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

Cancellous
bone
Acrylic bone
cement
Cortical bone
pressure
Thermal and
mechanical symmetry
heat convection
Fig. 2: Material regions of the finite element mesh (left) and double symmetric finite
element model including boundary conditions (right)
By the help of the described FE-model, an exemplary investigation of the thermal
behaviour of the curing process and a subsequent mechanical analysis of a uniform
loading condition is examined. In the first load step the curing process of acrylic bone
cement right after its injection into the vertebral body is explored. Within this load step
no mechanical loads are applied and the bone cement region is assigned with an initial
temperature of θ0 = 22°C.
The thermo-mechanical simulation of the first load step results in the following
behaviour: Due to the exothermal chemical reaction of the acrylic bone cement, heat
gets dissipated and leads to an increasing temperature. Since heat flow is allowed to
ambient regions, the peak values of temperature decrease over the time until a uniform
temperature of 37°C is reached again. The temperature evolution over time has been
plotted for different locations in Fig. 3 (left). The figure shows that the temperature in
the centre of the bone cement reaches values of about 60°C. However, temperature
values beyond the region of bone cement just slightly rises over 37°C. This behaviour is
also confirmed by the temperature field plot on the geometry of Fig. 3 (right).
Fig. 3: Temperature evolution at different locations (left) and contour plot of the
temperature field at the time t = 5 min (right)
Within a second load step, a mechanical load has been applied to the upper shell of the
vertebral body. In order to achieve a uniform distribution of the loading, a pressure of
p = 0.5 N/mm 2 has been applied to the upper cortical shell. This corresponds to a
resulting total force of F ≈ 700N in vertical direction. The resulting vertical
displacements of the treated vertebral body are compared with those of an untreated
one. Fig. 4 shows the contour plots of the displacements for both cases. Due to the
stiffening of the core of the vertebral body treated by vertebroplasty, lower values of the
displacements are achieved for the treated vertebral body. As an additional result,
differing stress distributions between both models have been observed. This can be
1
3
2