Page 176 - Visions of the Future Chemistry and Life Science

Exploring human organs with computers 157

9.4 Bone and skin
Perhaps the most obvious biological application of finite-element model-
ling, given the popularity of the technique in mechanical engineering, is in
bone mechanics. The structural properties of bone are determined by non-
cellular organic and inorganic components. It is only these components that
are included in the simplest models. The potential exists to assess quantita-
tively an individual patient’s risk of bone fracture, which has significant
clinical implications in an ageing population. Currently, estimates of this
risk are limited by the inability to allow for complex structural features
within the bone. However, if the internal structure of a bone was determined
in vivo, using X-ray-based computed tomography, an accurate finite-element
model could be built to estimate the maximum load that can be borne before
fracture. Finite-element models can aid in surgical spine-stabilisation proce-
dures, thanks to their ability to cope well with the irregular geometry and
composite nature of the vertebrae and intervertebral discs.
The acellular structure of real bone is modified continuously accord-
ing to the internal stresses caused by applied loads. This process, which
represents an attempt to optimize the strength-to-weight ratio in a biolog-
ical structure, is achieved by the interaction between two types of cell, one
that absorbs bone and the other that synthesises new bone. New bone is
added where internal stresses are high, and bone is removed where stresses
are low. An accurate finite-element model of this combined process could
be used clinically to determine the course of traction that will maximise
bone strength after recovery from a fracture.
Another well-established area of mechanical finite-element analysis is
in the motion of the structures of the human middle ear (Figure 9.3). Of
particular interest are comparisons between the vibration pattern of the
eardrum, and the mode of vibration of the middle-ear bones under normal
and diseased conditions. Serious middle-ear infections and blows to the
head can cause partial or complete detachment of the bones, and can
restrict their motion. Draining of the middle ear, to remove these products,
is usually achieved by cutting a hole in the eardrum. This invariably results
in the formation of scar tissue. Finite-element models of the dynamic
motion of the eardrum can help in the determination of the best ways of
achieving drainage without affecting significantly the motion of the
eardrum. Finite-element models can also be used to optimise prostheses
when replacement of the middle-ear bones is necessary.