Biomedical Engineering Reference
In-Depth Information
3.5 Effects of Processing on
Biomechanical Properties of
Musculoskeletal Allografts
culoskeletal allografts is comparable to that
from blood transfusion [
]. Terminal steriliza-
tion by gamma-irradiation or lyophilization of
musculoskeletal tissues can further diminish
HIV and HCV infection rates, but at the cost
of a decrease in the mechanical properties of
the allograft. Until more tissue-screening tests
become available, active surveillance and audit
of stored nonimplanted allografts will provide
further assurance for the quality control of
musculoskeletal transplants.
Using fresh, fi bular strut or femoral ring cor-
tical bone allografts from younger, nonosteo-
porotic donors permits the surgeon to maximize
the structural integrity of reconstructive pro-
cedures [
10
Gamma-irradiation and lyophilization (freeze-
drying), two commonly used techniques for
terminal sterilization of allografts, lead to
weakening of grafts (Table
3
.
3
). Gamma-irra-
diation of at least
megarads is required to
inactivate viruses, whereas
3
.
0
1
.
5
to
2
.
5
megarads
can inactivate bacteria [
]. Radiation
dose weakens the biomechanical properties of
musculoskeletal allografts. Gamma-irradia-
tion at virucidal levels signifi cantly increases
fatigue while decreasing failure strength,
failure energy, and stiffness [
20
,
45
,
47
]. The osteoin-
ductive ability of bone allografts can be
maximized by selecting demineralized, freeze-
dried grafts from younger donors [
11
,
15
,
17
,
39
,
46
,
52
,
55
5
,
13
,
19
,
23
,
45
,
54
,
].
In the future, new methods or modifi cations
of existing tissue-processing techniques may
be developed to maximize the osteoinductive
and osteoconductive properties of bone
allografts. Possible approaches to augment
bone allograft performance may include the
use of bone morphogenetic proteins (BMPs) or
other local or systemic mediators of growth
and infl ammation. For example, the use of
structural cortical bone allografts with osteo-
conductive and structural capabilities could
add to the osteoinductive ability of the graft
[
37
,
50
68
]. Lyophilization reduces screw pullout
strength and the maximum limits of strength,
torque, and torsional stiffness and diminishes
absorption energy [
]. Lyophilized
allografts need to be rehydrated before trans-
plantation; the quality of rehydration can also
affect the mechanical parameters [
28
,
41
,
54
].
Terminal sterilization with ethylene oxide does
not signifi cantly weaken screw pullout strength
[
28
,
41
,
61
]. Musculoskeletal tissues subjected to
boiling or autoclaving exhibit signifi cant reduc-
tions in strength, but freezing does not reduce
their strength [
54
7
].
]. Although promising, the use of
BMPs in conjunction with allografts needs
further study.
25
,
40
,
56
3.6 Conclusions
Musculoskeletal allografts are an alternative to
autografts without the associated morbidities
[
References
]. Allografts are widely available
in a variety of preparations, and their trans-
plantation is a safe, comprehensively regulated
practice with a low incidence of HIV [
6
,
18
,
30
,
35
,
59
1
. Hepatitis C virus transmission from an antibody-
negative organ and tissue donor-United States,
2000
-
2002
(
2003
) MMWR
53
:
273
-
276
.
],
HCV, and bacterial infection. The risk of infec-
tion is further decreased when musculoskeletal
allografts are obtained from AATB-accredited
tissue banks that practice comprehensive
donor screening and tighter tissue procure-
ment and employ more testing than required
by the FDA [
2
,
32
,
53
2
. Transmission of HIV through bone transplantation:
case report and public health recommendations (
1988
)
MMWR
37
:
597
-
599
.
3
. United States Census Bureau, Statistical Abstract of
the United States (
2001
) No.
168
Organ Transplants
and Grafts,
1990
to
2000
.
4
. Update: allograft-associated bacterial infections-
United States (
2002
) MMWR
51
:
207
-
210
.
]. The use of tissue banks that
perform PCR analysis and/or histomorpho-
metric testing of donor tissues further mini-
mizes the risk of viral or bacterial transmission
[
67
5
. Anderson MJ, Keyak JH, Skinner HB (
) Com-
pressive mechanical properties of human cancellous
bone after gamma irradiation. J Bone Joint Surg Am
74
1992
:
747
-
752
.
6
. Banwart JC, Asher MA, Hassanein RS (
) Iliac crest
bone graft harvest donor site morbidity. A statistical
evaluation. Spine
1995
]. The risk of HIV infection from
fresh-frozen, non-terminally-sterilized mus-
47
,
48
,
51
,
58
20
:
1055
-
1060
.
