Biomedical Engineering Reference
In-Depth Information
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Galt cycle (%)
(a) Patellar tendon
Galt cycle (%)
(b) Popliteal depression
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(d) Anteromedial tibia
(c) Anterolateral tibia
FIgure 13.7
Peak resultant shear stresses on residual during the whole gait cycle (the finite element model
considered the pre-stresses after donning the prosthetic socket).
The real procedure of socket donning was not simulated in this investigation. Instead, the final
deformation state of the limb immediately after donning into a socket was studied. Simulation of
socket donning is challenging because donning involves wiggling the limb into the socket and these
large relative motions are difficult to define. If the wiggling motions are not simulated and the limb
is forced straight into the socket, severe distortions of the limb could happen due to the complicated
shape of the socket. In addition, large sliding actions between the limb and the socket require large
computational resources.
Three main factors, the geometry of the models, material properties, and loading/boundary
conditions, determine the accuracy of an FE model. In this model, the accuracy of the geometric
representation was assumed within a tolerable level as the geometries of the residual limb (with
bones) and the prosthetic socket were obtained from MRI images and a prosthetic digitizer, respec-
tively. However, the temporal variation in the residual limb volume and shape was not consid-
ered. Previous studies have shown that when the prosthesis is not worn for 30 minutes or more the
measured interface stress can change significantly (Zachariah and Sanders, 1996; Silver-Thorn and
Childress, 1997). This suggested drastic changes in shape and volume may occur in the residual
limb, particularly when unloaded.
The residual limb was assumed to be linearly elastic, nonviscoelastic, homogeneous, and iso-
tropic. Better characterization of the material properties of different locations of the residual limb,
considering time dependency and loading direction as well as nonlinear material properties of the
soft tissue, is desired. In addition, it was assumed that the soft tissue was a passive structure. In
reality, the muscles, particularly at the posterior cuff of the residual limb, would have some degree
of contraction during walking. Muscle contractions, leading to stiffness changes at different regions
of the limb, could alter the stress distribution at the limb-socket interface. Little is known about
the effect of muscle contractions on interface stresses. The difference in interface stress between a
passive soft tissue structure and a soft tissue with muscle contraction deserves further investigation.
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