How are Monopolies prevented in small, highly specialised markets?

I'm now studying a course on public health, and had a case study on a man needing an implant. The state would cover the price of the visit and basic medicines but the implant is NOT covered by them, Afaik the market is not regulated by the government.

And i got curious about how you'd prevent a monopoly/price gouging in a market that only consists of a handful of manufacturers continent-wide, especially with a demand as inelastic as possibly dying patients