You need devices which can objectively measure the strength of something like an ankle in multiple vectors of motion. And then algorithms which can combine the data into a meaningful index.
That doesn't sound very difficult. You could measure the angle that someone could extend before feeling pain, or ensure the ankle could apply a certain amount of pressure before the patient feels pain, etc.
I doubt you would need to get the FDA involved at all.
There is basically no risk of harm from such a thing. If all you are doing is using it verify whether a surgery worked or not, then it's not actually a treatment is it? Surgeons make their own tools, jigs and tests all the time.
And if for some reason you had to, there are different grades of hard with a medical regulating body. For instance, it is super easy to develop medical tools. Harder again to do implants, harder again to do medicines.
So I want to reiterate. Not hard at all.
- source. Worked for a medical device company that did implants, wound care and medical tools. Surgeons would often ask for custom tooling or jigs through us that we would get made up for them.