The 67 y.o. patient had 5 recurrent fractures of the lower extremities without considerable cause within 5 years. O-shaped deformity of the legs in the knee joints has appeared. The skull, pelvis and lower extremities X-Ray shows the thickening of flat bones. In the long bones there is a hyperostosis along the bone axis.The blood test has not revealed any inflammation activity. Serum calcium is normal. What disease do you consider in this case?