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The present paper intends to check the possibility of improving convenience of the osteoporosis model from ovariectomy in the rat by anticipating the operation to the 40th week rather than the 52nd week of age, thought by some authors to be the optimum model. To this end two parameters have been examined: 1. the bone mass variation determined with the vertebral photon densitometer and weighing of the femur ashes; 2. the chemico-crystallographic characteristics of the bone determined through chemical analysis and X-ray diffractometry. The results obtained induce us to believe that ovariectomy carried out on rats of 40 or 52 weeks of age determine bone mass losses that are statistically comparable. The variations in the dry bone weight/ash bone weight ratio are superposable and the structural chemical characteristics, due to the increase in bone turnover are testified by the increase in osteocalcin. Moreover, it was observed that the most reliable evaluation of bone mass loss can be obtained with the vertebral photon densitometry. Thus, it is thought that the model which foresees ovariectomy of rats 40 weeks after birth compared with those operated 52 weeks after birth, represents a good model of osteoporosis from estrogen deficiency which, due to the evident practical advantages deriving from the use of younger animals it associates a decrease in the use of laboratory animals with statistically reliable results.

To evaluate the relative rates of bone mineral content loss in postmenopause due to both estrogen deficiency and ageing, three groups of women were studied by computerized bone densitometry at the radius mid-point and at the distal point, modified according to the Abwrey technique. All women were in apparent good health and never had estrogen therapy. In the first group there were 64 women aged between 30 and 50 who were ovariectomized between 25 and 35 years of age. The second group was made up of 309 women between 50 and 55 years. In the third group there were 136 women aged 30-50 with normal ovaric function. The ordinary functions of linear polynomial regression were used to describe the variations in density with age. The percentage of postmenopausal bone loss was determined by calculating the BMC value at the start of the menopause and again twenty years later, according to the linear regression equation of postmenopausal period of each group of women in the study. The women who had natural menopause showed an average bone loss per year of 1.63% at the mid radius and 1.0% at the distal point. The ovariectomized women had an average loss of 0.85% at the mid point and 0.66% at the distal point. No significant decrease of bone mass was found before menopause. From a comparison between the two groups of women with analogous periods of menopause, it comes out that, during the first 20 years of natural menopause, estrogen deficiency is responsible for 52.5%-66.4% of the bone mineral loss, the remaining amount being attributable to other causes, connected with ageing. Estrogen deficiency is therefore, the principal factor causing bone mineral loss in natural menopause.