Many private health plans cap prosthesis coverage at $2,500 or $5,000 a year, or pay for just one device per limb in a lifetime. But now amputees and prosthetic-device makers are pushing state legislatures throughout the United States to pass laws that mandate prosthesis coverage. Prosthetic devices are among the biggest-ticket items affected by the effort to curb rising health costs by asking patients to pay a bigger percentage of their medical bills.