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.