ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

4. RESULTS AND DISCUSSION
Convergence was achieved after the 20 th iteration with all femoral strains within
physiological limits, in agreement with Phillips [9]. The predicted hip and knee contact
forces were in the range of the forces measured by Bergmann [17] and D’Lima [19],
respectively, for all activities modelled.
Iteration Number
% Walking (red) Upstairs (green) Downstairs (blue) Influence
Frame 1* 2 3 4 5* 6 7 8* 9
Position
1 24.92 1.45 1.57 9.43 0.52 36.31 10.88 6.69 8.25
2 20.26 1.20 1.36 13.21 0.25 48.87 6.53 2.61 5.71
3 17.16 1.22 1.82 12.20 0.29 54.61 5.28 2.57 4.85
4 18.66 1.56 1.62 13.06 0.28 54.22 4.41 1.53 4.66
5 16.91 1.36 2.06 12.03 0.31 56.07 4.99 1.79 4.47
10 17.77 1.49 1.56 12.6 0.33 56.15 4.54 1.35 4.2
20 16.43 1.39 2.01 12.54 0.41 57.75 4.09 1.57 3.8
Table 1 – Influence (%) of different frames in the material properties distribution for the femur for
different iterations; anterior/posterior view of the influence for the last iteration (right). * - peak frame
Table 1 shows the percentage of influence of each frame modelled in the material
properties calculated for four stages of the iterative process and their topological
influence for the last iteration (right). A frame was considered to influence the element’s
material properties when it produced the maximum absolute value of 11, 22 or 33. It
can be observed that different loading configurations influence the optimisation process
in different regions. All three daily activities have an impact in the optimised structure,
particularly in the proximal femur. Going upstairs seems to be the load case
predominantly driving the adaptation process in the distal half of the femur, apart from
the anterior side of the condyles, where walking is dominant. This might result from the
joint action of high hip and knee contact forces, muscle forces and the tibio-femoral
flexion angle. Table 1 also highlights the importance of considering multiple instances
of the activity load cycles. It is shown that consideration of only the frames where peak
contact forces have been measured does not create an envelope that represents the full
driving stimulus of the adaptation process [6, 8]. Table 1 also highlights that the
consideration of multiple activity load cycles is critical in bone optimisation simulation
and that an attempt to model more frames for each load case and more extreme load
cases will result in a more accurate stimulus distribution. Activities with high flexion
angles such as sit-to-stand might play an important role in producing accurate models of
material properties distributions, particularly for the distal part of the femur. However,
some modifications have to be implemented to the model in order to simulate such
activities, such as the inclusion of the possibility of muscle wrapping.