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.

It would be very difficult to bring that to market as an FDA certified medical device. And then repeat the process for every other joint.

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.

I'm not sure if that was sarcastic or not, but you mean devices like hanging scales and string, and a protractor?

Ah, but it's a medically certified protractor! Disposable (for safety of course) and $500 a pop.

It seems to me that squat, deadlift, and an agility test would probably cover that.

well....get to work?