Biomedical Engineering Reference
In-Depth Information
table 14.2
effects of Changing bone's young
'
s modulus (e) and length and multiplying load on
maximum Principal Strains (mPS) at Push-off Phase of the gait
Iv: Change of bone
length while normal
walking load was
applied and e = 3,000
mPa
I: Change of e while
normal walking load was
applied
II: Change of axial loads,
while keeping
e = 15,000 mPa
III: Change of axial
loads, while keeping
e = 3,000 mPa
load
multiple
load
multiple
bone length
(mm)
e (mPa)
mPS
mPS
mPS
mPS
15000
0.09
1
0.09
1
0.38
180
0.37
9000
0.13%
2
0.18
2
0.79
200
0.38
3000
0.38
4
0.36
4
1.56
220
0.39
14.4 aPPlICatIonS
In this study, FE modeling was used to compare the effects of different loading cases, bone densities,
and limb lengths on stresses in the bone implanted with a transfemoral osseointegrated prosthesis.
Loading B, corresponding to the actual forces and moments measured by a triaxial load transducer,
produced less uniform stress distribution than loading A, with peak stresses being up to three times
higher than in loading A. This indicates that the rehabilitation program controlled by visual feed-
back from the weighing scale may in fact be producing much greater stresses than expected. These
higher stresses may cause intolerable pain, which typically delays the weight-bearing exercise and
rehabilitation program. Loading C represents a series of loads applied on the abutment during walk-
ing. Although the magnitudes of the peak stresses predicted in loading C were comparable to those
occurring when the subject was asked to transfer nearly the full body weight in loading B, high
stress occurred at different locations of the implanted region.
The lengthy rehabilitation program has been perceived by amputees and the clinical team as
a shortcoming. The results of this study imply that one possible way to refine the weight-bearing
exercise is to incorporate a triaxial load transducer, which provides feedback on three-dimensional
forces and moments to the patients and clinicians. At the early phase of the rehabilitation, the trans-
ducers can be used to increase the likelihood of forces being applied predominantly along the bone
axis. This distributes the stress evenly and reduces the magnitude of peak stresses. The uniformly
distributed stress allows the quality of the bone to be improved along the entire interface and the
lower stress magnitude will likely reduce the sensation of pain. Three-dimensional loads can then
be applied progressively in proportion to the loads that would be expected during walking and other
activities of daily living.
The model also suggests the importance of assessing the bone quality of amputees who are to
receive implanted prostheses prior to the implantation procedure. With a bone loss of 40% and the
application of a load that corresponds to four times the push-off loading, bone failure will occur.
This model simulates a realistic scenario. Previous studies have indicated that the density of the
bone of a transfemoral amputee drops by 40% after 6 months of using a socket prosthesis (Meerkin,
2000). The bone loss is due to disuse atrophy as the residual bone experiences lesser forces. An
increased force of up to four times normal loading would likely occur during an accident, such as
a fall. In daily activities like sit-to-stand, stair climbing, or squatting from a standing position, the
compressive loading at the knee joint can reach 2.5 times body weight.
The FE model has certain limitations. The geometry of a generic femur was used. Use of a
standardized femur has the advantage of allowing precise control over the implant-bone interface,
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