A lot of the time in situations where there isn't access to a Vet that can do proper diagnostic testing, such as an X-Ray, Ultrasound, CT Scan, Blood-Work or other Lab Work, etc., then in situations like the one your bird has where it's an orthopedic/neurological issue, the Vets will do something called a "Treatment-Trial". This simply means that they put the patient on a prescription medication that is meant to only make the negative signs/symptoms subside, such as pain-killers, anti-inflammatory meds, Corticosteroids, muscle relaxers, etc., and if a certain medication relieves the symptoms very effectively, then this will give them the same diagnosis that the diagnostic testing would, such as an X-Ray...
You mentioned that they put your bird on Antibiotics and Corticosteroids already, and this was actually a "Treatment-Trial"...With Corticosteroids, if they relieve the symptoms of the issue long-term, then typically the issue is nerve-related and not related to an orthopedic problem, such as a bone, ligament, tendon, or muscle issue...However, if the Corticosteroid only relieves the symptoms temporarily or not at all, then it's typically not an issue with the sympathetic nervous system, as Corticosteroids only help orthopedic issues for a very short time and not in any type of drastic way, unless you are able to put the patient on them for a very long time...And putting a bird on long-term Corticosteroids (or anyone for that matter) can cause some very nasty side-effects and actual long-term or permanent syndromes and diseases, such as Cushings Disease....
In-contrast, another "Treatment-Trial" for this type of situation that is much safer for long-term use (if it is effective in relieving the symptoms) would be simply putting your bird on a prescription NSAID daily for around a month...If the issue is in-fact swelling/inflammation and irritation of the sheath around the tendons or the tendons themselves that cause that specific muscle or muscle groups to perform this flexion motion that your bird is constantly in, then you would know that this is exactly what the issue is, and then you can go from there...There are a few prescription NSAIDS that work extremely well for Tendon and Tendon-Sheath inflammation, and if this is in-fact your bird's issue, you should see your bird getting some relief pretty directly, followed by more and more progressive improvement as the days go on. Metacam/Meloxicam is one of these NSAIDS, probably the safest for the bird's Kidney's and the most effective for relieving Tendon and Muscle inflammation and irritation.
I forgot to ask you if there is any visible swelling in your bird's leg down through the ankle joint to the foot (is there a size difference or visible swelling/inflammation/redness when compared to the opposite, non-effected leg/ankle/foot)? If there is visible swelling, especially in the lower leg and upper foot area, or the area around the ankle joint (specifically on the top of the ankle joint and lower leg), then this is another sure sign that the Tendons that are responsible for the flexion motion are very irritated and inflamed, probably from overuse or improper use over-time.
***If an X-Ray isn't possible, or at least isn't possible right away, I would ask your Vet if they would put your bird on a daily dose of Metacam/Meloxicam or other similar Anti-Inflammatory Drug (not a narcotic pain-killer or Corticosteroid), and then simply watch your bird's leg and see if that flexion starts to subside over-time...If so, then you have your answer, and your bird will also get some pretty immediate relief that even narcotic pain-meds won't help because they do nothing for inflammation.
***The "cure" for this issue with the Flexor-Tendons in that area is actually pretty simple and non-invasive, and is exactly the same as the "surgery" that people have done for Carpal Tunnel Syndrome in their wrists...I don't know if you've ever known someone who has severe and long-time Carpal Tunnel Syndrome, but in addition to the constant, dull, achy pain they usually experience in their wrist and hand, the other common symptom is a constant feeling of and/or actual contraction of their fingers into a fist. We all have an extremely large group of Tendons that run right through our wrists that control our hands and specifically our fingers and thumb movements. It's amazing to see, because we all have over a dozen different Tendons that run through our wrist joints, and because our wrists are not very large and there is not much space at all for all of these Tendons to run through, the sheath that is wrapped around all of them and encases all of them often becomes quite swollen with overuse or misuse. Over-time people can end-up with their fingers constantly curled-up into a fist, and if the overuse/misuse isn't stopped, eventually the Tendons become so compressed that they actually start to "bunch-up" inside of that sheath that encases all off them, and then they lose the ability to open their fingers back up, and they have a permanent fist all the time...And the surgery to fix this issue (in addition to stopping the action/misuse that is causing it in the first place, along with anti-inflammatory meds) is simply a 15 minute-long surgery where a very small incision about 2" long is made in the skin over the wrist joint, which exposes the sheath that encases all the Tendons...Then a very tiny incision is made in this sheath, and then a retractor is placed inside of this tiny incision, and then the retractor is used to spread-out/stretch-out the sheath, creating a much loser sheath and a lot more room for the Tendons to run through, and this instantly relieves the bunching-up and constriction of the Tendons. The incision in the sheath around the Tendons is left open, and the incision in the skin is sutured closed, and then the wrist/hand is put in a cast or a metal brace with the fingers all extended for about 4 weeks, and this usually is enough time to allow all of the swelling of the Tendons to go down, and it allows the sheath around the Tendons to stay permanently opened-up and stretched-out, creating ample room for the Tendons to run through...The whole thing takes about 15-20 minutes, and I got to do a bunch of them during my surgical internship during graduate school...
The exact same procedure is done on birds with constant-contraction of the Flexor Muscles to their toes, but only if a daily dose of anti-inflammatory drugs doesn't relieve the swelling causing the contraction of the Tendons...A lot of the time with birds the surgery isn't needed because the anti-inflammatory drugs are all that is needed to relieve the issue...The difference between birds and people is that people unfortunately tend to keep doing the motion/behavior that is causing the issue to begin with, such as typing on a computer keyboard every day in an improper position or with improper wrist support...With birds, they typically are smart enough to stop doing what causes them pain, lol...
So I would talk to your Vet ASAP about simply starting your bird on a low-dose of an anti-inflammatory drug, such as Metacam, once daily, and see if you see any change in your bird's foot and toe-contraction after a couple of weeks on the anti-inflammatory drugs...
