Biomedical Engineering Reference
In-Depth Information
Fig. 25.6
Full-field deformation maps of the native ACL and tissue engineered BLB explant dur-
ing the uniaxial loading (Ma et al.,
2012b
)
25.4 Results
Full-field deformation maps of the native ACL demonstrate that the native ACL is
inhomogeneous and functionally graded with the most compliant region near the
tibia insertion (Fig.
25.6
). The contralateral tissue engineered BLB explant shows
a similar full-filed deformation map. The micromechanical model proposed herein
captures the uniaxial load-unload responses of the native ACL and the engineered
BLB (Figs.
25.7
A and C). With the same sets of the parameters, the model predicts
the stress relaxation responses (Figs.
25.7
B and D).
The parameters used to predict the engineered BLB response are very similar to
those used to predict the native ACL response (Table
25.1
). A total of 7 parameters
are needed to capture these responses, but unlike the previous models discussed in
the Introduction, the current formulation is 3D and additional model parameters are
not needed to describe other deformation states.
25.5 Discussion
The conventional methods of strain measurement in soft tissues either record the
displacement output from the testing machine itself or add fiduciary markers on the
tissue surface as optical displacement trackers. Although gripping systems have be-
come more advanced, tissue slip in the grips is still difficult to avoid. Experiments
have shown that ligaments and tendons are highly nonlinear within their physiolog-
ical range, which is a strain level of less than 0
.
05. Therefore, the displacements
attributed to machine and grip compliances would largely affect the accuracy of
mechanical characterization of these tissues. Therefore, we have employed optical
strain measurements in our analysis. The ACL has a very complex geometry: the
ligament portion consists of multiple fiber bundles twisting together and the inser-
tions to the femur and tibia have irregular footprints. Using a speckle pattern and
digital image correlation methods described elsewhere (Ma et al.,
2012b
), we are
able to accurately capture the strain contours of the entire ACL surface. This infor-
mation is critical for 3D finite element implementation. The full-field deformation
