FRESNO, Calif. --Dr. Stephen Tower is an orthopedic surgeon. When the arthritis in his hip became too hard to bear, he opted for a metal-on-metal hip. But the new hip, he says, caused him so much pain, exhaustion, and depression that he had to have it removed. The tissues around the hip had been destroyed, and metal debris was spreading down his thigh. Tower says normal friction in the artificial joint caused cobalt, a toxic metal, to be released into his body, leading to serious tissue, heart, brain, and thyroid problems.