As said for obtaining 510(k) the only requirement is to show that the instrument is similar that was developed before 1976 and they are classified under class 2 devices, but for PMA there has to be more trials on humans or animals and documents about its efficiency and safety has to presented supporting the experiments or we say trials done on humans. PMA is harder both economically and time wise to complete the whole process so many companies clear their devices through 510(k).Answering your question I think that necessarily the new device should be as safe and efficient as the predicate one.There may be something in common like use and technological characteristics or common use but different technological characteristics though having no question of safety and efficiency. Claiming substantial devices does no actually means that they should be identical. It is established in regards to the use, energy gained-required-delivered, materials used, manufacturing process, performance, effectiveness, safety, biocompatibility, standards and many other.