Biomedical Engineering Reference
In-Depth Information
indications for donations include deaths
resulting from trauma with large resuscitation
volumes, with or without blood or blood prod-
ucts, and deaths resulting from poisoning or
related to toxic overdoses [
ation of structural proteins [
]. In addition,
heat sterilization may not inactivate bacterial
spores [
7
27
]. Gamma-irradiation at the level of
1
.
5
to
2
.
5
megarads or above [
8
,
13
,
36
,
42
,
45
,
].
Upon identifi cation and screening of an
acceptable donor, appropriate consent must be
obtained from the donor or nearest relative
prior to tissue and/or organ procurement. Mus-
culoskeletal allografts may be obtained from
living donors, multiorgan donors, and cadav-
ers. Harvesting of a musculoskeletal allograft
from a living donor (such as a femoral head
allograft harvested from a total hip replace-
ment) is performed in a sterile operating room,
as is harvesting from a multiorgan donor.
Cadaveric musculoskeletal tissues must be pro-
cured within
67
62
] is believed to inactivate bacterial contami-
nants and HCV, but not HIV [
].
Gamma-irradiation, moreover, weakens mus-
culoskeletal allografts [
20
,
45
,
47
]. Lyophilization,
i.e., freeze-drying, is a process by which water
is removed from the tissue to the point where
cellular activity is no longer supported. This
process involves partially freezing the tissues
to allow sublimation of water, followed by
further drying with the aid of other techniques.
As a result, HIV and HCV are inactivated and
the risk of transmission is minimized in the
infected blood products and bone marrow [
14
,
44
].
This technique may, however, reduce the
strength of the musculoskeletal allografts [
53
hours of death, with the time
interval between death and refrigeration not to
exceed twelve hours. Harvesting of a musculo-
skeletal graft from a cadaver is performed in an
approved, aseptic environment. Musculoskele-
tal allografts can be categorized as (
24
14
,
44
] . W i t h p r o p e r s t o r a g e , f r e e z e - d r i e d a l l o g r a f t s
retain biological activity for several years.
Chemical sterilization with proprietary solu-
tions or ethylene oxide has also been used for
terminal sterilization. Adverse reactions, such
as moderate infl ammation from residual ethyl-
ene oxide in the allograft, have been reported
[
) bone
with soft-tissue attachments (such as a bone-
patellar tendon-bone allograft), (
1
) bone devoid
of soft-tissue attachments (such as a femoral
head), or (
2
) an isolated soft-tissue allograft
(such as a meniscus). After the tissue is har-
vested, the donor serum and allograft are cul-
tured for microbial contamination. The
allograft is then cleaned, soaked in an antisep-
tic solution such as BioCleanse (Regeneration
Technologies, Alachua, FL), and irrigated with
or without pressurized lavage or by ultrasonic/
mechanical cleansing techniques. The allograft
is then frozen and may be terminally sterilized
(described below). In some cases, freezing is
replaced by cryopreservation techniques to
retain cell viability and possible osteogenic
ability.
Freezing cannot substitute for sterilization
and at best may only prevent bacteria, fungi,
spores, or viruses from growing. As a result,
some tissue banks perform terminal bacteri-
cidal and virucidal sterilization that includes
heating, gamma-irradiation, chemical steril-
ization, and lyophilization. These procedures
further reduce the risk of infection and allo-
genic response by musculoskeletal tissues.
Some tissue banks routinely “pasteurize” or
autoclave allografts [
3
]. Proprietary solutions may contain
particular bactericidal, virucidal, and fungi-
cidal agents, but there is no industry-wide stan-
dard for their usage.
Allogenic bone can be machined and
separated into cortical, corticocancellous,
and cancellous preparations. Cortical and cor-
ticocancellous allografts are used for structural
support and have limited osteoconductive
capability, with no osteoinductive properties.
Cortical and corticocancellous bone grafts
undergo slow resorption in the host secondary
to limited vascular invasion; this decreases the
structural properties of the graft. The cortical/
corticocancellous allograft is incorporated by
the host through creeping substitution in con-
junction with slow bone remodeling. These
grafts are available in several forms: morsell-
ized “bone chips,” short segments of diaphy-
seal rings from femora or tibiae, iliac crest
bone wedges, cortical struts, and whole
bones en bloc, such as a fi bula. Large areas of
nonincorporated necrotic bone often remain in
a patient for years after implantation. Cancel-
lous allografts provide limited structural
support and osteoconductivity that can be
enhanced with demineralization. In the course
of bone remodeling, cancellous allografts
8
,
54
,
60
,
62
]; the resulting
increase in temperature eliminates the biologi-
cal activity of the cells, but may decrease the
strength of the grafts as a result of the denatur-
36
,
42
