Biomedical Engineering Reference
In-Depth Information
Different cases of falling of transfemoral amputees with a conventional shaft
prosthetic were developed and analyzed. Based on the analysis of video sequences,
five particular scenarios of falling (Table
7.1
) were selected for the study. These
scenarios were repeated by a healthy subject (185 cm, 75kg) without the harness.
The subject was protected with hockey pads to prevent injury and the study design
was approved by the local ethics committee.
Forces and moments as determined per multi-body simulation about the antero-
posterior (ap), mediolateral (ml) and proximodistal (pd) axes are reported in the left
rigid femur segment 240 mm superior to the knee joint axis (Fig.
7.1
b). This location
was chosen in consultation with experienced orthopaedic surgeons as representative
of the height of above-knee amputations.
All trials for each scenario were averaged with the force peaks aligned at time
∼
0.1 s. The duration of the load peak was defined as the time interval in which the
load was
50% of the peak load to enable comparisons between scenarios.
The results showed that all forward falling scenarios displayed a distinct force
peak, but not the backward falling scenario. For S1, falling from a stationary standing
position with ground contact on both knees, a mean peak force of 2014
>
±
528N over
11
2ms was observed. For S2, from the same position but with ground contact on
one knee, the mean peak force was higher (2614
±
±
346N) and the mean duration was
longer (35
21ms). The largest internal force in all five scenarios were observed in
G1, falling on one knee from normal gait (Fig.
7.4
). The mean peak force was 3274
±
±
519N over a loading period of 35
±
21ms with a resulting moment of 176
±
55Nm
±
over a loading period of 267
150ms. The largest component of the resultant force
was directed axially along the bone. The mean peak force for G2 was 2234
±
659N
with a mean peak moment of 109
37Nm. The reduced force compared with G1
was due to the support of the contralateral leg. The lowest peak force of 1099
±
±
269N was observed for the only backwards-falling scenario, G3. Conversely, the
largest peak moment, 187
177Nm, was observed for this scenario (Fig.
7.4
).
According to the load limits reported to be 190Nm in a related study of bone
failure of osseointegrated prosthesis fixation [
80
], the findings of this study suggest
the possibility of fracture of the affected leg in every investigated scenario. Further
work is warranted to determine the load magnitudes that will cause damage to the
bone-implant interface.
For the first time, the bending moments for cases of unsupported falling that
could be relevant for the design of multi-axial safety elements has been reported.
These findings will be of interest in combination with finite element (FE) analysis
to determine real stresses in the osseointegrated bone to allow conclusions about
location and risk of fracture under high loading situations.
±
