Last year, Cigna Corp. and the New York hospital system Northwell Health discussed developing an insurance plan that would offer low-cost coverage by excluding some other health-care providers, according to people with knowledge of the matter. It never happened.

The problem was a separate contract between Cigna and NewYork-Presbyterian, the powerful hospital operator that is a Northwell rival. Cigna couldn’t find a way to work around restrictive language that blocked it from selling any plans that didn’t include NewYork-Presbyterian, according to the people.

Dominant hospital systems use an array of secret contract terms to protect their turf and block efforts to curb health-care costs. As part of these deals, hospitals can demand insurers include them in every plan and discourage use of less-expensive rivals. Other terms allow hospitals to mask prices from consumers, limit audits of claims, add extra fees and block efforts to exclude health-care providers based on quality or cost.

The Wall Street Journal has identified dozens of contracts with terms that limit how insurers design plans, involving operators such as Johns Hopkins Medicine in Maryland, the 10-hospital OhioHealth system and Aurora Health Care, a major system in the Milwaukee market. National hospital operator HCA Healthcare Inc. also has restrictions in insurer contracts in certain markets.

The U.S. spends more per capita on health care than any other developed nation and will soon spend close to 20% of its GDP on health. Americans aren’t buying more health care overall than other countries. What they are buying is increasingly expensive. Among the factors driving spending is the opaque way the price of health care is set, a problem exacerbated by the hidden details in agreements between insurers and health-care providers.