The two spellings are orthopedics and orthopaedics.
* Orthopedics is the preferred spelling in American English.
* Orthopaedics is the preferred spelling in British English.
This difference stems from the Greek origin of the word. "Ortho" means "straight," and "paedos" means "child." In British English, the "ae" digraph is often used to represent the Greek "ai" sound, which is why the "ae" spelling is preferred.
However, in American English, the "ae" digraph is less common, and the "e" spelling is considered standard.
Ultimately, both spellings are technically correct, but it's best to use the spelling that's consistent with the region or style guide you're following.