The MRI machine and the CNC machine working in concert produce joints that are the perfect fit for a person, and it's that perfect fit that allows people to walk normally again, and the ability to give a person back their mobility is what drives up the demand for artificial joint implants.
If your doctor knows that the joint isn't going to work very well, he's not going to recommend it. He'll keep his arthritic patients on pain killers instead, so that they can at least walk, albeit with difficulty. But, if he expects his patients will be delighted with the artificial joint they get, he will recommend it, and there will be a very much increased demand for artificial joints and hence both MRI machines and CNC machines and the people that make and service them. And, of course, there will be a greater demand for orthopedic surgeons to put them in.
Economics is simple. You just have to be selling what people want, and technology makes that possible.