Knee airbags have little effect on injury risk and could increase it in some cases, according to a new study.

Insurance Institute for Highway Safety (IIHS) researchers looked at injury measures from more than 400 frontal crash tests it performed as part of its vehicle ratings program to see if injuries were less likely in vehicles with knee airbags.

The results found knee airbags had “only a small effect on injury” in the drivers-side small overlap front and moderate overlap front crash tests. While the small overlap test actually increased injury risk for lower leg and right femur injuries, head injury risk was slightly reduced. The researchers found no effect on injury measures in the moderate overlap test.

“There are many different design strategies for protecting against the kind of leg and foot injuries that knee airbags are meant to address,” said Becky Mueller, an IIHS senior research engineer and co-author of the paper. “Other options may be just as, if not more, effective.”

The IIHS said one reason some manufacturers have been installing knee airbags is to help vehicles pass federally mandated tests with unbelted dummies and said it was possible that knee airbags would help unbelted occupants in real-world crashes.

“The IIHS study didn’t look specifically at crashes in which people weren’t using seat belts, and dummies are always belted in IIHS vehicle ratings tests,” the report added.