Final answer:
Arthrography is the essential projection used to evaluate arthritic knees, providing detailed images of soft tissues as well as bones within the joint.
Step-by-step explanation:
The essential projection used to evaluate arthritic knees is arthrography. Arthrography is a specialized form of medical imaging that employs a contrast agent to make soft tissue structures visible on X-rays. Unlike a standard X-ray, an arthrogram shows both the bony structures and the surface of the soft tissues lining the joint. This is particularly useful as it allows for early detection of degenerative changes in the joint cartilage, which can occur in arthritis before the bones show significant changes. Additionally, arthroscopic surgery is a minimally invasive procedure that is useful both for diagnosis and for treatment of knee problems, including those related to arthritis.