Biomedical Engineering Reference
In-Depth Information
Activated macrophages that produced high levels of ROS accelerated Ti ion release
from cpTi, pointing to ROS-induced corrosion [
62
]. All this indicates the possibility
of metal corrosion occurring even on materials considered to be relatively inactive
from a biological point of view.
4.3
Integration of Metal Implants and the Role
of Endothelial Cells
Upon implantation tissues around the implant respond with reactions similar to the
wound healing process. This process can be divided into three phases—inflammatory,
proliferating and remodelling or maturation phase [
28,
48,
57,
60
] . During the fi rst
phase inflammatory cells, such as neutrophils and monocytes, are recruited to the
site of implantation to remove cellular debris and possible sources of infection [
93
] .
The inflammatory phase is followed by the proliferative phase, characterised by the
formation of granulation tissue. It consists mainly of fibroblasts that proliferate and
produce extracellular matrix (ECM). To supply the newly formed tissue with nutri-
ents and oxygen, vascularisation is induced in the form of granulation tissue [
17
] .
Tissue vascularisation at the interface of an orthopaedic implant is crucially impor-
tant for implant integration and stability. Endothelial cells play a central role in the
formation of blood vessels in the newly formed peri-implant tissue in a process
called angiogenesis. Moreover, they take part in the inflammatory phase of wound
healing after surgery. They do so by releasing cytokines (for example, IL-8 and
MCP-1), which are involved in the recruitment of leukocytes to the wound area.
Furthermore, adhesion molecules (e.g. E-selectin and ICAM-1) expressed on the
surface of endothelial cells allow the attachment and extravasation of inflammatory
cells into the peri-implant tissue [
47
]. Wound healing around an implant is con-
cluded with the maturation or remodelling phase, during which provisional ECM is
replaced by mature ECM and the cell density in the peri-implant tissue is reduced
[
60
]. Close proximity of endothelial cells to the surface of metal implants enables
their exposure to metal degradation and corrosion products. While the reactions of
endothelial cells to metal particles and metal ions have been relatively well studied,
their response to electrochemical processes, especially to processes associated with
cathodic partial reactions on the metal surface, has not yet been examined.
4.4
Oxidative Stress: ROS Formation and Antioxidant
Defence Mechanisms
Molecular oxygen possesses two unpaired electrons with the same spin in the exter-
nal p* orbital that is responsible for the ease with which O
2
forms radicals, or ROS
[
23
]. ROS, such as singlet oxygen (
1
O
2
), hydroxyl radical (
·
OH) and superoxide
radical (
·
O
2
−), are a group of free radicals with high reactivity [
10
] . H
2
O
2
is another
non-radical product of incomplete oxygen reduction. ROS and H
2
·
O
2
are products of
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