抄録
We have analyzed the trabecular bone of the ilium from six postmenopausal women by using a quantitative histomorphometry, to distinguish osteoporosis from physiological osteopenia in aging. Three of these 6 subjects, with mild Osteopenia and relatively slow rate of bone loss, probably representing physiological osteopenia in aging, showed a decrease of mean wall thickness (MWT). Relative thickness of interstitial bone between two packets in the trabecular bone (%IBT) tended to increase with advancing age. Osteoblastic lifespan, estimated by the bone formation period (sigma f), was shortened in proportion to a decrease of MWT. These findings suggest that the shortening of osteoblastic lifespan and the resulting decrease of bone formation are responsible for the bone loss in physiological Osteopenia in aging.
In contrast to these 3 subjects, the remaining 3 with considerably small bone mass and rapid bone loss, were thought to have osteoporosis as a pathological process. They exhibited thin trabeculae (small MTT) with a concomitant decrease of mean thickness of interstitial bone (MIBT). One of them, with multiple compression fractures of the vertebrae, had typical lowturnover osteoporosis characterized by the absence of tetracycline apposition. Since there was no difference in the MWT between “osteoporotics” and “physiological osteopenics”, the thin trabeculae in the bone of osteoporosis was apparently due to a decrease in the amount of interstitial bone representing the remnant of the pre-existing bone resorbed by osteoclasts. This evidence suggests that osteoporosis is characterized by the enhanced resorption of pre-existing bone (interstitial bone). Furthermore, the findings in low-turnover osteoporosis which showed smallest MIBT among the subjects despite scanty resorption surface, might indicate that osteoporosis, regardless of the current activity of bone remodeling, should have undergone a period of high-turnover phase, sometime in the past.
In conclusion, the mechanisms of bone loss appear to be different between postmenopausal osteoporosis and physioloigcal osteopenia in aging. Postmenopausal osteoporosis may therefore be regarded as a discrete clinical entity rather than an extreme condition of physiological bone loss with aging.