Remodeling
Once bone formation (the acquisition of bone mineral density "BMD")
has stopped (somewhere about 28 years of age) peak bone mass is maintained by a process
called "remodeling". Remodeling is a process which occurs 24
hours a day, 7 days a week and involves the continuous breakdown and re-formation of bone.
Remember, bone is a live tissue just like the kidneys, heart, and other organs of the
body. This live tissue continuously remodels itself to maintain maximal BMD and to repair
any small (or large) damage (such as fractures). Remodeling is under the control of a
number of hormones.
Remodeling consists of tearing down small parts of the bones, and then re-forming them.
The breaking down portion is termed "resorption" and is
performed by large cells within the bones called "osteoclasts".
Osteoclasts live in the central portion of the bone. They are continually removing small
(microscopic) portions of bone at the edge of the bone surface. Nearby, bone forming
cells called "osteoblasts" begin to fill in the holes left
behind. Here is the big problem: remodeling does not seem to be a perfect give and
take of bone mass. The osteoblasts are less efficient at making bone than the
osteoclasts are at removing it. Although the difference is slight, this is what
accounts for the gradual loss of bone density (BMD) as a person ages. Any factor which
causes a higher rate of bone remodeling will ultimately lead to a more rapid loss of bone
mass and thus more fragile bones.
Amount of Strong Bones
Formed During Youth
- subtract -
Amount of Bone
Lost to Remodeling
Over Many Years
- equals -
Level of Bone Mass
as an Adult (or degree
of osteoporosis)
Factors Involved in Maintenance of Adult Bone Mass
The same factors which encouraged bone formation as a youth and teenager affect the
maintenance of bone mass during the adult years. The most important influences are calcium
intake, reproductive hormone status, normal parathyroid
gland function, and physical activity. Each of these are
discussed on separate pages.
