Medicine For People!

February 2010

Your Bones – What Gives?

When I was a child, people never sat down to a meal without a glass of milk. All our lives we were told to drink our milk because we needed calcium for strong bones. Women are still advised to take calcium supplements to shore up their bones and prevent fractures and bent spines.

So naturally, when a test comes along to measure the calcium in our bones, we take it seriously. The test is called a DEXA scan. DEXA stands for Dual Energy X-ray Absorptiometry. To the dismay of many women, despite years of drinking milk and taking calcium supplements, the test tells them their bones lack calcium and they are at high risk for a fracture! What gives?

Well, not your bones, I hope.

This newsletter begins a short series on bone health, which will cover DEXA scanning and treatment of osteopenia and osteoporosis. In writing about bones, I will also be talking about other bodily systems. The crucial point is that our bones are not simply hunks of calcium. They are complex, alive, and ever changing. Since the DEXA scan primarily measures calcium (along with trace amounts of other minerals), the test can be misleading as we'll discover next month.

A Bone is More Than A Mineral

Figure 1: http://kcfac.kilgore.edu/kcap1/hip%20joint%201.jpg

Here is a picture of dried up old skeleton. This one has been wired together for display, but it's not much different from one you might find in the desert, the remains of a long-dead bandito. Separated from a living human body, these bones have become more mineral than animal. They are made from calcium, the same mineral as in limestone. Imagine though, that your femur was no more than a piece of limestone. Would it support your weight? Could you do jumping jacks on it? Not hardly – it would crumble. The strength of your bones comes not just from calcium but also from the microscopic structure, which the next diagram will reveal.

The Secrets of Structure

Let's start with this diagram of an entire bone. Ignore the labels for now, and scroll down to detail view and parts list below.

Figure 2: http://en.wikipedia.org/wiki/Bone

Figure 3: http://en.wikipedia.org/wiki/Cancellous_bone

Here is a cross-section of that bone, a long bone such as the femur. It is made up of several different sub-structures.

The periosteum: a tough layer of fibrous tissue encasing the hard bone, with its own nerves and blood vessels.

Compact bone: Also called the cortex, the quarter-inch-thick outer layer of the bone is what we usually think about when we imagine a bone. Compact bone carries most of the load.

Spongy bone: Inside the cortex resides the softer more central part of the bone, called the spongy, cancellous or trabecular bone.

Marrow cavity: Medullary cavity in doctor-speak, this structure contains tissues that create blood cells, immune cells, and platelets (for blood clotting), or it may contain fat.

You can read more about bones in the Wikipedia article cited above. [http://en.wikipedia.org/wiki/Bone]

Bones as Engineering Marvels

Bones, like medieval cathedrals, show a masterful balance between stresses and structure.

Figure 4: http://www.biomedical-engineering-online.com/content/5/1/12/figure/F3?highres=y

Here is an engineering analysis of the forces on that femur while standing. The blue lines show the lines of compressive force as the femur transfers the weight of your upper body down to the lower part of the bone and toward the ground. If an engineer were to replace the bone with cathedral arches, this is the shape they would take. The red lines of tension show the stretching of the outer part of the bone required to carry the load. Were the bone a bridge, these lines of tension would correspond to steel girder or cable bracing, required to prevent the masonry from pulling apart. Not illustrated in figure 3 above is the tough connective tissue permeating the microstructure of the bone, aiding in withstanding tensile stress.

Here's a drawing of an actual femur, once again showing the strong outer cortex (light brown below) and a mesh-work of supporting structure inside, the spongy trabeculae (in black, below.) The trabeculae, like supportive beams inside a cathedral dome carry the loads pictured above in blue and red.

Figure 5: http://www.itmonline.org/arts/femur.htm

Spongy trabecular bone grows in patterns that match the patterns of stress on the bone. Stressed bones create electric fields (a piezoelectric effect, such as used to create light flashes in children's shoes). Stimulated by these electric fields, bony replacement tends to occur along these field lines, 1 strengthening the bone as a natural consequence of physical exercise. A major problem with space flight is that without gravity, astronaut's bones weaken at a rapid rate. 2 Astronauts must stress their bones in space to prevent this.

This balance of stress and structure would not work without flexibility. Just as airplane wings are designed to bend, just as skyscrapers have some built-in give to allow them to withstand wind gust, healthy bones also have elasticity. This elasticity allows bone to withstand shocks and carry loads without breaking.

Bone strength, therefore, depends upon elasticity, microscopic structure, protein reinforcement within the bone, and the massive outer cortex supported by delicate and precisely arranged inner bracing, the trabeculae.

Soft Tissue Supports the Bones

Surrounding tissue contribute to bone strength. Muscles help the ligaments hold our bones in place, and the stronger and better balanced these muscular forces, the more protection the bones have against fracture. Even fat plays a role. While a thin person might have just a half-inch of fat and soft tissue covering the outside of her hip bone, a heavy person might have two inches or more of cushioning to rely on if he falls.

The Role of Nerves

Nerves critically support the skeleton by allowing us to move correctly. When the nerves to the foot are intact, constant feedback that you are not aware of governs the muscles that operate the foot. This feedback moves the foot and plants it on the ground in a way that limits stress on the bones. Diabetes can destroy nerves, especially the long ones serving the feet, and people with diabetes frequently develop numbness of the foot. When the numbness extends to the ankle, the person can develop a "Charcot joint"- a malformed joint resulting from excessive force on the bones. Without nerve feedback, the person may think he is walking normally, but those subtle protective adjustments are not made, and the bones of the foot are subtly pounded into the terrible deformity known as Charcot joint. 3

Nerves wear out with age. We more often see younger women who fall reflexively break the fall with their arm and hand, resulting in a broken wrist. An older woman's nervous system may not respond as well, so that she falls onto her hip instead, risking a fracture.

Bones are All This and More

Bones are not passive shapes of calcium. They are living, vital organs that manufacture blood and immune system components as well as help regulate acid-base status among many other metabolic functions. They can serve you long and well when you maintain good health habits. You can take control with thoughtful nutrition, vigorous regular exercise, and an understanding of how bones carry out their task.

Coming Months

  • Aging Bones
    • We will review a DEXA report and analyze the limitations of this particular type of scan.
    • We will give you tools to estimate your risk of osteoporotic fracture.
    • We will discuss pharmaceuticals and other measures to strengthen bone and reduce fracture.
  • Multiple vitamins
  • Colon cancer screening

Endnotes:

2 Bone loss during long term space flight is prevented by the application of a short term impulsive mechanical stimulus. Goodship AE, Cunningham JL, Oganov V, Darling J, Miles AW, Owen GW. Acta Astronaut. 1998 Aug-Sep;43(3-6):65-75.

Royal Veterinary College, University College London.

In long term space flight, the mechanical forces applied to the skeleton are substantially reduced and are altered in character. This reduced skeletal loading results in a reduction in bone mass. Exercise techniques currently used in space can maintain muscle mass but the mechanical stimulus provided by this exercise does not prevent bone loss. By applying an external impulsive load for a short period each day, which is intended to mimic the heel strike transient, to the lower limb of an astronaut during a long term space flight (5 months), this study tests the hypothesis that the bone cells can be activated by an appropriate external mechanical stimulus to maintain bone mass throughout prolonged periods of weightlessness. A mechanical loading device was developed to produce a loading of the os-calcis similar to that observed during the heel strike transient. The device is activated by the astronaut to provide a transient load to the heel of one leg whilst providing an equivalent exercising load to the other leg. During the EUROMIR95 mission on the MIR space station, an astronaut used this device for a short period daily throughout the duration of the mission. Pre- and post-flight measurements of bone mineral density (BMD) of the os-calcis and femoral neck of the astronaut were made to determine the efficacy of the device in preventing loss of bone mineral during the mission. On the os-calcis which received the mechanical stimulus, BMD was maintained throughout the period of the flight, while it was reduced by up to 7% on the os-calcis which received no stimulus. Post-flight, BMD in both the stimulated and non-stimulated os-calcis reduces, the extent of this reduction however is less in the stimulated os-calcis. For the femoral neck, the mechanical stimulation does not produce a positive effect.