The bones are living tissue that are prone to a wide range of diseases just as with any other part of the body. One of these conditions is osteoporosis where the bones become weak and brittle. Osteoporosis does not occur overnight – it develops over years. Although it is more frequently seen in seniors, it usually started years before. Often it is not detected until a person goes for certain medical investigations for other conditions. Falls in the elderly can often bring to light the condition as even minor falls leads to severe bone fractures. In addition, fractures may not heal as expected and this can cause long term disability.
The outer part of bone is the hard compact matrix known as the dense bone. It is formed by a compound known as hydroxyapatite – calcium phosphate crystals. Hydroxyapatite is laid down by specialized bone cells – osteoblasts. The inner tissue is soft and spongy cancellous tissue known as the trabecular bone. Within the central canal of the long bones lies the bone marrow. Despite most of the inner bone being delicate tissue, it is well protected by the outer dense bone.
Most people think that the bones are not living tissue since it can remain for decades or even centuries after death. However, this is untrue. The bone is similar to other tissue in that it needs a constant supply of oxygen and nutrients to stay alive. In addition the supply of minerals like calcium and phosphates needs to be ongoing even into adulthood as the outer bone structure is not static.
Our bones start off as cartilage from the time of birth. This is softer tissue yet rigid and flexible. Gradually this cartilage becomes ossified – meaning that it changes to hard bone. This involves the deposition of certain minerals such as calcium and phosphate in a manner that makes it compact and hard. Peak bone mass is reached around 20 to 40 years of age.
The density of bones is determined several factors such as :
Even though this hard bone tissue is laid down, it constantly undergoes remodeling. Some cells take up some of the bone and other cells lay down new bone. This process of bone deposition and resorption is kept in balance at all times. However, this balance is upset in osteoporosis. More bone is resorbed than deposited and this causes the bones to become weak and brittle.
The reason why this shift in balance between bone resorption and deposition occurs is not always known (idiopathic) and is termed primary osteoporosis. This means that there is no underlying disease that causes this shift. It can also occur with old age (senile osteoporosis) and after menopause (post-menopausal osteoporosis). Even in these instances where the contributing factors are understood, it is still not due to any disease. However, osteoporosis may occur as a consequence of certain diseases and it is then known as secondary osteoporosis. These diseases may not be directed at the bone specifically but can upset the hormones and nutrients needed for healthy bones.