactions. The physiochemi-
cal properties of titanium have been associated with the unique tissue response
to the materials; these include the biochemistry of released corrosion products,
kinetics of release and the oxide stoichiometry, crystal defect density, thickness
and surface chemistry [Healy et al., 1992b].
As seen above, in general, the titanium passivating layer not only produces
good corrosion resistance, but it seems also to allow physiological fl uids, proteins,
and hard and soft tissue to come very close and/or deposit on it directly. Reasons
for this are still largely unknown, but may have something to do with factors such
as the high dielectric constant for TiO
2
(50 to 170 vs. 4-10 for alumina and dental
porcelain), which should result in considerably stronger van der Waal's bonds
on TiO
2
than other oxides; TiO
2
may be catalytically active for a number of
organic and inorganic chemical interactions infl uencing biological processes
at the implant interface. The TiO
2
oxide fi lm may permit a compatible layer of
biomolecule to attach [Br å nemark, 1985 ; Kasemo et al., 1985 ].
5.3.2 Mechanical Compatibility
Biomechanics involved in implantology should at least include the nature of the
biting forces on the implants, transferring of the biting forces to the interfacial
tissues, and the interfacial tissues reaction, biologically, to stress transfer condi-
tions. Interfacial stress transfer and interfacial biology represent more diffi cult,
interrelated problems. While many engineering studies have shown that variables
such as implant shape, elastic modulus, extent of bonding between implant and
bone, and so on, can affect the stress transfer conditions, the unresolved question
is whether there is any biological signifi cance to such differences.
The successful clinical results achieved with osseointegrated dental implants
underscore the fact that such implants easily withstand considerable masticatory
loads. In fact, one study showed that bite forces in patients with these implants
were comparable to those in patients with natural dentitions. A critical aspect
affecting the success or failure of an implant is the manner in which mechanical
stresses are transferred smoothly from the implant to bone. It is essential that
neither implant nor bone be stressed beyond the long-term fatigue capacity. It is
also necessary to avoid any relative motion that can produce abrasion of the bone
or progressive loosening of the implants. An osseointegrated implant provides a
direct and relatively rigid connection of the implant to the bone.
This is an advantage because it provides a durable interface without any
substantial change in form or duration. There is a mismatch of the mechanical
properties and mechanical impedance at the interface of Ti and bone. It is inter-
esting to observe that from a mechanical standpoint, the shock-absorbing action
would be the same if the soft layer were between the metal implant and the bone.
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