You probably have never heard of the Os Trigonum, and the odds you have one are slim.

But Andrew Luck does have this extra little bone behind his left ankle, according to the Indianapolis Star, which suggests the “triangle bone” could be the source of the Indianapolis Colts quarterback’s mysterious leg pain that will keep him out the rest of the preseason.

The Colts still don’t know the exact nature of Luck’s injury, the Star’s Gregg Doyel reports, despite general manager Chris Ballard calling it a high-ankle issue after owner Jim Irsay said in a radio interview that Luck had a bone problem. Approximately 5 to 10% of humans have the Os Trigonum, which is considered useless. Luck had a shot to numb the area where that bone is, but it didn’t solve the issue.

Luck originally was diagnosed with a calf strain in March and has only practiced three times during training camp. The 29-year-old missed time during the 2015 season with a lacerated kidney and missed the entire 2017 season after shoulder surgery.

The start of Luck’s 2019 campaign is now up in the air thanks to this pesky bone.