Biomedical Engineering Reference
In-Depth Information
Table 25.1
Model parameter comparison between native ACL and engineered BLB
Tissue type
MacKintosh spring
Fast spring
Fast dashpot
Slow spring
Slow dashpot
nkΘ
A
L
c
l
p
nkΘ
B
η
B
nkΘ
C
η
C
(kPa)
(-)
(-)
(kPa)
(kPa/s)
(kPa)
(kPa/s)
Native ACL
5000
12
.
5
8
5000
1000
8000
280000
Engineered BLB
5000
12
.
4
8
3500
1000
2000
150000
tutive model are in agreement with the previous biological and mechanical studies.
The model parameters used to capture the native ACL and engineered BLB response
show the three parameters for the MacKintosh network are indeed very similar, indi-
cating the engineered BLBs have remodeled in vivo and have developed mechanical
properties that are very similar to that of the native ACL. The BLB parameters in-
dicate a more compliant response than do those of the ACL, indicating a longer
recovery time may be needed for our BLBs to fully develop to native ACLs.
The seven model parameters needed to capture the nonlinear response of these
tissues are intended to capture the full 3D response of these tissues, however they as-
sume initial isotropy. The modeling framework does allow for initial anisotropy, as
seen in Fig.
25.2
, without adding several more parameters. A transversely isotropic
ligament or tendon would incur an additional parameter, which is the ratio of the rep-
resentative volume element dimensions in the direction of the fibers versus perpen-
dicular to them. In ongoing work we intend to implement this model into a commer-
cially available finite element program to simulate the complicated inhomogeneous
response of the knee during an anterior tibial translation, the motion that results in
ACL failure. We will assess the initial anisotropy requirements needed to capture
this response.
25.6 Conclusions
Experimental results show the mechanical response of ligaments and tendons is non-
linear, viscoelastic and functionally graded. Moreover, engineered ligaments used as
an ACL replacement rapidly develop in vivo to obtain similar structure and function
to native ACL. Our micromechanical computational model of connective tissue has
been used to explore the rich mechanical response of native and engineered liga-
ments from a microstructural point of view and is in agreement with the findings in
histological studies.
Acknowledgements
We thank MICHR and Coulter Foundation for their generous financial sup-
port.
