THERE ARE NUMEROUS causes of osteoporosis, the most frequent being lack of estrogen. About 30% of all postmenopausal women develop osteoporosis within 10–15 years of menopause. (1, 2) The annual costs caused by osteoporotic fractures in 2003 in the United States were estimated at up to $16.7 billion. (3) In Germany, the entire costs for osteoporotic therapy amount to 10 billion Euro a year. (4) Furthermore, osteoporotic bone heals ∼30% more slowly(5, 6) than normal bone.
Hormone deficiency leads to a reduction of bone structure, and even minor trauma can cause fractures. These fractures are predominantly located in the metaphysis of long bones (i.e., the distal radius, the proximal femur, and the lumbar spine). BMD measurements to evaluate the degree of osteoporosis are therefore carried out at the femoral neck and the lumbar spine.
The ovariectomized rat is an established model of osteopenia and osteoporosis. (7–9) Animals develop substantial osteoporosis within a few months of ovariectomy such that, after 6 months, ∼50% of bone mass has been lost. (10, 11) From previous studies it is well known that the degree of osteoporosis is best determined at the metaphyseal femur or tibia of the rat, (11, 12) trabecular density being the key parameter. The diaphysis of long bones consists of cortex and bone marrow. The growth plate does not close during the lifetime of the rat. Osteoporotic changes do not manifest in the diaphysis because there is no trabecular bone. The diameter of the diaphysis can increase only slightly. Nevertheless, most of the experimental mechanical tests to evaluate osteoporosis or osteoprotective substance have been carried out in the diaphysis of the femur or tibia. (13, 14) In this area of the long bones, both a three-point bending test and even a four-point bending test are possible. These are easier to manage than a standardized metaphyseal fracture. In contrast to metaphyseal BMD or histomorphometrical measurements determined in the same animal and time, (15, 16) the results of the diaphyseal bending or break tests are not able to reveal significant differences in osteoporosis.
We postulate that mechanical testing of the metaphysis of long bones is necessary to evaluate the special properties of osteoporotic bone. BMD measurement is an indirect parameter only, which provides poor evidence for the real stiffness and actual resistance to applied force. Low BMD without a fracture would not be a problem. The manifest fracture is the crucial point in osteoporosis. In the osteopenia model of the rat, the metaphyseal tibia is the most reproducible fracture area.