Just to clarify a couple of things in case there is any confusion. Osteoporosis and osteoarthritis are very different medical conditions. Osteoporosis can't turn into osteoarthritis. Osteopenia if untreated can turn into osteoporosis.

Dystrophy doesn't affect bones. It affects muscles. Muscle dystrophy is loss of muscle other than due to disuse. It can be caused by things like lack of blood flow or nutrients which cause parts of the muscle to die but it is not caused by lack of use and is not reversible where there has been death of a muscle. Its not the same as muscular dystrophy which is collective name for a group of quite different inherited genetic disorders.

Muscle atrophy is caused by lack of use or lack of movement. The muscle bulk is reduced because the fibres are not asked to work. Muscle atrophies at a rate of about 4% per week - even more if there is a change from total use to total disuse. Muscle atrophy is usually reversible once a muscle is used again (by using the limb or in physiotherapy). Even in cases where all of the motor signalling from the central nervous system has been lost such as in spinal cord injury which has resulted in paralysis, atrophied muscle can be rebuilt using artificial electrical stimulation.