Doctors have struggled to prevent hearing loss that is common among premature babies placed in protective incubators to grow and on artificial ventilators to breathe.

But a hearing specialist with Children’s Hospitals and Clinics of Minnesota might have just cracked the case.

Until now, efforts have focused on preemies’ exposure to noisy, hissing ventilators. Solutions have included quieter ventilators, noise insulation around their motors and little mufflers over infants’ ears.

“It hasn’t done a bit of good,” said Children’s Dr. James Sidman. “It’s one of those well-kept, not well-acknowledged secrets in the neonatal world.”

Trouble is, these solutions focused on the form of hearing that is most commonly understood: sound waves traveling through the ear canal that are converted with the help of three bones in the ear into vibrations that the brain ultimately interprets.

Sidman said they ignore a secondary form of hearing known as bone conduction — the process by which sound waves rattle the skull, and similarly convert to vibrations that travel to the inner ear.

Such sound waves often reach the skull through the thin tissue of a baby’s gums, which happens to come in close contact with ventilator tubing that pipes oxygen into preemies’ developing lungs.

Sidman reasoned that air noise from this tubing could pepper the skull with vibrations and tax preemies’ ears from the inside. So he used decibel meters to measure noise produced at the juncture of ventilator tubing that would normally rest near the gums.

The results were alarming.

“This study … shows that kids are living in an airport for weeks and months of their young life,” Sidman said.

The results, published in the Laryngoscope, don’t prove a single cause. Fragile preemies also might just be predisposed to hearing loss.

But the results should encourage the creation of ventilators that address this noise route, Sidman said. Research also should re-evaluate mufflers over infants’ ears, because they might trap in noise produced by bone conduction even while preventing noise from entering the outer ear.

“That’s where we thought the problem was coming from,” he said. “Nobody thought about the skull vibration part of it.”