<prompt> Answer this question truthfullyWhat is the classification system for osteoporosis?  <response>: <reason> There are three types into which osteoporosis is sub-divided. They are:Involutional Osteoporosis - otherwise called primary osteoporosis. No underlying cause is identified. Secondary osteoporosis in which the underlying cause eg, steroid use is known. Rare forms of the disease, such as juvenile, pregnancy-related, and postpartum osteoporosis.There is a spectrum of disease ranging from osteopenia milder and earlier bone loss to osteoporosis later more severe bone loss.Osteopenia which is defined as bone mineral density ranging from less than -1.0 and more than -2.5, can be a precursor condition to osteoporosis. In osteoporosis the bone mineral density BMD is reduced, bone microarchitecture is disrupted, and the amount and variety of non- collagenous proteins in bone is altered. Osteoporosis is defined by the World Health Organization WHO in women as a bone mineral density 2.5 standard deviations below peak bone mass 20-year-old sex-matched healthy person average as measured by DXA; the term established osteoporosis includes the presence of a fragility fracture. 