FDA does not approve all medical devices. FDA has an application process that is specific to the type of device and the potential for serious harm. There are two types of applications that must be submitted to the FDA, (1) Premarket submission; and (2) 510(k) application. Whether a medical device requires a premarket application or not and which type of premarket application is dependent on how FDA classifies the medical device. A Class 1 device is the least likely to cause harm. Many Class 1 medical devices have been exempt from the premarket application process.
By way of example, a toothbrush is a Class 1 exempt device. Class 2 devices require a 510(k) application to be submitted and "approved" prior to putting the medical device in the US market. Class 3 devices and devices that are not classified must go through the PMA application, these are life-sustaining devices.
As noted above, not all devices need FDA approval prior to going into the US market. Moreover, FDA will note that for 510(k) applications they do not "approve" the medical device. FDA simply clears the device to go to market. In fact, FDA specifically disclaims the use of the phrase FDA approval for these medical devices. However, industry and laypeople still refer to a 510(k) clearance as FDA approval.