Bone has two parts; a hard part (measurable as bone density) made of minerals, mostly calcium and phosphate, and a softer, flexible part made of a protein called collagen.

Bone is a living structure that provides strength and resilience so the skeleton can absorb impact without breaking.

Let's start with some show-and-tell.

Don't worry about all the big words - you only need to know a few of them - I'll let you know what they are.

The words aren't nearly as important as getting the concepts, the big picture.

There are two major parts to bone.

• One is the hard, outer-shell called compact or cortical bone, or the cortex.

• The second is the softer, spongy middle called trabecular bone.

For now, as we learn about osteoporosis, we only need to deal with the second one.

Trabecular bone is naturally hollowed out so there can be room to perform other functions in addition to holding us up and having a place to hook muscles to.

Notice all the holes and hollow spaces in the picture.

Healthy Bones Are Like A Subway System

For example, there are tubes or canals running all through trabecular bone - just like the tubes and tunnels in a subway system.

Also, there are also holes and cavities... these holes and cavities are like the platforms at the subway stations - the tunnels interconnect all the platforms.

In other words, these tubes, holes, and cavities (the hollow spaces) are made up of the trabecular bone itself.

This is what we are interested in. We need walls, platforms, and ceilings to make up the tubes, holes, and cavities.

Plus, we need pillars and supports. All of this must be structurally sound and strong in order to keep the roof from caving in and the walls from falling down.

The Tunnels

The tunnels have three major functions.

• Tunnels carry supplies into the stations (the arteries)

• They carry trash out of the stations (the veins)

• They carry important messages to and from the stations (the nerve system)

You can see the veins and arteries (blue and red) running through the canals in the picture of bone above.

A very important side note:

• These blood vessels carry calcium in, and they carry calcium out, as well.

• If the pH of the bloodstream is a little too acid - as is found after drinking a dark cola, or having a high-protein meal - the bloodstream washes calcium away from the lining of the tunnels.

• The calcium is taken from the bone in order to buffer the acidity, so a normal pH balance can be restored.

• Over time, these tiny losses of calcium add up to the major bone loss of osteopenia, and osteoporosis.

And although the nerve system controls every thing that happens in the body - including what happens in the bone - the nerves are really too small to see, and are not shown in the picture.

Nerves carry the important messages into and out of the stations, instructions, status reports, etc, and generally tell things when to switch on or off.

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The Platforms

The platforms are where the action is. Platforms are where things are stored and built, like marrow and blood cells. Also in the platforms are workers to keep things clean and functioning properly.

The Pillars

The pillars need to be strong enough to support the weight and stress loads of everything above, plus some extra to be ready for surprises like earthquakes.

Well, not an earthquake really, but our bones need to be strong enough to hold up under normal activities, such as being able to bear the additional weight and stress that occurs when you jump from a high step down to the floor.

It takes something out-of-the-ordinary to create a failure of the design - something like a fall or other injury is required for a fracture to occur.

In the case a acute injury such as a fall the stress overload on bone is sudden and unexpected - like this:

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In the case of a chronic health condition, like low bone density and osteoporosis, it is the slow breakdown of the internal structure that causes weakness.

Like the subway pillar supporting the ceiling in the picture below, there can be a slow erosion taking place, reducing the pillar's ability to do its job.

Bone density doesn't change in one day,
it happens over time.

There are two things that can happen:

• If left alone, the pillar will eventually crumble and and the ceiling will collapse.

• If the maintenance crew is given the time, tools, and materials, it can repair the structure, allowing it to last many more years without a problem.

But wait, just a minute... although there is some corrosion on the subway pillar, it hardly looks like it is ready to crumble and fall apart...

hmmm... maybe that is like osteopenia - mild bone loss. Low bone density, perhaps, not quite perfect, not like when it was at its peak - but still structurally sound. Yes, it needs a little attention, some bone density improvement, but this is nothing to be afraid of.

Osteoporosis and Osteopenia

There is another page on this site with some detail about low bone density, but this seems to be a good place for a brief definition and a picture.

Remember, we began looking for normal; now lets look at what is abnormal.

First, let's find out what the words mean.

From the Greek origins of the words:

• Normal - means - met the standard

• Osteo - means - normal bone

• Penia - means - reduction, deficiency, less than normal

• Porosis - means - porous or full of holes

• Holes - means - empty space

So,

• Osteopenia means Bone reduction or deficiency - low bone density

• Osteoporosis means Bone full of holes - lower bone density

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This picture from the Surgeon General's report on Bone Health and Osteoporosis, taken from a classic study reported in the Journal of Bone and Mineral Research in 1986 demonstrates rather extreme low bone density very well:

This is classic study showing normal bone structure on the left, and severe osteoporosis on the right.

The word 'severe' is added only in the presence of fracture.

I first read this study around 1990 or so, and if I recall correctly the 'Normal' on the left is of a lady who was, I believe, 76 years old; and the osteoporosis on the right was of a lady who was 47 years old.

In the example on the right, note the bridges that fail to reach the other side; and the micro-fracture exactly in the middle.

If there is to be any fear about osteoporosis, I would think this is it.

This would demonstrate what we imagine as the condition of weak, fragile, delicate, brittle bone that we want to avoid.

I can't find a photo of Osteopenia, but you can imagine it would be somewhere in the middle closer to 'Normal' and far from 'Severe'.

These 1986 study is interesting for a three reasons:

• First, it shows us clearly what fragility fracture means.

• Second, it shows us that osteoporosis is not necessarly an age-related condition as we used to be taught.

• Third, it demonstrates that we have known this for more than 20 years.

This is a common problem in medicine - keeping up with current information. The Surgeon General writes "Too little of what has been learned thus far about bone health has been applied in practice."

Doctors continue to advise patients with twenty or thirty year-old information.

Anyway, with osteopenia, and osteoporosis, your bone density is lighter, less substantial, not as sound or dense as we would expect from a young normal (25 to 35 year old, T-Score normal) woman or man.

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Let's end with this:

• Our bodies innately know how to design, manufacture, and maintain the skeletal system and structure.

• The maintenance system works so well, that the only time we have a problem is when something unusual happens - an acute injury.

• It is only when the structure is not maintained that chronic problems, bone density changes such as osteoporosis, can occur.

• With proper maintenance, the skeletal sysem and every part of it can last for decades - after all, it was designed to do so. Bone density need not be an issue.

Our job is to make sure we provide the necessary ingredients to the maintenance crew - water, good nutrition, regular exercise - so they can keep the system - the skeletal system - running smoothly.

That includes adequate bone density, and flexibility, both minerals and collagen.

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