Jordon,
Thanks for taking me in the private message to come review this thread. This sure is a quite a difficult problem for you. Sorry to hear that you're going through this. Just to review, you had your surgery in September 2015 and underwent a manipulation under anesthesia which sounds at least partially successful. You've then had a second opinion showing a bone scan with moderate uptake. Your second opinion surgeon was thinking that your continued problems exist from either a loose prosthesis, malposition of the components, or infection.
Please feel free to correct me if this synopsis is not correct.
First of all the bone scan typically will be "hot" for about 1-2 years following a knee replacement. This type of scan is of only limited value within this time from a knee replacement. It sounds like your second surgeon is considering multiple different possibilities. However, I (if I were you) would want some more definitive evidence for which is the cause of your ongoing stiffness and pain before considering your next step which may be surgical management.
It does make a lot of sense to do a workup for infection just to make sure. Your surgeon should be able to tell whether the swelling in your knee is due to an effusion. If there is actual swelling inside of the joint then they can take a sample of that fluid and send it to the lab. That is one of the most definitive ways of determining infection. Some other options include checking blood serum values for ESR, CRP, and WBC. They could also consider a white blood cell tagged scan of the knee which would be more definitive for infection.
A review of your x-rays would be needed to help determine any malposition of the components. A very subtle sign of loosening may not be easily visualized on x-ray. The use of advanced imaging such as a CT scan or MRI is of limited value due to metal artifact from the prosthesis.
If you're problem were due to continued adhesions or scar tissue, then it's not unreasonable to consider an arthroscopy of the knee. In the right patient this type of procedure can be successful to help achieve further flexion of the knee. This surgery is less successful in achieving further extension.
Best of luck.
Orthodoc