Only class 3 medical devices require clinical trials and post market reporting, similar to the way new drugs are approved. There are class 2 devices that require post-market surveillance but they do not have the pre-market human trials requirement of class 3.
Class 2 devices represent the largest share of new devices coming to market each year - for example, in fiscal year 2013, the FDA cleared 2,895 class 2 devices and approved 44 class 3 devices.
Class 1 devices are for the most part exempt from quality systems regulations but still need to be manufactured according to current Good Manufacturing Practice (cGMP) and the manufacturer must list and register all devices manufactured in the USA or in foreign countries. Manufacturers and initial importers/distributors also must keep a complaint system for class 1 devices.
Back in 1976 when the current classification system started, the FDA grandfathered in devices that were on the market, and if a manufacturer of an existing device made modifications due to advancements in technology or cost savings, the idea for a class 2 device (moderate risk) was to prove “substantial equivalence” to the current device. The practical implementation of this idea was and is the FDA’s “510(k)” application to inform the FDA of intent to market a device. A 510(k) application has sections applicable to biocompatibility and safety testing, software testing (if applicable) and bench testing for physical performance features, but there is no requirement for clinical (human) testing. So the tests are carried out by manufacturers using cell culture in a test tube or petri dish (in vitro), in animals (in vivo), to physical standards for performance, and simulations to test software. The tests can be performed in house if the manufacturer has the resources, or most commonly at independent research labs. The lab will produce a report making reference to the standards and protocols used in running the test, and the manufacturer will include the report in the 510(k) application. When the FDA accepts all the manufacturer’s claims in the 510(k) application after review and discussion by the FDA’s device, the the FDA issues a letter stating the device is now “cleared” to market.
Class 3 devices are “approved” by the FDA after a process that includes a premarket application (PMA) including clinical trial data and specific post-market reporting requirements. I have not myself worked on PMA device filings because the costs to bring such devices to market are high and out of reach for smaller manufacturers. Most PMA devices come from larger companies or spinoff ventures of a research institution.