Dentists are apparently among the group of workers who lack the skills necessary to compete in the modern economy, who then turn to the government to protect their jobs and wages. This is in effect the story told in this Washington Post news article about the power of the American Dental Association (ADA).
The piece focuses on the ADA's efforts to block other professionals from doing work that is now done by dentists. While the piece doesn't mention this fact, the ADA also blocks foreign-trained dentists from practicing in the United States. Dentists cannot practice in the United States unless they have a degree from a U.S. dental school. (Since 2011, graduates of Canadian dental schools have also been allowed to practice here.)
As a result of this protectionism, the pay of dentists averages $200,000 a year, roughly twice as much as their pay in other wealthy countries. This costs the country $20 billion a year (roughly equal to the TANF budget) in higher dental expenses.
It's striking that the protectionism for dentists gets so little attention relative to much less costly forms of protectionism, like tariffs for steel, cars, or other items. Perhaps it has something to do with the people reporting on the topic identifying with the beneficiaries. I discuss this in chapter 7 of Rigged: How Globalization and the Rules of the Modern Economy Have Been Structured to Make the Rich Richer (it's free).