Breaking Bones

Bone has two major functions as a lever and as a spring.

Humans have flexible bones in order to absorb energy through deformation without cracking. Bone stiffness resists bending without cracking.

The ability to do this depends on the unique material composition of bone and its three dimensional conformation.

Different bones have different function and position in the body   and so the material composition varies depending on the location.

Long bones act as levers that are light and rigid with the cortex replaced by a marrow cavity. Vertebral bones act as springs and have a honeycomb structure.

Bones are more fragile if there are abnormalities in bone mineral density (BMD).

Some people have fractures because they have very low mineral density. The bone breaks when it bends too much. Others break bones because the mineral density is too high and the bone is too stiff.

Older people are more likely to get fractures because as we age the load on the bone decreases because our muscle mass decreases. The stress on the bone increases because  the area of bone that is to tolerate that load also decreases.

There is a natural turnover of bone, but as we age the bone balance goes awry. After the menopause women are in particular vulnerable to fracture.

There is a compromise in bone strength with increased bone loss.

Conor Caffrey is a writer on Science and Medicine.



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