A few months ago, I was thinking about the opportunity to use 3d printing/imaging to drastically lower the cost of these types of devices. Ultimately the biggest hurdle is going to be getting an extremely accurate scan (not sure if you could do some combination of using dental x-rays, along with 3d imaging tech from mobile cameras to build the mouth model for each customer).

Part of the solution could be in home try-on (similar to Warby Parker). Ie send 10-20 sets of 3d printed retainers, all slightly different but potential fits. The customer selects which one fits best & the full set is based on that starting model. Since you're not actually creating molds to base the retainers on, the cost of each slightly different model should be pennies.

Anyway, that's about as far as I got... then I went back to building enterprise software.