Buyers beware if you are looking to by a BONE GROWTH STIMULATOR.  Most of these devices have internal chronometers.  They are only good for 270 treatments or 270 days form the day of activation.  Most of these devise look like they are new.  They can not be reset and are completely worthless.  Make sure you ask the seller what the first date of activation was and how many treatments have been used.  You can find information and manuals on the internet.  Check them out before buying  a unit.