Bridges and bones, girders and groans
by Carl Wieland
Some years ago, while driving across the Sydney Harbour Bridge, my (then) small
daughter* asked me why the bridge was made
with all those funny poles and cross-cross things. Why not in one smooth piece?
I asked her to imagine beginning with a bridge of solid steel, strong enough so
that it wouldn’t buckle and collapse as cars drove over it. I pointed out
how heavy and expensive it would be. So we had to cut pieces out, in our imagination,
to make it lighter and cheaper. Which pieces would be the best ones to leave behind,
so as to stop it from crumbling? In time, playing with these ideas, the two of us
non-engineers began to see how and why trusses supporting garage roofs, for example,
could keep most of the strength of a solid, more heavy and expensive beam just by
‘eliminating the pieces’, in a sense, that were not actually acting
as braces against the load.
Several years later, owing to a major car accident, I was walking around with a
massive pin running right down the centre of my thigh bone (femur) (See Figure 1,
right). Because the fracture in that bone was not healing, all the weight of my
body was being supported by the pin, locked in place by sturdy horizontal screws
top and bottom. The metal in the pin and screws was the finest space-age steel alloy.
So why was the orthopedic surgeon advising yet another major operation to try to
get the bone to heal? After all, I was able to walk around. Why not just let the
massive steel rod carry my weight for the rest of my life? Surely man’s high-tech
metals are just as good as some old bone!
Trusses and braces
The surgeon well knew from experience that the finest metal would eventually fatigue
and give way in time—yet not so the average person’s bones. (In fact,
within a few months, signs of excess strain on the metal had already shown up on
X-ray. The amount of repetitive stress placed on the leg bones during walking is
remarkable.) What is it about bone that makes it so special, so incredibly strong,
yet light, and so resistant to stress and fatigue that it puts space-age metallurgy
to shame?
The X-ray (see original Creation magazine article; not possible to be reproduced
here) showed lots of denser (whiter) fine lines inside the bone substance. These
are like ‘braces’ inside the bone—areas of increased strength
for load-bearing like the criss-cross members in a truss, so that the remaining
areas can be lighter. Like our Harbour Bridge, this gives maximum strength for minimum
weight. The ‘braces’ in bones are placed so that they are exactly co-ordinated
with the lines of stress, the directions in which the weight is transmitted through
that bone. In itself, that is a beautiful example of clever engineering design in
bone. But there is more—much more!
The bridge that is continually rebuilding itself
If it were only a matter of clever engineering, man could design a similar structure
for a leg bone with all sorts of internal bracing, which would make it as light
as bone—able to bear the same load—at least at first. But even that
would wear out after several years. So why is it that an ordinary thigh bone (for
all practical purposes, and in the absence of diseases such as osteoporosis) will
never wear out like a metal structure?
The answer lies mainly in the fact that bone, a living structure, is continually
dismantling and rebuilding itself. It’s likely that the bones you now have
are not the same as you had 10 years ago! They have all been ‘removed and
replaced’, brick by brick, as it were. Certain cells in your body have the
job of devouring the old bone, while others lay down new bone in its place. Long
before any fatigued areas can ‘give way’, they will be replaced with
brand ‘new girders and trusses’. If that happened to the Sydney Harbour
Bridge, it would last forever. But the marvels of bone engineering do not stop there.
Not only rebuilding, but redesigning
Bones and bridges cannot be compared exactly from an engineering viewpoint. A bridge
always takes stresses along the same lines, between the same points, throughout
its lifetime. But the situation for the human body is different. Throughout their
lifetime, people change in the way their body weight is distributed (looked in the
mirror lately?). For instance, they may, because of arthritis or some disability,
change they way they walk and the exact way in which they put weight on the limb.
So when the lines of force transmission through the limb change so that the existing
‘girders’ or ‘braces’ are no longer in the right place,
why does bone not eventually fatigue? The fascinating answer is that the bone is
not only rebuilding itself, but redesigning itself as the lines of stress change.
Remember our imaginary Harbour Bridge that is continually replacing its girders?
Imagine that it was often being shifted on to different pylons and tilted at different
angles, for example, so that the areas that have the greatest stress are continually
somewhat different. Now we would find that replacing existing girders was not sufficient.
They must be put into new positions according to precise engineering principles.
Those that are no longer usefully bearing stress must be removed and replaced with
others at the correct angle. And that is exactly what happens in bone, incredible
as it may seem! Programmed in the DNA instructions that are in every cell of our
bodies is the marvellous capacity for our bones to continually remodel themselves
so that their internal engineering is always lined up so as to exactly cope, in
the most efficient possible way, with the precise forces acting upon them. In fact,
if the forces get larger (for example, a one-legged man who supports the weight
of his body on the one limb all the time) the bone will actually become thicker
and stronger.
Dissolving space men
This explains why weightlessness, which looks like such fun, is a major problem
for would-be space travellers. No weight means no stress on bone, so the body’s
mechanisms have nothing to ‘guide’ their construction. Old bone is still
being chewed up, but there is no way of knowing where the new ‘girders’
should be placed. The net result is that the bones tend to ‘dissolve’
and become porous.
Sloppy surgeons?
All of this also explains why a doctor setting a fracture doesn’t have to
be anywhere near as precise as you might think. Figure 3a (above) shows a broken
bone—let’s say that Figure 3b shows the same bone after the young intern
in casualty has had a go at getting it in the right position and has put plaster
on it. Along comes the senior bone specialist whose job it is to check the X-ray.
Does he say ‘hold it’ and demand that the two halves of this bone be
repositioned so that they are in a perfectly straight line and end-to-end? Not at
all, because he knows that this bone will heal (Figure 3c) and will in time ‘remodel’
itself in the way we have described (Figure 3d).
In the
first chapter of Romans, we are informed that men and women are ‘without
excuse’, since the evidence of God’s power and wisdom is all around
them in creation. How much more is this so in our age of tremendous advances in
knowledge, which have revealed ever more astonishing marvels of complexity and design
in the living world? The glory and honour of such engineering marvels do not belong
to ‘nature’, but to Jesus Christ the
Creator of all.
* Ed. note: both Dr Wieland’s daughters
are now married, and he is now a proud grandfather. Return to text.
(Available in Chinese (Simplified),
Chinese (Traditional) and Spanish.)
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