Adams, 51, lost his right leg to cancer 40 years ago, and he has worn out more legs than he can count. He picked a gold plan on the Colorado health insurance marketplace that covered prosthetics, including microprocessor-controlled knees like the one he has used for many years. That function adds stability and helps prevent falls.

But when his leg needed replacing last January after about five years of everyday use, his new marketplace health plan wouldn’t authorize it. The roughly $50,000 leg with the electronically controlled knee wasn’t medically necessary, the insurer said, even though Colorado law leaves that determination up to the patient’s doctor, and his has prescribed a version of that leg for many years, starting when he had employer-sponsored coverage.