[Osteoporosis--concept, classification and epidemiology].

Though vertebral fractures were required to make the diagnosis of osteoporosis prior to the advent of methods for accurate bone measurement, osteopenia is readily defined by a decrease of bone mineral density by 2 to 2.5 SD from the peak bone density. After excluding other metabolic bone diseases such as primary hyperparathyroidism, osteomalacia, renal osteodystrophy, multiple myeloma and tumor metastases by means of X-ray studies and biochemical studies on serum and urine, by far the largest proportion of patients with osteopenia are usually found to have osteoporosis. Primary osteoporosis is found in males and females after middle age, and secondary osteoporosis at any age with definite causes such as corticosteroid excess, immobilization, rheumatoid arthritis or vitamin C deficiency. Estrogen withdrawal in young women is classified as secondary osteoporosis, but postmenopausal osteoporosis with similar cause is usually classified into primary osteoporosis, creating a confusion. Rapid bone loss occurring only during a few years after menopause should be clearly distinguished from the life-long process of bone loss common to males and females and should not be classified as a "type" of osteoporosis.