Biological Architecture

Your bones bear loads like a building, but it's more complicated because you move and the loads change. Like a building, your body tries to be efficient and not waste strength where it's not needed. Bones are the right shape for their load, and even their internal microstructure is efficient of strength. If you look closely at slices of bone, you can see bone is of two types: Cortical, solid bone, and Trabecular, a porous rigid 3-D framework of bone, filled with marrow, as shown in these magnified images. The trabecular stuff is found in the spine and at joints. Your big leg bones are trabecular at their ends and cortical in the middle. All bone is full of living cells, and it can heal and adapt to its environment. In particular, the cells will rebuild the trabecular structure to adapt to the load it carries: it can change after an fracture that heals out of position, so that the load is different.

This has been known for some time: here's an old photo of a cross-section of the top of a human femur, the upper leg bone, including the ball part of the ball-and-socket joint of the hip. Next to it is a engineering diagram of the force lines, and the lines at right angles to the forces. The microstructure of the trabeculae follow the force lines, officially called stress trajectories, just like in the catenary arch, and the right angle lines are side braces to prevent buckling movement. This is Wolff's Law in the medical textbooks, even though Julius Wolff in 1870 got the reason wrong! He argued the pattern was inherited, like overall body shape, but actually it's a dynamic process. The living cells in bone are constantly breaking the bone down in little areas and rebuilding it, a process called bone remodeling. This fixes any deterioration, allows the bone to grow, and aligns the structure with the current force lines. This gets pretty complicated and is the subject of ongoing research, especially for diseases like osteoporosis that weaken the bone so it's more likely to break. A classic readable book on subjects like this is D'Arcy Thompson's On Growth and Form, still in print after generations. (The femur illustrations are from this). At a sophisticaled conceptual level, there's a famous dispute in theoretical biology on architectural spandrels.

. . . Farewell!
The day frowns more and more. Thou'rt like to have A lullaby too rough: I never saw The heavens so dim by day: A savage clamour!
Well may I get aboard! This is the chase. I am gone for ever! [Exit, pursued by a bear]

Return to CPO Home Page