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Osteoporosis   p 2
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2. What is osteoporosis? Link to the Medical Research Council web site
Normal and osteoporotic bone micrographs
Figure 3. Comparison of normal versus osteoporotic bone.


Normal and osteoporotic bones
Osteoporosis is a disorder of bones. The disease manifests itself by causing fractures, particularly of the hip, wrist and spine. The word ‘osteoporosis’ literally means ‘porous bones’ (Figure 3). The bone becomes so fragile that it breaks following even trivial trauma - such as a fall from standing height. You can see that there is less bone and some of the cross struts are thinner or missing.

To understand the underlying causes of osteoporosis we will first take a detailed look at the structure and physiology of bone. Bone is a living tissue that is formed, removed and replaced throughout life with a balanced cycle of loss and formation.

2.1 Normal bone
In adults there are two main forms of bone, cortical and trabecular.
Cortical bone

Cortical bone is dense and compact; you may find it described in A-level textbooks as ‘compact bone’. Cortical bone runs the length of the long bones, forming a hollow cylinder (Figure 4).

Interactive cutaway of hip joint
Figure 4. A long bone (femur) showing cortical and trabecular bone. The bony trabeculae are arranged in the directions of tension and compression.




Trabecular bone

Trabecular bone occurs in the expanded head of the long bones. It also makes up most of the bone tissue in the vertebrae. It has a light, honeycomb structure in which a three-dimensional network of trabeculae act as struts. It is sometimes called ‘spongy bone’.

This type of structure is optimised for strength and lightness. The trabeculae are oriented so that they can take up the stresses in the bone. Figure 4 shows the head of the femur and you can see how the trabeculae are arranged to give the bone strength exactly where it is needed. The spaces between them contain blood vessels and bone marrow.

Bone turnover and osteoporosis
The turnover of trabecular bone is greater than that of cortical bone because the honeycomb structure has a large surface area. This is fine if the cycle of bone loss and replacement are in balance. However, if the cycle gets out of balance, then trabecular bone is lost rapidly.

Osteoporosis occurs when so much bone is lost that the strength of the bone is reduced. The parts of skeleton made from trabecular bone are more susceptible to failure because of their higher turnover rate.

Question 1
Osteoporosis is a disorder that results in the skeleton becoming so fragile that it breaks very easily.

What are the main functions of the skeleton?

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