Bridges and bones, girders and groans
Some decades ago, when my daughters were both still small, we were driving across the Sydney Harbour Bridge. One of them asked me why the bridge was made with all those funny poles and criss-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 large, strong, steel rod carry my weight for the rest of my life? Surely man’s high-tech metals are just as good as some bit of 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?
If you look closely at the X-ray of human bone here (not mine) you can see 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 the 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.
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 2a (right) shows a broken bone—let’s say that Figure 2b 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 2c) and will in time ‘remodel’ itself in the way we have described (Figure 2d).
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.
Re-featured on homepage: 20 June 2023