Why was orthopedic surgery called Orthopedic?

Ortho is from the Greek, orthos, meaning straight. Pedic comes from the Greek pais, meaning child. It's the same root as pediatrics.

So, ortho-pedic could be translated as "straight child". Orthopedics is the specialty of straightening (or fixing) disorders of the structure of the body, which in the earliest days of the specialty tended to be congenital defects of children.

Source: Taber's Cyclopedic Medical Dictionary, 13th ed.