Biomedical Engineering Reference
In-Depth Information
always different in morphology and composition from the bulk. Differences arise
from molecular rearrangement, surface reaction, and contamination. Third, for
biomaterials that do not release nor leak biologically active or toxic substances,
the characteristics of the surface govern the biological response. And fourth,
some surface properties, such as topography, affect the mechanical stability of
the implant/tissue interface [Wen et al., 1996].
On a macroscopic level (roughness
m) roughness infl uences the
mechanical properties of the titanium/bone interface, the mechanical interlock-
ing of the interface, and the biocompatibility of the material [Ratner, 1983; Baro
et al., 1986]. Surface roughness in the range from 10 nm to 10
>
1 0
μ
m may also infl u-
ence the interfacial biology, since it is the same order as the size of the cells and
large biomolecules [Kasemo, 1983]. Microroughness at this level includes mate-
rial defects, such as grain boundaries, steps and kinks, and vacancies that are
active sites for adsorption, and therefore infl uence the bonding of biomolecules
to the implant surface [Moroni et al., 1994]. Microrough surfaces promote sig-
nifi cantly better bone apposition than smooth surfaces, resulting in a higher
percentage of bone in contact with the implant. Microrough surfaces may infl u-
ence the mechanical properties of the interface, stress distribution, and bone
remodeling [Keller et al., 1987]. Increased contact area and mechanical interlock-
ing of bone to a microrough surface can decrease stress concentrations resulting
in decreased bone resorption. Bone resorption takes place shortly after loading
smooth surfaced implants [Pilliar et al., 1991], resulting in a fi brous connective
tissue layer, whereas remodeling occurs on rough surfaces [Gilbert et al., 1995].
Successful clinical performance of machine/turned CpTi implants has resulted
in a wide-spread usage of them. However, in bone of poor quality and quantity,
the results have not always been good, motivating the development of novel types
of osseointegrated implants. The development of Ti implants has depended on
new surface-processing technologies. Recently developed clinical oral implants
have been focused on topographical changes of implant surfaces, rather than
alterations of chemical properties [Deporter et al., 1999; Buser et al., 1999; Plamer
et al., 2000; Testori et al., 2001; Sul, 2003]. These attempts may have been based
on the concept that mechanical interlocking between tissue and implant materials
relies on surface irregularities in the nanometer to micron level. Recently pub-
lished
in vivo
investigations have shown signifi cantly improved bone tissue reac-
tions by modifi cation of the surface oxide properties of Ti implants [Ishizawa
et al., 1995; Larsson et al., 1997; Skripitz et al., 1998; Fini et al., 1999; Henry et
al., 2000; Sul et al., 2001; Sul et al., 2002a; Sul et al., 2002b].
It was found that in animal studies, bone tissue reactions were strongly rein-
forced with oxidized titanium implants, characterized by a titanium oxide layer
thicker than 600 nm, a porous surface structure, and an anatase type of Ti oxide
with large surface roughness compared with turned implants [Sul et al., 2001; Sul
et al., 2002a]. This was later supported by work done by Lim et al. [Lim et al.,
2001] and [Oshida, 2007b], who found that the alkali-treated CpTi surface was
covered mainly with anatase type TiO
2
, and exhibited hydrophilicity, whereas the
acid-treated CpTi was covered with rutile type TiO
2
with hydrophobicity. Besides
μ
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