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

instead the femur was supported by a system of muscles (18) and ligaments (5)
modelled as simple connector elements. Each point not lying on the femur was assumed
to be fixed at its origin in space (Fig. 1b and 1d).
Muscle and ligament origin/insertion points for the lower limb were obtained from a
recent study [14]. These points were then scaled to suite the geometry of the composite
femur using techniques described by brand et al 1982 [15]. Due to size and location
some muscles had to be represented with more than one origin point in space (e.g.
Gluteus maximus). A number of beam elements were also employed with a high
stiffness in compression to facilitate the transfer of the patella-femoral force from the
muscles and ligaments to the distal femur. A linear elastic force-displacement
relationship was considered for each connector element in this study, with muscle
stiffness values being determined for each connector element based on available data
from a previous study [7].
4. RESULT
a b c d
Figure 1: Restrained (a, c) and musculoskeletal boundary conditions (b, d) for
intact femur and TS implanted femur with 4mm lateral offset.
To investigate the influence of FE boundary conditions on the stresses in the intact
femur a number of transverse sections at 50mm intervals were taken along the shaft of
both models (Fig. 2). These transverse sections showed that the femur restrained at the
mid-shaft was subject to significantly higher cortical stresses (particularly in regions 3-4
close to the fixed boundary) when compared to the femur supported by muscle and
ligaments structures, which had a much more uniform stress distribution along the shaft.
In contrast the stress distribution in the distal femur was observed to be very similar for
both models suggesting that the fixed boundary conditions have minimal influence in
regions close to the point of load application. Cancellous bone stresses were also found
to be quite similar for both models, due to the majority of the cancellous structure being
located in the distal most region of the femur.