Bone Structure and Growth

Anyone who has owned something made of fibreglass appreciates the advantages of complex materials and the deliberate arrangement of the fibrous and crystaline components to reflect expected stresses. The trabecular structure of the upper metaphysis of the femur is a good example.

Victorian engineers have found that hollow iron bridge piers resist bending stress while reducing their weight. If you roll-up a sheet of paper into a tube and compress it with your hands, you can model the failure of a simple tubular structure. This failure is resisted better, if the tube were filled with material, which need not be stiff, even (very) compressed air would resist the formation of those crumpling waves that preceed collapse (Victorian engineers found it easier to use brick or rubble). Cancellous bone can support more complex structures, while marrow in the spaces can perform both a mechanical and blood-forming function, all this while reducing the required amount of structural material.

Most students are familiar with the Haversian systems of concentric rings of dense bone matrix around a central canal. The bone structure, thick cortex or small separate trabecula, is fabricated from these adjacent columns of bone. Numerous micro-channels cross the concentric ring towards each central canal. Researchers have found that dentine or cortical bone with these fine micro-channels has greater resistance to impact forces than a solid material from the same compounds. It is thought that the forced movement of 'fluid' through the micro-channels reduces the impact, converting the stress from a square-wave to a less damaging rounded waveform and matching the force, within the structure, better to the elasticity of the material.
Incidentally, the shear through those micro-channels may strip electrons from components of the 'fluid', providing a stimulus to the osteoclasts, additional to the piezo-electric potentials from crystaline distortion and providing another argument for some impact forces in the prevention of osteoporosis.

There is nothing profound in the conclusion that the geometry of bone will affect its strength and response to disease. Do you remember the party trick with a hen's egg?
The congoscenti will compress the egg in their hands at its rounded ends. Those who compress the egg across its broad curves will get egg on their hands and figuratively, on their faces.
The underlying principle is that bones, of the same material and similar microstructure, bearing on each other, will resist the same stress better if their bearing-surface has a smaller radius of curvature and will develop fewer microfractures.

Makers of prosthetic bones in the young are continually reminded of the problems of maintaining a structural supportive function, while increasing the size of the bone. You may face similar problems, if you want to change the foundation of your house. Texans with their mixture of clay and sand provide much work for engineers, who jack-up the houses to work underneath.

Another way might be to raise the house with a bag of water, but the instability of fluid-filled structures would soon make for an expensive new home replacement. If we fill the bag with quick-setting cement and fill the bag at the top, it might be possible to gradually raise the house, while also building-up the foundations.

Cartilage performs this function in the growing bone and the mechanism is similar to those tree roots that may elevate the roadway near your home.

The eiphyseal plate separates the epiphysis from the immature bone. Smaller molecules are converted into larger ones and these accumulate within an enlarging polymer, including some fluid. The force for expansion of the polymer comes from osmotic difference in the degrees of freedom of newly fabricated large molecules inside the cartilage and smaller molecules outside. Fortunately for the model, real life involves intermittent, rather than continuous forces. The cartilage accumulates and is converted to bone at the junction with the metaphysis. Predictably, the mechanical strength is not ideal and bone injuries in the young frequently involve the epiphyseal plate and this provisional newbone at the junction with the metaphysis.

I have simplified the drawing, returning the last image to its original appearance in a longer bone to indicate the beginning of the next 'incremental' stage of cartilage growth. In real-life, cartilage and bone growth are continuous, but may have periods of greater or lesser activity, growth spurts.

Cartilage is avascular; this imposes asymmetry on the blood supply to bone ends and epiphyses. The combination of two materials and non-uniform perfusion means that growing bones respond differently to trauma.