As the United States struggles to rein in its growing $2.7 trillion health care bill, the cost of medical devices like joint implants, pacemakers and artificial urinary valves offers a cautionary tale. Like many medical products or procedures, they cost far more in the United States than in many other developed countries. Makers of artificial implants — the biggest single cost of most joint replacement surgeries — have proved particularly adept at commanding inflated prices, according to health economists. Multiple intermediaries then mark up the charges.