The death toll eventually reached 59 babies — and still no clear explanation for what went wrong, the CPSC said.

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But a new study by an outside expert hired by the CPSC suggests that babies died in inclined sleepers for exactly the reasons that pediatricians and safety advocates had been warning about for years. Its findings highlight CPSC staff’s failure to understand the risk and undercuts arguments from industry officials that the product category could be made safe with minor changes to voluntary safety codes.

The study, led by Erin Mannen, assistant professor of orthopedic surgery at the University of Arkansas for Medical Sciences who specializes in infant biomechanics, found that the product’s design is dangerous.

Babies are especially susceptible to suffocation in an inclined sleeper because the products appear to make it easier for babies to roll into an unsafe facedown position and puts them in an exhausting fight to maintain a safe posture. The study examined how 10 infants moved in the devices and monitored their blood-oxygen levels.

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None of the inclined sleep products that Mannen tested were found to be safe for infant sleep.

“I think this confirms what we’ve been saying all along,” said Nancy Cowles, executive director of the safety advocacy group Kids in Danger.

Inclined sleepers have been controversial since Fisher-Price invented the category 10 years ago. The devices allowed babies to sleep at a 30-degree angle, violating the American Academy of Pediatrics’ safe-sleep guidelines that said babies should sleep on flat surfaces. And Fisher-Price had invented its inclined sleeper without medical safety testing or input from a pediatrician, according to a Washington Post investigation.

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Mannen was hired by the CPSC last year as the number of deaths associated with inclined sleepers grew, but staff struggled to pinpoint the problem.

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Her study was included in a new CPSC proposal for mandatory safety rules for inclined sleepers. The new rules would effectively ban current models of inclined sleepers.

New sleepers would be limited to a 10-degree incline, the same limit for bassinets. Mannen’s study found that the suffocation risk to infants disappeared below 10 degrees.

Mannen said in an interview both regulators and companies could benefit from biomechanical studies.

“This type of research should be done before products get to market,” she said.

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The roots of the recall and subsequent outcry over why it took regulators so long to act can be traced back to critical decisions Fisher-Price made a decade ago that first allowed the inclined sleeper into people’s homes, illustrating how the nation’s product safety system relies heavily on manufacturers — rather than regulators — to protect against dangers in new products.

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“People assume we bless a product before it comes to market,” longtime CPSC commissioner Robert Adler told The Post earlier this year. “That isn’t the case.”

The Post’s investigation found that Fisher-Price developed its revolutionary product based on faulty beliefs about infant sleep, with no clinical research into whether it was safe, and, rather than seeking the advice of pediatricians, consulted just a single doctor — a family physician from Texas whose expertise had already been doubted by judges and who would eventually lose his medical license.