What is the main difference orthroscopy and an arthroscopy?

Arthroscopy: It appears that orthroscopy is a misspelling of the term arthroscopy. So there is no difference.

There is none: Both refer to joint procedures involving small incisions and cameras. Small incisions are used to enter the joint with a camera, fluid and small instruments. The fluid is used to fill-up the joint, creating more working space. The camera is placed into the joint to visualize the contents of the joint and the small instruments are used to repair and/or remove structures within the joint.