Found on Reddit:

I'm a dentist and I can tell you exactly what he likely meant. Slovenia of course was never a part of the USSR but the USSR had largely friendly ties with Yugoslavia, which Slovenia was a part of. And part of that was knowledge exchange in Healthcare between the Soviets and Yugoslavia/Slovenia.

The USSR developed a number of... Interesting... dental techniques independently of the west. To be clear, these techniques DO work, but they are distinctive and a little bizarre to us westerners. So your dentist likely looked in your mouth, saw some of those techniques -- which are occasionally done in the US as well, largely from ex-Soviet/ex-communist dentists -- and made the connection. As for why he asked Slovenia specifically? A lot of Slovenian dentists migrated to the US and brought their know-how over.