Dental care is a peculiar niche of the U.S. healthcare system. Even though teeth and gums are just as much part of the human body as kidneys or elbows, they are insured differently — a lot differently.

When the Patient Protection and Affordable Care Act was written and debated, comprehensive dental insurance never really became a focal point. Lawmakers ultimately created a few provisions that may boost access to oral care, but dental coverage still escapes the grasp of millions of Americans.

Dental plans garnered national attention after it was discovered that HHS overstated 2014 enrollment figures in the ACA's insurance exchanges. The government included almost 400,000 stand-alone dental plans, which are much cheaper and separate from standard health plans. After accounting for those, the number of people who were enrolled in full-service medical plans was 6.7 million. A House committee plans to grill CMS Administrator Marilyn Tavenner on the numbers Tuesday.

Lost in that discussion, however, is the question of how much the law has done to advance dental care. Not enough, advocates argue.

The Affordable Care Act mandated pediatric dental services as one of the 10 essential health benefits for health plans, but adult dental services were excluded. In addition, all health plans must cover oral health risk assessments for children up to 10 years old with no copayment, coinsurance or deductible. The law also allowed states to expand Medicaid and its related dental benefits to more low-income children and adults.

But large gaps in coverage remain, primarily for adults who don't qualify for Medicaid. “More children have been enrolled (in dental plans) through the Affordable Care Act,” said Maxine Feinberg, president of the American Dental Association. “However, it really only helped adults in a minimal way.”

About 187 million people have some form of dental insurance, according to the National Association of Dental Plans. Coverage is provided through two main outlets: employers or public programs like Medicaid and the Children's Health Insurance Program.

A majority of people who have dental insurance get it through their employer. Almost nine in 10 employers with 200 or more workers and about half of all companies offer dental benefits, according to the Kaiser Family Foundation. The most common forms of coverage are like “prepaid gift cards,” Feinberg said. Routine cleanings and other preventive services are completely covered, and all other dental care needs are covered up to a yearly maximum figure.

But that leaves about 130 million Americans who have to pay for their dental care completely out of pocket or rely on supplemental dental policies. That figure includes millions of Medicare beneficiaries. Traditional Medicare does not cover dental care unless it's an emergency procedure during a hospital stay.

Cost and a lack of dental providers are cited as the key barriers for obtaining care. In some instances, the results have been lethal. The most famous case was Deamonte Driver, a 12-year-old boy in Maryland who died in 2007 after bacteria from an infected tooth spread to his brain. Deamonte's family lost its Medicaid coverage. More recently, in 2011, Kyle Willis, 24, died in Ohio after a wisdom tooth infection forced him to the emergency department. Mr. Willis had no insurance and couldn't afford antibiotics.

Ultimately, the Affordable Care Act is expected to bring some kind of dental coverage to 8.7 million kids and 17.7 million adults by 2018, according to an ADA-commissioned analysis conducted by actuarial consulting firm Milliman. A vast majority of those gains will be through Medicaid expansion, and some asterisks apply.

Medicaid dental benefits for adults vary widely in each state. Some states like Connecticut and New York offer extensive coverage that includes preventive cleanings and restorative services like fillings and crowns. But others offer zero dental coverage, or only cover emergency services that relieve tooth pain and infection. That means many people who live in states expanding Medicaid eligibility may only benefit marginally, and some others in non-expansion states won't benefit at all. The ADA study said of the 26 states expanding Medicaid, nine provide “extensive” adult dental benefits.

The scenario also assumes patients can find dentists accepting Medicaid. Only one-third of practicing dentists take Medicaid patients due to lower reimbursement rates.

Dr. Richard Manski, a dentistry professor at the University of Maryland who has studied dental insurance said the state programs that prioritize dental care actually offer “robust” coverage. But “the problem with the Medicaid plans is there's always a fixed pot of money,” he said.

Dental benefits are often the first to get cut when states need to get their Medicaid budgets in order. Even the federal government has encouraged state Medicaid programs to tinker with their dental care benefits when money gets thin. In 2011, then-HHS Secretary Kathleen Sebelius wrote letters to governors saying that limiting or eliminating dental care benefits is an effective way to save Medicaid funds.

The impact of the ACA's exchanges on dental care is similarly cloudy. Although dental benefits for children up to age 19 are required for all health plans sold on the individual and small-group markets, each exchange can take a different approach, said Colin Reusch, senior policy analyst at the Children's Dental Health Project. Some exchanges require health insurers to embed pediatric dental coverage. Others allow the benefits to be sold in stand-alone policies, requiring people to pay a separate premium.

The average cost differential between a medical policy with embedded dental coverage and a medical policy without dental coverage on the federally run exchanges ranges from $33.45 per month for a family with one child to $70.05 for a family with three or more children, said Evelyn Ireland, executive director of the National Association of Dental Plans.

Mr. Reusch said he's hopeful the gap between dental and medical care can be bridged, even though the ACA will leave many without dental insurance and nothing has changed with Medicare. Providers in accountable care organizations or patient-centered medical homes are now somewhat responsible for the oral health of patients, especially if dental issues ultimately lead to more complex health problems.

“In the long term, that's really beneficial in terms of shifting the oral healthcare delivery system towards integration, which is where we want to go,” Mr. Reusch said.

Bob Herman writes for Modern Healthcare, a sister publication of Business Insurance.