Osteoporosis is a disease characterised by a loss of bone mass, leading the bone to become lighter, more porous, less dense, and therefore more prone to fractures and deformity.

To understand osteoporosis, you first need to understand bone.
A typical bone is made up of a shaft and two ends, which are known as the extremities.  The outer shell of at typical bone is known as compact bone.  This layer is hard and covers most of the surface of the bone.  The two extremities consist of spongy bone.  This is made up of plates that form a porous network.

The spaces within this network are usually filled with bone marrow which is a soft, fatty substance.  Inside the shaft is the medullary cavity which is a hollow that is filled with bone marrow.  The marrow is red in young animals but gradually turns yellow.

Some bone ends are involved in joint movement.  Where this occurs the extremity is covered with a thin layer of smooth cartilage.  This cartilage is called the articular cartilage and its job is to provide a friction-free surface to aid movement.

Around the entire surface of the bone - except where there is articular cartilage, is a thin, fibrous membrane called the periosteum.  Bone-forming cells are located here and are responsible for laying down bone to increase the width of long bones.  It also lays down bone in response to healing at places where fractures have occurred.

Between the shaft and extremity is a disc of cartilage called the epiphysial cartilage.  Osteoblasts which are bone forming cells, are located in this disc and lay down bone which makes the bone longer.  This disc is only active in the human and animal until mature size is reached.  After this, the disc ossifies.  In humans this happens in the late teens or early twenties.

About one third of the weight of bone consists of fibrous tissues and cells which make a framework.  Two thirds consists of the inorganic salts which are deposited within the frame work to make bone tissue hard.  These salts are chiefly calcium and phosphorus - in fact, calcium phosphate accounts for some 80% of salts deposited in bone. Other salts include calcium carbonate and magnesium phosphate.

Fractures and Fracture healing
A fracture is simply a break in the bone.  If the broken ends of a fractured bone are brought together and immobilised, the normal process of healing will take place.  As the fracture occurs, some blood vessels are ruptured.  This causes blood to pour around the broken ends of the bone.  A blood clot forms which is invaded by connective tissue cells.  These cells set about forming granulation tissue and new blood capillaries.

At the same time, the osteoblasts from the surface of the bone - the periosteum divide rapidly and produce a massive amount of osteoid tissue which is called a callus. The callus completely encircles the broken end of the bone and also penetrates some way into the marrow cavity within the shaft.  The callus thus forms an effective splint and prevents movement of the two segments while the fracture heals.

As soon as the callus becomes calcified and hard, it has changed into true bone.  The callus now reorganises itself to form a typical bone shaft with a marrow cavity.  The healing process is now complete. The process above is straightforward when the break in the bone is clean i.e. there are two level surfaces being knitted together.  If the break is a compound one i.e. there are several breaks, or if the bone has been shattered buy crushing, healing is much more difficult and there is a risk that the healed bone will not be straight.

Osteoporosis is more common as you age

Osteoporosis is an age-related disease, predominant in post-menopausal women over 50 years of age. This is due to decreases in estrogen post menopause - estrogen plays a role in maintaining bone health.

Other factors that can contribute to osteoporosis is lack of exercise (bone-stress from exercise increases bone density), a diet lacking in calcium and protein, smoking, and hormonal abnormalities.