When does one qualify for total knee replacement?

Pain/disability. When to have a knee replacement has to be an individualized decision. Moderate to severe pain and/or loss of the ability to function to one's satisfaction are reasons to have a knee replacement. Failure to get relief with nonsurgical measures is also a reason. Severity of arthritis on an x-ray should not be the primary factor, as many with severe x-ray changes have little pain and good function.
Qualify TKA. Pain and deformity with x-ray changes failing conservative treatment plan.