@Chao ****
A tourist visa gives you 60+30 so not sure what dental apt takes more than 90 days. Remember visa's in Thailand are aligned with your intention. Get the visa that suits your intention and reason for being here. If you're trying to game the system to get a 5 year visa for a dentist appointment, that isn't it's intention and they will be sure to make sure you are on the right visa if you need to extend or deal with immigration.