Once the bone has reached its adult size and shape, the old bone tissue is constantly destroyed and new tissue is formed in its place, in a process known as remodeling.
Remodeling occurs at different speeds in various parts of the body. For example, the distal portion of the femur is replaced every 4 months; The bones of the hand are completely replaced during the whole life of the individual. Remodeling allows worn or injured tissues to be replaced with new, healthy tissues. It also allows the bone to serve as a calcium reserve for the body.
In a healthy adult, a delicate homeostasis (balance) is maintained between the action of osteoclasts (resorption) during calcium removal and that of osteoblasts (apposition) during calcium deposition. If too much calcium is deposited, bone callus or spurs may form, causing interference with movement. If too much calcium is removed, the bones weaken, making them flexible and subject to fracture.
Normal growth and remodeling depends on several factors.
- sufficient amounts of calcium and phosphorus must be present in the individual's diet;
- Sufficient vitamins should be obtained, especially vitamin D, which contributes to the absorption of ingested calcium.
- The body needs to produce the hormones responsible for bone tissue activity:
- Growth Hormone (Somatotropin): secreted by the pituitary gland, responsible for bone growth;
- Calcitonin: produced by the thyroid, inhibits osteoclastic activity and accelerates calcium absorption by bones;
- Parathyroid hormone: synthesized by parathyroids, increases activity and number of osteoclasts, increasing the rate of calcium in the bloodstream;
- Sex hormones: are also involved in this process, helping in osteoblastic activity and promoting the growth of new bone tissue.
With aging, the skeletal system suffers calcium loss. It usually starts at age 40 in women and continues until 30% of the calcium in the bones is lost by age 70. In men, the loss does not occur before age 60. This condition is known as osteoporosis.
Another effect of aging is reduced protein synthesis, which decreases the production of the organic part of the bone matrix. As a consequence, there is an accumulation of inorganic part of the matrix. In some elderly individuals, this process causes a weakening of the bones, which become more susceptible to fractures.
The use of orthodontic appliances is an example of bone remodeling, in this case resulting in dental arch remodeling.
Braces exert different forces from those to which the teeth are naturally subjected. At points where there is pressure, bone resorption occurs, while on the opposite side there is matrix deposition. Thus, the teeth move through the bones of the dental arch and occupy the desired position.