Heather Lange knows the value of regular dental checkups for her kids, but making it happen isn't always easy. Appointments for four children get expensive, and only one dentist near her home in Red Wing accepts her MinnesotaCare coverage.

"When I went to make an appointment, [I was told] to not even attempt it because there was a six-month waiting list," said Lange, 35. "There's nothing to do unless I pay out of pocket."

Cases like hers are sparking concern over the thousands of Minnesota families who lack regular dental care because of high costs or low state payment rates to dentists. Some 400,000 preschoolers turned up at Minnesota hospitals with severe oral complaints from 2007 to 2012, according to the Minnesota Department of Health. Overall, Minnesotans racked up $80 million in hospital bills over five years for oral care that could have been avoided by regular dental visits.

"Preventable cases are the ones that are always bad news," said Jon Roesler, an epidemiologist at the Health Department. "If they were getting proper dental care, they shouldn't be showing up at the ER."

This year the University of Minnesota is asking the Legislature for $1 million to fund a mobile dental clinic that has served needy communities while training the U's dentistry students. The 43-foot-long bus with faces of smiling children splashed across its side served more than 1,000 adults and children each year, but it has been idled since late 2015 for lack of funding.

The bus was subsidized for many years by UCare Minnesota, the large health insurer, until UCare lost a large chunk of its revenue base in state contracts last year. The future of the clinic is still in flux, a UCare spokeswoman said.

Leon Assael, dean of the U's dentistry school, calls oral health problems a hidden epidemic. "It's not [that] there [aren't] enough dentists. … It's that dental services are costly and not included in other health insurance," Assael said.

The Minnesota Dental Association has recognized the problem for years, offering an annual "Give Kids a Smile" event, when hundreds of dentists open their offices for a day or two of free care. At this year's event, held Friday and Saturday, nearly 500 dentists and hundreds of volunteers treated roughly 5,000 children.

Stacie Bergstrom of Buffalo, Minn., brought her two sons, Riggs, 3, and Soren, 10, to Park Dental Ridgepark in Minnetonka for an exam. After her husband had an unexpected surgery that drained the family's health savings account, she said there wasn't much left for dental care this year.

"The choice was either skip it for this year or wait to get caught up," Bergstrom said. "I'd hate to wait because we've always made it a priority."

Undiagnosed cavities are what Dr. Elias Kouvalis often sees in children because cavities form more frequently in kids than adults.

Still, Kouvalis, who practices at Park Dental in Plymouth, said he's noticed fewer problems in kids recently because pediatricians are encouraging low-sugar diets and pushing parents to bring their children to a dentist early.

"The kids that are most neglected are from parents who don't have insurance and don't have the means to go to the dentist," he said. "It's more of an access problem than anything."

Youssef Rddad is a University of Minnesota student on assignment for the Star Tribune.