A 69-year-old female presents with the recent onset of mild back pain. Otherwise she has no other pains and quite good in her daily activities and active in her NGO field activities. Physical examination reveals kyphosis, while an x-ray of her back reveals a compression fracture of a veebral body in the dorsal spine, with spondylotic changes and mildly reduced bone denisty. Serum calcium, phosphorus, alkaline phosphatase, ESR and parathyroid hormone levels are all within normal limits. This woman's bone changes are most likely due to