ABSTRACT
Functions of Bone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 Bone Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492 Bone Remodeling and Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494 Fracture Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494
Gender Differences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496 Fractures during Infancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497 Fracture during Childhood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497 Fractures during Adolescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497
Eating Disorders and Fracture Risk, a Special Consideration during Adolescence . . . . . . . . . . . . . . . . . . . . . . . . 498
Fractures during Young Adulthood . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499 Fractures in Middle Adulthood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 Fractures in Older Adults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500
Exercise for Bone Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 Nutrition for Bone Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502
Bones provide mechanical function, which is rigidity and stiffness. This is necessary to move about on dry land, while resisting the natural forces of gravity. There is a fine balance between a skeleton that must resist these gravitational forces and one that would be too heavy to carry around. This homeostasis that needs to exist is directed by a negative-feedback loop. Mechanical sensors detect the exact amount of bending that occurs in bones during the forces and loads that the body sustains. When the level of bending is higher than the set point, a signal is sent to adjust the balance of resorption and formation, and initiate the bone
remodeling process. Routinely, bones bend 0.1 to 0.15% in all dimensions.1 If the amount of bend is a larger percentage than this range, the remodeling apparatus is signaled to increase the proportion of bone formation, so as to provide the increased rigidity necessary to resist gravity, as well as reduce the likelihood of fracture under normal forces. Conversely, if the percentage of bend is lower than this, the negative-feedback loop signals for a higher proportion of bone resorption, or breakdown, so that the skeleton is not too bulky and heavy for the normal functions that it is expected to perform.
