Biomedical Engineering Reference
In-Depth Information
to the implant. Bone loss contributes to the loosening of implants. Thus, immune
response can affect the performance of the graft.
6.4.5 Infection
Biomaterial-associated infection is the second most common cause
of implant fail-
ure. The infecting micro-organisms are either introduced during implantation of a
prosthesis device or are carried to the biomaterial surface by a temporary infection
of the patient. Although sterilization techniques (described in Section 6.3.5) are
used to prevent bacterial, fungal, and viral contamination, implanted biomaterials
are more vulnerable to microbial colonization. Surfaces well colonized by healthy
tissue cells tend to resist infection by virtue of cell membranes and eukaryotic ex-
tracellular polysaccharides. The high incidence of infection occurs because host
defenses in the vicinity of the prosthesis are generally impaired in the absence of
cells. Micro-organisms arriving at the surface, through introduction with the de-
vice or transient infection, are not eliminated by the immune system. Infection of a
prosthetic implant will almost always result in reoperation, amputation, or death.
Combined rates of death or amputation from infected cardiac, abdominal, and ex-
tremity vascular prosthesis may exceed 30%.
Adhesion-mediated infections develop that are resistant to antibiotics and
host defenses and tend to persist until the biomaterial or foreign body is removed.
Clinical retrievals of prosthetic devices indicate that a few bacterial species such
as
Staphylococcus epidermidis
and
Staphylococcus aureus
, dominate biomaterial-
centered infections. However, other bacterial species including
Escherichia
coli
,
Pseudomonas
aeruginosa
,
Proteus microbilis
,
b-hemolytic
streptococci
, and
ente-
rococci
have also been isolated. Bacteria are surface-adherent organisms and 99%
of their biomass exists on surfaces rather than in floating forms. The hemodynamic
interactions required within a device create fluid eddies and tissue damage that are
favorable to clotting cascades and the initial events of microbial adhesion. Surface
disruption by wear, corrosion, trauma, toxins, biosystem chemical degradation, or
bacterial mechanisms establishes appropriate environmental conditions for oppor-
tunistic bacteria within a microenvironment.
6.4.5.1 TheoriesofBacterialAdhesion
When biomaterials are introduced into a patient, their surface may become an
adhesive site for bacterial colonization. The process of bacterial adhesion is bio-
chemically parallel, and may be competitive or mutually exclusive to the process of
tissue integration. A number of interactions may occur depending on the specifici-
ties of the bacteria or biomaterial surface characteristics. The process of bacterial
colonization can be broadly classified into attachment, adhesion, agregation, and
dispersion at the substratum surface (Figure 6.10). A bacterium swims towards the
prosthetic material using polar flagella and forms random loose attachments to the
prosthetic material. Initial attachment depends on the general long-range physical
characteristics of the bacterium, the fluid interface, and the substratum. Subsequent
to attachment, specific irreversible adhesion could occur as a time-dependent chem-
ical process involving chemical binding, hydrophobic interactions, and interactions
