University of Illinois Extension

Background on Bones

There are two basic types of bone: trabecular and cortical. Trabecular bone is most active in being rebuilt - and it is the most sensitive to changes due to osteoporosis. Trabecular bone is found in the vertebrae and forearms. Cortical bone changes very slowly and is not affected as much by changes in calcium intake. Cortical bones are found in the limbs.

The long bones of the body stop growing at about age 20 and bone density reaches its peak between ages 25 and 30. More scientifically, the cells involved in building bone are called osteoblasts. The cells involved in tearing down bone are called osteoclasts. Osteoblasts and osteoclasts work together in the bone marrow to keep forming and reforming bones.

When the balance between osteoblasts and osteoclasts is uneven, loss of bone results. The building cells (osteoblasts) cannot keep pace with the osteoclasts resulting in a condition called osteopenia (reduced bone mass). As this process continues, more bone is being cleared than built and the osteopenia develops into osteoporosis.

Osteoporosis is defined as a thinning of bone caused by the loss of minerals, mostly calcium. The thin bones can lead to fractures of the wrist, hip, or development of the characteristic "Dowager's hump". It can also lead to collapse of vertebrae in the spine so that people "shrink" or lose height.