| Bone mineral density is highly correlated with bone | | | | accelerates sharply. Estrogen receptors, present in |
| strength and with fracture risk. Skeletal weakness, | | | | bone, directly affect bone cell function by inhibiting |
| which develops in osteoporosis, results in part from | | | | osteoclastic bone resorption. With the decline in |
| structural and quantitative abnormal connections or | | | | estrogen levels, osteoclastic and osteoblastic activity |
| alterations in bone quality. These include changes in | | | | increase, but excess osteoclastic resorption occurs, |
| bone turnover and in rate of repair, as well as loss of | | | | causing a net bone loss of up to 2 to 3 percent per |
| connectivity of the trabecular elements that comprise | | | | year for approximately 5 years. Total lifetime losses |
| cancellous bone. Such changes contribute to skeletal | | | | may equal 30 to 40 percent of peak mass for |
| weakness; however, some of these changes in bone | | | | women and 20 to 30 percent for men.Bone loss may |
| quality are the direct result of bone loss.Bone is a | | | | occur in either trabecular or cor. tical bone from an |
| living tissue in a constant state of turnover and | | | | imbalance of skeletal remodeling that favors bone |
| renewal. Bone remodeling maintains healthy bone with | | | | resorption. Trabecular bone is spongy or cancellous |
| an ability to store calcium essential for bone density, | | | | and comprises 80 percent of the adult skeleton while |
| bone strength, and other vital body functions. Bone | | | | cortical bone is com. pact or tubular and comprises 20 |
| responds to physiologic demands and repairs | | | | percent of the adult skeleton. Cortical bone |
| microstructural defects. Two major multinucleated, | | | | predominates in the shafts of long bones, whereas |
| monocyte/macrophage lineage cell types, osteoclasts | | | | trabecular bone is concentrated in the vertebrae, |
| and osteoblasts, are involved with bone remodeling. | | | | ends of long bones, pelvis, and other flat bones. |
| Osteoclasts resorb bone and osteoblasts synthesize | | | | Trabecular bone has a greater surface area than |
| new bone matrix. Osteoid (the organic matrix of | | | | cortical bone and is therefore metabolically more |
| bone) subsequently calcifies to fill in defects resulting | | | | active. Bone remod. eling is a dynamic process |
| from osteoclast activity. Overall bone turnover is | | | | necessary to provide calcium for extracellular |
| determined by the skeletal summation of metabolic | | | | function, for repair and removal of old bone, and to |
| activity generated by osteoclastic/osteoblastic | | | | maintain skeletal elasticity. Bone remodeling occurs at |
| coupling During growth and development, net bone | | | | about one million bone sites at any given time and |
| formation exceeds resorption. The critical years for | | | | within a given year, remodeling occurs in 25 percent |
| building bone mass begin in preadolescence. Peak | | | | of trabecular and 3 percent of cortical bone. The |
| bone mass occurs at about age 20 for the hip and | | | | complete bone remodeling cycle (bone resorption |
| during the early thirties for the spine. During the | | | | followed by formation of bone matrix and then |
| fourth decade in both genders, skeletal bone loss | | | | mineralization of the matrix) takes up to 8 months to |
| begins. At menopause, the rate of bone loss | | | | occur. |