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.

Slovenja mentioned!!!!!! 🔥🔥🔥🔥🇸🇮🇸🇮🇸🇮🇸🇮🇸🇮🇸🇮🇸🇮🇸🇮🇸🇮

I wonder if it's the modern equivalent of a hairdresser checking someone's roots and asking "have you been to Turkey lately?"

European health tourism is a thing after all, and my understanding is that the budget option dental work or hair replacement therapy leaves telltale signs of treatment styles that generally come from that nature of procedure.

That, or they found a "Made In Slovenia" sticker on the back of one of your dentures 😂

When he was working, did you notice a small 4"x4" red booklet in his pocket?

Hmm. I got dental work done in the UK in the 80s. Guess my US dentist are just too polite to mention the shit show ;)

Slovenia! Now on dentistry!

Slovenia was always quite close to the west, so were the dentists. And the prices were never so low, that we would have dentist tourism. Not lower than Italy for instance.

Goy's Teeth